Wikipedia:Reference desk/Archives/Science/2012 August 5

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August 5

botox and spoiled food

okay, let's say i a come across some improperly stored food that has become contaminated by botulism toxin. can i harvest it from the can of food and use it to reduce the appearance of wrinkles in my face.

note that the question is not SHOULD i do that but can i, in theory, do that? or is the kind you put in your face a *different* type of botulism. — Preceding unsigned comment added by 24.228.83.134 (talk) 01:36, 5 August 2012 (UTC)

There are seven different types of botulinum toxin, and the one used to reduce wrinkles (along with treating many conditions) is naturally found in improperly canned food. Someguy1221 (talk) 01:51, 5 August 2012 (UTC)

But as far as harvesting the toxin, the preparation you find in a cosmetic surgeon's office was purified from a bacterial culture, and not from spoiled food. Purifying it out of food would likely be extremely difficult, and purification is necessary to prevent allergic reactions that would result from injecting random crap under your skin. Someguy1221 (talk) 01:56, 5 August 2012 (UTC)

Organic group abbreviations

I can't seem to find any information (on WP nor on the wider Internet) about abbreviations for organic groups. Our article methyl group mentions that that group is often abbreviated as "Me". I see other abbreviations from time to time (for example, "Pr" for the propyl group). Is there any commonly-accepted list of these abbreviations, or are they simply ad-hoc shortenings? — This, that, and the other (talk) 03:11, 5 August 2012 (UTC)

We have a (probably incomplete) list at Symbol_(chemical_element)#Other_symbols_that_look_like_element_symbols. That list is sadly unreferenced, so I'm left with no clue as to whether there is an accepted standard or if people just make them up as desired. Someguy1221 (talk) 04:01, 5 August 2012 (UTC)

Thanks :) — This, that, and the other (talk) 05:40, 5 August 2012 (UTC)

Here is a list of commonly used abreviations. I don't know who the authority is on this though. IUPAC doesn't seem to have much to say on the topic. 112.215.36.172 (talk) 10:18, 5 August 2012 (UTC)

Please explain electricity consumer earthing

In the multiple earth neutral system Keit described in response to an earlier question, how many of these household ground rods with "several hundreds of ohms" resistance each are typically combined in parallel? Is it only to the neutral from the local transformer, or are the secondary neutrals of multiple transformers connected? Are metal cold water pipes not also used as grounds (granted some places use nonmetallic pipe, and the US lately seems to require a driven ground rod in addition to and water pipe ground))? Is a secondary neutral carried all the way back to the substation? Ive seen lots of installations (US) where there is a ground rod at the transformer and one at each house served by the transformer, without a "lighting secondary main" of phases and a neutral connected to other transformers along the line. It is all too common for there to be a fault in the transformer or external to it from the primary to a secondary conductor. If such a fault from the primary to secondary occurred and there were tenor 20 houses with 200 ohms ground resistance each, all the wiring in the houses would be elevated to quite a high voltage, until the primary fuse blew. Recent US code called for a supplemental ground if the first driven ground has over 25 ohms resistance. Common US practice is to bury the rod and the connection of the wire to it, making the "wiggle test" difficult. Also it is common for the connectors in the neutral line to develop a loose or high impedance condition, with normal load current having to flow to earth through the ground rod or water pipe at the house, until the problem is fixed. In a loose neutral condition, the remote grounds would not help much. Edison (talk) 20:17, 4 August 2012 (UTC).

As Edison's question requires an answer that covers several related topics that will be off-tack for the OP of the earlier question, I have moved his query to here - I hope Edison and everbody else does not mind. Edison has asked so many questions in his lengthy paragraph above that an answer must also be pretty long. So I intend to answer it all, but different parts of the answer will come over the next day or so - I do have lots of other commitments. As it appears that Edison has some misconceptions about what house/dwelling earth stakes/electrodes are supposed to do, and what they are not intended to do, I shall include an explanation on that, as well as answering Edison's specific questions & comments. Keit60.230.207.82 (talk) 04:11, 5 August 2012 (UTC)

I applaud your moving my questions to a separate topic and welcome your enlightenment on the topic of earthing practices. Grounding/earthing problems are a frequent root cause of power quality complaints. Edison (talk) 00:38, 6 August 2012 (UTC)

• The discussion below is divided according to the following topics:-

• Terminology
• Number of paralleled earths in a typical MEN area
• Connection of MEN neutral runs together
• Connection of utility water pipes in earthing
• Use of extra neutrals
• Risk to comsumers from HV faults
• Issues with burying earthing components
• Loss of supply neutral to consumer premises
• What consumer earths are intended to do
• What consumer earths are NOT intended to do
• Principal risk in HV faults - Earth Potemtial Rise & Step & Touch Hazard
• What Power Companies do about EPR & S&T
• Need for Power Coordination, what is Power Coordination
• What Power Companies do about Power Coordination - impact on earthing
• Summary / Conclusions.

• Terminology

As in any technical field, using the correct terminology aids considerable in understanding the topic. The following terms are used in the following text (text by Keit):

Multiple Earth Neutral (MEN) system: A system of local electricity distribution at Low Voltage comprising 4 conductors/wires - one for each of three phases (the "active" conductors) and a neutral conductor/wire that carries return current. The neutral is nominally at zero volts tension. At each metered customer connection, the neutral is earthed by a customer supplied earth electrode - a short (1.5 m typical) metal stake driven into the ground.

Common Multiple Earth Neutral (CMEN) system: The connection of the neutrals of more than one MEN system together.

