Immersion Thermostat wire burns out

[brixtonia]

I have a 4kW immersion heater in a thermalstore - very recently replaced by a plumber as the previous one was leaking. Supplied with a cotherm thermostat and a second pocket for a safety cutout into which the safety cutout from the previous immersion was fitted and seems to be working fine.

I smelled burning and removed the cover to find that the insulation on the wire between the cutout and the stat was melting. The end closest to the stat was clearly more affected. The stat has deformed around the point the wire connects to it and the stat has ceased to work.

The wire between the safety and the stat is exactly the same as the wire from the spur to the immersion which has been in place since I can remember. The connection seemed sound but hard to tell now.

One thing I had noticed after it was installed is that it was cycling on and off quite rapidly.

Just a faulty stat?

 

[marconi]

Usually, the electric immersion heater is rated at 3kW. You mention the IH is now rated at 4kW after it was replaced by the plumber. This would be something to clarify first.

A 4kW IH draws more electric current - about 16Amps - whereas a 3kW one draws about 12Amp. It is important that the cabling, connections, thermostats , switches and supply from the consumer unit are suitably rated for the power of the IH.

If you send me details of the thermal store I will look up the power of the IH which should be fitted.

 

[brixtonia]

Thank you. It has replaced a 4kw immersion and the stat is / was 20A rated. I've replaced the stat and it does not cycle in the same way. I think it must have been a fault with the stat.

 

[ColinACJL]

I have a 4kW immersion heater in a thermalstore - very recently replaced by a plumber as the previous one was leaking. Supplied with a cotherm thermostat and a second pocket for a safety cutout into which the safety cutout from the previous immersion was fitted and seems to be working fine.

I smelled burning and removed the cover to find that the insulation on the wire between the cutout and the stat was melting. The end closest to the stat was clearly more affected. The stat has deformed around the point the wire connects to it and the stat has ceased to work.

The wire between the safety and the stat is exactly the same as the wire from the spur to the immersion which has been in place since I can remember. The connection seemed sound but hard to tell now.

One thing I had noticed after it was installed is that it was cycling on and off quite rapidly.

Just a faulty stat?

You can get clamp meter for a about £25 online.

 

[brixtonia]

So the new stat has also burned out. The live connects first to the safety cutout, then into the 20A temp control stat, then to the element (using 2.5mm wire). It's always the wire between the two stats which seems to overheat. The wire insulation melts and the terminal melts at the point at which it connects to the second stat (20A temp control).

The 4kw immersion element seems fine with resistance testing at about 14-15ohms. The immersion replaces another 4kw immersion.

Any suggestions?

 

[johntheo]

Have you checked the voltage, also suggest (when repaired) check voltage drop between two above mentioned terminals.

 

[brixtonia]

Have you checked the voltage, also suggest (when repaired) check voltage drop between two above mentioned terminals.

Thanks. I will check when I have a new stat. Just noticed that the safety cutout (in line before the variable temp stat) is 16A rated. It operates fine and has been in there for years, but could its rating possibly be causing a problem for the stat?

 

[johntheo]

I don't think so as its not doing any switching normally, your measured 14 ohm resistance means a current flow of 16.43A at a nominal 230V (if that's the UK voltage), normally, heat damage with fairly high loads is caused by lose connections but I think that is very unlikely, now at any rate, I would measure the volt drop (immersion on) between the furthest away terminals first, if no volt drop then problem elsewhere, maybe the control stat is rubbish and causing very high resistances while switching (unlikely) but if you get a volt drop of say 5V then this will result in ~ 80W heat being generated somewhere between the two stats, you can then narrow it down by probing separately across each set of stat terminals, you say it's the wire between the two stats that's overheating which might point to a control stat problem but see what readings you come up with.

 

[brixtonia]

I don't think so as its not doing any switching normally, your measured 14 ohm resistance means a current flow of 16.43A at a nominal 230V (if that's the UK voltage), normally, heat damage with fairly high loads is caused by lose connections but I think that is very unlikely, now at any rate, I would measure the volt drop (immersion on) between the furthest away terminals first, if no volt drop then problem elsewhere, maybe the control stat is rubbish and causing very high resistances while switching (unlikely) but if you get a volt drop of say 5V then this will result in ~ 80W heat being generated somewhere between the two stats, you can then narrow it down by probing separately across each set of stat terminals, you say it's the wire between the two stats that's overheating which might point to a control stat problem but see what readings you come up with.

