Advantages of a type C MCB

[Mike2007]

What are the advantages of a type C MCB over the usual type B MCB ?

 

[ricicle]

The advantage is that you can have greater start up surges on the circuit. ( approx. 10x In as opposed to 5x In for a type B)

The disadvantage is that your Zs has to be half the value of a type B for an mcb of the same value.

 

[JohnW2]

The advantage is that you can have greater start up surges on the circuit. ( approx. 10x In as opposed to 5x In for a type B). The disadvantage is that your Zs has to be half the value of a type B for an mcb of the same value.

... although, if I'm interpreting the regs correctly, 411.4.4 allows one to rely upon an RCD for fault protection in a TN system if the Zs is too high. That seems to make sense, since with a TT system, in the absence of parallel paths, one has no choice but to rely on RCDs for fault protection.

Kind Regards, John.

 

[EFLImpudence]

... although, if I'm interpreting the regs correctly, 411.4.4 allows one to rely upon an RCD for fault protection in a TN system if the Zs is too high. That seems to make sense, since with a TT system, in the absence of parallel paths, one has no choice but to rely on RCDs for fault protection.

I think you are (interpreting the regs correctly) in respect of TT where there is no alternative.

However, with TN it should be emphasised that RCDs may only be used for additional protection and where Zs for the MCB cannot be satisfied i.e. IMPOSSIBLE- not just isn't satisfied. 531.3.1

If that were the case, then all requirements for MCBs could be ignored merely by the inclusion of an RCD.

A type C or D MCB should not be fitted if Zs is too high even if an RCD is present.

After all the R1 may be the reason the maximum Zs cannot be met and this with Rn may result in the MCB not operating under L - N fault conditions.

 

[ericmark]

The MCB is in fact two units in one.
The thermal trip which will after some time trip at the rated value.
The magnetic trip which depending is B, C, or D will trip at 5, 10, or 20 times rated value.
The trip will protect both with a short to earth and across lives and to operate the magnetic part the impedance must be low enough.
Oddly we seem to refer to impedance with respect to earth and PSC with respect of across lives but in real terms they are the same the prospective short circuit current is worked out by finding the impedance. In theory the PSC is which ever is the higher line to earth or line to neutral as it is measured to ensure the MCB can handle the current likely to be involved.
But there is little point in having a MCB or RCBO with the magnetic part if it could never operate and I would consider where RCD protection is added then the impedance line to neutral is still a factor when selecting B, C, or D MCB's.
With the top of the range Robin meter the PSC and ELI readings were line - neutral and line - earth but with the cheaper meters there was no auto swapping within the meter all it did was work out the maths.
I did have a lead made up so I could measure the line - neutral and I would always note both readings. But the recommended form from the IET does not have provision for the line - neutral reading. So in the main it's only taken and not recorded.

There are two reasons to take the line - neutral reading. As well as insuring the current is neither too high or too low for the trip to work with. It also will work out the volt drop. Volts divided by PSC times rated current = volt drop however it is total so at 0.35 the incoming volt drop could be 11.2 volt supplying at plus 10% and minus 6% the supplier can have a volt drop of 36.8 volt if it has an open circuit voltage of 253 volts.

But we want local volt drop which is permitted 5% so 11.5 volt so for example at 32A we are allowed 0.36 ohms add to that the supply impedance and 0.71 ohms is the likely pass figure. Now a B type will trip at 1.44 ohms so well within the limits. C type will trip at about 0.72.
Now it all begins to make sense. Since at 32A the loop impedance is approx the same to trip a C type MCB as permitted for the volt drop there is little point in fitting a B type RCBO it may as well be a C type. That is assuming a TN-C-S supply so the earth loop at the head will be same as the neutral loop which has to be 0.35 ohms or better.

So for a ring main with a TN-C-S supply where all sockets are protected by an RCD there is no point using a B type circuit breaker.

So lets look at lights with a 6A circuit breaker. 6.9 volt drop so 1.15 ohms plus the 0.35 = 1.5 ohms impedance. With a D type allowed 1.9 ohms so with lights we can safely use a D type as if within volt drop then we are well within the limits to trip a D type MCB.

