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 TM 55-1905-223-24-17
(b) If breaker has been tripping, reset it and check current through A-A1, B-B1 and C-C1 with clamp-
on ammeter. Check amp rating of breaker. It is usually stamped on breaker lever or body.
Compare amp reading with breaker rating. If current through breaker is higher than rating,
either a temporary overload exists or the breaker is improperly sized.
(c) If breaker is tripping at a current below its rating or not tripping at a higher current, replace it.
(3) Contactors.
(a) Check the contactor relay coil as follows:
1
Turn OFF all power to unit. Remove wires from coil. Spread them apart to prevent accidental
shorting or grounding.
2
Use an ohmmeter to check across coil terminals. If the meter indicates a circuit, the coil is OK.
If a very high resistance is indicated, the coil is open. Replace coil or complete component. If
a low resistance is indicated, place one meter probe to ground or to unit frame. Touch other
probe to each coil terminal. If a circuit is indicated from either terminal to ground, replace the
contactor.
3
Check coil voltage. It is usually indicated on the coil body, component nameplate or body.
Turn ON unit power. Establish voltage both coil wires. Use a voltmeter to check voltage
between coil wires. It should be the same as indicated coil voltage. If not, either coil supply'
voltage is incorrect (determine by further checks) or the incorrect coil is being used.
(b) Check the electrical contacts in the contactor:
1
Before checking, determine if contacts should be normally open or normally closed when coil is
deenergized.
2
Turn OFF all power to unit. Remove wire leading to the load-controlled device. Use an
ohmmeter to check across each set of contacts. Normally open (NO) contacts should read
open or an infinite resistance. Normally closed (NC) contacts should read zero resistance. If
an incorrect meter reading is obtained, remove all wires leading to component and observe if
reading still indicates improper contact position. If so, inspect the contacts. Look for welded or
burned contacts, metal spatter, and carbonizing. Replace the contactor if these conditions
exist.
2-38
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