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 MIL-HDBK-1023/4
(5) Insulation-Resistance Test. Perform an insulation-resistance test on all field
circuits. Record the readings, and compare them with the previous readings. For series circuits,
the insulation resistance may be measured by simply removing the ends of the loop from the
power supply. For parallel circuits, all connections must be removed before insulation resistance
may be measured. The maintenance supervisor should consider troubleshooting the circuit
resistance, using the guidelines provided in Table 9 High voltage insulation resistance test on
each series and multiple underground circuit to determine complete freedom from grounds.
Whenever possible, these tests should be performed when the ground is thoroughly wet. Circuits
which pass insulation resistance tests during dry weather may fail after a heavy rain. A 10 to 20
percent annual decrease in insulation resistance value (in mega-ohms) is normal. An annual
decline of over 50 percent, or 4 percent decrease, from one reading to the next indicates the
existence of a problem that should be troubleshot. Consistent testing and good record keeping is
the essential. The test procedure is as follows for each circuit:
(a) Disconnect both leads from the regulator output terminals. Support both
leads so there are air gaps of several inches between bare conductors and ground. Make sure the
cable sheath is clean and dry for a distance of at least 1 foot (300mm) from the end of the cable.
Also make sure exposed insulation at the end of the cable is clean and dry.
(b) Test each circuit immediately after installation according to "First Test for
New Circuits," as listed in Table 10 Test any circuit installed for 60 days or more, even if it has
not been operated, according to "Succeeding Tests and Old Circuits."
(c) The maximum acceptable leakage current, in microamperes, should not
exceed the values in subpar. (f) below.
(d) When additions are made to old circuits, test only the new sections
according to "First Test on New Circuits." Test the complete circuit at the reduced voltages to
ensure reliable operation.
(e) Connect both conductors, and apply the test voltage shown for 5 minutes
between conductors and ground. The above tests must be performed with a suitable high voltage
tester which has a steady, filtered output voltage. The high voltage tester must have an accurate
voltmeter and microammeter for reading the voltage applied to the circuit and the insulation
leakage current. All high voltage tests on airfield lighting circuits must be carefully supervised
by qualified personnel to ensure that excessive voltages are not applied to circuits.
(f) During the last minute of the above tests, the insulation leakage current in
microamperes for each complete circuit must not exceed the following value calculated for each
circuit:
1. Allow 2 microamperes for each 30/45, 100, 200, 300 and 500W series
2. Allow 3 microamperes for each 1000 feet of cable. This value includes
allowances for the normal number of connectors and splices.
3. Add the above values to determine the total allowable microampere
leakage for each complete circuit.
(6) Input Voltage. Measure the input voltage to the vault. This measurement
should be repeated every few hours throughout the day and night since the demand on the
commercial power network varies throughout the day. The input of each phase should be
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