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 MIL-HDBK-1023/4
(2) The sectional isolation method for pursuing grounds divides the load circuit
into subsections that are tested separately. Megger insulation testing of subsections identifies
those subsections that are grounded or have low resistance to ground for further subdividing and
testing.
(3) When isolating grounds in the circuit, the load circuit is broken at several
locations, an equal distance apart along the load circuit. Each section is then tested
independently to ground with a megger. Bad sections with low resistance readings are identified
and broken again into several suspect sections. Megger testing then resumes on the suspect
sections, further identifying the location of the ground or grounds. This process of elimination
continues until the remaining section is small enough to investigate directly.
(a) Figure 30 illustrates a grounded load circuit that has been manually opened
in three locations, creating four separate sections of load circuit. The four sections of load circuit
are subjected to four megger tests at two locations in the vault and field. Megger tests 1, 2, and 3
all passed. However, test 4 indicates a grounded section. These first four tests have narrowed
the ground location to 25 percent of the load cable.
MEGGER TEST 3
MANUAL OPEN
MANUAL OPEN
MEGGER TEST 4
GROUND FAULT
MANUAL OPEN
MEGGER TEST 1
MEGGER TEST 2
LOAD IS DISCONNECTED FROM REGULATOR
= ON
= DIM
= OFF
Figure 30
Sectional Isolation Method - Megger Tests Locate Grounded Section of Load
(b) Notice: The load has been disconnected from the regulator preventing
possible damage to the regulator and false readings through lightning protection devices.
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