RCBO curve

[Greenarmy]

I’m probably going to look stupid here, but it will be worth it if I get the answer , as I can’t fathom it out.
I’ve never really thought of it before, but…..
A circuit breaker comes in curves B, C, and D. Now we all know it’s mostly to do with the in rush of current ( layman terms) but the ELI value has to be lower to achieve tripping times.
However, with a RCBO , your tripping time will be around let’s say 18 milli seconds. So why are all RCBO not at least C type? What’s the point of a B type if you have already achieved the task of the device tripping in the desired time under fault conditions?
No doubt someone will tell me what I’m missing . I look forward to it .

 

[Rockingit]

The OP is clearly misunderstanding the three different fault types under discussion here. It’s also unwise to assume that an RCD (or residual component of an RCBO) will disconnect faster than an MCB under an ADS or overload situation - type A & B RCD’s are now often in the >100mS X1n area which makes disconnection under ADS times very achievable. Yes, fault protection can be afforded by an RCD however it’s rare to be able to do away with overload protection as well.

 

[Greenarmy]

The OP is clearly misunderstanding the three different fault types under discussion here. It’s also unwise to assume that an RCD (or residual component of an RCBO) will disconnect faster than an MCB under an ADS or overload situation - type A & B RCD’s are now often in the >100mS X1n area which makes disconnection under ADS times very achievable. Yes, fault protection can be afforded by an RCD however it’s rare to be able to do away with overload protection as well.

I'm not suggesting to do away with overload protection. I'm talking about changing a D16 amp RCBO to a C16 amp RCBO . Both offer 16 amp overload protection! , and both offer instant tripping under fault conditions..
I don't see any advantage changing them over.

 

[newfutile]

you may want to change if maximum ZS does not comply or if you require a faster disconnect time, 80% values for a D16 is only 0.54 ohms. i would say its good practise to design circuits to trip on the mcb portion (if possible) then the (RCD) Residual Current Device part is just for additional protection

 

[Pretty Mouth]

I think if the MCB's rating is at or below the cable's CCC then the trip time is not an issue, as they have similar/coordinated thermal overload characteristics. Meeting ADS time is a different matter of course!

I had in mind that, where protected for overload by an MCB, a live conductor could still exceed it's thermal limits under adiabatic conditions. I'm not 100% on that, but it's the only way I'm able to explain some of the data in table B7 in the OSG. (not that that backs up my earlier post, as it seems higher fault currents are more onerous)

 

[DefyG]

Apparently the RCBOs are £100 a go and he would need around 10 of them.

What are the loads as just changing them to C type may cause further issues. D type must have been specified for a reason ..... especially 10 of them, unless the original installer got a job lot and the attitude of 'that'll do'!

 

[Rockingit]

This is a common area to be incorrectly noted on an EICR, there is no issue whatsoever. It should be itemised fault protection is afforded by the RCD. Unfortunately our industry is plagued by non competent people carrying out these works.

I don't believe the OP has specified what rating the residual values are? Because if these are upstream of a cascade they could easily be 100-300mA in which case they wouldn't, necessarily, be affording fault protection.

I go back to the basics here that there was a reason a designer decided to install type D RCBO's (which let's face it aren't an everyday back of the van kind of item). If the circuit Zs is too high(long) now then logically it was too long to start with, so it may well be RCBO's were installed to overcome that issue (assuming 30mA for touch voltage limiting, etc) but it could just as easily be that they are delivering to a fixed device where the MI's say that's what to use and has nothing to do with Zs values. It could also be that the manufacturer of that RCBO states a different max Zs to the standard table value or even that the circuit was just over the line a few years ago on Zs before the figures got revised down in BS7671 a while back

(Hence, total agreement with you about people 'inspecting' who don't have the depth and breadth of knowledge to make these judgments).

 

[davesparks]

However, the rcbo will see the fault condition and disconnect in milli seconds would it not? Even in a live to neutral short circuit ?

The overcurrent element of the RCBO will disconnect on a short circuit, the residual current element will not react to it.

So for example if you have a d curve rcbo , and your ZS is .1 ohms too high, changing to a c curve rcbo of the same amp rating, is not going to make it trip any quicker under fault conditions

It could change the tripping time for a short circuit fault.

 

[Julie.]

If they are standard RCBOs the MCB aspect could be b,c, or d , but the RCD aspect is almost certainly 30mA instantaneous.

If the MCB aspect does not clear in time, ADS would be met by the RCD aspect - in this case the installation has RCD as FAULT protection not additional protection.

There would be no advantage in changing the curve from what it is.