Ground: That dirty stuff we walk on when outside in the sunshine. In USA it means something diffrent - see below.

Earth: A system of at least one electrode driven into the ground and it connecting wire(s) intended to sink electric current and thereby minimise the voltage on whatever is connected.

Great Body Of Earth: That big round thing all and all the animals live on, and used as a reference point by electrical Engineers. The great body of earth provides a very low electrical resistance between any two points - measurement and theoretical studies have shown that it provides 0.049 ohm/km (known as the Carson Resistance, named after an American telephone engineer who first proved its' magnitude) between any two points, regardless of geology. However connection must be made by earth electrodes, and due to the concentration of current near each earth electrode, the resistance at each electrode is very high and dependent on local soil resistivity unless multiple electrodes are driven to considerable depth. Great Body of Earth is not an officially recognised term, but I have used here to make the following text clearer for non-electrical engineer people.

Low Voltage (LV): A voltage used at power company customer premises and distributed in a MEN system. Low voltage is defined as over 32 V AC and below 600 V AC. Typically 115/120 V phase to neutral (North & South America, Japan) and 230/240V in almost all other countries.

High Voltage (HV): A voltage over 600V and equal or below 132 kV AC. In most of the world, HV is standardised at 6.6 kV, 11 kV, 22 kV, 66 kV, and 132 kV phase to phase.

Extra High Voltage (EHV): any voltage higher than 132 kV.

Transformer: A device that convert from one voltage to another. In particular, used to convert HV into LV for ditribution to customers

Transmission Line: An infrastructure of wires or cables used to carry large amounts of power at HV or EHV to large substations.

Feeder: An infrastructure of wires or cables used to carry electrical power at HV from a substation to another or to an trasnformer.

Ring Main Unit (RMU) A three way switch carrying HV. The power company can use it to isolate the incomming feeder, the outgoing feeder, or the local load as may be required to isolate faults or "deaden" a feeder or load so that linesman can make repairs.

Substation: A substation is a facility for isolating and connecting transmission lines , feeders, and transformers to meet operating requirments vis-a-vis changes in load and/or system faults. Substations can range from just an RMU plus a transformer, up to large installations covering thousands of square meters of space. Substations usually incorporate protection - automatic devices such as circuit breakers that isolate faults in order to minimise damage to power company infrastructure.

Earth Potential Rise: The local rise in ground voltage around an earth electrode system.

NOTE: USA terminology varies from the above. In particular, USA uses the term ground and grounded to mean, in this context, an electrical earth, and electrically earthed. The meaning varies with context, whereas the non-USA terms are constant in meaning - they do not vary with context. Unfortunately, due to the wide availability of American textbooks, the terms ground and grounded have crept in everywhere, causing some confusion and the need for more convoluted techical wording.

Keit124.182.53.95 (talk) 13:02, 7 August 2012 (UTC)

• In the multiple earth neutral (MEN) system, how many of these household ground rods are typically combined in parallel?

It varies considerably.

In accordance with standards enforced by the Authority, each property that has a consumer's meter (for electricity charging) must have ONE earth electrode. The electrode must provide a standard buried depth (which is not varied, regardless of soil conductivity or any other factor). The electrode is connected by a wire (known as the earthing conductor) to the meter box neutral bar (a copper bar to which all the house wiring neutrals are connected). The neutral bar has the neutral wire from the supply authority's street distribution connected to it. The supply authority/power company's neutral condutor running down the street thus connects all the house/consumer earth electrodes in parallel. The number connected in parallel in the MEN system may range up to several hundred or more.

The supply authority MUST ensure that the system is safe in all cicumstances. In a well developed street with a hundred or more consumer earths paralleled, there is no problem. However, in a new land development, the first house completed may, for a while, be the only house completed. There are some mitigating factors, but with only one functioning comsumer earth, the supply authority must install a good enough earth electrode system at the distribution feed point (transformer, transformer & ring main unit, or whatever), and may, if soil conditions are not favorable, install additional spaced earth electrodes, AND/OR, interconnect the neutral with that of an adjacent distribution area with a good number of installed consumer earths i.e., implement a CMEN. What is a "good enough" earth system? That is a complex issue of its' own. It is clear that Edison has some misconceptions. If you want to know more about this sub-topic, post a new question.

• Is a MEN only to the neutral from the local transformer, or are the secondary neutrals of multiple transformers connected?

Often, the neutral, and thus the earthing system, for a distribution (240V supply for a street or small area) is isolated from the neutral & earths of other areas. However, the supply authority/power company may elect to electrically tie the neutrals of two or more distribution areas together, either directly, or via the high voltage (6.6 kV and higher) neutral and earthing system. This is known as the CMEN system (Commoned Multiple Earth Neutral). Again, the reasons for tieing MEN systems together or not tieing them together are a complex subject on its own, but reasons for a CMEN may include: a) insuficient consumer earths in a particular MEN area, b) poor soil conductivity, c) high voltage system configuration, to name a few. There are both pros and cons, depending on circumstances, regarding implementing a CMEN or just having seperate MEN runs.

Keit60.230.207.82 (talk) 04:43, 5 August 2012 (UTC)

• Are metal cold water pipes not also used as grounds?

The basic answer is "No, but...". They have been used in the distant past, and it is possible that one could find an old house that still has as its consumer earth as metal water pipe.

