3-28
Where conditions are such that the ship’s structure cannot be used as the return path, one of the other
conductors (known to be good) may be used. For example, to check D, you can connect a jumper from
pin D to pin A of plug 1 (female) and the ohmmeter leads to pins D and A of plug 2 (female). This
technique will also reveal the open in the circuit.
TESTING FOR GROUNDS
Grounded circuits are caused by some conducting part of the circuit making contact either directly or
indirectly with the metallic structure of the ship. Grounds can have many causes. The two most common
are the fraying of insulation from a wire and moisture-soaked insulation. The fraying of insulation from a
wire allows bare wire to come into contact with the metal ground. Moisture-soaked insulation causes
reduced insulation resistance (also classified as a ground).
Grounds are usually indicated by blown fuses or tripped circuit breakers. Blown fuses or tripped
circuit breakers, however, can also result from a short circuit other than a ground. A high-resistance
ground can also occur when current is increased significantly but not enough to rupture the fuse or trip the
circuit breaker.
CAUTION
Before testing any circuit, ensure the circuit under test has been de-energized
and checked with a safety shorting probe.
In testing for grounds, you may use a megger or an ohmmeter. Measuring the resistance to ground
from points in a circuit determines if the point is grounded. Referring again to figure 3-20, you can see
one possible means of testing a cable for grounds. If the jumper is removed from pin D of plug 1 (female),
a test for ground can be made for each conductor in the cable. You can do this by connecting one meter
lead to ground and the other to each of the pins of either of the plugs. A low resistance indicates that some
part of that conductor or one of the plug assemblies is grounded. Both plugs must be removed from their
units; if only one plug is removed, a false indication is possible because a conductor may be grounded
through the unit.
TESTING FOR SHORTS
A short circuit, other than a grounded one, is one where two conductors touch each other directly or
through another conducting element. Two conductors with frayed insulation may touch and cause a short.
Too much solder on the pin of a connector may short to the adjacent pin. In a short circuit, enough current
may or may not flow to blow a fuse or open a circuit breaker. A short may occur between two cables
carrying signals but might not be indicated by a blown fuse.
Shorts occur in many components, such as transformers, motor windings, and capacitors. The major
test method used to detect shorts in such components is to measure resistance. The indicated resistance is
then compared with the resistance given on schematics or in the equipment technical manuals to
determine whether the measured value is within specifications.
An ohmmeter is the device used to check for shorts. You can use the ohmmeter to detect a short
between two conductors by measuring the resistance between them (be sure electrical power has been
disconnected). A low resistance reading indicates a short. You can test the circuit in figure 3-20 for a
short by first removing the jumper and disconnecting both plugs; you then measure the resistance between
the two suspended conductors.
