# Motor Short/Cir. Overloads?



## alpha3236 (May 30, 2010)

Motor OL's operate because of excess heat build up do to an over amperage condition and take a while to trip. Whereas short circuit/ground fault protection is theoretically supposed to function in something less than a full cycle ( less than 1/60 of a second). OL's are not designed to take care of a problem occurring in that amount of time.


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## guest (Feb 21, 2009)

What Alpha said, plus:

Yes, a high enough short circuit current can totally melt or vaporize the overloads...I think BBQ or someone posted pics of just that in a thread a while back. 

I once saw (the remains of) a 480v, 3-phase pump starter where the farmer bypassed the fuses because they kept blowing (grounded winding in the motor). The contactor, overloads and control transformer were completely vaporized, all that was left in that starter cabinet was a lot of burned, molten metal. I wish I had taken pics. (No camera/camera phone at the time.) 

That;s why the NEC requires a properly sized circuit breaker or fuse ahead of the overloads.


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## mrmike (Dec 10, 2010)

Cletis said:


> Hey Motor Gurus'. I'm trying to wrap my head around something (the in phrase I guess).
> 
> I'm refreshing myself on 430 today. The general teachings are the overloads are for overloads and fuse for short circuit protection.
> 
> ...


 
Yes, Indirectly it is. The fuses or Breakers are sized including the starting current- so some ground faults don't exceed that rating. And sometimes breakers are faulty & will not trip on a Short ciruit.. 
So, If any of these conditions exist, Beaucoup amps will go thru those O/L heaters & may Burn them open before the O/L trips. I have seen this many times........
I have also seen where O/L' would not trip either, and actually had one blow up like a grenade right next to me. It was a good thing that I saw it glowing orange and I had moved away from it......................


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## di11igaf (Jan 1, 2012)

I guess technically if a condition arises where the load becomes higher than conductors can handle and the fuse or breaker doesn't trip, then the overloads could protect the conductors. The fault would have to be in the motor, or at least after the overloads though. So even though they are supposed to protect the motor, in some cases it could do more than that. If a fault is before the overloads, then they will do nothing, the ocpd will have to 'handle' it.


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## electures (Oct 23, 2008)

OL's protect the motor. Fuses protect the branch circuit wiring.


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## Tom45acp (Sep 6, 2011)

I think part of the confusion on this subject comes from the fact that we are used to installing short circuit, ground fault and overload protection at the beginning of a circuit. Short circuit and ground fault protection must be at the beginning of a circuit to protect what comes after the protection. Overload protection will provide protection upstream and downstream as long as it is installed before the load. 

Part IV of article 430 deals with short circuit and ground fault protection. You can size the breaker (generally) at 250% of the amp values given in tables 430.247 to 430.250. You could easily end up with a #12 conductor on a 40 amp breaker. Obviously, this does not protect the conductor from overload.

Part III is for motor and branch circuit overload protection and the overloads are usually sized based on nameplate amps. If you have correctly sized the conductor, then the overload device protects the motor and the conductor.

I'm in agreement with the others about blowing up overloads during a short circuit, it can be spectacular.


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## mrmike (Dec 10, 2010)

electures said:


> OL's protect the motor. Fuses protect the branch circuit wiring.


 
 WHAT ????????


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## don_resqcapt19 (Jul 18, 2010)

You also have to look at the manufacturer's instructions, especially for IEC equipment. There will be a maximum size of OCPD specified and it will often be smaller than the maximum permitted by the NEC.


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## frenchelectrican (Mar 15, 2007)

mrmike said:


> WHAT ????????


 
Oui, Sérieusement.

( Yes, Seriously ) 

If you read Tom's comment on the Art 430 that was pretty much clear cut on that.

And keep in your mind if the O/L is functioning properly they will do it as they are designed. but if not working properly you can see other members mention a pretty ugly senie (view ) I have couple did do pretty good damage when the O/L did fail.

Also if you suspect if was on hevey fault it will be wise to check everything with fine comb ( normally I will just suggest to replace it due some case you may not find hidden damage along the way )

( Just look at the short circuit rating you will be suprised why some just can't take it [short circuit ])

Merci,
Marc


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## mrmike (Dec 10, 2010)

frenchelectrican said:


> Oui, Sérieusement.
> 
> ( Yes, Seriously )
> 
> ...


 

Coming from an industrial plant, I have seen many overload faults & short circuits and that is where I base many of my posts from....
O/L's and Fuses both protect the wiring & the motor, besides what they are designed for. 
O/l's over current-over time and fuses//breakers instantaneous. They don't care if it is the wiring or the motor-thus my "what"


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