IMMersion heater problem

[litody]

Always happens at christmas.

main fuse box tripped off and have narrowed it down to emersion heater on timer. Either lower element or timer unit.

If I switch the timer unit off I can reset the main fuse box trip switch. But with timer unit switched on reset on main fuse box trips immediately.

Its the 15amp switch on main fuse box trip.

The whole house goes goes onto Economy 7 at night and the timer emersion on during night time.

So how do I tell if its the element or the timer unit which is shot?

I have no testing kit but may be able to borrow one if neighbour is around today.

Can I test across the main element ? I figure if its blown it will be earthing through the water to tank to earth connection.

Any help/suggestions much appreciated?

 

[dcawkwell]

Usually economy 7 cylinder have a second element to heat the water during the day.

Anyway disconnect the economy 7 immersion at the immersion and connect it to a flex and plug it into a socket. If it is the element it will blow trip there as well.

 

[Madrab]

Chances are is the timer, an immersion element is just a heating element so it either conducts electricity or when broken, it doesn't, very unusual for them to short circuit, which is what's causing the breaker to trip. That's usually caused by crossed, frayed wiring or a problem with something else, e.g the timer.

You can bypass the timer by taking the feed wires from the FCU that runs to the timer and disconnect and run them directly to the immersion element, kinda like what DC mentions but rather that plugged into the ring main, it's still on it's own fused radial circuit.
Testing the element itself is easy, circuit tester, isolated the element, disconnect the wires from it, test for resistance, if it comes back a high resistance then element is fine, if it's a zero then it open therefore broken.

 

[matt1e]

Chances are is the timer, an immersion element is just a heating element so it either conducts electricity or when broken, it doesn't, very unusual for them to short circuit,

Sorry I have to disagree It is not unusual for them to short to the case at all

Matt

 

[Gasop]

an immersion element is just a heating element so it either conducts electricity or when broken, it doesn't, very unusual for them to short circuit

You can bypass the timer by taking the feed wires from the FCU that runs to the timer and disconnect and run them directly to the immersion element

Testing the element itself is easy, circuit tester, isolated the element, disconnect the wires from it, test for resistance, if it comes back a high resistance then element is fine

Some quite erroneous and potentially dangerous advice here.

It is not at all unusual for element to short out and trip/blow fuses.

An immersion timer is not usually fed direct from a fused consumer unit,
not a good thing to mess about with these.

A high resistance across an element would normally indicate a fault
around 18-21 ohms is what to look for on a 3kw element.

By passing any electrical device can be very dangerous if one is not
competent or confident with such work.

Best and safest to get someone in to put things right.

 

[litody]

Now I'm confused.

What I did was disconnect the brown and blue wires from the bottom element, the one I thought came on at night. Switched the main fuse on and then switched the timer unit on. It did NOT trip the fuse. But when I turned the booster dial to on it tripped the fuse. But bear in mind with it connected turning the the timer on it did trip (that was during night on time.)
Earth wire was still connected to lower unit.

So I reconnected lower element and then disconnected upper element blue and brown wires leaving earth connected. Switched back on. Fuse did NOT trip. Then switched on booster dial and did NOT trip. Booster light came on.

So it looks to me as though it is the upper element which is causing the problem.

This is very confusing because that element should only come on when booster is switched on which it wasn't when the fuse went originally.

So what do you think? Is it the top element which is shot? Are the elements connected the wrong way round. i.e. top to bottom and bottom to top. It's been that way for the last twenty years.

I have always thought hot water is limited before it runs out. i.e. One long (10-15 mins) power shower and thats your lot. (only small house with small tank)

Any more thoughts?

Thanks.

 

[litody]

Update.

Just turned main time dial so it comes on and I can hear it heating up water. Only bottom element is connected so that's definitely working and it MUST be the top element that is causing problem.

Still confused because if that's the case then WHY did fuse trip in the first place. I haven't used the booster dial for months and so top element shouldn't have been used for months. So why would it suddenly trip fuse?

This suggests to me that it is the timer unit which is shorting?

Thoughts please.

p.s. haven't got hold of tester as neighbours not there today but at least I can have hot water until after the holidays.

 

[Madrab]

Some quite erroneous and potentially dangerous advice here.

It is not at all unusual for element to short out and trip/blow fuses.

An immersion timer is not usually fed direct from a fused consumer unit,
not a good thing to mess about with these.

A high resistance across an element would normally indicate a fault
around 18-21 ohms is what to look for on a 3kw element.

By passing any electrical device can be very dangerous if one is not
competent or confident with such work.

Best and safest to get someone in to put things right.

I must disagree, the advice was not erroneous nor potentially dangerous if conducted correctly. Granted I should have stated only proceed if the OP was confident in working with an isolated circuit.

