Quantcast Clamp-on Ammeter

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b) Due to the high sensitivity of this test set, a balance can often be obtained with a
good conductor. (The fault location will be indicated as the center point of the conductor). Such
a balance would be due to normal cable leakage current and would result in a reading of
approximately 50 percent in a cable of uniform insulation quality at a uniform temperature. For
this reason, the existence of a fault should be established by insulation resistance measurements
before attempting to determine actual location of the fault.
Safety. Before attempting to make any connections, make sure that all exposed cables
are de-energized.
Figure 5
Typical High-Resistance Fault Locator
Clamp-on Ammeter
General. The true RMS (root mean squared) ammeters (true RMS) shown in Figure
6measure alternating current. Some models are provided with plug-in leads to permit the
instrument to be used as a voltmeter or as an ohmmeter. When checking current, use a current
clamp probe, as shown in Figure 6 The ammeter is the airfield electrician's most important tool,
and should be a true RMS ammeter. Other ammeters (averaging and peak indicating) are
inadequate for airport lighting use. Averaging and peak indicating ammeters will not measure
the non-sinusoidal waveforms correctly and will indicate current levels below actual current
levels. Only true RMS ammeters are capable of reading non-sinusoidal waveforms that are
present on constant current regulator outputs and airfield load circuits. The typical true RMS
ammeter used by airfield electricians consists of a true RMS multi-meter connected to a current
clamp accessory. The current clamp accessory allows current measurement without interrupting
or directly coming in contact with the circuit being measured. Electricians should avoid "Hall
Effect" current clamp accessories.


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