Quantcast Figure 4 Insulation Resistance Tester (Typical).

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Figure 4
Insulation Resistance Tester (Typical).
a) When preparing to make an insulation-resistance test, first make a complete safety
check. This includes making certain that equipment to be tested is disconnected from all power
sources. All safety switches should be opened, and other control equipment locked out so that
the equipment cannot be accidentally energized.
b) If neutral or ground conductors must be disconnected, make sure they are not
carrying current and that, when disconnected, no other equipment will lack protection.
c) Observe the voltage rating of the tester and take suitable precautions.
d) Large equipment and cables usually have sufficient capacitance to store a
dangerous amount of energy from the test current. After taking resistance readings, discharge by
leaving the tester connected for at least 30 seconds before touching the leads.
e) Do not use the tester in an explosive atmosphere. An explosion may result if slight
sparking is encountered when attaching or removing test leads, or as a result of arcing through or
over defective insulation.
High-Resistance Fault Locator
a) The high-resistance fault locator, Figure 5 utilizes a modified wheatstone bridge
circuit in which the two sections of the faulted conductor (one on each side of the fault) comprise
the two external arms of the bridge. The remaining two arms of the bridge are contained in the
instrument. By using a detector circuit of extremely high input resistance, it is possible to locate
high-resistance faults. With this bridge arrangement, faults having resistances from 0 to 200
megohms can be located within an accuracy well within 0.5 percent. A typical error would be
6 inches (15 cm) in 500 feet (150 m) or 0.1 percent.


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