Thanks. I will try all of this once I get a new stat. Only doubt I have about it being a faulty stat is that this has been a consistent problem with various stats. Even on the previous immersion. But then what else could it be? I don't suppose there is any mileage in changing the order of the cutout and the temp stat? Or even putting the cutout after the element?

 

[johntheo]

Is the old stat(s) actually burned out or can you hear it switching in/out if you turn the dial?.

 

[marconi]

Post some pictures please which includes details of the burned out link wire and its terminals.

Did you make up the link wire? If you did what did you use?

Also send a picture or details of the IH element, control stat and safety stat.

The cotherm stats I have looked up are rated at 16A which is suitable for a 3kW IH element but unsuitable for a 4kW element. Are you sure the two stats are rated at 20A?

 

[brixtonia]

Is the old stat(s) actually burned out or can you hear it switching in/out if you turn the dial?.

No switching sound but when measured with a multi meter at ambient temperature it shows no circuit up to about 60C then about 3 Ohms resistance.

 

[brixtonia]

Post some pictures please which includes details of the burned out link wire and its terminals.

Did you make up the link wire? If you did what did you use?

Also send a picture or details of the IH element, control stat and safety stat.

The cotherm stats I have looked up are rated at 16A which is suitable for a 3kW IH element but unsuitable for a 4kW element. Are you sure the two stats are rated at 20A?

The link wire is the positive wire taken from a new section of 3 core 2.5mm2.

The stats I have used are Cotherm TSD 00706 and TSD01106 - both rated 20A (and printed on the side) and, importantly, variable to 80C (as this is a Thermalstore).

The safety is a Sunvic VKL 2352 with manual reset (16A). It's been in place for a while but seems to work fine.

Can't get a photo just now and it's dismantled but as previously described mains live runs into Sunvic, then into Cotherm, then to immersion terminal. Neutral fixes to the opposite terminal. Contacts are good.

 

[johntheo]

No switching sound but when measured with a multi meter at ambient temperature it shows no circuit up to about 60C then about 3 Ohms resistance.

Don't think that means alot because if the closed contacts of the thermostat had a resistance then they would be generating ~ 550watts in the stat itself when wired in series with the 14 ohm element.

 

[marconi]

Is there a reason why the tank is fitted with a 4kW element rather than the typical 3kW element? Could you use a 3kW element instead or are you in a hurry to heat up the tank?

20A rated safety and control stats such as those Cotherm types are on their limits to switch and pass 20A for long periods of time and will inevitably deteriorate through concentrated electrical heating of their contacts and connections. The physics of electrical heating is that it is proportional to the current squared not simply the value of the current. This means Ohmic heating as it is sometimes called rises rapidly with each increment of current eg (read as I/Isquared) 1/1, 2/4, 3/9, 4/16...16/256....20/400.

Those 4 kW elements require much more robust stats rated at say 30A - 40A to give reliable service. For 4kW and higher elements , ordinary stats like the cotherm ones you have are used to switch an electromagnetic switch called a contactor on and off. The stats only then have to handle the very small current required to power the contactor's electromagnetic operating coil; the contactor has substantial switch contacts and surfaces to safely and reliably close and open the immersion heater circuit through which flows 4000/230 = 17 Amps. In the early stages of heating cold water the current is likely higher than 17A because the element's resistance is lower than at warmer water temperatures.

One also needs to employ the correct power cable to the element which has heat resistant insulation suited to higher operating temperatures. It is sold as 'immersion heater cable'. eg:

Doncaster Cables Immersion Heater Cable 3183TQ 2.5mm2 x 1m Coil - https://www.travisperkins.co.uk/cable-lengths/doncaster-cables-immersion-heater-cable-3183tq-2-5mm2-x-1m-coil/p/759014

Finally the terminations of the flex must be made using ferrules or ring crimps to maximise conductivity of electricity.

I believe then the combination of 4kW IH element and those Cotherm stats is unsuitable. One could stick with the Cotherm stats if you swapped to a 3kW element. To use a 4kW element you require a 30-40A contactor.

I have not looked up your profile but if you are not an electrician then this is a job for one to complete for you. Hope this helps.