Regardless of the presence of any "in ground" metal, an earth electrode of prescibed length MUST be provided. However, the principle is that the house earthing system must present the people in and around it with the lowest available voltage (i.e., closest to the potential of "great body of earth"). If fortuitously earthed metalwork is available, e.g., metal water pipe, gas pipe, the steel of a steel framed building etc, and due to high local soil resistivity such metal work could be a "better earth" than the prescibed length stake, and as such offer a lower voltage, then the house earth shall be electrically connected to this metal work. Rules on connecting to water pipes vary regionally, as it has some definite disadvantages - it can present a risk to plumbers and can "export" voltage to other properties/consumers - but in no case shall metal work be relied upon for electrical earthing. If a building is multi-storey or surmounts a hill top, it may be subject to lightning strikes as defined by relavent standards. In such cases, there will be lightning ariels (pointy rods on the roof that provide the most attractive place for a strike), down-conductors, and a lightning drain earth system. Lightning drain earthing systems will gnerally comprise multiple deep electrode systems to provide a specified low electrical impedance to "great body of earth" and as such far outperform any earth stake provided as part of the building electricity system. Again, you still have to provide the regulation prescribed short length electricity consumer earth electrode, and the lightning drain earth is connected to it (at one point only), and both earth systems must be clearly labelled as to which one they are. Keit121.215.143.169 (talk) 10:06, 5 August 2012 (UTC)

• Is a secondary neutral carried all the way back to the substation?

Essentially, No. There is only one neutral in principle. In open wire street distribution (i.e., unisulated wires strung on poles), there are four conductors comming from the transformer - one for each of the three phases, and the neutral wire. However, in underground distribution, or bundled aerial cable, the cable may be screened, and may be steel wire armoured. Some power authorities earth the armouring at one end only (this avoids heting due to current in the armour and thus maximises the current carrying capacity of the phase conductors. Others earth the armour at all connection points - experience has shown this to minimise lightning-caused faults and is becoming standard practice with all administrations.

In high voltage transmission lines, there is normally no neutral wire at all, but there will be an earth conductor of some sort. In open wire practice, there is normally an earthed wire above the phase wires to provide a more advantaguous place for lightning to strike, and for power cordination purposes. In HV underground transmission, each phase conductor will be surrounded by a coaxial copper screen, and the whole surrounded by steel wire armour. The screens and armour will be earthed at both ends, and thus will provide a return path for current. Mutual inductance between the phase conductors and the screens and armour will force most of the return current to go in the screens and armour and not the great body of earth, regardless of the relative impedances.

See discussion on CMEN above.

Keit121.215.143.169 (talk) 10:39, 5 August 2012 (UTC)

• It is all too common for there to be a fault in the transformer or external to it from the primary to a secondary conductor. If such a fault from the primary to secondary occurred and there were ten or 20 houses with 200 ohms ground resistance each, would not all the wiring in the houses be elevated to quite a high voltage, until the primary fuse blew?

Such primary to secondary faults certainly do occur. However, in a correctly implemented MEN (or CMEN) system there is no dangerous rise in voltage in the houses etc. The MEN consumer earth system is not intended to deal with such faults and quite normally will not be capable of sinking the current. It would be prohibitive in cost to make consumer's earths capable of sinking HV-sourced fault current - especially in many areas of Australia, where the ground is essentially dry sand and/or rock to great depths and the electrical resistivity very high. Power authorities/companies provide other means for managing and sinking fault currents originating in the primary/HV side. I will explain this in another paragraph or 2, or 3.

Keit121.215.143.169 (talk) 11:20, 5 August 2012 (UTC)

• Common US practice is to bury the rod and the connection of the wire to it, making the "wiggle test" difficult.

In my answer to the OP in Ref Desk http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Science#Earthing, I intentionally omitted some factors he didn't need to know.

At one time in Australia, for instance, it was sufficient, in terms of regulations, to drive the house earth stake into the ground near the meter box, and attach a green earth wire of the prescribed gauge, one end to the neutral bar in tehg meter box, and the other end to the earth stake with a clamp. As part of their electricity supply contract with the power company, consumers/householders were required to maintain the wire, clamp and stake in good order and condition (not actually themselves - they should hire an electrician when required). Installing electricians commonly left some excess lenghth in the wire, coiled up, to facilitate re-terminating should it become necessary. However, because the wires were exposed, they tended to occaisonally get damaged. I myself once stripped one such earth wire right off accidentally while operating a large lawnmower. Most folk would eventually notice damage and call an electrician, but some might not. Current regulations require the wire and its attachment to be "protected against damage from reasonably expected events" - this means running the entire wire length in conduit. Commonly, nowadays, the entire setup is buried, but in this case there must be a small pit housing the top of the stake and its' connection, so that it can be easily located and checked. In such cases, the wrigle test is still possible. The lids of the pits are inscribed "MAIN EARTH". Any other earth present, such as a lightning drain earth, ham radio earth, etc must NOT be labelled "MAIN EARTH". Completely hiding it by burying it in soil without a pit is very bad practice and not acceptable. It must be possible to visually inspect it. Keit124.182.149.216 (talk) 12:23, 5 August 2012 (UTC)

• It is common for the connectors in the neutral line to develop a loose or high impedance condition, with normal load current having to flow to earth through the ground rod or water pipe at the house, until the problem is fixed.

Edison's claim that this is common is very surprising - it would only be common if installation practices are very poor and/or the installing electricians incompetent. Never the less, such a fault is obviously possible.

