Resistive element OCPD

[SUNRAY]

Yesterday I baby sat while the DNO/energy supplier fitted a main isolator in advance of an overhaul of the installation and while power was off took the opportunity to disconnect and remove 2 redundant fuseboxes.

Today I got the call alleging I'd created a tripping problem on a circuit. The circuit in question looks like 7/0.044", is a 30A fuse (presumably) in one of these:

followed by a 32A RCBO in one of these:

It has absolutely nothing to do with the work I did and checking voltages of the supply I measured 404,400,405,234,231,233. Still extremely close to yesterdays.
When I got there the RCBO was tripped. I disconnected and using a multimeter (Not wanting to risk any damage by using a meggar) I found 3MΩ between N&E, but L&E & L&N were O/C. In other words my initial test found nothing to trip the RCD.


Looking at the other end of the circuit I found 3x 30A DP versions of this style of isolator:

Each supplying a bench top 2,6KW oven, more precisely these are dated lab grade units with a volume equating to about a 200mm cube. Very basic; power switch, thermostat and elements.
We've had a number of threads, mostly about 3KW ovens, where the advice given is to wire to the typical 32A cooker circuit without any additional overload protection. This is a practice I've criticised several times but accepting it's permissable in the regs I repeated the test with all 3 ovens switched on. Still using the Lidl multimeter I had 3MΩ between N&E & L&E, and 5Ω L&N.

The bells didn't immediately ring in my head so I reterminated & repowered the circuit and just managed to measure nearly 70A with a clamp before the RCBO tripped.

Changing over to my Robin meter the loop was 3.48Ω or the 3 ovens individually 20.35Ω, 20.17Ω & 5.30Ω.

Disconnecting the offending oven I found the 1.5mm heat resisting flex:

The top pic is of the section trapped between a tubular metal leg and sawn wood shelf upright, the cable still meggars OK. The heating element measures 5.1Ω

 

[SUNRAY]

From another thread:

What would be the point? You will not accept the explanation of what the the regulations state.

You never did explain how the oven element (I think it was) you encountered managed to draw four times its design current seemingly quite happily.



What would you say if I replied to Eric's comment by stating that NO appliances require RCD 'protection'.

I fully understand the regulation. Whether I accept that it is safe to run a 15A flex on a 30 or 45A OCPD is my opinion only. However my personal experience comes into it as well. Having experienced significant overload on such a cable, my opinion is heavily swayed in the direction of safety.

I cannot explain how the oven ran at an elevated current, other than to say it did. The damage caused to the 1.5mm² heat resistant flex it shown above for you perusal, I wonder what your opinion is regarding the damage caused by that 'impossible occurance'? and further by the additional potential damage that could have occurred if left?

RCD's? well in a perfect world we wouldn't need them would we? Ah but this isn't a perfect world is it?
I stick RCD's on almost everything domestic and have done so since before they were popular.

 

[Lectrician]

And if that flex had finally melted through, causing a direct short, do you think a 16amp RCBO would trip quicker than a 32amp?

 

[SUNRAY]

And if that flex had finally melted through, causing a direct short, do you think a 16amp RCBO would trip quicker than a 32amp?

Well surely the difference is the 16A OCPD would have prevented such a high current flowing in the first place and prevented that fire risk.

 

[Lectrician]

Well surely the difference is the 16A OCPD would have prevented such a high current flowing in the first place and prevented that fire risk.

NOPE. Very unlikely in normal instances, but with your earth loop, it wouldn’t trip quickly enough.

That’s the difference between fault current and overload current. The magnetic side of the RCBO would take over. A 32amp B type RCBO needs 160amp to operate “instantly”. Your earth loop is suspiciously high. Almost “class room”.

And why random voltage readings? What do they prove?

 

[SUNRAY]

NOPE. Very unlikely in normal instances, but with your earth loop, it wouldn’t trip quickly enough.

That’s the difference between fault current and overload current. The magnetic side of the RCBO would take over. A 32amp B type RCBO needs 160amp to operate “instantly”. Your earth loop is suspiciously high. Almost “class room”.

And why random voltage readings? What do they prove?

I haven't got my notes here to quote the Z's & PSC but the sub is electrically only ~60m from the 'ovens' which includes ~40m between transformer and 400A C/O (My belief is the dedicated cable was run {upgraded from 100A & live redundant C/O still exists, only a few years ago} with the expectation of upgrading the fuses in the future. The PSC is measured in 1000's.

The low resistance readings shown above are effectively the element resistances of the 3 'ovens', the trip time of the 32A RCBO is nigh on instant, only just enought time to see the ~70A.

 

[JohnW2]

And if that flex had finally melted through, causing a direct short, do you think a 16amp RCBO would trip quicker than a 32amp?

Well surely the difference is the 16A OCPD would have prevented such a high current flowing in the first place and prevented that fire risk.

Are you not perpetuating the common misconception that protective devices (any standard protective device) can limit the magnitude of a fault current?. If there is a 'direct short' the fault current will be determined solely by the impedance of the fault path - all a protective device can do is limit the duration of the fault, not the magnitude of the fault current.

Kind Regards, John

 

[bernardgreen]

all a protective device can do is limit the duration of the fault

which does significantly reduce the risk of a self sustaining fire occurring.

 

[JohnW2]

which does significantly reduce the risk of a self sustaining fire occurring.

Indeed - but, as you know, I was responding to ...

... the 16A OCPD would have prevented such a high current flowing in the first place ...

... which, as I said, is likely to perpetuate the misunderstanding about OPDs in general.

In any event, in the face of a 'direct short' (which what was being talked about) the difference (if any) in (very short) disconnection times between a 16A and 32A OPD would have no noticeable impact on 'fire risk', would it?

