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- A source of electric current,
- A volt meter,
- A switching matrix used to connect the current source and the voltmeter to all of the contact points in a cable.
In addition to these parts a cable tester may also have a microcontroller and a display to automate the testing process and show the testing results.
A cable tester is used to verify that all of the intended connections exist and that there are no unintended connections in the cable being tested. When an intended connection is missing it is said to be "open". When an unintended connection exists it is said to be a "short" (a short circuit). If a connection "goes to the wrong place" it is said to be "miswired" (the connection has two faults: it is open to the correct contact and shorted to an incorrect contact).
Generally, the testing is done in two phases. The first phase, called the "opens test" makes sure each of the intended connections is good. The second phase, called the "shorts test" makes sure there are no unintended connections.
There are two common ways to test a connection:
- A continuity test. Current is passed down the connection. If there is current the connection is assumed to be good. This type of test can be done with a series combination of a battery (to provide the current) and a light bulb (that lights when there is a current).
- A resistance test. A known current is passed down the connection and the voltage that develops is measured. From the voltage and current the resistance of the connection can be calculated and compared to the expected value.
There are two common ways to test for a short:
- A low voltage test. A low power, low voltage source is connected between two conductors that should not be connected and the amount of current is measured. If there is no current the conductors are assumed to be well isolated.
- A high voltage test. Again a voltage source is connected but this time the voltage is of several hundred volts. The increased voltage will make the test more likely to find connections that are nearly shorted since the higher voltage will cause the insulation of nearly shorted wires to break down.
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