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 indicator. It has a maximum indication of 50,000 megohms to infinity and is available in
different voltage ratings, usually 500, 1000, or 2500 V. Performance of an insulation resistance
test on electrical insulation (prior to placing equipment in service or during routine maintenance)
will provide a good indication as to the condition of the insulation. Wet insulation can be
detected very readily. A high reading, however, does not necessarily mean that equipment can
withstand the rated potential since the instrument does not apply rated potential. A
high-potential test, in addition to the resistance test, may be desirable but not practicable. In lieu
of a high-potential test, a dielectric-absorption test, using a megohmmeter, may be performed.
Recording and plotting periodic resistance readings will show trends and may often predict
insulation failure.
5.2.9.3 Ground Ohmmeter. A ground ohmmeter measures the resistance to earth of
ground electrodes. Calibration is usually zero to 500 ohms. Some types of ground ohmmeters
also include provision for measurement of soil resistance.
5.2.9.4 Low-Resistance Ohmmeter. Low-resistance ohmmeters are available for small
resistance measurements such as electric conductors, joints, contact surfaces, and electric
windings. The low-resistance ohmmeter is not considered a high-precision device, but it is
useful in field testing. Low-resistance ohmmeters contain a source of direct current and a meter
to read the resistance in microhms.
5.2.9.5 Ground Detector. A ground detector indicates a ground path on an ungrounded
system. Direct-reading types are available. An incandescent lamp ground detector connected
either directly to low-voltage systems or to potential transformers on medium-voltage systems is
sometimes used. The voltage rating of the lamp is selected so that the lamp glows on the normal
system. Whenever a ground fault occurs on one phase of the system, the lamp connected from
that phase to ground will go out or dim and the other two lamps will glow brightly. With a fault
of sufficiently low resistance on one phase, the lamp glow difference can be detected visually.
A voltage relay and an alarm device could be installed to sound whenever a ground fault occurs.
5.2.9.6 Oscillograph. An oscillograph is an instrument for observing and recording
rapidly changing values of short duration, such as the waveform of alternating voltage, current, or
power. The oscillograph has many uses, such as determining transient characteristics of voltage,
current, and other phenomena which occur too rapidly for measurement, by indicating meters.
Frequencies of up to approximately 10,000 Hz can be measured.
(a) Most magnetic-type oscillographs consist of a galvanometer (which gives
deflections closely proportional to the instantaneous value of current or voltage), an optical
system (using a light beam from a mirror rather than usual pointers), and the recording device
(film or light-sensitive material which can be moved rapidly). Multi-element oscillographs are
available for recording several different values simultaneously, such as, three-phase current,
voltage, and power.
5-5
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