Garage RCBO tripping large machinery

westie101 said:

Don't forget that as far as the DNOs are concerned the ELI at the cut-out for a TNCS system can be as high as 0.8. These figures were set many years ago by the DNOs

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Indeed, and that's obviously a potential problem as far as ADS is concerned. If the EFLI at the origin of an installation were 0.8Ω, that would mean that the PEFC of a final circuit could not possibly be greater than about 273A (at 218.5V), even if the final circuit itself had a negligible impedance. That means that the largest possible MCBs which would satisfy disconnection times (with a negligible impedance final circuit) would be a B50 (not B63), C25 or D10. In practice, of course, the final circuit would have some impedance, so those MCB sizes would be two great to provide the required disconnection times. With just a modest extra 0.3Ω in the final circuit, the PEFC would be around 198A, in which case the largest possible MCBs would theoretically be a B32, C16 or D6.

Kind Regards, John

westie101 said:

Don't forget that as far as the DNOs are concerned the ELI at the cut-out for a TNCS system can be as high as 0.8.
These figures were set many years ago by the DNOs

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I thought that was TN-S, and that the max for TN-C-S was 0.35Ω?

What's the max for TN-S then?

ban-all-sheds said:

westie101 said:

Don't forget that as far as the DNOs are concerned the ELI at the cut-out for a TNCS system can be as high as 0.8. These figures were set many years ago by the DNOs

Click to expand...

I thought that was TN-S, and that the max for TN-C-S was 0.35Ω? What's the max for TN-S then?

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IIRC from previous discussions, DNOs tend to regard 0.8Ω as the maximum for either ... I'm not too sure where the (so commonly quoted) 0.35Ω comes from.

Kind Regards, John

JohnW2 said:

westie101 said:

Don't forget that as far as the DNOs are concerned the ELI at the cut-out for a TNCS system can be as high as 0.8. These figures were set many years ago by the DNOs

Click to expand...

Indeed, and that's obviously a potential problem as far as ADS is concerned. If the EFLI at the origin of an installation were 0.8Ω, that would mean that the PEFC of a final circuit could not possibly be greater than about 273A (at 218.5V), even if the final circuit itself had a negligible impedance. That means that the largest possible MCBs which would satisfy disconnection times (with a negligible impedance final circuit) would be a B50 (not B63), C25 or D10. In practice, of course, the final circuit would have some impedance, so those MCB sizes would be two great to provide the required disconnection times. With just a modest extra 0.3Ω in the final circuit, the PEFC would be around 198A, in which case the largest possible MCBs would theoretically be a B32, C16 or D6.

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Well - people who will not learn and equip themselves to test their installation will just have to stick to those maxima, won't they. Assuming, of course, that they are aware of, and understand, the issue.

But then what does it matter if they don't? Why should anybody here be concerned about "accepting" that people might do things which result in an unsafe situation?

JohnW2 said:

I'm not too sure where the (so commonly quoted) 0.35Ω comes from.

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It's the maximum value stated in the OSG.

Right now ICBA to try and find anything which validates that.

ban-all-sheds said:

Well - people who will not learn and equip themselves to test their installation will just have to stick to those maxima, won't they.

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I'm surprised to hear you advocating such a potentially dangerous practice. As you say, I was quoting "absolute maxima", which would only apply to final circuits which themselves have negligible impedance. For a real world circuit, and a person "who will not learn and equip themselves to test their installation (or, at least, undertake some calculations)", the only totally safe 'working maximum' would surely be zero.

Kind Regards, John

ban-all-sheds said:

JohnW2 said:

I'm not too sure where the (so commonly quoted) 0.35Ω comes from.

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It's the maximum value stated in the OSG. Right now ICBA to try and find anything which validates that.

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Attempting to 'validate' things in the OSG is usually a fool's errand!

Kind Regards, John

One could ask the IET for the provenance of the figure.

ban-all-sheds said:

One could ask the IET for the provenance of the figure.

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If it matters to you, you could.

Kinds Regards, John

I was told at collage one should allow for a TN-C-S installation to rise to 0.35Ω and a TN-S installation to rise to 0.8Ω that does not mean they are those figures, but you should allow for changes in the road to cause the figures to rise.

I questioned this as at the time working where we had a 350A three phase supply from a transformer on the premises although I had no access and I was using moulded breakers which would not trip with a 0.35Ω figure on the magnetic part of the trip.

The lecturer then modified his statement and said for a domestic supply which since consumer units are only rated at 100A can never be more than 100A.

Taking 100A and a volt drop of 36.8 we have an absolute maximum of 0.368Ω so as far as a line - neutral figure goes to keep within the volt drop permitted there is a maximum of 0.368Ω now we can see where the 0.35Ω figure comes from. Since the earth wire can be smaller than the neutral with TN-S we get 0.8Ω of course with a 60A supply it could raise to 0.613Ω. So with my 350A supply the maximum would have been 0.105Ω.

So it does not matter if TN or TT the line - neutral impedance with a 100A supply is taken as 0.35Ω it's just simple maths from the +10% ~ -6% limit on the supply voltage.

As to allowing for the loop impedance to change not so sure. What they were saying if the incomer was 0.25Ω and you installed a shower on a B45 MCB then using old book figures the 1.022 - 0.1 = 0.922Ω that one should be measuring to allow for the supply loop impedance to change. I suspect now we have a new figure that allows for variation in the supply loop impedance?