Moving the other way lets look at a shower with a 45A supply. Now we move the other direction and likely we will need a B type as for volt drop looking at 0.6 where to trip a C type needs 0.5 so B type at 1 ohm may be required.

So with a TN-C-S supply and all RCD protection we could likely see a mixture of D6, C32 and B45 MCB's fitted there is no reason why we should have a B6 for lights.

But this is for a TN-C-S supply with a TT supply the supplier is allowed 36.8 volt drop at 100A this 0.368 ohms. Oh what a surprise now we see why 0.35 ohms is the limit. But hang on what with a 60A supply now the impedance could be 0.61 ohms so yes one may need to use B type after all.

However when we complete any circuit we inspect and test. It takes seconds to test the loop impedance. So no need for all the calculations we just stick the meter on and job done.

But still raises the question when the neutral - line impedance is so important why no provision in the paperwork to record it?

 

[westie101]

That is assuming a TN-C-S supply so the earth loop at the head will be same as the neutral loop which has to be 0.35 ohms or better.

It does not "have to" it "should be"!

A figure of up to 0.8 is acceptable as the DNO 100A fuse will still operate at that figure.
This figure was taken from a Supply Industry Specification, which decided on the figure of 0.35 as that is what would be expected where the neutral and earth were combined.
Unfortunately BS7671 jumped on this figure without fully explaining it so it has become slightly abused in it's use!

 

[EFLImpudence]

In theory the PFC is which ever is the higher line to earth or line to neutral

A slip of the 'pen'

 

[ericmark]

That is assuming a TN-C-S supply so the earth loop at the head will be same as the neutral loop which has to be 0.35 ohms or better.

It does not "have to" it "should be"!

A figure of up to 0.8 is acceptable as the DNO 100A fuse will still operate at that figure.
This figure was taken from a Supply Industry Specification, which decided on the figure of 0.35 as that is what would be expected where the neutral and earth were combined.
Unfortunately BS7671 jumped on this figure without fully explaining it so it has become slightly abused in it's use!

I will agree but is it so hard to use a meter?

 

[westie101]

There is no problem as long as those who use that figure are fully aware of it's origin, meaning and use.
We constantly get calls about high loop impedance on TNC-S supplies quoting figures between 0.35 and 0.8 where we then have to spend time explaining that it is acceptable, which is difficult if it is the customer and they have had a PIR that raises it as a "fail"

 

[JohnW2]

I think you are (interpreting the regs correctly) in respect of TT where there is no alternative.

Indeed. As I said, it's obvioulsy what has to happen with TT, since there is no alternative.

However, with TN it should be emphasised that RCDs may only be used for additional protection and where Zs for the MCB cannot be satisfied i.e. IMPOSSIBLE- not just isn't satisfied. 531.3.1

Well, all that emphasising, underlining and capitalisation is yours, not that of BS7671. 531.3.1 merely talks of situations in which 411.4.5 "cannot be satisfied", without any such emphasis. I would say that it's anyone's guess as to how the authors intended "cannot be satisfied" to be interpreted ....

If that were the case, then all requirements for MCBs could be ignored merely by the inclusion of an RCD.

... which was, as you might guess, going to be the basis of my next question! TT systems exist and are allowed - which presumably means that they are considered to be acceptably safe. So, if it's considered safe for L-E fault protection to be dependent upon RCDs in TT installations, why not also in TN ones? [see below - I may have answered my own question!]

After all the R1 may be the reason the maximum Zs cannot be met and this with Rn may result in the MCB not operating under L - N fault conditions.

L-N faults are obviously a different matter, and no different for TT and TN systems. As you say, it's then R1+'RN' (i.e. usually 2*R1), plus the external L-N loop impedance which matters, and that has to be sufficiently low to achieve the required disconnection time.

However, as above, I think I've probably answered my own question. Logic suggests that (despite your emphasising), reliance on an RCD for disconnection in the case of an L-E fault ought to be as acceptable with TN systems as with TT ones. However, that doesn't alter the requirements for disconnection with L-N faults - and if the R1 and 'RN' are low enough to satisfy that, they are pretty likely (although not guaranteed) to also result in a sufficiently low Zs for adequate L-E fault protection from the MCB.