However, perhaps conter-intuitively, a relatively high house earth is actually safer. Remember that the purpose of a house earth (consumer's earth) is to ensure that appliance metal work is at the lowest available voltage, and NOT to ensure the actual voltage is minimal. It matters not a whit if the appliance user, touching exposed metal of the appliance, is at (say) 100 V above true earth, so long as he can't touch anything at a lower voltage. See other paragraphs on this. However, if the house earth, and therefore the house neutral, is, say, 50V above true earth due to high earth system resistance, then that is 50V less to run the appliances. Lamps will be very dim and flicker badly, and power appliances and electronics will not work properly, if at all. This will make the householder immediately contact an electrician to get it fixed. If a house earth was good enough to sink the load current and keep the neutral voltage low, then lamps and appliances will still work ok - so the householder in ignorance will not call an electrician. The open or high impedance neutral connection to the street wiring/cabling may become a residual fire risk. Keit120.145.32.129 (talk) 14:16, 5 August 2012 (UTC)

. Please note that the correct spelling is "neutral" not "nuetral." In the US, at least, overhead or underground connections from a transformer or a lighting secondary main are typically made by crimped connections, which can develop a higher than desired resistance over time. This may be in a phase or a neutral. A "loose neutral" has undesired effects, such as the side of 120/240 distribution with a high load having a low voltage while the other side has a high voltage. It is painfully common. Edison (talk) 00:36, 6 August 2012 (UTC)

I have corrected the spelling. Crimped connections are used here in Australia too. However, connection trouble is very rare. I have been in the electronics and electrical game for many years and have never experienced it, only heard about it. Of course, as we use double the voltage, the effect of any connection resistance will be a lot less. For instance, a 1 ohm resistance in a circuit feeding a 240 V 1 kW resistive load will drop the load voltage by 2% - hardly noticeable. In a 120 V 1 kW circuit, the load voltage will drop 9.4%. Even so, I think you must be mis-informed, either that or crimping practices in your area are poor. The practice of splitting a phase into two, with 120 V (one side earthed) used for lighting and domestic appliances, and 240 V (balanced to earth) for heavy draw items and industrial equipment is pecular to the USA and certain other "110/120 V" countries. We use 240V (single phase) and 415 V (3 phase) for all consumer equipment. However a dropped neutral will still result in a similar problem at reduced degree - a heavy load on one phase will increase the voltage on the other 2 phases. Keit58.169.250.192 (talk) 02:12, 6 August 2012 (UTC)

Texts on power quality and my own utility experience negate your claims I am "misinformed" with regard to the incidence of high resistance connections in the conductors from the utility to the consumer, and I doubt that workers are peculiarly careless in North America. Edison (talk) 03:43, 7 August 2012 (UTC)

Where I have used the term "misinformed", I meant in connection with what you appear to believe is the function and performance of the various sorts of earthing used. However I do find your claim that supply neutrals are commonly high resistance or open surprising. It certainly is a rare problem in Australia. I too doubt that elecrical workers are especially careless in North America - in fact over the years the USA has, among professionals in the electricity supply field, a reputation of having a safe and reliable system - at least until the US Govt started interfering with pricing policy and wholesale regulation. So perhaps you are misinformed or have misunderstood what you have seen. Just what is your utility experince? Keit124.182.53.95 (talk) 13:19, 7 August 2012 (UTC)

What the consumer earths in an MEN (or CMEN) system are intended to do:

The consumer earths are principally there to ensure that a person who is holding or touching exposed metal-work of an appliance, and thereby has his body at the neutral voltage, is at a voltage low enough in comparison to any other conductive thing (water tap, damp floor, gas stove, or whatever) he/she could simulataneously touch, that he/she will not percieve an electric shock and is not at risk of electrocution.

An example should make this clear:- Let us say that, due to a certain resistance in the supply authorities earthing system, together with the load current on the street distribution or fault condition existing in the absence of consumer earths, an appliance metal work is at 35 V with respect to the great body of earth. If the consumer (in bare feet perhaps) is standing on a bare damp concrete floor, or is simultaneously touching a gas-main fed oven, then he will get a shock, as the floor or oven will be at zero potential. There is in this case 35 V potential difference across him.

Now, assume each house has its earth stake. If the soil in the area is low in resistivity, small currents will flow to great body of earth via these stakes, thereby forcing the neutral voltage down - making it safe.

Let's now say the area is like many areas in Australia, essentially a thin layer of top soil, and under that dry sand to a depth of 30 m or more, overlaying bedrock. In such a case, the geology is high resistivity and even many parallled consumer earths won't do a terrific job of bringing down the neutral voltage. The consumers, though, are still safe - why? Because everything conductive they can touch is either fully insulated, or is in contact with topsoil, which has all these earth stakes poked into it. If the nuetral is 35 V above true earth, then everything else is nearly the same. If everything the consumer can touch is at the same 35 V, 100V, or even an impossible 350 V, it doesn't matter. There is no potential diffrence across him and he can't get a shock.

Keit58.169.250.192 (talk) 02:59, 6 August 2012 (UTC)

What the consumer earths in an MEN (or CMEN) system are NOT intended to do:

1. Consumer earths are not intended to carry significant current. For both normal loads and fault conditions in street distribution, house wiring, and appliances, the current in the earth stakes is very low. The bulk of load and fault currents is carried back to the transformer in the supply authority/power company's neutral wire. This is only partially dependent on the electrical resistance of stakes to great body of earth vis-a-vis the lower resistance of the neutral. The return current is largely forced into the neutral by mutual inductance between the active (phase) conductors and the neutral. Essentially, this means that the active wire, being very close to and physically in parallel with the neutral, forms a transformer with the neutral, so that whatever current flows in the active wire causes a more or less equal return current in the neutral. In general, a current of more than a few milliamps in the earth stake is an indicator of incorrect or faulty installation.

