Schneider RCBO's

[EFLImpudence]

Yes, but as we've discussed before, 411 actually appears to allow that 'primary' automatic disconnection of L-E faults can be provided by an RCD (the footnote to Table 41.1 says "Where compliance with this regulation is provided by an RCD ...."). Hence, it would appear that an RCD can provide both 'primary' and 'additional' fault protection - or am I misinterpreting?

"Where... "(not you can) I would think this means when you have had to rely on RCD for reasons above.

As above, the regs seem to imply (actually, say) that an RCD can be used as a 'usual method' of satisfying 411.3.2.2 for primary L-E fault protection.

Not CAN be used - only if unavoidable.

No argument there. I would personally actually say that that it was always bad practice to use an RCD 'because the installation was faulty', even if were a mansion!

Perhaps 'faulty' wasn't the right word to use but if MCB disconnection conditions cannot be met (in 'normal' property) there must be a fault or faults.

Perhaps there should be a note under each regulation, and under each note, stating what was actually meant.
Perhaps this is why they are not statutory.

 

[JohnW2]

Yes, but as we've discussed before, 411 actually appears to allow that 'primary' automatic disconnection of L-E faults can be provided by an RCD (the footnote to Table 41.1 says "Where compliance with this regulation is provided by an RCD ...."). Hence, it would appear that an RCD can provide both 'primary' and 'additional' fault protection - or am I misinterpreting?

"Where... "(not you can) I would think this means when you have had to rely on RCD for reasons above.

As above, the regs seem to imply (actually, say) that an RCD can be used as a 'usual method' of satisfying 411.3.2.2 for primary L-E fault protection.

Not CAN be used - only if unavoidable.

Those 'CAN's and 'only if unavaoidable' are all yours - they do not appear in 411.3.2.

Furthermore, despite what you may expect, nowhere in 411.3.2 (automatic disconnection in the case of a fault) is there any mention of 'MCB', 'fuse' or anything else specific - only 'protective device'. You may feel that the spirit of 411.3.2 is that fault protection should, whenevr possible, be provided by an MCB/RCBO/Fuse, and that RCDs can only be used for that purpose 'when unavoidable', but there is absolutely nothing in the words of that part of the regs which appears to say this.

Perhaps there should be a note under each regulation, and under each note, stating what was actually meant. .... Perhaps this is why they are not statutory.

Yes, I've often wondered about that. For the regs to be Statutory would surely require that they would have to be written far more clearly, unabiguously, explicitly and probably more exhaustively than they currently are - and maybe that's what has put people off the idea of making them Statutory.

Kind Regards, John.

 

[EFLImpudence]

Those 'CAN's and 'only if unavaoidable' are all yours - they do not appear in 411.3.2.
Furthermore, despite what you may expect, nowhere in 411.3.2 (automatic disconnection in the case of a fault) is there any mention of 'MCB', 'fuse' or anything else specific - only 'protective device'. You may feel that the spirit of 411.3.2 is that fault protection should, whenevr possible, be provided by an MCB/RCBO/Fuse, and that RCDs can only be used for that purpose 'when unavoidable', but there is absolutely nothing in the words of that part of the regs which appears to say this.

No, but if you have adhered to 415.1.2 and so have applied 411 to 414 and consequently have met the appropriate values in the fuse and MCB/RCBO tables then, in my opinion, the note to table 41.1 only refers to installations where these have been impossible to achieve.

 

[JohnW2]

No, but if you have adhered to 415.1.2 and so have applied 411 to 414 and consequently have met the appropriate values in the fuse and MCB/RCBO tables then, in my opinion, the note to table 41.1 only refers to installations where these have been impossible to achieve.

I agree that's all very sensible but, as I think we have agreed, it's not actually what the words of the regs say. Put another way, if compliance with BS7671 (as it is) were a statutory requirement, I don't think that any court of law could find against you for believing that an RCD could be used as the sole means of L-E fault protection, even if it were not 'unavoidable'.

This is to a large extent academic since, even with an RCD, one still has to have an MCB or fuse to provide protection against overload and L-N faults. As we've agreed, the only practical relevance is that one should not really use an RCD for L-E fault protection simply because one hasn't designed the installation to achieve an (achievable) sufficiently low Zs for the MCB to provide that protection.

Kind Regards, John.

 

[EFLImpudence]

I don't think that any court of law could find against you for

Even if it did you may appeal successfully thus rendering the first judge and the prosecution WRONG. Oh, well.