If that it correct, then, if there is RCD protection in place, the requirement in relation to L-N fault protection really ought to be in terms of L-N loop impedance, not Zs/EFLI.

Kind Regards, John.

 

[EFLImpudence]

I think you are (interpreting the regs correctly) in respect of TT where there is no alternative.

Indeed. As I said, it's obvioulsy what has to happen with TT, since there is no alternative.

As in a previous thread it was, I think, agreed that your system was better protected either by the service pipes or your neighbours' systems.
Lately however some posters have commented that they would feel safer with a TT system rather than relying on the supply neutral. I am not clear about this myself but I suppose the better would be both.

However, with TN it should be emphasised that RCDs may only be used for additional protection and where Zs for the MCB cannot be satisfied i.e. IMPOSSIBLE- not just isn't satisfied. 531.3.1

Well, all that emphasising, underlining and capitalisation is yours, not that of BS7671. 531.3.1 merely talks of situations in which 411.4.5 "cannot be satisfied", without any such emphasis. I would say that it's anyone's guess as to how the authors intended "cannot be satisfied" to be interpreted ....

I emphasised the words that mean you can't rely on RCDs and ignore the MCB requirements in TN systems.
I take this to be in the event of a fault in an existing system; the cause of which cannot be determined and renewing the circuit would be impracticable (perhaps temporarily).
Obviously it would not apply to a new design which was badly engineered.

If that were the case, then all requirements for MCBs could be ignored merely by the inclusion of an RCD.

... which was, as you might guess, going to be the basis of my next question! TT systems exist and are allowed - which presumably means that they are considered to be acceptably safe. So, if it's considered safe for L-E fault protection to be dependent upon RCDs in TT installations, why not also in TN ones? [see below - I may have answered my own question!]

Perhaps it may be revised as I suppose it still applies to the single RCD protecting the whole TT system deemed to be acceptable because unavoidable yet not ideal.
This is unnecessary for TN systems.

After all the R1 may be the reason the maximum Zs cannot be met and this with Rn may result in the MCB not operating under L - N fault conditions.

L-N faults are obviously a different matter, and no different for TT and TN systems. As you say, it's then R1+'RN' (i.e. usually 2*R1), plus the external L-N loop impedance which matters, and that has to be sufficiently low to achieve the required disconnection time.

However, as above, I think I've probably answered my own question. Logic suggests that (despite your emphasising), reliance on an RCD for disconnection in the case of an L-E fault ought to be as acceptable with TN systems as with TT ones.

As above, the MCB will clear the L - E fault in TN. Was that considered preferable to one fault blacking out the whole installation?

However, that doesn't alter the requirements for disconnection with L-N faults - and if the R1 and 'RN' are low enough to satisfy that, they are pretty likely (although not guaranteed) to also result in a sufficiently low Zs for adequate L-E fault protection from the MCB.

More likely the other way around.

If that it correct, then, if there is RCD protection in place, the requirement in relation to L-N fault protection really ought to be in terms of L-N loop impedance, not Zs/EFLI.

As Eric mentioned, there is no provision for this to be recorded but it is taken into account in the PFC recorded. If the worse case figures satisfy then the system is adequately protected.

A lot of posters express disquiet about the reliability of the RCD.
Perhaps they (RCDs) are not regarded, by the powers that be, as safe enough yet to do as you are suggesting.

 

[JohnW2]

As in a previous thread it was, I think, agreed that your system was better protected either by the service pipes or your neighbours' systems.

Indeed, and that illustrates what I regards as one of the major strangenesses/ inconsistences about the regs/GNs. I am not allowed to 'rely upon' service pipes as my TT earth (unless they are from a private supply) seemingly because of bthe risk that someone will creep around one dark night and turn them into plastic without my noticing. However, when it comes to testing my installation, it seems accepted and expected practice to measure Zs with all parallel paths in place. As a result, for the reasons you mention above, the measiured Zs throughout all the circuits of my installation is low enough to provide the required fault disconnection times with MCBs alone. However, if that hypothetical person with a lorry full of plastic pipe appeared one dark night, and worked very quietly, my Zs figures would suddenly all become about 75Ω without my realising.