2. Consumer earths are not intended to sink current due to faults to the High Voltage side of the transformer. Such fault currents are up to 3000 Amps, and to expect any consumer to pay for an earth to handle that would be utterly prohibitive. The power company must provide other means to manage and sink HV fault currents - this will be described below. In practice, the paralleled consumer earths in a well developed MEN or CMEN area can assist in sinking fault currents arising from HV to LV breakdown.

3. Consumer earths are not intended to sink significant load current should the supply neutral be high resistance or broken/open circuit. To provide consumer earths do so would in many, if not most, areas would be expensive, and would mask a neutral fault. With the prescribed single short earth stake, failure of the supply neutral will result in lights and appliances not working properly, and, usually, not working at all, resulting in the consumer/householder urgently seeking an electrician, as mentioned elsewhere. Keit60.230.199.55 (talk) 05:35, 6 August 2012 (UTC)

Principal risk in fault currents from High Voltage transmission lines & feeders - Earth Potential Rise

Each MEN area providing low voltage (115/120/230/240 V) to consumers is feed via a step down transformer from a high voltage (between 6 kV and 132 kV) line or feeder. The principal interest here with faults on HV lines or faults in substations/RMU's/transformers fed from HV is what is called "Step and Touch hazard". When there is a HV fault causing a current to flow to earth via the substation/RMU/transformer earth electrodes, the current leaves the electrode(s), passing through the soil, locally radiating out in all directions, prior to finally heading downwards into the great body of earth. The soil has an electrical resistance - this means that the soil current causes the earth electrode(s) to raise in voltage above the deep earth level. As the current leaves the electrode(s) and spreads out to lower and lower current density, on the surface of the ground there is a contour of decreasing voltage.

A given electrode might (say) be raised to 1200 V above deep earth level, and at 1 meter away the soil surface be at 1000V, at 2 m away 500 V, and so on, until at a large distance the voltage is negligible. This is called Earth Potential Rise. Due to the high voltage available on HV lines, the current into earth electrodes can be substantial, and so Earth Potential Rise can be substantial.

In my example just given, a person standing with one foot 2 m from the electrode, and the other foot 1 m away from the electrode, would have a voltage difference between his feet of 1000 - 500 ie 500 V. Unless he has dry shoes with very good insulation properties, that 500 V difference, known as the step voltage, will result in hazardous current to flow thru his body and may very well kill him.

In my example above, let's say the substation has a metal/wire fence around it, or a brick or concrete wall around it. Brick and concrete must be regardled as conductive. Due to the voltage contour on the ground/soil surface, if a person touches the fence or wall, his hand will be at a higher voltage with respect to his feet - this is known as "Touch Voltage". Touch voltage can be more dangerous than an equal step voltage.

When Engineers design a substation, RMU, or transformer installation, they calculate the gound surface contour of voltage, and calculate the step and touch voltages, and then take action to ensure the step and touch voltages are within safe limits (which assume a 2m high human) regardles of worst case fault conditions.

For any given substation/RMU/transformer. one can draw a more or less circular line, on the ground surface, around the substation/RMU/transformer which is the closest distance a human can safely stand. For a small street trnansformer of RMU, the safe distance can be zero. For a major substation fed from a major HV or EHV transmission line, it can be 50 m or more.

Earth potential rise in LV (120/240 V) distribution due to faults to an LV conductor are a non-issue as the available voltage is not high enough to hurt anyone before a circuit breaker or other protection trips.

Keit124.182.38.215 (talk) 11:34, 6 August 2012 (UTC)

What power companies do in order to reduce Earth Potential Rise & hazardous Step & Touch voltages

There are a multitude of methods, all with their own pros & cons, but the main methods are:-

(a) Reducing the HV fault current by inserting an impedance in the earth connection at the source end of the HV line/feeder - either a resistance or inductance.

(b) In underground HV cabling, utilising the screens and steel wire armouring to carry the return current. As stated elsewhere, mutual inductance between the phase conductors and their screens and steel wire armour forces most fault current to return via the screens and steel wire armour. The ferromagnetic properties of the steel wire armour usefully enhances the mutual inductance effect. Open wire (ie wires slung on poles or towers) trasnmission can use the lighning protection conductor or power coordination conductor for this purpose.

(c) Install an effective earth electrode system at the substation/RMU/transformer to sink the fault current not returned in the screens & armouring. This "left over" current can be hundreds of amps or more, requiring a resistance to earth as low as an ohm or even less. so such earth systems can be very substantial, comprising multiple deep driven electrodes. I have acted as consulting Engineer on projects where substation earthing has cost over $1 million - much more than the substation hardware.

(d) Install a fence or wall at a safe distance, so that humans cannot approach within a Step & Touch hazardous area.

