Wiring a Contactor

[mic2876uk]

I'm looking at installing a contactor so that I can control 3kW of electrical heating load with a standard central heating controller.

The contactor i've found uses 240v to activate.

I just need confirmation on how to wire it in. I'd also assume i'd need to fuse down the switching power to 3amp so a fault wouldn't fry the heating controller (and allow me to use 1mm cable to the controller)

(https://www.screwfix.com/p/british-...C0LnffNPEUo2IvfQKfAaAsr_EALw_wcB&gclsrc=aw.ds)

To add is the contactor selected the most suitable for the job? to add I may end up using 2, 2kW heaters making a total max load of 4kW

 

[ericmark]

It depends on the anti-hysteresis used, or how often the contactor works, we found with shrink rap machines we were forever it seems changing the relay working the heating elements, they did not seem to last long, so we moved to solid state, however then you need solid state fuses as a trip is too slow, so with a convector heater the relay may need to work once every 2 minutes, but with a oil filled radiator once every 10 minutes to get same control as the oil damps the output.

I would use the relay you show unless it burns out quickly and only then move to solid state. Solid state can cost 4 times what a standard relay costs, and so worth trying with standard type first.

Personally I would use a thermostat designed for electric under floor heating, they are normally rated at 16 amp.

 

[AndyPRK]

It's says dual pole.
I would use 1 contact per heater, as that would spread the load nicely.

or there is a double width 40A one produced

https://www.bgelectrical.uk/circuit-protection/consumer-units/devices
select controls tab

 

[mic2876uk]

Great thanks, I will try and contact them and find out what sort of duty cycle they are designed for.

I'd prefer to use a standard heating controller as i'd like to use the same brand of smart thermostat I use for my boiler in the house.

In terms of actual wiring could anyone produce a quick schematic? especially for contactor drive. Or is it as simple as providing positive at one terminal and negative at other? (with control being given by switching the positive feed on and off) and would you recommend fusing down the switching supply to 3amp?

I'd also say that both the power for the heaters and switching power should be provided by the same MCB?

Whats the reason for suggesting a 40amp contactor? as 4kW load at 230v is 17.3amps and I was planning on using a 20amp MCB for the supply for the heaters

I was also planning on using oil filled radiators (if i can find some wall mountable ones that aren't £300! especially when SF sells portable ones for £36.99) Although i'm sure they could be made to be wall mountable

 

[SimonH2]

Yes, apart from there being no +ve or -ve, this is AC where they swap round 50 or 100 (depending how you look at it) times a second - you just want live to the controller, switched live to the contactor coil, and other side of contactor coil to neutral. As you've already said yourself, the control circuit should have it's own fuse/MCB appropriate to the rarting of the controller and the cable size used. If you're using a contactor fitted in the CU, then the obvious option is a low rated MCB or RCBO feeding the controls, and one or two MCBs or RCBOs supplying the heating loads via the contactor contacts. Just consider the space requirements as that could be up to 5 modules worth of space.

 

[mic2876uk]

Yes, apart from there being no +ve or -ve, this is AC where they swap round 50 or 100 (depending how you look at it) times a second - you just want live to the controller, switched live to the contactor coil, and other side of contactor coil to neutral. As you've already said yourself, the control circuit should have it's own fuse/MCB appropriate to the rarting of the controller and the cable size used. If you're using a contactor fitted in the CU, then the obvious option is a low rated MCB or RCBO feeding the controls, and one or two MCBs or RCBOs supplying the heating loads via the contactor contacts. Just consider the space requirements as that could be up to 5 modules worth of space.

Hi I only work it out to be 3 modules, 1 20amp MCB, 1 20 Amp Contactor and 1 3 amp MCB

Also found out from the manufacturer the contactor is rated to 2,000,000 cycles (24hrs x 60 mins = 1440 minutes a day, say it switched off and on every 10 minutes (unlikely) that would be 144 cycles a day. therefore 2,000,000 / 144 = 13888 days before failure or ~38 years and that's assuming heating it required all year around)

I get what ericmark is saying about the solid state relays as perhaps with something like a eurotherm controller where the call for heat on off it often less than 1 second to precisely control the process temperature. A domestic heating controller is not going to have that many on off cycles per hour. For starters I don't think a combi boiler would like that kind of frequency either.

I will however report back if it fails in less than a year. also the smart thermostat i'm using will report how often it calls for heat.

 

[SimonH2]

Well I did say "up to". Potentially 2 modules for a contactor (depends which one you use), plus 3 RCBOs if you decided to split the heating load over two circuits. As you say, with the smaller contactor and only one heating circuit - 2 modules.

 

[mic2876uk]

Not entirely relevant to the post, but we've just had an SSR fail at work, a 100amp SSR protected by a 160amp fuse...

The cables next to it are the remains of the supply cables

My role isn't electrical I just use the facility

 

[SimonH2]

I imagine that fault wasn't hard to track down
I'm guessing that the fuse protection wasn't within the spec for the device, and so a downstream fault took the SSR out with it ?

 

[mic2876uk]

I imagine that fault wasn't hard to track down
I'm guessing that the fuse protection wasn't within the spec for the device, and so a downstream fault took the SSR out with it ?

Hi SimonH2,

Upon further investigation the service engineer believe the output / load cable from the SSR to the heaters was loss which overtime generated significant heat slowly degrading the cable to the point of failure. At the point of the failure the resultant heat from the arc as the cable failed fried the SSR's circuit board opening the connections and thus removing the power to the damaged cables.

In total this facility has 2 SSR's controlling 74kW of heating load!