However we have gone well above the original question by now the poster should be really scratching his head. Pobodies Nerfect and even electricians make mistakes but at least with inspection and testing we hope we will highlight errors before the circuit is energised. However without inspection and testing these errors will not be found. So with any DIY there is a possibility because of lack of equipment and knowledge that errors will be missed. So we come back to the old question "What should DIY people be doing?" I would say people a lot better than me (As I can't see the point in half what is required) have written the Part P and similar laws which seem to say DIY people should stay out of the consumer unit.

So simple answer if it requires one to alter the consumer unit get an electrician to do it. How does that sound?

ericmark said:

Taking 100A and a volt drop of 36.8 we have an absolute maximum of 0.368Ω so as far as a line - neutral figure goes to keep within the volt drop permitted there is a maximum of 0.368Ω now we can see where the 0.35Ω figure comes from.

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Maybe that helps you to see where the 0.35Ω comes from, but I have to say that it doesn't really help me to! You appear to have divided the maximum permitted range of variation of supply voltage (253V-216.2v = 36.8V) by 100A to get the 0.368Ω - but, as my grandmother would probably have asked "what has that got to do with the price of fish?"!!

Interestingly, what your approach does remind us is that even if one's 'normal' supply voltage, with no load, were close to the maximum permitted (253V), then the maximum external L-N loop impedance to prevent that voltage falling below the minimum permitted supply voltage (216.2V) would be about zero, not 0.368Ω or 0.35Ω!! Even with no-load supply voltages much lower than 253V, the L-N loop impedance would still have to be lower than 0.368Ω to prevent the voltage falling to below 216.2V at 100A.

Kind Regards, John

JohnW2 said:

Interestingly, what your approach does remind us is that even if one's 'normal' supply voltage, with no load, were close to the maximum permitted (253V), then the maximum external L-N loop impedance to prevent that voltage falling below the minimum permitted supply voltage (216.2V) would be about zero, not 0.368Ω or 0.35Ω!! Even with no-load supply voltages much lower than 253V, the L-N loop impedance would still have to be lower than 0.368Ω to prevent the voltage falling to below 216.2V at 100A.

Kind Regards, John

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Not sure where you get zero from? However it would likely be lower than 0.368Ω as with micro generation the no load voltage can't be set at 253 volt or all the grid tie inverters will lock out on over voltage. I remember the problem on the Falklands when the temporary accommodation block was built powering it from 10 x 500kVA and 2 x 750kVA generators in the main generator shed. The only way was step-up and step-down transformers and 3.3kV links between them. Even so volt drop was a problem. Every transformer has an impedance as well as the cable and size does matter. With narrow boats there is a problem when using isolation transformers with a 3.5kVA transformer even with no cable it is near impossible to get 2.87Ω. For anything to blow a 13A fuse when supplied with an isolation transformer is near impossible.

However to avoid electrolysis either isolation transformers or diodes in the earth are required. Or kiss goodbye to the sacrificial anode.

So the question is if it's really that important to keep within the limits for line - neutral when we know there are many times it is impossible to get the figures required. With the blue isolation brick often there is a simple overload device with a little red button to reset it which is purely thermal. With a D16 it may never trip the magnet part but the thermal part will still trip at 18A although it may take some time. However it will trip before cable melts and that's the bit that matters. As far as line - earth the RCD does that bit.

What's happening?

ericmark said:

With a D16 it may never trip the magnet part but the thermal part will still trip at 18A although it may take some time.

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No it won't. (16 x 1.13 = 18.08)

ericmark said:

Not sure where you get zero from? However it would likely be lower than 0.368Ω ...

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If your supply voltage were 216.2V with no load in your installation, then, if the external L-N loop impedance were anything above zero, then any current drawn by your installation would cause the voltage at the origin of your installation to fall below the minimum permitted figure. However, do people really see such large fluctuations in their supply voltage? I cannot say that I have personally noticed it.

I'm not really sure how the the rules about 'permitted supply voltage' are meant to work - over what range of loads does the supply have to remain within 230V +10% -6%? As were perhaps implying in your previous post, if the requirement were that the supply should not fall below 216.2V with a 100A load, that would imply that the external L-N loop impedance would have to be below 0.368 to avoid the supply voltage rising to above 253V with no load.

However, you make me realise that something is going on that I don't fully understand. If one assumes for a moment an external L-N loop impedance (regardless of whether TN-C-S, TN-S or TT) of about 0.35, then when someone switched on, say, a 10.5kW shower one would expect the voltage at the origin of the installation to fall by about 16V (not to mention the drop within the installation, as far as the shower was concerned). Would such a sudden drop not cause a noticeable 'dimming of lights' (at least, 'old-fashioned' ones!) etc, which just doesn't seem to happen?? ... and that's all before one starts considering the loads of all the other installations which are sharing at least part of the supply path that gives rise to the external L-N loop impedance that one 'sees'.

Kind Regards, John

EFLImpudence said:

ericmark said:

With a D16 it may never trip the magnet part but the thermal part will still trip at 18A although it may take some time.

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No it won't. (16 x 1.13 = 18.08)

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Does I1=1.13In (and I2=1.45In) still apply to Type D MCBs? - I've only ever seen those figures quoted in relation to Type Bs, and have never had reason to discover whether the same figures apply to Type C and D ones.

Kind Regards, John