120v on each leg no 240 when contactor makes

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Dpeck2

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Looking at a single phase water pump outside controlled by a contactor relay I measured each phase to ground at 120 and phase to phase 240v when contactor closes I still measure 120v in phase 2 ground but loose the 240v phase to phase throughout entire circuit including the 100 amp sub panel mains and motor doesn't do anything
 
Welcome !
I may be overlooking something but my first thought is a poor connection in the feeder to the panel (perhaps a bad breaker or fuse).
I have seen both (breakers & fuses) allow enough current through to allow a reading but zero out once a load is applied.
 
180507-1107 EDT

Dpeck2:

I would agree with augie47.

I assume you are using a high input impedance meter, and that is fine.

You need to work back further toward the source of power.

First, at your motor location with the contactor closed measure:
1. The working hot phase to a screwdriver in the earth nearby. Should be near the 120 value.
2. Same for the other phase. Probably reads 120 V from voltage thru the motor.
3. Now from neutral to the same earth connection.
4. Then from from EGC to the earth.
5. You have already said that with contactor closed phase to phase is 0.


These are just so you have something to use for comparison.

Keep the contactor closed. This keeps a load on the hot lines.

Next go as close to the source of power as possible and measure each hot line to earth at that location. Should be 120. Read hot to hot and reading should be 240. If phase to phase is still near 0, then the problem of a high resistance is somewhere before this location.

If voltages are good at this location, then the problem is between here and the motor, and since you indicated the problem exists at some subpanel, then problem is between here and the subpanel or within the subpanel.

.
 
Dpeck2 said:
Looking at a single phase water pump outside controlled by a contactor relay I measured each phase to ground at 120 and phase to phase 240v when contactor closes I still measure 120v in phase 2 ground but loose the 240v phase to phase throughout entire circuit including the 100 amp sub panel mains and motor doesn't do anything
Click to expand...

I am guessing a high impedance connection somewhere. Turn the contactor on and work your way back toward the service point until you find where you still have 240V.
 
Dpeck2 said:
Looking at a single phase water pump outside controlled by a contactor relay I measured each phase to ground at 120 and phase to phase 240v when contactor closes I still measure 120v in phase 2 ground but loose the 240v phase to phase throughout entire circuit including the 100 amp sub panel mains and motor doesn't do anything
Click to expand...
What voltage is the pump motor?
 
augie47 said:
Welcome !
I may be overlooking something but my first thought is a poor connection in the feeder to the panel (perhaps a bad breaker or fuse).
I have seen both (breakers & fuses) allow enough current through to allow a reading but zero out once a load is applied.
Click to expand...
Underground conductors sometimes do that when you lose one also - especially aluminum. Low impedance meter is enough load to drop it to zero volts reading.
 
If the contactor closes and you don't have 240v but you have it when the contactor is open then it sounds like a bad contactor
 
Dennis Alwon said:
If the contactor closes and you don't have 240v but you have it when the contactor is open then it sounds like a bad contactor
Click to expand...
But he also is measuring voltage before it closes and comparing to after it closes - so likely is talking about measuring line side of contactor voltage. Closing contactor places load across the 240 volts - but some impedance somewhere in the supply circuit is too high to allow it to remain at any significant level with the relatively low resistance motor load introduced into the circuit - one will read nearly full 240 volts across whatever that point is once it is found.
 
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