 

[JohnW2]

Even if it did you may appeal successfully thus rendering the first judge and the prosecution WRONG. Oh, well.

Quite. Such is the 'fun' of legislated regulations!

As a final thought .... one interesting thing about all this is that if one does have a circuit designed such that an MCB provides adequate 'primary' protection (via ADS) against L-E faults, if one then adds the 'additional proetction' of an RCD, it will very probably be the RCD that operates (first, or perhaps only) if an L-E fault arises. That causes one to think a bit about the meaning of 'primary'/'main' protection and 'additional' protection !

Kind Regards, John

 

[trevmai]

Schneider also own Square D, from them you'll be able to get type B breakers to fit merlin gerin boards. About £38 + vat from newlec.

 

[BigJon]

You'll also be able to get them for around £20 on ebay.

 

[mfarrow]

I agree that's all very sensible

I disagree. If it were sensible then it would be written into the regulations or OSG.

Most L-E faults will be captured by the earth leakage part of an RCBO first anyway. Unless there are grounds for not believing that an RCD is not reliable, then I see no reason why they shouldn't be used as a protective device for fault protection. I also believe a lot of the reasoning for not using them as such is historical - where RCDs protected a whole board or half a board then, really, they should not be used as such. But for final circuits, in the modern context of RCBOs, I believe they are a useful alternative to fuses/MCBs.

The main issue this presents is that your main design factors are now touch potential and volt drop.

 

[ban-all-sheds]

According to a certain Paul Cook:

In TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedence restrictions are the same, and a Type C or D RCBO may not provide a 0.4s disconnection time.

 

[JohnW2]

I agree that's all very sensible

I disagree. If it were sensible then it would be written into the regulations or OSG.

Sorry, I probably wasn't clear enough. What I was trying to say was that EFLI's interpretation of the regs might be a sensible interpretation of what the regs were trying to say, but was not what they actually said.

I agree essentially with the rest of what you say. My only uncertainty is that there appears to be this perception that RCDs are less reliable, particularly if not regularly tested/'sexercised'. I don't know how true that is but, if it is true, then this is obviously a consideration.

Indeed, wherever it is practical (obvioulsy not with TT systems), it would seem sensible to design a circuit such that the OPD (as well as the RCD) was capable of providing reasonable L-E fault protection - simply as 'belt and braces', particularly if there are question marks over RCD reliability.

Kind Regards, John.

 

[ericmark]

What we are looking at is speed of operation.
We have three parts to the RCBO
1) Magnet trip
2) Thermal trip
3) RCD trip
If we look at a 32A MCB then with an impedance of 7 ohms the trip will open with a short circuit. But that would take some time. With an impedance of 1.44 ohms it will be very quick.

For many years we measured the ELI and pre-RCD this was likely all we needed to measure as if the ELI was within limits then the line - neutral impedance was also likely between limits.

However now with the RCD I ask should we be measuring and recording the line - neutral impedance?

As pointed out using B, C, or D type RCBO makes no difference as far as ELI goes but it does as far as line - neutral faults. But do we need to disconnect within a very short time for line - neutral faults? The thermal release will open before cable overheats so why do we need the magnetic part of the trip.

BAS has kindly given us part of the answer with "Paul Cook" report and the need for voltage to work the electronics was redressed with the active RCD so it would always fail safe. RCBO's are however not active.

As far as line - neutral goes to comply with the 5% volt drop we need nearly the same impedance as we do to ensure the magnetic side will operate with a Type B. So using Type B would seem to be a standard method. But with the three phase board to fit a MCB/RCD takes up double the spaces to a MCB and one asks is it really something we should be doing combining RCD and MCB or would it be better to use separate units?

As to TT supplies the use of single pole switching RCD devices must be a problem. Yes we could fit a 100ma RCD which in turn feeds RCBO's but I have asked why we can't get the boards to take twin pole RCBO's? In the hotel while on holiday I see boards with twin pole RCBO's fitted so why not here.

We do it seems have to do things which seem wrong just because we can't buy the goods to do it right. Be this 55 - 0 - 55 volt split phase where we use blue instead of black for second phase with yellow Arctic flex or selection of RCD types.

 

[JohnW2]

According to a certain Paul Cook:
... RCBOs .... in RCD mode they .... need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, ....

Can anyone explain to me why the RCD functionality of an RCBO differs from that of an RCD in the manner described?

Also, the above seems to be talking about a situation in which there is simultaneously an open-circuit neutral and a L-E fault. If that's correct, isn't it a vanishingly improbable scenario?

Kind Regards, John.