Storage Heater Puzzle?

JOHNPT

Hello. I have got a real mystery with a storage heater. In apt there is 2 storage heaters one hall one living room. Hall works fine problem is living room storage heater keeps triping the fuse. There brand new storage heaters. I changed fuse too no avail. It seems to trip when contactor initial energises. I have energised the contactor myself to test storage heater and sometimes it trips straight away and sometimes it works fine and storage heat works fine.
Would it be possible to change the fuse to a C thpe Mcb. I was thinking maybe the initial starting surge is causing fuse to trip.
Any advice would really be appreciated.

Thanks in advance

davelerave

there is no surge so 'c' type not needed or allowed(for domestic) .check wiring and appliance. ,is it a 20amp

JOHNPT

Thanks for reply.

Ya its 20 amp. And i changed the fuse so it not faulty. When i energise contactor first time it will trip fuse. However when i do it second or third time fuse stays up.
However it always trips at 11pm at night when night time rate comes in.

davelerave

might be a fault.take a resistance reading of circuit with heater swiched on at wall .should be 20-30 ohms approx. also test insulation resistance with wall switch off/on.

JOHNPT

Thanks davelerave.

So do u think its heater maybe faulty?
If the resistance is above or below 20-30 ohms what would that indicate.
When i was testing heater i once connected it into a socket beside heater and it worked fine.
Dosent seem make sense if it was faulty wud it not trip MCB or ELCB.
Thanks a million for your advice

davelerave

no. wiring fault.

2011

take a resistance reading of circuit with heater swiched on at wall .should be 20-30 ohms approx

+1. This would be my first suggestion.

there is no surge so 'c' type not needed or allowed(for domestic)

I think that a C type is allowed. It is just that the disconnection times must be met which means that a lower earth fault loop impedence is required.

also test insulation resistance with wall switch off/on.

A good idea, but beware the spur outlet or the 20DP switch may have a neon indication lamp!


What KW rating is the storage heater???

However when i do it second or third time fuse stays up.

I assume you mean that it is possible to get the MCB to stay on with the heater on (eventually). If you have a grip on ammeter it would be good to measure the current being drawn. This may give an indication of what the problem is.


There is a chance that the MCB is faulty. Try another one just to see. If not the chance are it is a faulty heater or an insulation resistance issue.

davelerave

true .there might be a neon.prob an earth fault somehere though.as john has swapped mcb.assuming load is standard

2011

If the resistance is above or below 20-30 ohms what would that indicate.

Ohms law. For example if the resistance was 10 ohms then the current drawn would be 230/10= 23 amps. A loadd of this size suddenly switching in may well cause a 20A MCB to operate.

no. wiring fault.

The chances are davelerave is correct.

2011

true .there might be a neon.prob an earth fault though

Yes.
What I mean is that it is best to ensure the switch is in the off position rather than in the on position with the heater disconnected.

davelerave

the megger will show an earth(only) problem in heater separately to insulation tests on cable .even with neon.that was my thinking.L-N fault would be uncommon.

2011

the megger will show an earth(only) problem in heater separately to insulation tests on cable .even with neon.that was my thinking.L-N fault would be uncommon.

It is uncommon as you say, but it does happen.

All I am suggesting is to ensure that the DP switch or the spur is in the Off position when the megger test is carried out. If you are doing this test between live and earth you may as well bo it between live and neutral while you are at it. You never know perhaprs a screw has gone through the cable.

davelerave

we are confused.i'm suggesting the insulation tests with switch off as you say.but additionally megger the heater for earth fault with neon in circuit.

2011

I see what you mean.:D

JOHNPT

Thanks Davelerave and 2011 for replies.

Just a few questions. If I left double pole switch of heater off and waited till 10pm for contactor to energise and it didnt trip the MCB could i assume that the wiring to the double pole switch is ok?
Therefore it must be faulty heater?

davelerave

not really john.non-rcd protected circuits (especially) need to be meggered before use.i've never had any probs with storage heater mcbs tripping.sounds like a fault .visual check for damage, ammeter/loading and insulation tests should quickly show up any problem if it exists as you say you replaced mcb already.

JOHNPT

I dont have a megger just a multimeter. I suppose I need buy one soon anyways. Do u know where is good place to get one or are they expensive?

JOHNPT

Also do u knpw where good place buy clamp on ampmeter on do these meters show the max current on starting heater. If they do and it showed it was drawing a large current on switch on maybe this wud suggest a faulty element?

davelerave

there is no surge that i'm aware of as it's a resistive load .if the circuit was overloaded this would take a little time for the thermal trip .so i would guess a L-E fault if it's an instantaneous trip assuming you've tried swapping out mcb.as always though it could be something completely different:D

JOHNPT

Ya its instant trip the first or second time contactor energised. But then MCB stays up and it works fine thats what has me baffled. If it live to earth fault surely it wud trip everytime.
Could a faulty contactor couldnt cause this? - just want rule that out.

thanks

2011

Also do u knpw where good place buy clamp on ampmeter

Euroslaes, Kellighers or NEW.

Fluke make a great grip on ammeter/volt meter (AC & DC)/contunity tester/ohm meter all in one job.

It costs about €130

You will use it for many years so, get a Robin or Fluke, dont buy crap.
A meggar is big money in comparison!

Here is a simple idea:
Wire the storage heater with a different 2.5 cable just lying on the ground. See what happens. Then you know where the problem is, the cable or the heater.

davelerave

you've already eliminated the mcb ?so i would guess a L-E fault.screws and water, nicks damage etc. can produce varied outcomes .megger would eliminate it quickly anyhow.i don't like going too far out on a limb with speculation as i could be completely wrong

JOHNPT

Thanks for all advice. I will let u know how it goes.

JOHNPT

I connected heater to supply going to hall with an extension lead and still tripped so then took resisitance reading between live and earth and neutral and earth was 15 mega ohms but was only 2 megaohms between live and earth - it turned out that where flex was connected to heater the insulation on live was melted and was touching the earth of heater causing it to trip. All working fine now.
Thanks Davelerave and 2011 for all very good advice.

Just out curiosity what should the minimum resistance be between live and earth or live and neutral in 3kw heater like this?

2011

it turned out that where flex was connected to heater the insulation on live was melted and was touching the earth of heater causing it to trip.

Fair play to you! I bet you got great satisfaction from finding that. The next time you have a similar problem I bet you will find it much quicker.

BTW, the flex should be 3 x 2.5 HR (heat resistant) flex. I bet it was not! It feels different to normal flex, much stiffer and harder to strip. Make sure you use the correct flex!

Just out curiosity what should the minimum resistance be between live and earth or live and neutral in 3kw heater like this?

It should be over 10M Ohms which is 10 x 10^6 Ohms (or 10 million Ohms)! Generally you would get a reading of >100M Ohms, in other words higher than the meggar can measure. Be careful using a meggar on equipment, it can destroy electronic circuits. I saw a fire alarm panel destroyed ths way once!

davelerave

the heater could be meggered at 250v to locate a problem but the insulation resistance of these heaters can be low so it takes some judgement.as 2011 said care is needed when testing to avoid damaging equipment.obviously the downside of working without enough knowldge and meters is the risk to customer safety and insurance issues etc.