overload heater in motor starter

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hurk27

Senior Member
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Portage, Indiana NEC: 2008
marissa2 said:
It could be a safety thing, when they trip there is no power applied to them.
Lou
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When they trip there certainly is power on them otherwise they would not trip:happyno:

Joking as I knew what you meant:p

The heaters are not a contact device, they are just coils or strips of metal that are in series with the load, they do not open (unless damaged by to much current) they heat another part that opens a set of normally close contacts that opens the power to the contactor dropping out the power until it is manually reset, I don't know the real reason as to why they are after the contactor block but I would guess back in the day when safety was of less of a concern, one could change out the heaters because the power would be removed from them after a trip, also the control contacts were always in the grounded conductor side of the control circuit if the control circuit was grounded which also seems to point to allow the heaters to be changed without the removal of power to the starter, but with OSHA and safety today things have changed, and I guess they continued the design because it been done that way for so long, also not all control circuits were derived from a different source as some received their power from the line side of the motor starter which could mean you could have full motor supply voltage across the N/C contacts if the control wasn't a drop out stop start type.

Over loads are only there to protect the motor, they only have to be in series with the motor load to do this, so in respect they could be in the circuit anywhere, the only protection device that needs to be at the source of a circuit is the short circuit and ground fault device, as many motors had the overloads built right in the motor so the breaker feeding a motor circuit is only used for SCGF's this is why we can have smaller conductors on a motor circuit protected by a larger rated breaker.
 
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kwired

kwired

Electron manager
Location
NE Nebraska
hurk27 said:
Over loads are only there to protect the motor, they only have to be in series with the motor load to do this, so in respect they could be in the circuit anywhere, the only protection device that needs to be at the source of a circuit is the short circuit and ground fault device, as many motors had the overloads built right in the motor so the breaker feeding a motor circuit is only used for SCGF's this is why we can have smaller conductors on a motor circuit protected by a larger rated breaker.
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This is bottom line. Why it had become somewhat tradition to place overload devices on the load side of contactor, I have no idea.

Today it is more common to see overload devices on supply side of contactor, but the melting alloy heater style is almost always on the load side of the contactor. Electronic and bimetal types seem to vary more.
 
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