Solid State Relay - Electrician Talk - Professional Electrical Contractors Forum

simmo · 09-12-2008, 08:26 AM

Hi guys,

I've got a sticky problem, I need to replace a completely burnt out relay on a Bain Marie (Commercial type cooker)
This relay is burnt to a crisp and I cannot read off any voltage and load ratings from it. I do however know the input voltage is 240vac but I'm un-sure of the amperage rating.
Do I go for a replacement relay that equals the load of the thermostat that it's switching or something less, as the relay is taking the load before the thermostat anyway.

Thanks in advance.

John Valdes · 09-12-2008, 12:16 PM

The contacts on the relay are rated in amps. The load of each contact cannot exceed the contact rating. Relay contact current must be equal or rated higher than the load. Preferably higher in every case, especially if this relay cycles alot.

Billy_Bob · 09-12-2008, 12:29 PM

I like to overengineer things. I would get a relay with higher contact ratings than is necessary.

My thinking is that I like to fix things right, once, and for good!

Then I'm not so sure about this solid state relay thing. I'm an old timer who likes the good old mechanical relays. HOWEVER this sounds like a *very* dirty environment, so perhaps a sealed solid state relay would in fact be best???

randomkiller · 09-12-2008, 12:59 PM

Most of the single phase units I have worked on have four pole 50 amp contactors for dual elements. Size the contactor to the heater load, the thermostat would only have the coil as a load, thats the reason for the contactor.

kbsparky · 09-12-2008, 07:56 PM

Those solid state relays basically have no moving parts:

Billy_Bob · 09-12-2008, 11:40 PM

Quote:

Originally Posted by kbsparky

Those solid state relays basically have no moving parts:

Correct, but it is "fried". This has been my experience when solid state components attempt to replace good old fashioned mechanical relays.

But maybe they are making these things better these days?

simmo · 09-13-2008, 04:46 AM

Thanks guys,

I read a couple of the comments before I went to work this morning.
I replaced the relay with a new 45 amp one, wired it in and switched on the isolator and nothing worked. I then got my meter out and started looking at voltages across various things, and found the thermostat wasn't switching. So I'm up for a new thermostat as well. I removed it from the oven, and it smelt burnt as well.

Oh well - at least it killed an hour of my day today.
Thanks for your input.

John Valdes · 09-13-2008, 01:45 PM

Another thing regarding SS relays. Most only have one contact, leaving the other un-switched conductor hot to the load all the time. The only place I have seen and used them was for heating elements. They need a good load to close properly.
In most every case, when a SS relay fails it fails closed. My personal experience only.

PS.....SS relays get very hot. You must have a heat sink and some compound for proper operation.

simmo · 09-14-2008, 08:06 AM

Hi John,
You are correct they do get hot.
The replacement I got came with a small tube of heatsink compound.
They aren't all that common these days, the last time I had anything to do with SSR's was when working in the Plastics manufacturing industry on Injection moulding machines, for use with the extrusion heater bands to switch on and off.

John Valdes · 09-15-2008, 01:30 PM

simmo, Same here. We used many of them making polyethylene pipe. Some heater bands used contactors too. But the majority of the newer equipment used SSR's.
I had alot of fun working with plastics. Very hot though as you are aware. Winter was great.