Is this electrical fault diagnosis correct?

[Chris S]

I've got a storage heater on it's own circuit. It has an MCB rather than a traditional fuse. After the heater has been on for about five minutes the circuit trips.

An electrician checked the heater and said that there was nothing wrong with it. He also said that the circuit was earthing out somewhere. He showed me a tester beeping when he held it to the other MCBs but staying silent when he held it to this one.

If the circuit was earthing out then how could the heater come on for five minutes?

 

[raleighnut]

I've got a storage heater on it's own circuit. It has an MCB rather than a traditional fuse. After the heater has been on for about five minutes the circuit trips.

An electrician checked the heater and said that there was nothing wrong with it. He also said that the circuit was earthing out somewhere. He showed me a tester beeping when he held it to the other MCBs but staying silent when he held it to this one.

If the circuit was earthing out then how could the heater come on for five minutes?

Expansion within the heater?

 

[irw]

I've got a storage heater on it's own circuit. It has an MCB rather than a traditional fuse. After the heater has been on for about five minutes the circuit trips.

An electrician checked the heater and said that there was nothing wrong with it. He also said that the circuit was earthing out somewhere. He showed me a tester beeping when he held it to the other MCBs but staying silent when he held it to this one.

If the circuit was earthing out then how could the heater come on for five minutes?

Are you sure it's an MCB? An MCB trips on current overload, implying a possible short circuit. An RCD trips when it detects leakage to earth (by means of monitoring for an imbalance between live and neutral). An RCBO does both.

Not quite sure what his beeping tester was about- if it was voltage detection, all it means is there was power on the other breakers and not that one!

 

[Chris S]

Are you sure it's an MCB? An MCB trips on current overload, implying a possible short circuit. An RCD trips when it detects leakage to earth (by means of monitoring for an imbalance between live and neutral).

Yes - The electrician said there wasn't any RCD protection on the circuits so they would be MCBs.
I've used him a couple of times in the past and he seems OK. I may not have repeated what he said correctly. I was just wondering how the circuit could take 5 minutes to overload if the heater wasn't faulty.

 

[gbb]

Yes - The electrician said there wasn't any RCD protection on the circuits so they would be MCBs.
I've used him a couple of times in the past and he seems OK. I may not have repeated what he said correctly. I was just wondering how the circuit could take 5 minutes to overload if the heater wasn't faulty.

Depending on the type of element, they can behave completely differently cold to hot.
On our machines it's not unknown for a machine to trip when it reaches a certain temp. The difficulty then is finding which element...usually a process of disconnecting of individual elements.
None of this may relate to your problem, just an illustration of how elements behave.

 

[Chris S]

[QUOTE 4493750, member: 9609"]disconnect the night store heater and wire in something else in its place (just a regular electric heater) this should help to confirm if it is the heater or the circuit that is causing problems. Also depending on the wattage of the night store, (if below 2500w) you could wire that up to a 13amp socket and see if it works.

Obviously check with an approved electrician before a attempting any of the above to make sure it is safe.[/QUOTE]

Unfortunately the storage heater is 3.4 kW but I'll try wiring something else into the socket to see if it works.
I don't want to spend several hundred pounds having a new circuit installed only to find out it was the heater all along.
Perhaps the electrician was trying to cover this possibility with the beep test

 

[Chris S]

I put a 13 amp plug on the storage heater and it worked OK for 30 minutes. I then noticed that the plug and lead were getting very hot so I turned it off.
Is this what you'd expect if you put a 13 amp plug on a 15 amp appliance?

I'll try wiring a greenhouse heater into the circuit over the weekend to see if that blows the MCB.

 

[mk6golf]

If it is only an MCB then I will safely say it is the heater. As it is warming, the characteristics change and it seems to be demanding more current than your circuit is rated for, thus tripping the circuit. It'll be nothing to do with earth leakage unless an RCD or RCBO is fitted.

 

[Chris S]

I wired a 13 amp greenhouse heater into the circuit last night and the MCB also blew after about an hour. There's definitely a fault with the circuit, though that's not to say there isn't a fault with the storage heater as well.

 

[Chris S]

[QUOTE 4502014, member: 9609"] out of curiosity is it an older house that has been upgraded to MCBs.[/QUOTE]
The house was completely rewired in 2002. The fuse box was replaced by a consumer unit with MCBs.
I've just seen a video on YouTube showing how to replace these, I didn't realize it was so easy. I've just swapped the one that keeps blowing with one that doesn't, just to make sure it isn't faulty. I'll find out what happens tonight.

 

[midlife]

Back in my physics A level days the resistance of metals like nichrome rises with temperature.....will that have any effect in the current needed to keep on heating the element?

Shaun

 

[the_mikey]

Back in my physics A level days the resistance of metals like nichrome rises with temperature.....will that have any effect in the current needed to keep on heating the element?

Shaun

Ohms law says I = V/R

For I to increase R has to decrease (or V has to go up, but hopefully that is stable at 240V ish)


I have recently replaced an MCB at work as it was tripping at 2A when it was rated for 6A, although it's rare for that to occur, don't discount the possibility of a faulty MCB.

 

[slowmotion]

MCBs are designed to have two tripping mechanisms. If the fault is a gross overcurrent condition ( a much larger current than the rating of the MCB) such as a short-circuit, it is detected by an electromagnetic circuit that trips within milliseconds. If the MCB is passing a slightly larger current than its rating, it detects the fault thermally, and it can take many minutes to trip. My guess is that the MCB is carrying a slightly larger current than it was designed for.

See page 9 of this pdf for details of type B MCBs, the most common type used in domestic installations. (Graph on left of page)
https://www.tlc-direct.co.uk/Technical/DataSheets/MK_Sentry/Sentry_Tech.pdf

 

[Chris S]

If the MCB is passing a slightly larger current than its rating, it detects the fault thermally, and it can take many minutes to trip. My guess is that the MCB is carrying a slightly larger current than it was designed for.

It's probably a slightly larger current then, when I swapped the MCBs it took 3-4 hours to trip.

I don't want external cabeling so replacing the circuit means chiselling out plaster (by an approved contractor just in case it contains asbestos), replastering the walls and redecorating. I think I'll just leave this heater turned off until my next round of major maintenance.

Thanks for everybody's help.

 

[slowmotion]

If you want to see what the current passing through the circuit to the MCB actually is, use one of these....

http://www.screwfix.com/p/lap-digit...tracking url&gclid=CPqGl4XC0c8CFSMW0wodmDcOGg

All you have to do is open up the clamp and put one of the conductors ( Live or Neutral, but not both) inside.