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.

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.

I'm still not clear on this point - The discussion that has followed has focused on the types of overload that are feasible to occur,  but i am still not sure why the regulation that is related to overload is effectively broadened by the GN text to include overcurrent. 

Is the guidance from GN6 represented in BS 7671?  I can't see it in there.

I would guess it's because overload events are, by definition, a subset of overcurrent events, and protection against overload is, where required, provided by an overcurrent protective device.

chrisbryant said:

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.

IET have not 'chosen' this.

It simply occurs because 'overcurrent' (Chapter 43) comes in two flavours - FAULT CURRENT (Section 434) and OVERLOAD CURRENT (Section 433) ... BOTH are 'overcurrent'.

Specifically in this thread, the RCD is unable to protect against the thermal effects of either fault current or overload current. That is because it's not an overcurrent protective device, but a residual current protective device.

"'overcurrent' (Chapter 43) comes in two flavours - FAULT CURRENT (Section 434) and OVERLOAD CURRENT (Section 433) ... BOTH are 'overcurrent'".  

I get this, which is why i find the wording in the GN to be strange.  It is noted in GN6 as guidance against 536.4.3.2, which is "Overload protection of RCCB, switch, Transfer Switching Equipment (TSE) or impulse relay".  Overcurrent is not mentioned anywhere in that section.

chrisbryant said:

get this, which is why i find the wording in the GN to be strange.  It is noted in GN6 as guidance against 536.4.3.2, which is "Overload protection of RCCB, switch, Transfer Switching Equipment (TSE) or impulse relay".  Overcurrent is not mentioned anywhere in that section

Which Section of GN6 are you talking about in particular? There are a few that reference Regs in Reg Group 526.4.2 - that would help me to deal with this query.

Section 1.5 - the section in your response to the OP's original query.  The section is below:

chrisbryant said:

Section 1.5

Yes, agree this should  additionally be tagged against 536.4.2.4 relating to fault currents.

Hi Graham, Chris,

I am interested in the original question in this thread, but have been struggling to get my head around some of the definitions, as I thought that not all FAULT currents are necessarily OVERcurrents, i.e., if you have a TT supply with a line to exposed-conductive-part fault, the fault current isn’t necessarily going to exceed the rated value of the cable as it may only be a few amps, and so would not meet the definition of an Overcurrent. The definitions in BS 7671 for Overload and for Short Circuit both begin by classifying they both as “An Overcurrent….”. The definition of Fault current does not…..but I’m not sure how intentional that is.

Similarly this is also hurting my head: GN6 states that a Fault current is a type of Overcurrent. A short to an exposed conductive part is a Fault. Such a fault can be protected against by an RCD (and usually more slowly by an OCPD). But an RCD is not classed as an overcurrent protective device as it doesn't protect against any thermal effects. Is there another sub-category of Fault which is a residual current but not an Overcurrent (or perhaps two types of Earth Fault, high current faults that are Overcurrents, and low/residual/leakage current faults that are not Overcurrents?) I drew up the diagram below (based on GN6 1.2.x) to try to aid my understanding. Would a ‘residual current’ sit outside of this diagram?

Paul P Carroll said:

thought that not all FAULT currents are necessarily OVERcurrents

Correct. There are cases even in faults between live conductors, a specific OCPD is not required because of the nature of the source of supply.

Paul P Carroll said:

if you have a TT supply with a line to exposed-conductive-part fault, the fault current isn’t necessarily going to exceed the rated value of the cable as it may only be a few amps

This is where things start to get tricky. For ADS, we are required to consider worst-case conditions, which are taken to be the highest resistance of consumer's earth electrode, highest Ze from the distributor's supply, and NO parallel paths.

However, in a real fault to Earth, for protection against overcurrent, you need to take into account the highest prospective fault current, not lowest Zs. In a real installation, you will have parallel paths, and potentially a much lower resistance of consumer earth electrode. Worst case is the example shown in GN6 and EIDG, which shows a TT supply from a PME system, with shared extraneous-conductive-parts. Here, the prospective fault current is practically that of the TN system next door, but you can't rely on that for ADS ... see Figure 6.3 of GN6 9th Ed 2022

Paul P Carroll said:

A short to an exposed conductive part is a Fault. Such a fault can be protected against by an RCD (and usually more slowly by an OCPD).

Only for protection against electric shock, as discussed above.

This is where Table 41.xx won't help - because they are to achieve ADS.

You may also need to take into account the backup protection of the RCD, as the real prospective fault current (which varies throughout the year) will be higher than that values you used in calculating Zs for ADS in Chapter 41.

if you have a TT supply with a line to exposed-conductive-part fault, the fault current isn’t necessarily going to exceed the rated value of the cable as it may only be a few amps, and so would not meet the definition of an Overcurrent.

Or another approach - the definition is about the current exceeding the "rated current" - with a cable's CCC is only one example (the rated current of a conductor). There are other considerations too which you might have to consider when deciding that the rated current for a given situation is. For instance a c.p.c, or indeed the earthing system in general, will naturally develop a potential difference between its ends when carrying a current - if that exceeds some limit (say 50V) then safety is compromised regardless of whether the conductors start to overheat or not.  So you might decide that the maximum intended (or "rated") current that an earthing system can carry indefinitely  is substantially less than the CCC of the conductors that it consists of.

   - Andy.