4 pole contactor for oven.

[chris765]

Hello all,

I have installed an oven 3.5kW via a contactor, so as I can run 6mm supply in (futureproofing although by then the EU will have banned anything that doesnt't have a wind-up handle), that allows me to comfortably switch via an MK grid .

To the point, the contactor runs pretty warm, which concerns me?
It's a Wylex MESB-24NO and I'm using only 4 of the 8 terminals.
Because it's a 4 pole, do I need to do anything like parallel up the terminals or is this warmth normal. I'm using the first and the last terminal (to make wiring easier).

I've been struggling to find any literature and have hardly ever worked with contactors before.
Your advice will be much appreciated.

Chris.

Untitled

• chris765
• 5 Jun 2015
• 1

apologies in advance for neckache.

 

[flyingsparks]

From the pic it looks like the contractor is rated at 25A. What size fuse is on the circuit?. You could parallel up the live and neutral to share the load between the contacts, this will increase the life of the contactor. It is normal for them to run warm, just depends how warm yours is.
Why didn't you just use a 30A DP switch?

Ps. Expect a bo***cking from BAS soon!

 

[Taylortwocities]

From the pic it looks like the contractor is rated at 25A. What size fuse is on
Why didn't you just use a 30A DP switch?

Probably because he wants it to be switched from the MK grid. And they do not do a 30A DP grid switch.

 

[ban-all-sheds]

What is the function of the contactor?

 

[JohnW2]

From the pic it looks like the contractor is rated at 25A. What size fuse is on ... Why didn't you just use a 30A DP switch?

Probably because he wants it to be switched from the MK grid. And they do not do a 30A DP grid switch.

That, in itself, does not explain why a 30A switch is needed for a 3.5kW load - as far as that load is concerned a 20A one (even a 20A DP grid switch) ought to be adequate. Maybe flyingsparks was thinking about a possible change of load in the future, if the OPD is, say, 30/32A.

However, I have to say that, irrational though it may be, having a grid switch in the supply to an oven just doesn't sound/feel right to me (even if its current rating is adequate) - so I can sympathise with the use of a contactor!

Kind Regards, John

 

[chris765]

Yes, the reason for the contactor was because like John it felt wrong to switch the oven in the grid, and I wanted it switched from the grid. That would have also meant running 6mm to the grid as I run a 6mm supply to future proof any oven change.

I shall double up in the contactor, if as was said it will increase the life span of the contactor.
If running a little warm is normal then maybe we're ok
Thanks.

 

[Adam_151]

They will get warm, this is mostly resistance losses in the coil (you should notice it still warm when energised but no load being drawn.

If installing multiple contactors, it is normal to space them out by at least 1 module to prevent overheating

 

[bernardgreen]

Much of the heat is due to eddy currents induced in the iron core of the contactors solenoid. This is only a significant problem in contactors energised by AC

Not all contactors are designed to be permanently energised and these will get warmer than one that is designed to be permanently energised.

Contactors should not be fitted in small airtight boxes as they may require air flow to cool them.

 

[ericmark]

Use with other control devices to increase switching capacity
DIN rail mounting
Circuit indicator window allowing clear labelling of circuits
In accordance with IEC947-4-1, BSEN60947-4-1 IEC61095, BSEN61095
24A, (AC1) resistive switch capacity
230V AC 50Hz Coil
AC/DC rectified coil for silent operation

Not sure about eddy currents since DC coil but there must be some heat from the rectifier seems rather odd to say AC 50 Hz coil and then say rectified but all will give off some heat. Personally if I was doing it I would have used the MESB-40NO so I could supply from the standard 32 amp MCB.

I have a few times discussed the idea of contracts in parallel since it's unlikely any arching will be even between the two sets of contacts then for it to switch the extra current using two sets would not be proved. As to carrying the extra current yes likely the current would be even within the unit but unless a Y connector is used there would be extra wire to one of the contacts so not convinced it would double the rating. It would however double the chance of a contact sticking.

Since a DC coil the question must be asked if left energised for X years when coil is de-energised how quickly will it disconnect?

In theroy it may be a good idea but I would be far happier with a cooker isolation switch.

 

[Taylortwocities]

Eric does unearth a reasonable issue with this arrangement.

Normally a contactor is used to switch on a load, for most applications its normal state is OFF and the contactor is ON when the circuit is energised.

In Chris's application, the cooker switch is basically an emergency/functional isolator so the contactor coil is energised for 100% of the time, the load through the contacts only being present when the oven is being used to heat up the pizza!

My (electro-mechanical) telephone days stated that having a relay constantly energised was a bad design, due to overheating and early failure.

I am not aware of any stats/MIs regarding having a contractor energised all of the time, but Chris really should research this.

 

[chris765]

Yes I hadn't thought about the amount of time the contractor is going to be operational. It's a shame so little literature is available from manufacturer. I have fired an email off to electrum and will update when it arrives.

Thanks all.

 

[AndyPRK]

Couple of random thoughts:


1) If the contactor has NO & NC contacts, use the NC contacts, so you energise the coil when you want to switch the oven off in an emergency.

(should that fail you always have the CU)
Make sure the coil and load are supplied from the same MCB.

You do really need an isolator somewhere for maintenance though

2) You have a small oven now, and have future proofed the wiring, so ditch the contactor for now.

 

[JohnW2]

1) If the contactor has NO & NC contacts, use the NC contacts, so you energise the coil when you want to switch the oven off in an emergency. (should that fail you always have the CU)

In terms of 'switching logic' that is, indeed, the most obvious solution - but I don't really think it would be considered 'safe' as an emergency switch/isolator - if there is a perceived need for a local emeergency switch/isolator, that should really be 'foolproof', without reliance on the backup of upstream alternatives. Similarly, one could use some sort of latching contactor (again, energising it briefly to 'unlatch'), but that has the same problem.

2) You have a small oven now, and have future proofed the wiring, so ditch the contactor for now.

Indeed - but the OP seems to want to use a grid switch and, although a 20A one (which is available) would be adequate for his present oven, he shares my feeling that to have a gridswith directly controlling an oven supply just doesn't feel right!

Kind Regards, John

 

[EFLImpudence]

Couple of random thoughts:
1) If the contactor has NO & NC contacts, use the NC contacts, so you energise the coil when you want to switch the oven off in an emergency.

I think that the acceptability of that would depend on which circuit the coil is.

(should that fail you always have the CU)
Make sure the coil and load are supplied from the same MCB.

He has that anyway.

You do really need an isolator somewhere for maintenance though

The CU is always there and much more reliable.

2) You have a small oven now, and have future proofed the wiring,

Not sure if you can call installing a cable csa which has been used for ever can be called future-proofing.

so ditch the contactor for now.

Agreed. It is a daft idea

 

[bernardgreen]

Use a two coil contactor. One coil is only "strong" enough to hold the contactor in the ON state. The other is "strong" enough to pull the contactor in ( to pull it to the ON state )

Or use a NC contact on the contactor to by-pass a current limiter in series with the coil. When the contactor has pulled in the NC contact opens and the current limiter then reduces the current in the coil to the current needed to hold the contactor in. ( in = ON )