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sparkyparky

[ElectriciansForums.net] Contactor Query


Hi guys came across this today and it puzzled me slightly, it's a contactor for an extraction fan in the roof of a factory bay.

The white cable on your right goes up to a thermostat which appears to control the relay on the right which appears to control the contactor, just wondering why you'd nee the relay when the thermostat looks to be 240v (along with the contractor coil)?
 
A couple of possibilities spring to mind, the thermostat may be switching ELV because of its location but I don't see a transformer. Maybe the relay was installed to provide extra sets of contacts for a signal lamp or maybe a BMS signal, I can't see the wiring very clearly but it doesn't look like the other contacts are used......in which case I give up ;)

**edit**
Going by the half-baked way it's wired it could just be that the installer was completely out of his depth and clueless.
 
Possibly thinking about the surge on the contactor coil damaging the contacts on the stat,seen it before.

That would be my thought, I've wired therms to relays before when they were a bit small to be taking the whack every time the contactor pulled in. Also, maybe it was intended for the relay to also signal fan on/off at some point?
 
That contactor doesn't have any N/C contacts, so it could well have been used to provide this function as I'd imagine an aux block on the contactor would not fit in the enclosure.

I can't see on that picture if this is the case, and is just speculation.
 
Ah ok guys that does make more sense thanks, just as an aside is it alright having a motor starting automatically like that? I thought you had to have no volts protection for anything over 0.37 kw and I'd say this definitely is.
 
Ah right ok thanks, I thought they had to have protection against unexpected starting after a supply failure as well?

They do, but this falls under the risk assessment for the install. Is this fan going to harm anyone with an unexpected start? Like I say, I haven't seen the install so it's all speculative.
 
If it's under thermostatic control, almost by definition it must be acceptable for it to start without manual intervention. Power being restored unexpectedly is no different to the thermostat closing at some random instant.

It does look like the relay is simply relieving the stat of the contactor coil load - I can't see any other wiring to the relay cradle and both coils run from the red line and neutral so there can't be additional external switching. It might have been sufficient to fit the contactor coil with proper suppression instead.
 
Looks like its been wired by a plumber ... really unsure of the point of ID on just one cable, unsure the need of a relay TBH unless it has a higher inductive switching rating than the stat, although a quality stat would have adequate rating to switch the coil on the contactor and if it was a initial issue then a suitable low inrush contactor could have been used and little difference in cost..... agree with one post I read in whoever designed this is a house basher out of their depth.
 
Ok guys that's great thanks it really is a bit of a dogs dinner to say the least, any idea why they appear to have connected the neutral to 95?
 
The Lr2 D13 is a three phase thermal cutout.....would suggest the thermostat is switching the relay on elv, which powers the contactor to switch 3 phase via the thermal cutout to supply multiply or 3 phase fans.
 
thermostat is switching the relay on elv

Not the way it looks to me Tel. I think supply L1 is jumpered to A1 and the stat brown, stat blue SL feeds the relay coil which gets its N from the supply black at 95. Contactor A2 goes via the relay contacts to 96 and through the overload NC to N.

I wonder if the OCP on the motor circuit adequately protects the stat cable?
 
Closer look there is a red loop from L1 to A1 and the brown then goes of to the stat flex, it looks like some DF has put a relay in to monitor the O/L NC output to drop out the contactor if O/L operates because they do not understand how to incorporate it without the relay, I can pretty much follow all the wiring and does seem like its as I suggest so the stat is switching 230v and the relay is held in with the O/L.... hard to say without testing my theory.
 

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