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connected in parallel with the circuit to be measured, the internal meter circuitry is electrically connected
in series.
In making resistance tests, take into account that other circuits containing resistances and
capacitances may be in parallel with the circuit to be measured. Erroneous conclusions may be drawn
from readings obtained in such cases. Remember, a capacitor blocks the dc flow from the ohmmeter. To
obtain an accurate reading when other parts are connected across the suspected circuit, disconnect one end
of the circuit to be measured from the equipment. For example, many of the resistors in major
components and subassemblies are connected across transformer windings. To obtain a valid resistance
measurement, you must isolate the resistors to be measured from the shunt resistances of the coils of the
transformers.
Resistance tests are also used to check a component for grounds. In these tests, the component to be
tested should be disconnected from the rest of the circuit so that no normal circuit ground will exist.
Dismounting the component to be checked is not necessary. The ohmmeter is set for a high-resistance
range. Then the ohmmeter is connected between ground and each electrically separate circuit of the
component being tested. Any resistance reading less than infinity indicates at least a partial ground. You
can also check capacitors suspected of being short-circuited by measuring the resistance. To check a
capacitor suspected of being open, temporarily shunt a known good capacitor then recheck the
performance of the circuit.
CAUTION
To avoid possible damage to equipment during resistance tests, observe the
following precautions:
Always connect an ammeter in series—never in parallel.
Connect a voltmeter in parallel.
Never connect an ohmmeter to a live circuit.
Observe polarity when using a dc ammeter or a dc voltmeter.
View meters directly from the front. When viewed from an angle off to the side, an incorrect
reading will result because of OPTICAL PARALLAX. (Parallax was covered in NEETS, Module
3, Introduction to Circuit Protection, Control, and Measurement.)
Always choose an instrument suitable for the measurement desired.
Select the highest range first and then switch to the proper range.
In using a meter, choose a scale that will result in an indication as near midscale as possible.
Do not mount or use instruments in the presence of a strong magnetic field.
Remember, a low internal resistance voltmeter (low sensitivity) may shunt the circuit being
measured and result in incorrect readings.
