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.

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:

But surely standard domestic lighting circuits with standard bayonet / Edison fittings aren't fixed loads... There's no telling what shenanigans users might get up to!

(Try searching for "light socket splitter" for a start, though I've heard tell of adapters to BS1363-ish outlets...)

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

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

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

Unless you go down the 435.1 route - In ≥ Iz so overload protection is provided, and as long as the breaking capacity is suitable, fault protection may be assumed.

    - Andy.

AJJewsbury said:

In ≥ Iz so overload protection is provided,

Yes, with the important caveat being that would only work if there are no de-rating factors to apply to the 0.75 sq mm conductors, because I_z is the current-carrying capacity in the given installation conditions. (Hopefully, the cable is suitable for 6 A in the given conditions in this particular case, but there may well be cases where protection against overload by other means, or where overload protection can legitimately be omitted.)

Thanks Graham, I'm still digesting the info and trying to get hold of a copy of BS 7454, but in the meantime I have tried to answer your questions below:

gkenyon said:

Agreed - so what is protecting the 0.75 sq mm?

Paul P Carroll said:

OCPD: Schrack RCBO C6 30mA, type A, 6kA (backed-up by a 40A BS88-2 fuse)

gkenyon said:

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

Well, these cables are pre-installed in the roadside cabinet, flexible cables with IEC 13 connectors on the end, ready for the specific roadside controllers to be plugged in to them upon installation within the cabinet. In order to confirm the volt drop, EFLI and load within the calculation design software I need to include these, even if there was a get-out clause. I have assumed these cables are as per Table 4F3A for Flexible non-armoured cables. The manufacturer's CCC of 10A certainly matches the table. 

It turns out that the cabinet manufacturer has actually gone one size above the specification and is using 1mm2, which is better but it does not remove the issue. From the advice on here it sounds like I may be able to prove that the proposed scenario will not actually be a problem, however the design calculation software is still going to highlight a problem as it is working off worst case values and probably simplified calculations. But if I can satisfy myself that the scenario referred to in my original post (but with S now being 1mm2) will be compliant I will be happy.

Paul P Carroll said:

however the design calculation software is still going to highlight a problem as it is working off worst case values and probably simplified calculations. But if I can satisfy myself that the scenario referred to in my original post (but with S now being 1mm2) will be compliant I will be happy.

Agreed ... there are limitations to software that the experienced designer needs to use their experience to handle (and.or select the appropriate parameters or course of action).

0.75 sq mm can carry up to 10 A, but only under certain conditions - one of those being the length is limited to < 2 m, when used to supply appliances from a standard circuit to BS 7671 (for example, see Table 2 to BS 1363-1) which is roughly what you're describing here.