Quantcast Locating Opens in Field Circuits

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MIL-HDBK-1023/4
cables (L1-A and L1-B) presently connected to the 6.6 ampere regulator. Then connect the other
two load cables (L2-A and L2-B) to the 6.6 ampere regulator and test. Now, the on/off or
on/dim transition will be on the other side of the shorts and each short will be located.
WARNING:
Do not come in contact with the cable or meter while the circuit is energized. If
the meter must be handled or attached to the circuit while the circuit is energized,
use a hot-line clamp-stick.
6.6
Locating Opens in Field Circuits
6.6.1
Isolating Open Circuits. Before checking for open circuits, be sure that any grounds
are removed as described in par. 6.4. An insulation resistance test provides the quickest check.
If the insulation resistance of both feeders is satisfactory, the ungrounded open fault can be found
using the intentional ground and open circuit test, the insulation resistance tester, or the cable test
set.
a) Symptoms. Regulator trips off line when turned on either remotely or locally.
Regulator operates normally into shorted load. Megger tests across load circuit confirm that an
open load condition exists.
b) Troubleshooting. Check across the load with a Megger or ohmmeter. Verify that
the load has an open or high resistance across it. Label each output coming from the regulator
(L1 and L2) and Megger to ground. If one or both have a low resistance to ground, either the
"Intentional Ground" or "Ground Return" method can be used. If a measurement of resistance to
ground does not yield a reading, an "ungrounded open" exists and either the "Manual Ground" or
"Sectional" method will have to be used. See the instructions and illustrations on the various
methods in the "Airfield Troubleshooting Methods" section.
c) Causes. Opens are caused by the same reasons grounds occur, with the following
exceptions:
(1) Ice tends to pull circuits apart as it freezes in manholes, base cans, and conduit,
most often at connector kits which do not have heatshrink kits over connectors. However, in
some cases ice has been known to completely sever the conductor.
(2) Personnel working on a circuit made errors in reconnecting the load.
(3) Isolation transformer failures frequently cause opens.
(4) Ground settling after construction breaks connections in direct buried circuits.
d) Example Problems. The following are several examples of finding locations of
open circuits in series lighting circuits:
(1) Figure 37 illustrates troubleshooting a grounded open on the load circuit using
the "Intentional Ground Method." Prior to test, each load cable has been meggered. The
intentional ground is connected to the cable with the highest resistance to ground. The regulator
may likely trip out in an open circuit condition if the ground resistance at the open is too high.
Revert to other methods of load troubleshooting. In the example, the resistance to ground of
open circuit is low enough to allow for current flow. The open can be found quickly by driving
the load and finding the on/off light transition.
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