Please can someone tell me, is comparing 4mm and 2.5mm cable the same as comparing a chainsaw and a blender?

[JohnW2]

That's not strictly - or actually - true, is it? ... If the load cannot draw more than the CCC of the flex, then, as with most appliances in the home, the fuse in the plug isn't really necessary.

I was more than half-expecting such a comment

It would seem that many people are not aware of this potential exemption from the requirement for overload protection and/or choose not to invoke it. However, even for those who are 'aware', it comes down to an individual judgement as to what loads are "unlikely" to result in an overload current.

As you are aware, even in relation to purely resistive loads (usually heating elements) some people will argue that, in some circumstances (which I would probably regard as 'unlikely'!) an overload current could result. It is certainly less easy to argue that an overload current in unlikley when any motors (often fans) are involved, which applies to many things - e.g. fans i n general, hair driers, power tools and many cooking appliances.

It will be interesting to hear what the situation is in Australia, and whether theree is a corresponding exemption from the need for overload protection when the load is considered "unlikely" to result in an overload current.

Kind Regards, John

 

[bernardgreen]

If the load cannot draw more than the CCC of the flex, then, as with most appliances in the home, the fuse in the plug isn't really necessary

It might be that the flex is damaged between the plug and the fuse in the appliance then the cable must be able to safely carry the fault current until the 20 Amp MCB operates.

 

[JohnW2]

It might be that the flex is damaged between the plug and the fuse in the appliance then the cable must be able to safely carry the fault current until the 20 Amp MCB operates.

The requirement for adequate 'fault protection' remains even if one considers the load/conditions to be such that 'overload protection' may be omitted.

Kind Regards, John

 

[EFLImpudence]

It might be that the flex is damaged between the plug and the fuse in the appliance then the cable must be able to safely carry the fault current until the 20 Amp MCB operates.

Surely that would also be the case for a flex which is protected by an overload device.

 

[EFLImpudence]

It might be that the flex is damaged between the plug and the fuse in the appliance then the cable must be able to safely carry the fault current until the 20 Amp MCB operates.

Second thoughts:

Surely, if the appliance has such an integral fuse then that would be adequate to also protect the flex in any case - obviously it would not be a higher rating than one you would fit in the plug

 

[JohnW2]

Surely that would also be the case for a flex which is protected by an overload device.

As I said, this is all moot, since the requirement for fault protection doesn't go away even when it is felt that it can be argued that overload protection is not required.

Kind Regards, John

 

[JohnW2]

Second thoughts: ... Surely, if the appliance has such an integral fuse then that would be adequate to also protect the flex in any case ...

Not in the case that bernard was discussing (of a fault in the cable upstream of the appliance) - since none of the fault current would then flow through the appliance (or any fuse in it).

Kind Regards, John

 

[EFLImpudence]

Not in the case that bernard was discussing (of a fault in the cable upstream of the appliance) - since none of the fault current would then flow through the appliance (or any fuse in it).

Ah, ok. I think I must have misunderstood the significance of the cable damage.

Surely such an occurrence could be quoted against everything we advise anyone to do. "Yeah but what if the cable is damaged."

Almost as bad as having to state "method C of course" after every piece of advice re: cable size.

 

[JohnW2]

Ah, ok. I think I must have misunderstood the significance of the cable damage.

Fair enough, but I'm not sure what sort of cable damage you could have been thinking of other than a ('low impedance') L-E or L-N fault, upstream of the connected appliance (which I assume is what bernard was talking about).

Surely such an occurrence could be quoted against everything we advise anyone to do. "Yeah but what if the cable is damaged."

I don't really understand that comment. Surely the whole point of the requirement for fault protection is that the protective device should operate in the case of a ('low impedance') fault in the connected equipment OR in any of the cable supplying it, isn't it?

Almost as bad as having to state "method C of course" after every piece of advice re: cable size.

Well, for those who wish to comply strictly with regs, that caveat is important, isn't it? To say that, say, a 2.5 mm² radial can be protected by a 25A OPD, or that a 4 mm² radial can be protected by a 32A OPD without any such caveat would be potentially 'misleading', wouldn't it?

Kind Regards, John

 

[EFLImpudence]

Fair enough, but I'm not sure what sort of cable damage you could have been thinking of other than a ('low impedance') L-E or L-N fault, upstream of the connected appliance (which I assume is what bernard was talking about).

I thought Bernard was implying that omission of overload protection (what we were talking about) might be rendered unsatisfactory because the cable might be damaged and have a smaller CSA than was thought.
As I indicated, an occurrence that would apply to nearly everything we do.