Kind Regards, John

 

[SUNRAY]

Are you not perpetuating the common misconception that protective devices (any standard protective device) can limit the magnitude of a fault current?. If there is a 'direct short' the fault current will be determined solely by the impedance of the fault path - all a protective device can do is limit the duration of the fault, not the magnitude of the fault current.

Kind Regards, John

Indeed John and thanks for the correction.

 

[SUNRAY]

Indeed - but, as you know, I was responding to ...
... which, as I said, is likely to perpetuate the misunderstanding about OPDs in general.

In any event, in the face of a 'direct short' (which what was being talked about) the difference (if any) in (very short) disconnection times between a 16A and 32A OPD would have no noticeable impact on 'fire risk', would it?

Kind Regards, John

Actually thinking back, I fell into the trap of answering your comments without referring back to previous posts. Yes I totally agree OCPD's per se do not restrict current, what they do is cease the current if it exceeds a tripping curve limit.

In the case of the '3 ovens' there is no earth fault (other than the 3MΩ measured with a multimeter) and no 'direct short' circuit, the fault is ~5Ω L/N on one appliance which on it's own was not low enough to blow the 30A fuse or trip a 32A RCBO, that only tripped if the other appliances were also in use (I didn't check if it required one or both). However if it (the cable) had been correctly protected with a 16A, or even better 12 or 13A, MCB the fault would have been found so much sooner and the safety/fire hazard would not have existed.
On that basis I'll vehemently disagree with:

disconnection times between a 16A and 32A OPD would have no noticeable impact on 'fire risk',

Indeed even the users were aware that appliance was different as it heated up quicker than the others and they preferred to use it.

 

[JohnW2]

In the case of the '3 ovens' there is no earth fault (other than the 3MΩ measured with a multimeter) and no 'direct short' circuit, the fault is ~5Ω L/N on one appliance which on it's own was not low enough to blow the 30A fuse or trip a 32A RCBO, that only tripped if the other appliances were also in use .....

Yes, I realise that. However, I've explained the reason for my comment - given what you quoted and what you wrote in response, I felt that there was a distinct risk that you might re-enforce the (common) mistaken belief in many people's minds that a protractive device (OPD or RCD) can limit the magnitude of a fault current - which, as you know, is not corrector.

However if it (the cable) had been correctly protected with a 16A, or even better 12 or 13A, MCB the fault would have been found so much sooner and the safety/fire hazard would not have existed.

Perhaps - it depends upon the actual situation. If the element whose resistance had fallen to 5.1Ω (corresponding to peak current of about 45A) was under thermostatic control, then the average current draw over any appreciable length of time may not have been high enough for a ('dedicated') 16A MCB to ever trip.

What does surprise me is the implication that part of a heating element was able to tolerate about four times its 'rated' current for an appreciable period of time - I would have expected it to fail ('melt') pretty early.

On that basis I'll vehemently disagree with:...

You are again taking my statement out of context. I was responding to your statement about fire risk which, in turn, related to Lectrician's reference (which you quoted in your response) to a 'direct short'. I hope you would agree that in the presence of a 'short' a 16A MCB would provide no noticeably greater protection against fire than would a 32A one?

Kind Regards, John

 

[bernardgreen]

A bifilar heating element can be crushed and if the two heating cores come into contact then the resistance is reduced.

Most heaters with mineral insulation can run with the core(s) red hot for long periods of time in an over current fault situation

 

[SUNRAY]

Yes, I realise that. However, I've explained the reason for my comment - given what you quoted and what you wrote in response, I felt that there was a distinct risk that you might re-enforce the (common) mistaken belief in many people's minds that a protractive device (OPD or RCD) can limit the magnitude of a fault current - which, as you know, is not corrector.
Perhaps - it depends upon the actual situation. If the element whose resistance had fallen to 5.1Ω (corresponding to peak current of about 45A) was under thermostatic control, then the average current draw over any appreciable length of time may not have been high enough for a ('dedicated') 16A MCB to ever trip.

What does surprise me is the implication that part of a heating element was able to tolerate about four times its 'rated' current for an appreciable period of time - I would have expected it to fail ('melt') pretty early.
You are again taking my statement out of context. I was responding to your statement about fire risk which, in turn, related to Lectrician's reference (which you quoted in your response) to a 'direct short'. I hope you would agree that in the presence of a 'short' a 16A MCB would provide no noticeably greater protection against fire than would a 32A one?

Kind Regards, John

When I did the current clamp test, all 3 ovens were cold, operating the 32A RCBO to on, only gave just enough time for my clamp meter to give 2 or 3 ascending current readings before tripping... say 5 seconds tops but it felt much quicker than that, I didn't attempt to establish what the actual current draw was as seeing 70A was enough for me to start looking elsewhere. On that basis and that basis alone I'll guess removing the 20A of the other 'ovens' a 16A version will trip in the same time... a guess at under 5 seconds. I haven't looked at tripping curves but I'll hazard a guess it will show a B16 tripping in ½minute or so at 45A.

I'd hoped I worded the response in a way that I disagreed only in that particular situation, otherwise I'd done this:

and fully agree with what you say about direct shorts, assuming of course the loop impedance and voltage are adequate

 

[SUNRAY]

A bifilar heating element can be crushed and if the two heating cores come into contact then the resistance is reduced.

View attachment 255716
Most heaters with mineral insulation can run with the core(s) red hot for long periods of time in an over current fault situation

In fact mineral heater elements in air handling units often are glowing, very dim but visible in the darkness of the duct. The first time I saw it I was sure it was incorrect & got the engineer in there PDQ.
Red face for Sunray...