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.

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.

This section/excerpt has confused me somewhat.  The text referred in GN 6 is related to 536.4.3.2. 

This section of BS 7671 is "Overload protection of RCCB, switch, Transfer Switching Equipment (TSE) or impulse relay" and is relation to overload and not the broader category of overcurrent. 

The text in the regulations makes sense to me, but the text in GN6 confuses me.  Should the word "overcurrent" in that section of GN6 not have actually been "Overload and some fault currents"? 

Overloads are small fault currents - in the sense of things like a heater with a shorted turn half way along the element drawing 150% of expected, as opposed to proper faults in the '7671 definition, which are assumed to be a dead short, somewhere in the installation, usually at one end of the cable or the other, whichever is worst case. As a point of order a true dead short fault creates no heat light or sound, and is quite rare  - real faults have a finite impedance at the point of contact., and so do get hot and do damage, but also do not pass as much current as predicted.(hence jokey reference to silver spanner faults)

Mike

Hi Mike,

Thanks for the response.  I get the difference between the two, but wondered why the IET have chosen to use the term overcurrent in the Guidance Note in relation to a section of 7671 that is associated with overload.

The regs state the definition for an overload current in part 2 (an overcurrent occurring in a circuit that is electrically sound). Therefore 'overloads' are not faults at all and can only really occur in inductive circuits i.e. motors that have jammed. By definition, a resistive circuit cannot have an overload current as a fault must occur before an overcurrent happens. This is why resistive circuits that are not liable to overload conditions, need only fault protection and as such we can 'pass' circuits that initially seem dangerous (a 3kW convector heater on a 40A MCB wired in 2.5mm 6242Y for instance). As long as Zs and the CPC are sufficient then that circuit complies. 

Along with the standard confusion over earthing/bonding, I'd say overload current is the most commonly encountered misunderstanding I come across from time-served electricians in the industry.  

Er, final circuits containing sockets can easily become overloaded without any fault, be it a resistive load or otherwise. Just plug too many heaters in.

Very true, another viable overload condition. Which is why socket circuits would be subject to protection against overload current as well as fault current. Unless the total number of sockets limits the current flow to below the rating of the cable (i.e. a single 13A socket wired in 2.5mm cable) - I don't see why overload protection would need to be offered in that instance.  

Some of us like fairly close protection  upstream fuses anyway - just in case. The rest of the world is not always as well behaved as it should be...

 

Mind you that bit of 2.5mm2 can be overrun quite a lot for quite a while before it fails, assuming it was designed for a 20 year life at 70C copper temp and halving life every 8 degree or so... table from the 2004 commentary on the amendment to 6,1,3 , no longer sadly free to download.
Note that temperature rise more or less relate to the square of current - so if 27A gets 2.5mm T and E  to 70C from a 30C ambient (40 degree rise) then twice that is an 80 degree rise (to 110 degrees) and takes root 2 times the current or just under 39 amp...

Mike.

Therefore 'overloads' are not faults at all and can only really occur in inductive circuits i.e. motors that have jammed. By definition, a resistive circuit cannot have an overload current as a fault must occur before an overcurrent happens.

As well as sockets or other situations were end users can change the rating of the load (e.g. by replacing lamps with higher wattage ones), simple resistive loads can develop faults that have all the characteristics of an overload as far as the fixed wiring is concerned - e.g. shorted turns part way along a heating element (or fault to earth part way along a metal sheathed element, especially where there's no RCD upstream).

  - Andy.