Totally stumped. Night Storage Heater tripping out MCB, but ok on daytime circuit.

[Steve Poulton]

Please help. I have a basic understanding of electirics, but 5 Electricians are stumped and I don't know where to go next. My mother in law heats her house by 4 Night Storage Heaters, all of which are wired to a Economy 7 supply/fuse board, with no RCD just MCB. The largest of the Night Storage Heaters began to fail and trip the MCD. To cut a long story short, she ended up having a new one fitted. Unfortunately this new one also now trips the MCB. The electricians have tried swapping the various supplies to the various heaters around in the E7 fuse board and the same heater is tripping the different MCB. It would be an obvious thing to consider that there is a fault on the circuit between the board and the night storage heater. However, they have now wired it into the day time board and it is not tripping. Has anyone got any ideas as we need to get it wired back into the E7 circuit??

 

[ferg]

Well TBH it puts my smart arse comment into perspective.

Good find.

I don't think I'd have found that one quite as quickly as I made out I would.

My apologies for contributing to the stick you got and thank you for posting the fault.

 

[KEV 1 N]

Glad you’ve found the fault, it looks as if it’s been caught in time before it caught fire!
I think had there been power present at the time of investigating then the fault would have been found sooner. I think the E7 supply should have an override function for testing purposes.

 

[telectrix]

I was going to suggest fitting a gas boiler and rads.................

thinki did that, just before i grew the beard.

 

[Lucien Nunes]

Interesting find, that switch. But I'm struggling to work out exactly how it caused the tripping. A high resistance connection or even an arc fault in series with a mainly resistive load will only lower the average current. So if the MCB would hold with a good supply, it would still hold with an arcing one. For a transformer, motor or SMPSU load things are different - reduced voltage can increase the current and repeated inrush can also trip a borderline breaker. I'm not familiar with current production heater innards but I would be surprised if they have an element that would present an overload in the absence of active regulation by phase angle or duty cycle. So I am not convinced that the relationship between the bad switch and the MCB trips was low voltage as such.

Looking at that voltage drop, let's consider the load on the switch and its destructive capability. I don't know what the total connected load was at the time, but if we say 2kW per heater for a total of 8kW or 35A rated, reduced to 210/235x35=31A. With 235V supply and only 210V reaching the busbar, the power dissipation in the switch would be 31x(235-210)=875W. That's nearly a 1-bar electric fire, so even with much of the heat conducted away along the cables, the whole lot would be burnt to a crisp by now.

So, what's the connection? Inrush (into what?) Arcing causing a load regulation device in the heater to go bonkers (and what would be the load at 100% duty?). Heating in the board caused by the switch conducted along the busbar (at least seems theoretically possible but would probably only affect nearest MCB).

Suggestions on a postcard?

 

[Darkwood]

Appreciate the feedback always nice when we get closure, seems I was spot on with my suggestion the problem could be upstream of the mcb given the info given, It will be one experience that hopefully will give you a heads up next time you get similar issues, try not to get too complacent that all problems with a circuit have to originate downstream of the protective device.

 

[Deleted member 9648]

will done LT21,if that had been done in the beginning then it would not be a 6th visit would it not .

Easy to be an expert with hindsight on your side.

 

[marconi]

Lucien in your last post you wrote:

Looking at that voltage drop, let's consider the load on the switch and its destructive capability. I don't know what the total connected load was at the time, but if we say 2kW per heater for a total of 8kW or 35A rated, reduced to 210/235x35=31A. With 235V supply and only 210V reaching the busbar, the power dissipation in the switch would be 31x(235-210)=875W. That's nearly a 1-bar electric fire, so even with much of the heat conducted away along the cables, the whole lot would be burnt to a crisp by now.

I think you have misinterpreted what LT21 said:

While testing incoming voltage with multimeter I had clamp meter on out going supply. Voltage fluctuating between 235v and 210v resulting in outgoing supply to heater going up and down between 13 and 16amp causing MCB to overheat.

There was no measurement of the busbar voltage to which the MCBs are connected.

 

[Darkwood]

@Lucien Nunes

Arcing creates mains transients, the associated voltage and magnetic field spikes could in theory operate a mcb, those with heavier loads may be more prone? - either that or it was adjacent to the main switch but surely a visual inspection of the mcb when it was swapped out would show scorch marks....
If you ever have a radio near a arcing fault and your on a particular bandwidth then you can hear the range of interference crackle across your fav' tune.
I would like to know whether the mcb's were swapped over or the circuit moved over, it is plausible the conducted heat through the side wall of the mcb dramatically derated the mcb's value if the circuit remained in the same position with just a different mcb.

 

[Lucien Nunes]

Well, yes, arcing can cause transients. But fluctuating voltage and current, no matter how spiky, won't affect an MCB unless the average over the thermal or magnetic time constant gets too high. To trip it thermally, you've got to produce a significant overall increase in mean current, which transients into a nearly pure resistive load won't do. Magnetically, and you've got to achieve very substantial peaks which again I can't see if the load is mainly resistive. So I suppose my question is, if the MCB tripped purely electrically, what aspect of the heater converted a voltage that only fluctuated below normal, never above, into a current that rose substantially above normal. An SMPSU, yes, I can totally see that. But does the heater have an input regulation circuit that works like that? That would be a chunky and expensive piece of electronics for little purpose. Anyone taken one apart recently?

BTW I like your point about whether the alternative MCBs were moved to the same way in the DB where they could be thermally affected by the hot switch.

 

[marconi]

What I find interesting and worthy of further investigation is the fluctuation of mains incomer 235-210V for a circa 35-50A load? Further problems upstream?

 

[Midwest]

Interesting thread. And fair play to those 'on site' electricians being open and honest & coming on to this forum to post replies etc. If anyone suggests that they would of checked that main switch at first knockings, well Shirley they are deluded or even spaced out- even Lucien & Marconi are 'discussing' it. Hope this has rectified the fault.

PS I came across this thread, trying to track down that thread, where members post pictures of poor workmanship, where is it?

 

[Lucien Nunes]

I'm not sure either of us have understood the meaning of the voltage measurement, perhaps LT21 could clarify? If that was the incoming voltage, as per Marconi's interpretation, then it would have been lowest when the switch was making good contact, and therefore not the cause of the problem and/or the switch is not the cause. Plus, if the supply VD is such that full load pulls it down to 210V it's out of spec. And now, with a good switch, the voltage will still be pulled down just as much or more. OTOH if that was the busbar voltage, and the drop was in the switch rather than the supply, it will be good now but the old switch would have caught fire.

 

[elsparko]

im glad this is over and done with

 

[Darkwood]

Another avenue of thought but keeping on the arcing would be induced interference in the device, not only does the arcing create transients through the circuits but you have Electro-magnetic interference from the spark itself through the air (hence I mentioned the radio comment).. mcb's are not shielded devices so I can see a very local arcing issue could be inducing problem voltage spikes indirectly to the MCB ... it is an interesting fault tbh.

 

[Lucien Nunes]

I can't see it influencing the MCB. The arc carries the same current as the circuit (incomer, in this case) and behaves like a (poor) conductor, producing an equal field to a copper wire carrying the same current. Because of the way the gas ionises, the effective resistance fluctuates in ways that generate wideband noise, including at RF. But the energy radiated from the circuit picked up by the radio is a matter of milliwatts, and would not couple into the low resistance MCB heater or trip coil to any extent. To trip an MCB with an external field derived from a strictly limited current (there's a resistor in series with the arcing circuit) would require putting a substantial coil around it or carefully aligned next to it.

Does anyone think the MCB tripped magnetically, or thermally?