Common industry practice when planning a new HV route is to calculate the phase conductor current under worst case fault conditions (a Phase to neutral short in the substation/RMU/transformer). The maximum fault current in HV lines, due to protection coordination (ie ensuring that any circuit breaker trip due to a fault is the circuit breaker that causes power to be cut only in the system portion whewre the fault is) requirements, is of the order 18,000 Amps. However, normally, if it is 3000 Amps or less, fine. If over 3000 A, take action to reduce it to 3000 A, such as Method (a) above. Then, calculate what portion of that 3000 A or whatever it may be, will flow into the substation earth system, worst case. Then, use a suitable (ie, is economic, fits on the site, etc) combination of methods (b), (c), and (d) to get Step and Touch safety. Keit120.145.61.75 (talk) 02:42, 7 August 2012 (UTC)

Gosh, Keit, are you going for a record? So far you have used 8 different IP addresses in answering one question. Are you on the road? Edison (talk) 03:52, 7 August 2012 (UTC)

I've probably set a record as far as length of answer goes. But you asked a good question, and a good question deserves a good answer. The reason while my IP address keeps changing is 2 reasons: 1. My ISP uses dynamic IP - that is, they allocate an IP from a pool each time my PC boots up &/or connects. 2. I am developing certain software - when I want to test it, I disconnect from the public network and re-connect on a private network - otherwise via bug I could be a right pain to everyone. Sometimes, due to other software I've been testing, my PC crashes and has to be rebooted - dang, another different IP address. Keit124.182.16.69 (talk) 05:42, 7 August 2012 (UTC)

Need for Power Coordination

Energy from electricity infrastructure, especially HV and EHV transmission lines, can be trasnfer to other nearby conductors, including telephone company cabling, metal water pipes, metal sewage pipes, town gas pipes, railway lines, and even, in some cases, the steel structures of large area commercial buildings. Energy gets trasnfered in the following ways: If (say) a HV power company line, and a telephone company cable run physically parallel along a street for some distance, there will be mutual inductance between the two - in effect they form an elongated transformer - fortunately not a very efficient one. Fault currents due to substation short circuits and the like can, if the Engineers did not address the issue, cause voltages in the telephone cabling (which normally carry speech signals measured in mV, plus low voltage DC to power the phones) that can be lethal to telephone company linesmen & technicians. As insulation on telephones is designed to a reasonable price and appearance, bad cases can even be lethal to telephone users. Similarly, HV fault currents can casue voltages in pipes etc lethal to plumbers. As well as this, and even if lethal conditions do not exist, normal load currents in EHV, HV, and LV power company lines can cause "hum" in telephone company cables.

Earth potential rise in a substation can in some cases cause power frequency energy to get into the earthing system of a nearby telephone exchange (via the electricity supply cabling to the exchange - a large exchange may be fed with HV direct from the substation, for essential service reliability) - this can also cause hazardous voltages for both telephone company technicians and telephone company customers, because the phone cables radiating out from the exchange "export" the earth potential rise to all phone customer's premises. ("Exporting" of earth potential rise is one reason why in most countries it is not permitted to supply AC utility power to another metered property - e.g., an electicity customer can use an extention cord to power tools etc in his own yard, but may not use an extention cord to power devices in a neighbour's yard)

There are things power companies can do to remove the problem, or inadvertantly make it worse, and things that telephone, water and sewage, and railway companies can do to remove the problem, or make it worse. Thus there is a need for power companies and the other utilities to co-operate and co-ordinate their activities - this is called Power Coordination - and is a highly specialised branch of electrical engineering, and very rewarding, as I can tell you from personal experience. Keit124.182.16.69 (talk) 07:18, 7 August 2012 (UTC)

What power companies do as part of Power Coordination

The best and cheapest power codination is done at the planning stages - Since a problem arises by running power company lines and telephone/water/gas lines right beside each other for long distances down the same street, then try not to do it - or at least put the power company stuff on one side of the street, and the other ultilities on the other side. However, this is not always possible, and not always cost effective (e.g., if there is only one road/access to a community). There are many ways of solving the problem - below are the most common methods.

The power company can install a "shield conductor", earthed at each end, on its power line poles/towers, or in the same trench as its' power cables. Mutual inductance wil cause a current to flow in this extra conductor, and the magnetic field from this extra conductor will cancel out most of the field from the other conductors - in effect it acts as a shield for the other utilities. Shield conductors also, by carrying return current assist in mitigating earth potential rise during HV faults. The other utilities can also choose to install shield conductors, however this would be unusual.

It is possible that a substation/RMU/trasnformer earth system, although good enough to ensure Step and Touch is not a hazard, from a Power Coordination point of view, there is still a problem. If so, either the power company can install an extra low-resistance substation/RMU/transformer earthing system, or the other utilities can install earthing. Telephone companies often install enhanced earthing systems for power coordination purposes. It depends on who was first in the street (and who has the best negotiators).

Summary/conclusions

Hopefully, Edison has managed to wade thru all this, and it is of help/interest to Edison and other Ref Desk readers.

Hopefully, from all the above, it has become clear that:-

• The MEN and CMEN systems provide a sound, safe, cost effiective. electricity distribution that caters for all types of geology.
• The earth system at customer premises, need only, and should be, a prescribed short length single electrode - these electrodes carry only very small currents under both normal load and fault conditions. Their purpose is only to ensure that consumers touching exposed metal are thereby at the lowest available voltage at the site.
• The earth systems at power company substations/RMUs/transformers are designed to sink large fault currents and are intended to mitigate HV fault Step and Touch hazards and solve power coordination issues.
• Substation/RMU/transformer earthing systems are therefore typically substantial, and are individually on a case by case basis designed for specific perfomance ie a specified low resistance to great body of earth.

I have written all the above from a working knowlege of Australian standards, and practices, in particular AS 3000 Electrical Installations (formerly known as The Wiring Rules), EG-1 Substation Earthing Guide, and WA Electrical Requirements. Other standards apply. European standards are similar. The corresponding publications in the USA that cover some of the topics include the National Electrical Code (NEC), and IEEE STD-80 Guide For Safety in AC Substation Grounding. USA practice varies in detail. I have minimal knowlege of US practice although the technical libraies of employers I have worked/consulted for have copies of the NEC in their libraries. I do know that US standards can be seen as not as tight safety-wise as European and Australian standards. But it must be noted that: a) Large scale distribution of AC power occurred earlier in the USA. Those who come later can avoid early mistakes before it's too late to change. b) It's a lot harder to kill yourself with 120 V than it is with 240 V. However, basic fundamentals are just that - fundamental - they apply to everybody. So I think Edison was misinformed on some aspects, or perhaps misinterpreted local practice.