I don't really understand that comment. Surely the whole point of the requirement for fault protection is that the protective device should operate in the case of a ('low impedance') fault in the connected equipment OR in any of the cable supplying it, isn't it?

Well, yes, but overload protection (what we were talking about) would not have been omitted if there is a fuse in the appliance.

Fault protection is required whether overload protection is omitted or not.

Well, for those who wish to comply strictly with regs, that caveat is important, isn't it? To say that, say, a 2.5 mm² radial can be protected by a 25A OPD, or that a 4 mm² radial can be protected by a 32A OPD without any such caveat would be potentially 'misleading', wouldn't it?

Well, yes but is there nothing that we can assume the other person knows without have to cover all possible situations.

If we always include "method C of course" then why not "not bunched of course" and "not in a high ambient temperature of course" and "cable not damaged of course" etc.

 

[JohnW2]

I thought Bernard was implying that omission of overload protection (what we were talking about) might be rendered unsatisfactory because the cable might be damaged and have a smaller CSA than was thought.

Fair enough. As will be apparent, that's not how I read it.

Fault protection is required whether overload protection is omitted or not.

That's what I have repeatedly pointed out (to both bernard and you) and, although we (you and I) were talking about overload protection, it seemed clear (to me) that bernard was talking about a situation in which 'fault protection' would be needed.

Well, yes but is there nothing that we can assume the other person knows without have to cover all possible situations. If we always include "method C of course" then why not "not bunched of course" and "not in a high ambient temperature of course" ....

Yes, I agree that our 'caveats' are often tot comprehensive enough. Although I've probably as guilty of others of writing something like "Method C of course", I try to be more comprehensive by talking about "depending on installation Method", or something like that - often adding a 'probably, depending upon method of installation" when I talk about the adequacy of CCC of a cable of particular CSA.

However, you're right that we very often give no indication that things such as grouping and ambient temperature may also affect CCC. In fact, I suspect that (although often 'over-ridden by 'common sense') it is probably fairly unusual that a cable is strictly-speaking (pedantically?) "Method C with no de-rating factors" for its entire length, particularly since there is very often 'grouping' for at least a small part of the cable run

...and "cable not damaged of course" etc.

I really think that is different. We are talking about 'methods of installation' and very few people would be daft enough to knowingly install cable that was already damaged. What we are talking about is the need for 'fault protection' to address the situation of a fault arising in the cable subsequently (after installation).

KInd Regards, John

 

[EFLImpudence]

I really think that is different. We are talking about 'methods of installation' and very few people would be daft enough to knowingly install cable that was already damaged. What we are talking about is the need for 'fault protection' to address the situation of a fault arising in the cable subsequently (after installation).

Well, yes, so what should have been the correct reply to Bernard's post?

It might be that the flex is damaged between the plug and the fuse in the appliance then the cable must be able to safely carry the fault current until the 20 Amp MCB operates.

Yes, I know but that is a different matter and nothing to do with what we were talking about when I wrote:

If the load cannot draw more than the CCC of the flex, then, as with most appliances in the home, the fuse in the plug isn't really necessary.

 

[JohnW2]

Well, yes, so what should have been the correct reply to Bernard's post?

I would say ("I would, wouldn't I" ) the answer I gave - that, although you and I had been discussion 'overloads', bernard introduced a situation of a 'fault', and that fault protection is still required even when one can argue that overload protection could be omitted.

Yes, I know but that is a different matter and nothing to do with what we were talking about when I wrote:

As above, I agree. Bertnard introduced something different from what you and I had been discussing.

Kind Regards, John

 

[FrodoOne]

I was more than half-expecting such a comment

It would seem that many people are not aware of this potential exemption from the requirement for overload protection and/or choose not to invoke it. However, even for those who are 'aware', it comes down to an individual judgement as to what loads are "unlikely" to result in an overload current.

As you are aware, even in relation to purely resistive loads (usually heating elements) some people will argue that, in some circumstances (which I would probably regard as 'unlikely'!) an overload current could result. It is certainly less easy to argue that an overload current in unlikley when any motors (often fans) are involved, which applies to many things - e.g. fans i n general, hair driers, power tools and many cooking appliances.

It will be interesting to hear what the situation is in Australia, and whether theree is a corresponding exemption from the need for overload protection when the load is considered "unlikely" to result in an overload current.

