Quite simply because adiabatic is about protection against overcurrent, and RCCBs, or the RCD element of a combination protective device, cannot provide protection against overcurrent.

GN6 tells us (Section 1.5):

While residual current devices (RCDs) can provide protection against electric shock by automatic disconnection of supply, they do not provide protection against overcurrent. Residual current circuit-breakers (RCCBs) must always be backed up by a separate overcurrent protective device to protect against fault current (and, if required for the particular circuit, overload current). Overcurrent protection may be included in the same device, for example, residual-current circuit-breaker (with overcurrent protection) (RCBO).

This does bring into question how we approach the situation for TT systems. The important factor is that we can't assume the prospective earth fault current is determined by the earth electrode alone (in the way we do for Z_S for ADS), because extraneous-conductive-parts, or fortuitous earthing, may well reduce the overall effective earth electrode resistance ... and increase prospective fault current.

In the worst-case, prospective earth fault current could well be the same as L-N prospective fault current (see Section 6.4.3 of GN6), and therefore we ought to consider using the same approach for protection against overcurrent for earth faults in TT system earth faults, as TN system.

OK, so what sort of fault will exercise the insulation on the CPC to its thermal limit, without triggering the operation of any non-broken  RCD?

I agree that an RCD is not much  good against L-N or L-L faults.

Also  while an RCBO is built with contacts that can open the full 6kA, 10kA or 16kA or whatever, many RCD only devices have smaller arc chutes and may only be guaranteed to open a maximum Im of 1kA to 2kA without welding contacts or other internal damage.

Devices to IEC 1008 typically have a spec simiilar to this

Mike.

mapj1 said:

OK, so what sort of fault will exercise the insulation on the CPC to its thermal limit, without triggering the operation of any non-broken  RCD?

Perhaps life is the other way round ... for faults of rather high-current, the OCPD operates before the RCD ... that's why RCDs are tested (to the product standard) with their backup protection and have a conditional short-circuit rating, because of the "grey area" in the middle?

At the end of the day, regardless of what an RCD will or won't do, the standard for RCDs assumes (quite rightly) that there will be an OCPD as well. And the tests cover for the fact the RCD may operate first ... or not

And for designers using BS 7671, RCDs don't have a "let-through energy" or equivalent time-current curve to use with the adiabatic criterion ... fuses and circuit-breakers (including RCBOs) do ... but with RCBO's (and CBRs) the let-through energy is that of the mcb (CB) element, not the RCD element.

mapj1 said:

OK, so what sort of fault will exercise the insulation on the CPC to its thermal limit, without triggering the operation of any non-broken  RCD?

Perhaps life is the other way round ... for faults of rather high-current, the OCPD operates before the RCD ... that's why RCDs are tested (to the product standard) with their backup protection and have a conditional short-circuit rating, because of the "grey area" in the middle?

At the end of the day, regardless of what an RCD will or won't do, the standard for RCDs assumes (quite rightly) that there will be an OCPD as well. And the tests cover for the fact the RCD may operate first ... or not

And for designers using BS 7671, RCDs don't have a "let-through energy" or equivalent time-current curve to use with the adiabatic criterion ... fuses and circuit-breakers (including RCBOs) do ... but with RCBO's (and CBRs) the let-through energy is that of the mcb (CB) element, not the RCD element.