immersion heater fault?

[jaysparx]

ive got home from work the past few evenings to find my rcd main switch has tripped, checked my sky tv to see what time it tripped, it was approx 9 am. This also happened today (saturday) same time, so i pushed cartridge fuses on DB back in one by one and found the fault to be on the immersion heater,
what i dont understand is why it trips at this time only, and not when it comes on in the early hours!
can anyone help please!

 

[malcolmsanford]

How long as the immersion been RCD protected ? What makes you think it is the immersion that is faulty ? The only way you can really prove the immersion is faulty is by doing a IR test on it.

Have you gas by any chance? Is there anything timed to come on at 9am CH perhaps?

The immersion could be breaking down, perhaps not fully as of yet and when it comes on in the early hours, with no other equipment on at that time it's not leaking enough to trip the RCD. At 9am other things may be then operating and that then would be enough to trip the RCD.

The tihngs to do is an IR test on the immersion if you think that is the problem, then a ramp test on the RCD to see what that is tripping at. Unfortuantely an intermitent fault is the hardest to find and you will need to have the right test equipment, knowledge and patience.

 

[jaysparx]

Its always been rcd protcted, the immersion is on its own fuse and its this one that trips the rcd, we have storage heaters not gas and they are one a seperate DB.
Also we're always out of the house by 9am so i dont think its extra load.
unfortunatly i dont have a multi-function tester at the moment, only a continuity! but i will look in to hiring or borrowing one!
Thanks again for your time and advice.

 

[Octopus]

What happens if you turn the immersion off and back on again at a "different" time? i.e. when you're in.

 

[Guest123]

Can you alter the timer so it switches when you are present to see what happens???

Or,as you havn't the appropriate test gear, switch the immersion off at it's DP switch one morning before you go out and see if the RCD has tripped when you return. If not then you know the circuit is fine and the element is to blame.

 

[jaysparx]

i switched off dp switch on timer, popped fuse back in on the DB then switched timer on again, and it holds! so i have hot water now but i just dont understand why it always trips when it comes on in the morning! i will see what happens in the morning, im fairly new to the trade (2 years) so i might have to swallow my pride an get a proper sparks in! LOL
I'll keep you posted, thanks for the replys.

 

[malcolmsanford]

i switched off dp switch on timer, popped fuse back in on the DB then switched timer on again, and it holds! so i have hot water now but i just dont understand why it always trips when it comes on in the morning! i will see what happens in the morning, im fairly new to the trade (2 years) so i might have to swallow my pride an get a proper sparks in! LOL
I'll keep you posted, thanks for the replys.

If your in the trade Jay then you should be able to get hold of a Insulation Resistance tester, or to be honest you should have one. This is the only way to test that element and is a great tool to have for any fault finding.

If the element is holding in then chances are it's not that. Next thing to look at is that timer. Try leaving the timer out of the equation for a couple of days, just working the immersion/CH on the thermostat. If it dosen't trip then it may be that

 

[telectrix]

IMHO , IR test is the only way to find this fault. otherwise it's head scratching trial and error for the forseeable

 

[andyb]

How can it be head scratching trial & error?
An immersion circuit is one of the simplest circuits in the house, it's a simple radial.
Switch off the immersion & test the element, I think you'll find that that is where the problem is. It's always the element. Drain the tank & change the element.
If the element is ok test the circuit up to the time switch.

But I think you'll find it's the element, if you are using an immersion as your only means of heating water then it will only last a few years.

 

[jaysparx]

cheers malcom, my firm always gets someone in to do the testing so ive not had much call for one, but i can prob convince the missus that i need one now!
thanks again.

 

[ian]

I have seen this problem before, and I found the element was contracting when it cooled down and tripping the RCD. I could not get a fault when I tested because when I arrived it had sealed itself again. When I changed the element it was magically sorted. The problem is relatively common when the element it heating up and expanding, not quite as common when contracting.

Ian

 

[Jabbajaws]

Could l ask guyz, about the actual IR test itself on the immersion heater.?

Im a little bit confused about the test, with it being done on an immersion heater (Sadly l do not have test equipment at present).

If an IR test is done on the element itself, what will be the expected approximate reading if it is in working order? (Im assuming a high IR reading would indicate that the element is damaged in some way).

And why would an IR test be required and not a continuity test on an element? Isn't the element a bare conductor? And wouldn't that affect the IR reading, with the element not having insulation?

 

[telectrix]

sorry you are way out there. IR test should read high. rem. IR test is to earth! the higher the better. continuity should read around 18-20 ohms. the element is not bare, it's mineral insulated in metal.

 

[keniff]

I test by ir, l-n should just be dead short, for example 0 meg ohms on my megger, if I get a reading it's starting to deteriorate

 

[Jabbajaws]

Thank you for the feedback. One final concern l have regarding electrical immersion heaters.

Does anyone remember a few years ago, a fault occuring on a domestic immersion heater, in England, after which the thermostat failed, and the immersion continued to boil constantly, until it eventually exploded, with the tragic loss of a young infant, who was asleep on the floor below.

I noticed that on the news, it was announced that a simple part could have been fitted to the immersion heater to prevent this for a mere cost of approx £5.

What id like to know is What is the part that can be fitted, to prevent this from happening again? Does anybody know? Any info much appreciated...

 

[andyb]

This is a thermal cut out which is fitted as standard in most thermostats now.
What can happen without one is that in the event of a thermostat failure the water in the tank can boil & expand to fill up the storage tank in the loft. If the tank is plastic this eventually collapses allowing the boiler water to run into the room below.

 

[Jabbajaws]

Thank you AndyB. I assumed that the copper tank was what had exploded, due to pressure build-up with the hot water. It had never occured to me that the water would travel up to the cold water tank above it.

My mother has an immersion heater in her home, and it looks ok but im worried about this issue. If l replaced the thermostat on it (should it be an older one) and replaced it with a brand new one, would this prevent this sceanario from happening, in the event of thermostat failure?

My mum's immersion heater is situated in a dangerous place, with regards to the lower floor, should this ever happen...

 

[andyb]

It should do jabbajaws, I'm not sure if the if all new immersion stats have a thermal cutouts so I would check.
The danger is with plastic tanks or similar, metal ones should be ok.

 

[jpmiff]

i suspect that all plastic water tanks designed for domestic water uses as expansoin tanks for open vented CH systems will be able to withstand the maximum water temp of 100 degrees c

 

[telectrix]

i would definitely recommend trying a new stat with thermal overload. it's only 2 connections and drop in. you can tell it's got the overload as there is a small recessed button, similar to on a PC router.