Kind Regards, John

I seem to have stirred up quite a "discussion".
Re: -
"It will be interesting to hear what the situation is in Australia, and whether theree is a corresponding exemption from the need for overload protection when the load is considered "unlikely" to result in an overload current."
I cannot find a definitive answer in AS/NZS 3000, which is a "hard to read" document, available only in "paper" format, at the cost of about AUD$250 per edition - and is "copyright".

While I have only "scanned" the later comments, I must point out that any CU Circuit Breaker or Fuse is there solely to protect the "cabling/wiring" of the building concerned - and to prevent any fault causing overheating and (possibly) fire from such overloaded "wiring".

It is not there to protect any "equipment", nor any flexible "cords" therewith associated.

If a "piece of equipment" fails in such a way as to draw "excessive current", that "piece of equipment" has already failed.
It may have internal "overload protection" to limit the current drawn from the circuit concerned but, if not, the current may increase until it is limited only by the resistance of the conductors concerned, until the CU "protection device" operates - or
the internal (short circuit) fault becomes an "open-circuit", due to the absence of sufficient conductive (carbonised) material.

This is the "norm" in Europe (16 A), North America (15 A or 20 A) and Australia (20 A).
In the UK, a "ring" circuit could provide 32 A. However, the fuse in the UK Plug, limits the "overcurrent" (beyond that point) to 13 A - or to some lesser value.

Flexible "cords" operate in "free air" and are (or should be) visible.
With a "catastrophic overload" (short circuit), they would be required to carry the "overload" current for only a brief time, until the "overcurrent circuit protection" operated.
The worst case would be if a "device" developed a fault where the "overload current" gradually increased until it became a short-circuit.
During the period of "gradual increase" the "cord" may be overloaded and become "appreciably warm".

Fortunately, standards are such that a "cord" rated at (say) 7 A (on a circuit protected by a 20 A circuit breaker) will not "spontaneously combust"
if subjected to a 3-times "long duration" overload.
However, the insulation may become "soft", due to the additional heat produced in the conductors.

 

[JohnW2]

.... I must point out that any CU Circuit Breaker or Fuse is there solely to protect the "cabling/wiring" of the building concerned - and to prevent any fault causing overheating and (possibly) fire from such overloaded "wiring". .... It is not there to protect any "equipment",

We know that, although some people may 'argue'. In fact, provided only that the OPD is adequate to protect the fixed wiring, there's absolutely no reason why someone could not use an OPD rated to satisfy any perceived need for protraction of connected equipment - although that would often/.usually result in an unnecessarily restrictive' OPD rating.

.... nor any flexible "cords" therewith associated.

That's a bit different. Whilst the 'Wiring Regulations, per se, relate primarily only to the 'fixed wiring' of the installation, there are also requirements for adequate protection of flexible cables 'plugged into' that installation.

This is the "norm" in Europe (16 A), North America (15 A or 20 A) and Australia (20 A). In the UK, a "ring" circuit could provide 32 A. However, the fuse in the UK Plug, limits the "overcurrent" (beyond that point) to 13 A - or to some lesser value.

To be a little pedantic, it takes at least about 22A to 'blow' a 13A BS1362 (in-plug) fuse, so does not "limit the current to 13A".

Flexible "cords" operate in "free air" and are (or should be) visible. With a "catastrophic overload" (short circuit), they would be required to carry the "overload" current for only a brief time, until the "overcurrent circuit protection" operated.

Yes, but you are now talking ('catastrophic overloads') with what we call 'faults', not overloads. The potential problem (for the flexible cables) relates to 'non-catastrophic overloads', A 32A MCB will allow up to about *** to flow ndefintely, and higher currents than that for appreciable periods of time. If connected equipment results in a high overload current (e.g. an appliance with a 'jammed' large motor) then the flexible cable supply it could find itself carry high currents (much higher than its theoretical 'CCC') for at least long periods, maybe 'indefinitely'. Connection of such an appliance with nothing but a 32A MCB protecting its flexible cable would therefore not be regarded as acceptable in thee UK.

The worst case would be if a "device" developed a fault where the "overload current" gradually increased until it became a short-circuit.
During the period of "gradual increase" the "cord" may be overloaded and become "appreciably warm".

See above. I think that it could be worse than you suggest.

Fortunately, standards are such that a "cord" rated at (say) 7 A (on a circuit protected by a 20 A circuit breaker) will not "spontaneously combust"
if subjected to a 3-times "long duration" overload.

Probably not, but it is probably not a very desirable practice to rely on such 'safety margins.

Kind Regards, John