RCBO curve

[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.