For example, to find the required insulation
Circuits with a current capacity above 100
amperes cannot be measured on the ohm-
sheath of a cable 672 feet long:
2,640,000 672 = 3,928.57 megohms.
ohmmeter is 100,000 ohms (0. 1 megohm).
(2) In the case of a multiple-pair cable, to find
the minimum insulation resistance of the entire
of Electrical Instruments
cable to the cable sheath, divide the requirement
The insulation resistance between all connected
for one conductor by the total number of con-
electrical circuits of an electrical instrument and
ductors in the cable. For example, the total
the case should be no lower than 20 megohms. If
number of conductors in a 51-pair cable would be
the instrument has both a current and a voltage
51 x 2 or 102 conductors. Thus, the minimum
circuit, the insulation resistance between the
insulation ressitance requirement for a 51-pair
current circuit and the voltage circuit should be no
cable 672 feet long ( (1) above) would be:
lower than 5 megohms.
3,928.57 102 = 38.51 megohms.
3-15. Insulation Test of Tools and Appliances
3-13. Test of Wiring Installations.
Wiring systems are tested to see whether they are
pliances should be approximately the same as the
free from short circuits and grounds. Remove fuses
insulation resistance value of the wiring system to
and open switches before making tests.
which they are attached. Tests for short circuits
a. Testing for G. ounds.
and grounds are made in the same manner as tests
(1) Connect the GROUND terminal of the
ohmmeter to a good ground.
(2) Connect the LINE terminal to a wire of
The resistance value of resistors can be found with
the wiring system that is being checked for
the ohmmeter if within the megohm range. The
grounds. Measure one wire at a time. A low
resistor must be disconnected from its circuit and
reading indicates that the wire is grounded. A
all power turned off. Connect the LINE terminal to
high reading indicates that there is no ground and
one side of the resistor and connect the GROUND
that the insulation is good.
terminal to the other side. Operate the ohmmeter.
b. Testing for Short Circuits.
The reading will be directly in megohms.
(1) Connect the LINE terminal of the ohm-
meter to one wire of the wiring system.
3-17. Insulation Resistance Values
(2) Connect the GROUND terminal to the
Whenever possible, take minimum insulation
(3) Wire circuits of No. 12 or No. 14 AWG
resistance values of equipment being tested from
wire should have a resistance of approximately 1
the specifications covering such equipment. If
megohm. The insulation resistance of wires of
specifications are not available, use table 3-3 which
larger gauge is based on current capacity and
consists of accepted minimum requirements.
should read approximately as follows: