Proper way to wire overloads

[Gil47]

Thank you very much for your help. So we are out of compliance with the NEC and could be causing a potential problem of a fuse not popping or breaker not tripping? Or someone getting shocked because we have opened a grounding path? I just want some ammo to argue this with upper management to make them do the correct thing or at least let them know the dangers involved. We are using a grounded neutral on our transformers and all our cabinets are earth grounded.
Thank you very much for your help.

Maintenance Man, I feel you are on the right track,
here is a problem I see,
If the wire Nuetral from the NC on the overload and the coil gets shorted to earth then the overload contact position NC or NO will make no difference as the contactor coil will stay pulled in, now if you used the live supply then a short between the overload NC contact and the contactor coil would blow a fuse.

I believe that outside of the America's the opening of the Neutral would be against the code.

 

[Maintenance Man]

Thank you Gil 47 for responding to my question. I was told that in Europe that they switch the neutral instead of the hot wire for safety reasons. This might not be true at all; it is just what I was told.
Sincerely
Maintenance Man

 

[Matthew Modar]

Read the third post here: http://www.eng-tips.com/viewthread.cfm?qid=213024

If the overloads were wired on the line side, a shorted control circuit could fuse the contacts. Since they are normally closed, the problem may not be detected. The short is cleared, the motor is restarted. Weeks later the motor smokes or the factory burns down. Wiring the overloads on the neutral side reduces this possibility.

 

[leitmotif]

I dont argue a short in the control circuit could weld overload contacts which is why NEMA put them in the neut.

However the same argument would apply to the stop switch and the maintain contact when unit is running.

The other thought is what happened to overcurrent protection for the control circuit? Why did it not blow before enough power was developed to weld contacts?

Several comments were made at the other site
http://www.eng-tips.com/viewthread.cfm?qid=213024

that this is carry over from the old days. This is what I saw in Navy where we used (fused both sides) 480 control circuits. So that may be the reason.

I asked the question several years ago in a controls class and instructor did not know and have wondered about until today. When overhauling I generally put the overload contacts in the hot and not the neutral - it just seems better to me.
Dan Bentler

 

[HJTRBO]

Post 501...

And adding fuel to the fire...

So many American flags! Arghhhh

Never here any complaints from this side of the ditch, and we wire to a mongrel standard.

Happy posting!

 

[nettogrisen]

Question. What kind of overload breaker are we talking about?
As I see it there are 2 different ways of doing it.

One is to use this kind:

It doesn't require any extra wiring. It just breaks the power.


Another way is to use this kind:

You have to wire either A1 or A2 throug it as it doesn't automaticly break the power, wouldn't break the neut though. Over here it's just considered bad practice. You wouldn't do it that way on your lamp at home would you?

 

[Clay B.]

Maybe I missed something here. Having one side of the starter coil wired thru the contacts on the overload is pretty much standard. In most cases I have seen it done with the neutral side. My thought is if the overload tripps you want to turn the contact off so the chances of power being sent to the motor is reduced. As for welded contacts on the starter itself I do not see how the coil is wired is going to make much difference. On the panels I build I use the nc contact for the motor starter coil and the no contact for a status input to my plc. NOTE: the NC/NO Aux contact I use do not share a common.

In instances where the NC/NO contact shared a common I would wire the Hot Lead thru the contacts so I could still have my status to the PLC.

 

[Maintenance Man]

I appreciate all the good information from everyone. What we are controlling in the plant, is 3 phase motors where we have a contactor in which most of them are wired with 120 V AC going to A1 of the contactor coil that goes through a E-stop then stop button then to one side of the NO start button contact. When the start button is pressed, 120 v goes to A1 which energizes the coil. To latch ( holding memory) the circuit we have 120 V connected to one side of the NO terminal on contactor and another wire connected to other side of NO terminal on contactor that jumps to A1. So what happens is when the start button is pushed the contactor pulls in causing the N0 contact to close which passes voltage across it then jumps to A1 latching the start circuit. For the neutral side of coil we go through the NC contact on the overload then to A2 on the contactor coil. The overload is hooked directly to the bottom of the contactor with the 3 terminal pins that are on the overload to the 3 phase screws on bottom of contactor. The 3 phase voltage (3 wires) are connected to the top of contactor (L1, L2 and L3). We have 2 wires jump from the top of contactor to a transformer for the primary side and the secondary side of transformer is used for our 120v control voltage with a hot wire and grounded neutral. The 3 wires going to the motor are connected to the bottom of over load (T1, T2 and T3). When the contactor is pulled in and overload is not tripped the motor will run until the E-stop, stop button or overload trips or loss of power occurs. The contactor and overload we are using is a Telemaniac which has the dial to set the trip point you want for the size being used. I have seen overloads that actually have heaters with some kind of solder like substance that will cause it to melt when it gets to a certain temp causing a mechanical device to tune that trip the overloads. Now after all this ; I still have the question of what is meant by NEMA showing to wire a overload on the neutral side when the 3 phase current is what trips a overload from too much current going through any of the 3 wires. Is the neutral wire NEMA is talking about, the neutral wire going through the NC contacts on the overload? Sorry, I just want to be clear on this, that’s all.
Sincerely:
Maintenance Man

 

[Maintenance Man]

I have seen a lot of comments about breaking on the neutral side. When dealing with a 3 phase system we do not have a neutral wire; just 3 wires with around 277 V on each one to make our 480 V phase to phase. The only neutral we have is the neutral on the control circuit. Or maybe I do not understand as much as I thought I did. Thanks in advance.
Sincerely:
Maintenance Man

 

[KP_EENG]

When dealing with a 3 phase system we do not have a neutral wire; just 3 wires with around 277 V on each one to make our 480 V phase to phase. The only neutral we have is the neutral on the control circuit. Or maybe I do not understand as much as I thought I did.

Maintenance Man
[/quote]

If you have 277 VAC then you must have a 480 VAC, four-wire system, with one of the wires being a neutral. 277 VAC is the voltage between any leg of a 480 VAC Wye-connected transformer and the neutral line of that transformer. Divide 480 VAC by SQRT 3 (approx. 1.73)and you will get the 277 VAC.

Here is a diagram for the late RSDoran's website for a graphical representation:

http://www.patchn.com/index.php?option=com_content&task=view&id=16&Itemid=74

Regards

 

[OkiePC]

I have seen a lot of comments about breaking on the neutral side. When dealing with a 3 phase system we do not have a neutral wire; just 3 wires with around 277 V on each one to make our 480 V phase to phase. The only neutral we have is the neutral on the control circuit. Or maybe I do not understand as much as I thought I did. Thanks in advance.
Sincerely:
Maintenance Man

The comments about breaking the neutral are in reference to the coil power (control circuit), not the motor power.

 

[Maintenance Man]

I appreciate all the help everyone has given me on this issue. The conclusion I have come to, is that there are many ways to wire up a motor. Some have said that they also wire the neutral through the NC contacts on the overload to A2 on the contactor. The place I work wants it done this way so I guess I better do it to keep my job. Again thanks to everyone.

Sincerely:
Maintenance Man.