Why are overload contacts wired to neutral side of coil

[bfranklin6]

Most circuit diagrams and most, if not all, applications show the overload contacts wired between the starter coil and the neutral leg of the control circuit. Why?

 

[Wirenut]

Good question. I used to know the answer. I'll remember it tonight on my way home. I hope someone comes up with the answer before I leave. Wirenut

 

[wired1]

I think it's more convenience than design. Popular convention assumes "top-feed" for incoming power. On a built up starter, the coil terminals will be at the top, but ol contacts are below somewhere. If you have a remote ol relay, connect the coil to the neutral and put the ol contacts on the hot side.

 

[JBD]

According to a Square D publication (M-659 dated 5/87):

"Placement of the overload relay to the left of the coil was required by JIC and the automotive manufacturers for many years. NEMA has recommended placement to the right of the coil for many years. The NEC has no requirement one way or the other. All arguments against one position or the other, except in the case of reversing or multi-contactor controllers, are based upon a ground or a short circuit in an area of the controller circuitry in which it is not likely to occur.

In the reversing or other multi-contactor controller situation, the arguments against left mounting are compelling, since left of coil placement would require multiple normally closed overload contacts or an additional control relay circuit."

Concerning placement on the left of the coil, the publication also notes: "... short circuit current could weld overload contact closed. If not replaced it will not function in future overload situations."

 

[RavenJoe]

It's done purely for convienience. I have seen applications where the OL's were on the hot side and this is actually a better method. But is not the norm. Assume you picked up a ground on the neutal side of the coil prior to the OL's. Your motor would burn up if an overload condition exsisted. Most MCC's are prewired to a curtain extent and the manufacturer does not know what kind of controls you will be using to pick up a starter.It could be a PLC output where the holding circuit is programmed in the logic through an input.That would exclude the convience of having your sealing contact in such close proximity to the coil. But they do know you will need OL protection. So it's just basically easier to do it on the neutral side. Sorry I know it's a long winded answer. Hope it helps.

 

[EEIre]

IEC60439/60947 applicable in Europe requires us the put the overload on the live side of the coil.

The reason is the earth fault issue described by RavenJoe above

 

[bfranklin6]

Thanks for the replys! I hope to help others down the road!

 

[Wirenut]

I have seen control systems have overload contacts on both sides of the motor starter coil. Wirenut

 

[BJC]

The up stream coil is where additional controls ( stop-starts, limit switches etc. are added to a control circuit. OLs ( 49s ) are usually in every control circuit. It is probably easier to wire them in at the factory downstream as the OL relay is almost intergral with the starter.