Lately however some posters have commented that they would feel safer with a TT system rather than relying on the supply neutral.

Yes, I'm one of 'them', but ....

I am not clear about this myself but I suppose the better would be both.

I don't think so. That's essentially the situation I have (TT system plus the 'benefit' of TN-C-S via the neighbour), but I think that's probably the worst of both worlds - since my ('high resistance') TT earth would do nothing to protect my installation from a high voltage neutral if the neutral fault were in a bad place in the distribution cabling.

I emphasised the words that mean you can't rely on RCDs and ignore the MCB requirements in TN systems.
I take this to be in the event of a fault in an existing system; the cause of which cannot be determined and renewing the circuit would be impracticable (perhaps temporarily). Obviously it would not apply to a new design which was badly engineered.

I still don't think that the intended meaning of "cannot be satisfied" is anything like as clear-cut as you seem to believe. I particular, I doubt that it was meant to mean "would be totally impossible to achieve" - given only that ZE is not so high that it makes it literally impossible to attain a low enough Zs, one can always get a low enough Zs, even if it means using conductors the size of drainpipes!

As above, the MCB will clear the L - E fault in TN. Was that considered preferable to one fault blacking out the whole installation?

Who knows. However, that is less of an issue with more modern installations with multiple RCDs and/or RCBOs.

....if the R1 and 'RN' are low enough to satisfy that, they are pretty likely (although not guaranteed) to also result in a sufficiently low Zs for adequate L-E fault protection from the MCB.

More likely the other way around.

That's why "(although not guaranteed)" was there

If that it correct, then, if there is RCD protection in place, the requirement in relation to L-N fault protection really ought to be in terms of L-N loop impedance, not Zs/EFLI.

As Eric mentioned, there is no provision for this to be recorded but it is taken into account in the PFC recorded. If the worse case figures satisfy then the system is adequately protected.

It's fair enough to say that in relation to a TN system, but it is probably totally untrue of a TT system. With true TT, if Zs (i.e. EFLI) is all one measures, one has no idea whether the requirements for disconnection times for a L-N fault are satisfied or not - they certainly wouldn't be on the basis of the Zs measurements. Only an L-N loop impedance could demonstrate that.

A lot of posters express disquiet about the reliability of the RCD. Perhaps they (RCDs) are not regarded, by the powers that be, as safe enough yet to do as you are suggesting.

Possibly, but I have yet to see any concrete evidence that RCDs are any less reliable than MCBs. As discussed previously, I strongly suspect that the reason for this belief may be that faulty RCDs are detected (by householders pressing the test button or electricians undertaking RCD tests). If we could test MCBs, we might well find at least as many that had 'failed' - but there is no practical/ safe/ non-pyrotechnic way of testing an MCB, at least, not at the level of fault currents! MCBs might even be less reliable, because they rarely operate, and don't even get 'exercised' by people (occasionally!) pressing a test button.

Kind Regards, John

 

[westie101]

MCBs might even be less reliable, because they rarely operate, and don't even get 'exercised' by people (occasionally!) pressing a test button.

Certainly there is a lot to be said to "exercising" them.
But just musing there's a lot to be said for fuses in the reliability stakes! (OK I know there are other issues)

 

[ban-all-sheds]

I would say that it's anyone's guess as to how the authors intended "cannot be satisfied" to be interpreted ....

There is no ambiguity or imprecision in the meaning of "cannot" - look it up.

 

[EFLImpudence]

With true TT, if Zs (i.e. EFLI) is all one measures,

But it's not.

one has no idea whether the requirements for disconnection times for a L-N fault are satisfied or not - they certainly wouldn't be on the basis of the Zs measurements. Only an L-N loop impedance could demonstrate that.

L - N loop has to be measured to determine the PFC.

Edit - realised you mean Zs at accessories but the same answer applies if Ze and Zn are taken at the supply.