Immersion Thermostat and element - Burnt Out after 4 months!

[Newboy]

The way that the wire is connected to the immersion terminal looks (to my eyes) poor - there is uneven pressure on the stranded core. Where the connection is poor, there's the potential for a high resistance connection, that leads to heat being generated at the connection point and, as the core(s) are copper, the heat is conducted along the core leading to what you found on your first installation.

Personally I'd prefer the see the stranded conductor form a circle around the centre of the terminal so that when the clamp is tightened there is more even pressure over a greater contact area.

 

[Newboy]

I'd be interested if you could provide the Screwfix product codes for your latest purchase

 

[AdrianUK]

To the OP:

As a general rule, one would check the tightness of ALL screws and connections, including the factory fitted ones, when installing.

Did you check the tightness of the screw connecting that brown wire to the heater element when you fitted the immersion?

 

[bmouthboyo]

Thanks guys,

My latest purchase is the exact same as the original: https://www.screwfix.com/p/tesla-titanium-immersion-heater-element-27/75262

I imagine I would have checked all the screws the first time around as it would have popped out when I moved the thermostat around etc.

So sounds like might be worth replacing the stat to element wire with something without crumpets ends so I can wrap it around the terminal screw?

What gage wire would do? I have plenty of fine strand auto electric low AWG wire, or is that a big no no with AC?

 

[flameport]

so I can wrap it around the terminal screw?

That, or crimp a ring terminal onto the wire (only if you have a proper ratchet crimp tool though).

What gage wire would do?

1.5mm². Perhaps 2.5mm² - not larger, as it will be difficult / impossible to fit into the terminals.
auto wire may be suitable, depends if the insulation is rated to mains voltage, and it needs to be suitable for high temperatures as well.

After it is fitted, check all terminals are tight, then after a few days of heating / cooling, check the terminals again.

 

[endecotp]

I would say that ferrules - the crimped-on bits of metal that appear to be fitted to both ends of that wire in your new pictures - are not appropriate for a “saddle” type of screw terminal. You really want the wire to be wrapped in a U-shape around the screw so that the saddle is flat. If the saddle isn’t flat the screw won’t be straight.

Supplying it like that is far from ideal, IMHO.

Better link: https://www.screwfix.com/p/tesla-titanium-immersion-heater-element-27/75262
Interesting to read the reviews. Ignore the 4- and 5-star ones of course. There is one that talks about the live wire rusting, with a photo - is that you? (I guess not, it says 2015.) I can’t get the photo to enlarge.

I note that Tesla seem to be an actual company, not just a Screwfix imported-junk brand, so you could ask them what they think. http://teslauk.com/contact

I would not substitute different wire unless you convinced yourself that the current one is too thin. It is possible to measure it.

 

[bmouthboyo]

I would say that ferrules - the crimped-on bits of metal that appear to be fitted to both ends of that wire in your new pictures - are not appropriate for a “saddle” type of screw terminal. You really want the wire to be wrapped in a U-shape around the screw so that the saddle is flat. If the saddle isn’t flat the screw won’t be straight.

Supplying it like that is far from ideal, IMHO.

Better link: https://www.screwfix.com/p/tesla-titanium-immersion-heater-element-27/75262
Interesting to read the reviews. Ignore the 4- and 5-star ones of course. There is one that talks about the live wire rusting, with a photo - is that you? (I guess not, it says 2015.) I can’t get the photo to enlarge.

I note that Tesla seem to be an actual company, not just a Screwfix imported-junk brand, so you could ask them what they think. http://teslauk.com/contact

I would not substitute different wire unless you convinced yourself that the current one is too thin. It is possible to measure it.

Thanks for that. So to use the existing wire supplied I would need to cut the Ferrells off and strip back enough to wrap around the terminal. Would I need to do both ends or would the stat screw make better surface contact as?

 

[endecotp]

I suspect that the stat terminal is well-suited to take a ferrule.

 

[Newboy]

(only if you have a proper ratchet crimp tool though)

 

[bmouthboyo]

So today I attempted to remove the old immersion element but despite having it off only 4 months ago it is seized solid! I tried tapping it loose by moderately hammering the immersion wrench tool but it wont budge. Using my strength to unscrew it was just starting to warp the copper causing it to start to deform the header part of the tank so I immediately stopped. I think the heat that was generated has somehow made it seize up in the short space of time since installing.

What is strange is that the resistance between the element terminals is 20ohms which seems to be good, however my Timeguard FST77's output keeps instantly turning off 95% of the time upon being activated (boost or timed mode):

The rare times it keeps the output on the element seems to work fine, but as soon as the output is turned off it wont activate again and just keeps briefly flashing as in the video above. So now I am wondering if the timer is faulty. I changed the fuse just in case and no difference, reset it and entered time etc again and this didn't effect anything.

The burnout of the live wire must be linked to the timer issue. Either the immersion connection with high resistance caused the timer to break or the faulty timer caused the immersion connection to break. Even if the timer had been accidentally turned on constantly this shouldn't have happened surely?

Thanks for all the comments so far, reassuring to know people out there help like this.

 

[flameport]

The burnout of the live wire must be linked to the timer issue. Either the immersion connection with high resistance caused the timer to break

Possible, if the damaged wire shorted against the metal case of the immersion, although that should have blown the fuse as well.

or the faulty timer caused the immersion connection to break.

Not possible, it's a switch which is either on or off.

In any event that timer is clearly defective, so will require replacement.

Immersion heaters should not be powered with the cap removed, as several live parts are exposed.

 

[bmouthboyo]

Possible, if the damaged wire shorted against the metal case of the immersion, although that should have blown the fuse as well.


Not possible, it's a switch which is either on or off.

In any event that timer is clearly defective, so will require replacement.

Immersion heaters should not be powered with the cap removed, as several live parts are exposed.

Ordered another one and getting a refund for that so should be able to install tomorrow with existing seized immersion.

Is there any other checks I can do to ensure the element is OK? As said the resistance was 20 ohms but should I check the earth? I read someone mention the earth insulation resistance or something but a bit unsure where to measure this from or what this should be.

The cap was off just for the video to show the wiring and fault, always on usually.

 

[flameport]

All tests with power off:

For the element - resistance should be about 20 ohms between the two terminals.
Either terminal to the metal case / earth should be a very high resistance - many megohms at least.

Thermostat should be 0 ohms between the two screw terminals when set to high temp, and open circuit when set to low temp.

 

[AdrianUK]

I wouldn't attempt to re-use that immersion.

The element may well check out as being good, but that terminal has now oxidised and will never form a good connection. If you attempt to re-use it you WILL be back here again in a few weeks.

A few tricks to help you get the old immersion out:

Try using a hacksaw blade around the gasket to try & cut away any old sealant forming a lock
Heating the immersion heater head to crack the seal
Using a hammer to hit the immersion spanner (with the tank full of water to provide some weight)

I don't think that the original cause was the wire being too small. The immersion running current is only around 12A. Even a piece of 0.75mm2 is going to take this all day without getting hot to the point were it starts to oxidise. RS quote 0.75mm2 tri-rate as having a rating of 14A.