Current clamp
From Wikipedia, the free encyclopedia
In electrical and electronic engineering, a current clamp or current probe is an electrical device having two jaws which open to allow clamping around an electrical conductor. This allows the electrical current in the conductor to be measured, without having to make physical contact with it, or to disconnect it for insertion through the probe. Some types of current clamp are used to induce current in the conductor.
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[edit] Types of current clamp
[edit] Current transformer
The most common form of current clamp comprises a split ferrite ring. A wire coil is wound round one or both halves, forming the secondary winding of a current transformer. The conductor to be measured forms the primary. Like any transformer this type only works with AC or pulse waveforms, with some examples extending into the megahertz range.
This type may also be used to inject current into the conductor, for example in EMC susceptibility testing to induce an interference current. Usually, the injection probe is specifically designed for this purpose.
[edit] Iron vane
In the iron vane type, the magnetic flux in the core directly affects a moving iron vane, allowing both AC and DC to be measured, and gives a true RMS value for non-sinusoidal AC waveforms. Due to its physical size it is generally limited to power transmission frequencies up to around 100 Hz.
The vane is usually fixed directly to the display mechanism of an analogue (moving pointer) clamp meter.
[edit] Hall effect
The hall effect type is more sensitive and is able to measure both DC and AC, in some examples up to the kilohertz (thousands of hertz) range. This type is often used with oscilloscopes, and with high-end computerized digital multimeters.
[edit] Clamp meter
Pushing the large button at the bottom opens the lower jaw of the clamp, allowing the clamp to be placed around a conductor.
An electrical meter with integral current clamp is known as a clamp meter, clamp-on ammeter or tong tester.
In order to use a clamp meter, only one conductor is normally passed through the probe; if more than one conductor were to be passed through then the measurement would be a vector sum of the currents flowing in the conductors and could be very misleading depending on the phase relationship of the currents. In particular, if the clamp were to be closed around a mains extension or similar cord, no current will be measured at all as the current flowing in one direction will cancel that flowing in the other direction.
In practice, nearly all clamp meters are used by electricians, with the clamp incorporated into a general purpose multimeter. Such meters also often contain a mechanical pointer-locking device so that a current reading can be taken in locations where the meter pointer can't be seen, the pointer then locked, and the meter brought out to a convenient place for reading. For the meter shown in the picture, the white push-button marked "lock" provides this function.
Less expensive clamp meters use an average-detecting rectifier circuit calibrated to read in RMS units; assuming that the current is a sine wave of the local mains frequency - either 50 or 60 Hz. For other waveforms, incorrect readings may be obtained, so when such meters are used with non-sinusoidal loads such as the ballasts used with fluorescent lamps or high-intensity discharge lamps or most modern computer and electronic equipment, the readings produced by such meters can be quite inaccurate.
Typical handheld Hall effect units can read currents as low as 200mA, and units that can read down to 1mA are available.
The Columbia tong test ammeter, manufactured by Weschler Instruments, is an example of the iron vane type, used for measuring large AC currents up to 1000 amps. The iron jaws of the meter direct the magnetic field surrounding the conductor to an iron vane that is attached to the needle of the meter. The iron vane moves in proportion to the strength to the magnetic field and thus produces a meter indication proportional to the current. This type of ammeter can measure both AC and DC currents and provides a true RMS current measurement of non-sinusoidal or distorted AC waveforms. Interchangeable meter movements can be installed in the clamping assembly to provide various full-scale current values up to 1000 amperes. The iron vane is in a small cylinder that is inserted in a space at the hinged end of the clamp-on jaws. Several jaw sizes are available for clamping around large conductors and bus bars up to 4-1/2" (114mm) wide.