• Enjoy! Keit124.178.169.148 (talk) 08:36, 7 August 2012 (UTC)

Various minor typing errors and the like fixed. Keit120.145.72.208 (talk) 10:07, 8 August 2012 (UTC)

science/physics

Q1)an object of mass 1kg is tied with a 2m thread and it is rotated by velocity 5 m/s.calculate the centripetal force?

Q2)an object is having velocity 5m/s in east direction,now it turns to the north direction with a same speed and it takes 10sec.calculate the centripetal acceleration?

Q3)a rocket launcher launches a rocket of mass point 325 ton with velocity 50m/s and the launcher experiences a instant velocity of 3.25m/s.derive the mass of the launcher?

Q4)a bullet and a gun having velocity after shooting 500m/s and 8m/s respectively.derive the ratios of the masses?

Q5)two object of same mass,the ratio of there velocity is 1:3and ratioof there rotating radius is 3:1.derive the ratio of centripetal force? — Preceding unsigned comment added by Ekknoorkaur (talk • contribs) 04:37, 5 August 2012 (UTC)

Q6)Will we do your homework for you? I'll answer Q6. No. Looie496 (talk) 04:56, 5 August 2012 (UTC)

Genetically Modified Corn Cell

Ok so, the process of gene splicing has been used to create a recombinant plasmid. Then, this recombinant plasmid has been successfully inserted into a corn cell via a transformation method. From there, how does this genetically modified cell affect or create an entirely new crop, to carry out the newly desired functions? Any help would be GREATLY appreciated!! 220.233.20.37 (talk) 07:13, 5 August 2012 (UTC)

Presumably they don't just put it into any old corn cell, but into a reproductive cell. The corn plants produced from this reproductive cell would then contain that gene, and they would then pass it on to at least some of their offspring. As to how a gene changes the cell function, it's often accomplished by coding proteins, which then do the actual work. StuRat (talk) 07:51, 5 August 2012 (UTC)

Looking at [1], the popular methods seem to involve carrying out any of several transformation protocols on entire suspended embryos, at a very young stage. Someguy1221 (talk) 09:17, 5 August 2012 (UTC)

In theory, one can use any plant tissue [2] as plant cells can regress to regain their totipotentcy and from there can form a callus than roots and shoots and so on. That you don't need an embryo to genetically engineer in important as many crops produce worthless seed, ex Granny Smith apple trees are all descendant from a single branch through cuttings as their genetics are so heterozygous that any sexually derived offspring would be quite different from the parents. 65.95.22.16 (talk) 13:32, 5 August 2012 (UTC)

Grains like commercial wheat and corn seed are created differently from apples, though. There are intentionally out-bred. See hybrid vigor and Hybrid_(biology)#Hybrid_plants. (although neither is a very good explanation). Rmhermen (talk) 17:01, 5 August 2012 (UTC)

As 65.95.22.16 indicates, the transformation isn't usually done on reproductive cells, but rather on vegetative tissue or plant tissue culture callus. You then typically propagate the cells in cell culture (usually under selection so that only the transformed cells grow). At a certain point you can treat the callus with hormones which causes it to sprout roots and leaves, at which point you can transfer it to soil, grow it up, pollinate and collect seeds like a regular plant. This is for most plants. Occasionally you'll find other techniques, e.g. for the model organism Arabidopsis thaliana the easiest transformation method is in planta Agrobacterium transformation, where you basically dunk the whole flowering plants in the transformation solution and some of the seeds they then produce are transgenic. -- 71.35.119.233 (talk) 17:33, 5 August 2012 (UTC)

Priority of Köppen classification

My question is about the Köppen climate classification system. I am trying to determine the priority of calculation. For example, if an area is very dry and also very warm, it may well satisfy the conditions of being in Group A and Group B. The Sahara is mostly classified as Group B, hence B > A. Places in Antarctica, though, could be dry enough to satisfy Group B, but are classified Group E, hence E > B > A. Is this true? And where do C and D fit in? Thelb4^(talk) 16:23, 5 August 2012 (UTC)

It's not a question of priority. The groups are, at least in principle, non-overlapping. An area that is very dry (for the whole year) can't be in group A. It can be extremely dry for part of the year, as with savanna, but then it would have to be wet for another part of the year. Generally speaking A is hot and wet; B is dry all year and not too cold; C is temperate and not too dry; D is cold; E is polar. Looie496 (talk) 17:40, 5 August 2012 (UTC)

Because this will be archived, let me recalibrate that a bit. The basic point is valid, but here is a better description of the categories: A is hot and humid; B is dry (whether hot or cold); C is humid temperate; D is cold and humid; E is polar. Looie496 (talk) 15:36, 6 August 2012 (UTC)

HERBS in lab

hello Can you please explain the difference between oil and extract? Is oil a kind of extract? which one is the ethanolic yield from the seed? Thanks Simagoulou (talk) 18:43, 5 August 2012 (UTC)

