It is not entirely clear what your question is. RCBOs that only break the phase cores are common, certainly on 3 phase, but they are not a wonderful idea in systems with upstream RCD protection, delay or otherwise, as an NE fault (or an LE fault and a resisting LN load) is not disconnected by the operation of the RCBO, and the upstream protection fires as well.

Avoiding this sort of problem  is part of good system design. 

If you are really asking why do the regs not prohibit it, the answer is that there may be situations (not yours presumably) where the operation of the upstream RCD is desired for certain types of fault and there is no good reason for the rules to be overly restrictive - the regs are not a recipe book after all . It is a matter for the designer when capturing the system requirements and selecting the components to weigh up such factors as how much of a problem is nuisance tripping, versus the danger of a part-live part dead installation..

Some folk seem to struggle with protection on systems with TT earthing, but a tiered approach to both RCD trip thresholds and delay times, and yes L and N breaking at every tier, are the correct way forward.
Mike

Basically I’m coming across installs on tt systems where the time delayed RCCB is down stream and consumer unit has rcbos single pole installed. The faults are bypassing the rcbos and taking out the time delay RCCB. Where as if you have double pole rcbos installed it disconnects them under fault conditions. Only on tt systems only.

That is entirely possible, and  arguably phase only breaking RCBOs are not a great design choice for something like a dwelling , where normally you'd prefer to keep the lights on if the power goes off for example.

As above it is due to the fault pushing current in the neutral earth loop which is not interrupted. . Once the fault is cleared, do you measure the N-E offset voltage in these installations, or confirm that there is no significant standing current into the earth electrodes under normal operation ?

Large NE offset voltages can be an early warning of distribution side earth problems.

Mike.

Raymond Obrien said:

Why is not stated that the use of Double pole in  IET on these conditions.

It seems the powers that be are of the same mind - the DPC for AMD4 contains a new note to 536.4.1.4 ...

"NOTE 4: To achieve selectivity between an upstream Type-S RCD and any downstream RCD with respect to both a
line to earth and a neutral to earth fault, the downstream RCD shall switch all live conductors (including the
neutral)."

although to be fair it could be argued it was already implied - 531.3.1 demand that RCDs disconnect all live conductors (including N) with the exception of 531.3.1.201 that for protection against electric shock N need not be opened in TN and TT systems - which sort of makes sense as the system is safe from shock if the upstream device opens as well. But if you want discrimination (selectivity) then the consideration is not just for shock, so maybe the exception shouldn't apply. The new note is much clearer though.

   - Andy.

Raymond,

What is the time delay that is built into your "Time delayed" RCCB? Could it be that that device is still operating faster than the final circuit single pole RCBO's? What is the earth leakage setting of the "Time delayed" RCCB compared to the final circuit RCBO's?

Does one of the final circuit single pole RCBO's actually "Trip" same time as the "Time delayed" RCCB? If there is no sign of that could the "Time delayed" RCCB just be tripping due to summation of earth leakage on all of the final circuits?

I have worked in the world of TT earthing systems and flammable atmospheres for many years, and we utilise at least three diffrent levels of time/earth leakage current cascading, and yes 30mA instantanous final circuit RCBO's are all pole isolation. So we very very seldom hav a main or submain earth leakage device operating on a final circuit "Fault".

I would also think the original "Designer" also needs to take into account 314.1 (i) where you do not want numerous or all healthy circuits being disconnected by one faulty one.

Other reason we have all pole devices on our protection items is due to the requirement for hazardous area circuits for all "Live" conductors to be disconnected in event of a fault so that includes the neutral.

Maybe a few things for you still to look at perhaps.

Cheers GTB