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.

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.

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.

Is there a difference between PRC and XLPE generally?

The table and formulae are still reproduced in the latest version of the commentary, albeit with a hefty disclaimer from the BCA. Sadly IEC 943 does not seem to be available even as an obsolete document but judging by a preview of an Australian standard which remixed it is had some other interesting content... Does anyone know what has replaced it?

*sadly not revised since 17th Am3

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.

Indeed some faults can develop that have the characteristics of an overload current, but by definition these are not overload currents and protection can be omitted under 433.3.1(ii) using the definition in part 2. I've lost count of the number of EICRs I've seen where the testers have failed 1mm lighting circuits that are protected by a 16A MCB.  

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.

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.

Graham (and AJJewsbury),

Thanks very much for the feedback. The comment above regarding “in a real fault to Earth, for protection against overcurrent, you need to take into account the highest prospective fault current, not lowest Zs”, leads nicely in to the main query I wanted to ask on this thread:

I have a situation where a number of rural roadside technology cabinet sites were designed to be supplied by a TN-C-S DNO supply, but are now, at time of installation, to change to TT following further DNO feedback. When I update the design to reflect the alternative means of earthing, the smaller cables are failing the Adiabatic / Energy let-through calculation, and I can't change most of the elements as I have to work within the constraints of the client’s equipment, as summarised below:

Adiabatic Check:

K = 115 (can’t change, roadside cabinet is supplied by client to their own specification)

S = 0.75mm² (can’t change, roadside cabinet is supplied by client to their own specification)

I = 11A (TT earth electrodes have been specified to be max 20 Ohms. It may be possible to achieve less to get a slightly higher fault current, but no guarantee)

OCPD: Schrack RCBO C6 30mA, type A, 6kA (backed-up by a 40A BS88-2 fuse) (can’t change, roadside is cabinet supplied by client to their own specification)

t = would need to be <= 62.3s to satisfy the Adiabatic equation. Fig 3A5 indicates a time of over 400 seconds for the OCPD = Fail

Energy Let Through Check:

The cable withstand (K²S²) value is less than the energy let through (I²t) of the OCPD in EIDG table 8.6 = fail

I appreciate that the I²t values in table 8.6 are ‘generic’ and that manufacturer specific values are usually less, which may help my situation.

However, I have contacted the OCPD manufacturer, Schrack, and they have responded to say that they cannot provide such values (and their datasheet does not go any lower than 500A), and advised me to refer to IEC/EN 60898 for the maximum allowed I²t values – which effectively points me back to the generic/high values in EIDG table 8.6. So I am a little stuck.

Refs in GN6 6.4.3 and EIDG 8.11 reiterate what you have said about considering the highest prospective fault current. The original calculations were done assuming a TN-C-S supply with prospective fault current of 16kA and Ze of 0.35, and there was no problem. It is only after switching the supply type to a TT that the Energy let through issue has arisen. Does the fact that the design worked fine for a 16kA, 0.35 Ze TN-C-S arrangement not provide sufficient evidence that the design is compliant for the potential worst case fault current under a TT arrangement?

The referenced sections in GN6 6.4.3 and EIDG 8.11 don’t appear to mention any concerns over the prospective fault current being too low for an RCD, only too high, so could we not be satisfied that the RCD would be tripped by the 11A earth fault current, and any higher fault currents would be tripped by the overcurrent protection in the MCB element of the RCBO (as it has been proved in a TN-C-S equivalent calculation of the same design)?

Paul P Carroll said:

I appreciate that the I²t values in table 8.6 are ‘generic’ and that manufacturer specific values are usually less, which may help my situation.

However, I have contacted the OCPD manufacturer, Schrack, and they have responded to say that they cannot provide such values (and their datasheet does not go any lower than 500A), and advised me to refer to IEC/EN 60898 for the maximum allowed I²t values – which effectively points me back to the generic/high values in EIDG table 8.6. So I am a little stuck.

I can appreciate your difficulty here. Some manufacturers publish plenty of data; others less. As you say, the BS EN 60898 values are maxima. I think that you need to persevere - Schrack must have tested their devices otherwise they could not claim compliance with BS EN 60898.

Paul P Carroll said:

I = 11A (TT earth electrodes have been specified to be max 20 Ohms. It may be possible to achieve less to get a slightly higher fault current, but no guarantee)

Herein lies the problem ... you may well have a different prospective fault current.

Paul P Carroll said:

However, I have contacted the OCPD manufacturer, Schrack, and they have responded to say that they cannot provide such values (and their datasheet does not go any lower than 500A), and advised me to refer to IEC/EN 60898 for the maximum allowed I²t values – which effectively points me back to the generic/high values in EIDG table 8.6. So I am a little stuck.

Agreed - so what is protecting the 0.75 sq mm?

This is where I think you need to use adiabatic over the RCBO curve, because as the manufacturer says, the IEC/EN 60898 for the maximum allowed I²t values (or manufacturer's stated values) are only where the disconnection time is very short, i.e. for the "instantaneous trip" values, or t < 0.1 s (see second para of Reg 434.5.2 of BS 7671). That would cover you for high prospective fault current.

When we talk about trip times > 0.1 s, you need to plot the csa over the generic mcb curve in Appendix 3 (Fiig 3A4 for Type B)

This would appear to show that a B6 is unable to protect 0.75 sq mm cable unless you can achieve "instantaneous tripping").

HOWEVER, that's not the end of the matter ... I'm fairly certain that if you do a non-adiabatic line plot using the method in BS 7454 against the curve against the B6 curve, you will be able to demonstrate that the B6 always protects the 0.75 sq mm cable. Quite simply, it has to, because 0.75 sq mm copper generally has a basic current-carrying capacity of 6 A.

Paul P Carroll said:

S = 0.75mm² (can’t change, roadside cabinet is supplied by client to their own specification)

Is that specification also requiring you to conform to BS 7671? 0;.75 sq mm is only permitted (Table 52.3) for specific appliances (per the product standard) or in cables with seven or more cores