Overload Relays and Starter Neutral Wire

hanziou

Member
Join Date
May 2005
Posts
111
When I have looked motor starters in control panels, I have noticed that the overload relays will disable the contactor coil for the motor starter.

It seems that usually this done by running the neutral wire through contacts on the overload relay.
Why the neutral wire? Wouldn't it make more sense to interrupt the hot (ungrounded) wire?
 
It is one way to drop out all of the motors in the cabinet if you put the neutral through all of the O.L. contact blocks for all of the starters. If one overload trips, then all the starter coils will drop out.
 
glenncovington said:
It is one way to drop out all of the motors in the cabinet if you put the neutral through all of the O.L. contact blocks for all of the starters. If one overload trips, then all the starter coils will drop out.

I guess that makes sense. That sounds like that wiring is done to create a "Starter Neutral".

What I have commonly seen is the 'A2' terminal of the starter coil is wired directly through only its own O.L. relays in series to the neutral. That way if any single phase of the motor overloads, then the coil drops out, but it is only the one coil, not the rest.
 
It is actually very common for NEMA starters to use the neutral side wire for the overload contacts. However, more and more often we are seeing the placement of the OL contacts on the line side of the starter coil. The advantage of using neutral side of the coil is for reversing contactors and Wye-Delta starters the two coil neutrals can be combined into a single wire and then passed through a single OL contact. For single coil across the line starters however it really doesn't matter.
 
It is still a very bad practice though, to switch the neutral. Ever. It used to be very common, even 'the standard' for a very long time.

Switching the Neutral is BAD, as it is normally grounded, so a fault around the OL contact will still enable the coil.
 
rdrast said:
...It used to be very common, even 'the standard' for a very long time.

That is what I observed.


rdrast said:
Switching the Neutral is BAD, as it is normally grounded, so a fault around the OL contact will still enable the coil.

That is what I thought. That's why I was confused about the apparent 'standard'.
 
What is the likelihood of a fault around the OL contact? You're talking about a very short piece of wire in a panel with the starter. I've never had any problem with the OL contact on the neutral side.

Most of the starters I've used (especially reversing or two speed starters) have a wire routed to one side of the OL contact for you. If you protect the line side of a reversing starter, you need two OL contacts since the hot feeding each coil isn't common. If you just connect the neutral to the other side of the pre-wired contact, it will open either coil when an overload condition exists.

The biggest problem I've seen with motor starters is when the electrican leaves out the OL contact altogether. I recently found six machines with four starters each like that...it had been that way for 8 years before anyone checked the actual wiring! (I wasn't in that department then, so I don't know how it passed checkout.) So, when an overload occurred, the motor could single phase and let the smoke out. They had the neutral connected directly to the coil, instead of the built in common OL contact.

Those Telemechanique starters were internally connected "snap together" units that only had one screw terminal for the OL relay and it was numbered something odd. I can see where an electrician without the manufacturer's info would not know how to wire it. I had to get a new one from the storeroom and look at the instructions to figure it out. The other side of the contact was connected to both coils internally so you couldn't wire it to the line side anyway.

I have found other, common, easy to understand motor starters wired that way too by mistake.

I agree in general with the statement that switching neutral is bad, but this is a wise exception to that rule IMO.

Paul
 
Originally posted by OkiePC
What is the likelihood of a fault around the OL contact? You're talking about a very short piece of wire in a panel with the starter. I've never had any problem with the OL contact on the neutral side.
IEC 60204 has a specific exception to cover the case of overload contacts where "an earth fault is unlikely".
 
AutomaticLeigh said:
IEC 60204 has a specific exception to cover the case of overload contacts where "an earth fault is unlikely".

For close-coupled (single unit) OLR(MSP)/Contactor, I'll agree with you, but for some idiotic reason, I seem to have three different panel-builders now that are seperating the OLR(MSP) and the contactor. One very large, well known shop even went so far as to put the contactors on one panel, and the OLR's on a completely seperate SIDE panel.
 
I didn't quote all 60204 has to say on this because it never occurred to me that anyone would mount the O/L in another panel. The paragraph actually says the contactor and overload must be in the same panel for the exception to be valid.

Personally I've never wired the O/L contact in the neutral, but if you're looking for regulations or guidance, then IEC or EN 60204 would permit it if the other conditions are complied with.
 

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