Schrack RTE 24730 relay on nu-heat 4/ZDM-C PCB

[notb665]

I think this orange relay has got stuck on the 'on' position because *someone* connected the live supply and switched live output the wrong way around on this board for my underfloor heating:



Is there any way to reset it or is it fubarred?

 

[bernardgreen]

Either the relay has welded its contacts together or the control circuit has a fault.

The coil is 230 volt AC so unlikely a shorted transistor is holding it energised

http://uk.farnell.com/jsp/displayProduct.jsp?sku=1175041&CMP=KNC-GUK-FUK-GEN-EN-SKU

 

[notb665]

That is really great - thank you so so much.

There is no other control circuitry, so would connecting a live supply to the switched live out cause the welding of contacts together?

I am thinking of soldering a new one on rather than paying £120 for a new board. Do I need to follow any special procedures to safeguard the relay from the heat?

 

[Spark123]

Not really, as long as you can solder there shouldn't be a problem. You may need a solder sucker to remove the solder to release the old relay and clean the holes.

 

[plugwash]

I can't see how connecting a live to the switched live output would blow things if it is just a plain relay board (is there anything except the relay on the board?) unless you also had something like a neutral connected to the permanent live.

also that board appears to be double sided so you need to be extremely carefull when desoldering not to rip out the through hole plating.

if you break open the relay and it is indeed stuck you may be able to unstick it without having to replace it.

 

[notb665]

I can only see the relay on the board, this is the other side:



If it is not the relay I am at a dead loss of why the live and switched live on the board with nothing else connected buzz on a continuity test.

 

[ricicle]

Are you getting a zero ohm reading when you test between the live/switch live contacts on the relay.Some of these continuity buzzers will buzz at 'near zero' ohms continuity which could be reading through another component (or surpressors across the contact?)
You need to test with a continuity tester on the low ohms setting.

 

[Space cat]

There is no other control circuitry, so would connecting a live supply to the switched live out cause the welding of contacts together?

From what I can see, it says "24V IN", "24V SWIT OUT" and something that might be "24V CIRCUITS ONLY" near a terminal block that connects to the relay contacts. Where does LIVE come into this?

PS: I'm pretty sure that's a single sided board. The brown markings on the top are an exact match for the copper on the bottom.

PPS: £120 for five terminal blocks and a relay!!!

PPPS: How do you know it's not a normally closed contact? Does it open if you put volts onto the coil?

Edit: I've just had a good look at the back and the contacts in use are the normally open ones. So much for that theory. I've also looked at the data sheet. Top whack for those contacts is 30V DC or 240V AC. The two terminals bottom right with the red and yellow wires in them are also switched by that relay. If those are mains wires, as the markings seem to suggest, that's rotten design. A spike from an inductive load - a pump perhaps - could weld them nicely!

 

[bernardgreen]

The contacts ( according to the Schrack data sheet ) that are connected by PCB tracks are the normally open ones so either there is permanent 230 Volts on the coil or the contacts are welded. And it is not a remnance relay.

Disconnect everything from the board. if terminals 1 and 2 ( where the red and yellow are ) are still joined then the relay is shot with welded contacts.

 

[ColJack]

you can get relay bases that make the replacement of relays just a plug and play thing..

might be worth doing this as you then don't run the risk of cooking the relay soldering it in..

and get a spare relay for next time just in case..

 

[plugwash]

before you try anything as drastic as desoldering the relay get inside (by cutting/breaking if nessacery) the relay and see if you can find the welded contacts and reseperate them.

 

[bernardgreen]

before you try anything as drastic as desoldering the relay get inside (by cutting/breaking if nessacery) the relay and see if you can find the welded contacts and reseperate them.

If they are welded the contacts will be too badly damaged to be reliable. The surface plating of contact material will have been burnt off leaving the base metal which will not provide good switching contact.

 

[notb665]

Thanks for your replies, you all have been invaluble. I have ordered a new relay and I'll let you know how I get on.

In the meantime, this is the wiring diagram I am following:



I am concerned that the Radiator circuit zone valve could send a live via the orange swicthed live into the switched live output on the relay. The diagram is surely correct so have I been thinking about this the wrong way?

 

[Space cat]

I am concerned that the Radiator circuit zone valve could send a live via the orange swicthed live into the switched live output on the relay.

I think you're right! It's not easy when you can't pick the board up and turn it over but, if I understand this correctly, the purpose of that relay is to allow the floor heating stats and zone valves to control a boiler/pump combination which is already part of an existing CH/HW system.

The designer has put a relay in to prevent anything in the (unknown) existing system from feeding back to this one. That was a good idea but the relay is undersized for the job. It might have to switch a gas valve and pump in parallel - you can't assume that there'll be a pump over-run stat out there! The relay contact opens and breaks the supply to the pump. The pump inductance kicks back. The relay contacts arc over. End of relay.

If it was mine, I'd modify the board and fit a bigger relay. You're not using the 24V terminals so you don't need gold plated contacts. At the very least I'd put some suppression on it. Its a very simple circuit which could easily have been built using a plug-in relay and some terminal strip - but they'd never have got away with charging £120 for that!

 

[notb665]

I bought a new relay (with postage £9) and it didn't test continuous between the two contacts in question. All good. Took off old one and soldered in new one. Rewired. All works as expected!

Thanks again for all your help.