An oil is usually taken to mean a lipid that is in the liquid phase, or more broadly any hydrophobic liquid. An extract is, IIRC, an alcohol soluble scent or flavoring element from a food product. Many extracts contain what are called essential oils, which are usually the primary aroma components. To state it another way, the "extract" is all of the stuff you get when you soak the plant in alcohol. The "essential oils" are specific compounds which provide the scents for a foodstuff. Most extracts contain a mixture of essential oils, as many plants contain more than one specific essential oil. For example, many mint plants contain menthol, but they also contain other essential oils that give each a distinct flavor, which is why peppermint is not identical in flavor to spearmint, and thus while both extracts will contain a lot of menthol, they will also contain different things as well. --Jayron32 18:55, 5 August 2012 (UTC)

Are all extracts obtained by soaking in alcohol ? If so, I'd expect the result to be mostly alcohol. Do they then remove the alcohol in some way ? StuRat (talk) 19:12, 5 August 2012 (UTC)

Our article about Extracts mentions that some are used as the alcohol (or similar-solvent) solution whereas others are produced by other processes or somehow lead to the "pure" essence rather than a solution. Some of the terminology in this field is a bit convoluted (or not quite matching the standard modern scientific meanings) because the terms were used in this field well before there was a more specific understanding of the chemistry involved. DMacks (talk) 19:25, 5 August 2012 (UTC)

As DMacks notes, the terminology is a bit fuzzy, but usually when you buy something at the grocery store called "XXX extract" whether it is vanilla extract, peppermint extract, almond extract, etc. it is an alcohol extract, and yes it is more alcohol than essential oils. You can make water-based extracts as well, I suppose; Tabasco Sauce is basically a water extract of chili peppers. However, essential oils are strong juju; pure essential oils are rediculously pungent to the point of being unusable in pure form (from the pepper example, capsaicin, the essential oil of chili peppers, will blister your skin in pure form). Things get whacky when you start dealing with extracts and essential oils in pseudoscientific "new age medicine" bullshit like aeromatherapy and stuff like that; since they're just making it up as they go along the terminology gets contaminated from those fields. --Jayron32 19:34, 5 August 2012 (UTC)

The methods of extraction of essential oils from plant matter vary: steam distillation, or solvent extraction (where the solvent is removed prior to use), or use of carbon dioxide to extract the oils from the plant matter. Jayron, if you don't think essential oils work, then please don't use menthol or Olbas oil next time you have a cold: or peppermint oil if you have an acid stomach (Colpermin to give it its OTC name): or Friar's Balsam for a bad chest. Obviously none of these remedies work because, well, it's all pseudoscience isn't it. Actually the scientific study of essential oils is growing year by year. --TammyMoet (talk) 20:11, 5 August 2012 (UTC)

The issue is not with compounds that have medically verified theraputic effects. I am quite sure that many compounds which are essential oils also have theraputic effects. The issue is that, in the popular media, the real science is contaminated with bullshit. I'm all for double-blind placebo-controlled medical studies which show theraputic effects of any compound. That's fine. I use many compounds which are extracted from plants, and remedies which have a basis in such compounds. But the fact that there is good science sadly doesn't make the bullshit disappear. After all, my local pharmacy carries homeopathic remedies on the same shelves as the actual medicine. As long as that bullshit continues then we haven't won the war... --Jayron32 04:34, 6 August 2012 (UTC)

The usual result of pseudoscience is a core of products which actually work (mints are proven to have anesthetic properties, for example), accompanied by unsubstantiated claims far what can be proven. StuRat (talk) 20:17, 5 August 2012 (UTC)

Austrian window-box plant; ID, please

Could someone ID this plant, please: http://twitpic.com/afxhtc It was seen in many window-boxes in the Salzburg area of Austria in the last couple of weeks. The flowers are about half/ three-quarters of an inch in diameter. Andy Mabbett (Pigsonthewing); Talk to Andy; Andy's edits 21:04, 5 August 2012 (UTC)

Perhaps a Sanvitalia? Andy Mabbett (Pigsonthewing); Talk to Andy; Andy's edits 21:34, 5 August 2012 (UTC)

Or a Doronicum? Andy Mabbett (Pigsonthewing); Talk to Andy; Andy's edits 21:48, 5 August 2012 (UTC)

Sanvitalia seems correct. Likely Sanvitalia procumbens, which is widely cultivated and seems to have dozens of cultivars.-- OBSIDIAN†SOUL 09:45, 6 August 2012 (UTC)

Can mosquitos breed indoors ?

I have a spare bathroom I only use for the shower, and it tends to stay humid in there. Over the course of a week I killed a dozen mosquitoes in the bathroom, and only a couple in the rest of the house. Could they be breeding down the (rarely used) sink drain or in the toilet bowl ? I wouldn't think there would be any food for the larva there, but I might have rinsed out a bowl in there and left some food residue in the drain. I flushed the toilet several times, and ran the water in the sink for a bit, then poured bleach in both, and haven't had the problem since. Any thoughts ? StuRat (talk) 23:49, 5 August 2012 (UTC)

Googling suggests mosquito larvae in unused toilet bowls is a relatively common problem. E.g. Scrub all the fixtures with bleach. 70.59.11.32 (talk) 00:07, 6 August 2012 (UTC)

It doesn't take much. Mosquito larvae can make do on nutritionally weak food sources and, in some species, subsist entirely on algae. I wouldn't be surprised if the scummy water left in a drain might harbor enough bacteria to bring a handful of larvae to adulthood. Someguy1221 (talk) 05:13, 6 August 2012 (UTC)

Thanks. I was wondering how else they could all have gotten in. StuRat (talk) 19:17, 6 August 2012 (UTC)

Resolved