I think I have come across 1 or 2 elements in my time that are continually shorting compared to many many others that have blown and then have a zero resistance/conductive rating, so in my experience continually shorting elements have been rare.
All immersion elements should be connected to a radial or spurred circuit via a FCU - Fused Connection unit, not fused consumer unit as you stated, it should not be directly connected to the consumer unit without a separate fused unit inline.
I mentioned bypassing the timer as an immersion element can be directly connected to a fused connection unit, that way it may remove the element that is causing the tripping, that being the timer.
It depends on how you rate a high OHM reading, which is why i stated if it registers 0 then the element has an issue, Again happy to concede that I should have been clearer on this point.
Oh and a Merry Crimbo

 

[Madrab]

Sorry litody, didn't mean to rant in your post.

If that's the case then it could be just a bad contact in the timer unit and because it hadn't been used in so long then the contact arc'd and tripped the breaker unit.

Is it all working as it should now or is it still tripping when you boost the top element?

In my experience, the bottom unit is the unit that runs overnight on Economy7 and the top element is the element that is used as the boost, to give you a small amount of hot water quickly.

I think you need to investigate the circuit from the Fused Connection unit, through the timer to the immersion elements & thermostats.
To avoid any issues then please only attempt if you are familiar/comfortable with testing circuits, continuity and resistance and please ensure that any testing is done following all safety procedures when working with live circuits. If anything is of concern then please get a professional to do the work.

 

[litody]

Currently it's working but with the top element disconnected.

What I find odd is that with both elements connected and the main timer outside of its on period, there should be no current flowing to either element(with boost switched off). BUT switching on main fuse box trips the fuse box. This is why I think its the timer which is faulty. Otherwise why would the fuse box trip when there should be no current flowing to either element?

So question is, am I right in that if main timer is not in ON period of time, there should be no current flowing to either element (with boost off) and therefore it must be timer which is causing fuse box to trip?

I looked inside timer unit box and can't see anything that looks amiss. All wires in there look in very good condition and couldn't find any loose connections. But to check actual switching I would need to completely dismantle it and I'm not about to do that as it looks too fiddly and possibly easy to break (it's mechanical and not digital). I'd rather just replace complete timer unit (which looks simple if new one is as well labeled) but don't want to do that if its not the timer unit causing problem.

Will try and test top element resistance later tody.

 

[Madrab]

Yes, you're correct, logically, if the timer is in its off period then there should be no current flowing to either element therefore if the breaker trips then I would suspect the timer is at fault. That was why I suggested bypassing the timer and connecting the element directly to the fused connection unit (FCU) but I suspect that the timer is not fed from an FCU rather it is fed directly from the consumer unit?
If that's the case I wouldn't suggest that a DIY bypass is performed to rule out the timer as you would need to run a min 1.5mm cable directly from the CU and that's not a DIY job IMO.

 

[JohnD]

It did NOT trip the fuse.

When you say "fuse" do you mean "fuse" or do you mean "RCD?"

What letters and numbers are on the thing that trips?

(I think I know the answer, but we'll see)

Without wishing to be rude, I think Madrab is talking rubbish.

 

[EFLImpudence]

Just to clarify -

Immersion heaters are usually on their own 15A/16A circuit and therefore a 13A fuse is unnecessary and pointless - a 20A double pole switch is all that is required.

It can be argued that 13A fuses 'do not like' to be fully loaded continuously but that is a different matter and probably down to the quality of the FCU.

Even if connected directly to the ring final circuit fusing down to 13A is not really required as the immersion will not overload the conductors although fault current conditions must be correctly assessed.
DIYers will not be able to assess this.

Obviously if a socket and plug is used a 13A fuse would be unavoidable but this is not good practice.

 

[matt1e]

Currently it's working but with the top element disconnected.

What I find odd is that with both elements connected and the main timer outside of its on period, there should be no current flowing to either element(with boost switched off). BUT switching on main fuse box trips the fuse box. This is why I think its the timer which is faulty. Otherwise why would the fuse box trip when there should be no current flowing to either element?

So question is, am I right in that if main timer is not in ON period of time, there should be no current flowing to either element (with boost off) and therefore it must be timer which is causing fuse box to trip?

No you are not right
I take it that its an RCD that's tripping? (does it have a test button on it?)
It is likely tripping because it is detecting a neutral/earth fault and the neutral isn't switched via the timer (its permanently connected)
Regardless of what Madrab is saying This is MORE THAN LIKELY due to a failed element,he obviously hasn't dealt with enough of them, they can corrode through and short to earth like this

Matt

 

[JohnD]

I think that's most likely.