No overload protection for fixed load cable?

[JohnW2]

Hidden in a recent thread which many people probably gave up reading was the suggestion/implication that it may be acceptable for a cable not to be provided with overload protection if it is part of dedicated wiring for a single load, provided that adequation fault protection was in place. The example in question (from overseas) was a length 0.5mm² felxible cable in a circuit protected by a 16A OPD.

I realise that this may often be technically compliant, by invocation of 433.1.1(ii), but do people regard it as either sensible or good practice? Personally, I have my doubts.

To cite the example I mentioned in that other thread, what would people think (indeed, how would they deal with this in a PIR) if they found a dedicated circuit, say to an immersion heater, wired completely or partially (maybe just the final flex) in 1.5mm² cable, but protected with (for whatever reason!) a 32A or 40A MCB? Would you regard this as OK so long as the circuit's fault protection checked out satisfactorily?

I realise that 433.1.1(ii) only applies when 'overload' in the normal sense is not considered to be a likely possibility, but my main concern would probably be in relation to faults in the load which resulted in less-than-zero-impedance L-N faults (rather than the 'bolted short' fault considered by the regs) - against which the cable might have inadequate protection with the arrangement I have described.

Kind Regards, John

 

[sparkticus]

Hello johnw2, I am relatively new to this forum, its a rainy day and your question is an interesting one.


I would always be prudent and would protect the cable in a fixed installation for all circumstances with an appropriately rated MCB (or fuse) but ignoring regs and assuming an overload is highly unlikely then it may be reasonable to use the adiabatic equation to understand how hot the cable will get in the time it takes to disconnect.


Having said that, even fixed equipment can cause a sustained overload condition. An immersion heater may leak earth current in parallel with its normal functional load current. In a non rcd protected circuit the total current (load and leak)may cause a sustained overload leading to the 1.5mm cable lighting up.

 

[sparkticus]

JohnW2, I did not respond to the PIR question.

I have seen a short length (<150mm) of 1.5mm cable used to connect a standard socket (on a ring final circuit protected by a 32amp MCB) to a fused spur (FCU) which feeds a combi boiler. The 1.5mm cable was contained within the metal housing of the backing box surrounded by solid brick/plaster. If I remember correctly, I noted that on the PIR as a code 2 but issued a satisfactory PIR.

I included a calculation of the risk to the 1.5mm cable as an attachment to the PIR. I consulted the NICEIC for advice before issuing the PIR. True that I could have just upgrade the short length of cable but I felt (with advise and the circumstances) that it was acceptable.

 

[JohnW2]

I would always be prudent and would protect the cable in a fixed installation for all circumstances with an appropriately rated MCB (or fuse)

That's also my personal view

....but ignoring regs and assuming an overload is highly unlikely then it may be reasonable to use the adiabatic equation to understand how hot the cable will get in the time it takes to disconnect.

You can't use an adiabatic equation for the situation which concerns me (see below), since it's probably not going to be an adiabatic process. However, as I said, if the risk of 'overload' (per BS7671-speak) is considered highly unlikley, then the regs do allow 'overload protection' to be omitted.

Having said that, even fixed equipment can cause a sustained overload condition. An immersion heater may leak earth current in parallel with its normal functional load current. In a non rcd protected circuit the total current (load and leak)may cause a sustained overload leading to the 1.5mm cable lighting up.

As I said, even without any leakage to earth, I still have significant concerns in relation to 'partial' L-N faults (which the regs don't seem to consider) (e.g. a short across part of a heating element or a section of a motor winding). With the example I gave, a 40A MCB would allow about 45A to flow indefinitely, and best part of 60A to flow for quite a long time - and I seriously suspect that would 'do for' 1.5mm² cable!

Kind Regards, John.

 

[JohnW2]

I have seen a short length (<150mm) of 1.5mm cable used to connect a standard socket (on a ring final circuit protected by a 32amp MCB) to a fused spur (FCU) which feeds a combi boiler. The 1.5mm cable was contained within the metal housing of the backing box surrounded by solid brick/plaster. If I remember correctly, I noted that on the PIR as a code 2 but issued a satisfactory PIR.
I included a calculation of the risk to the 1.5mm cable as an attachment to the PIR. I consulted the NICEIC for advice before issuing the PIR. ....

Interesting. Since you're new to the forum, I should preface my comments by explaining to you that I'm not a qualified or practising electrician....

As before, if you felt it highly unlikley that the boiler could result in an 'overload' (as opposed to a 'fault'), then per 433.3.1(ii) that would actually be compliant. Even if you did not involke 433.3.1(ii), 433.2.2 would probably allow you to rely on the fuse in the downstream FCU to protect the 1.5mm² cable - so, again, probably compliant. Hence, if I were an electrician doing that PIR, I think that the regs would probably have obliged me to call it compliant, whatever I felt personally (and, in truth, I would personally be perfectly comfortable with what you describe, too).

Kind Regards, John

 

[sparkticus]

Ah yes, I see your point. Though in reality I have not seen many faults that produce a sustained increased load current (above the design current) across L=N or L1 - L2 - L3 without the situation rapidly evolving to a current which trips the protective devices. It is not impossible but is generally accounted for in the design of the majority of modern heating systems and motors etc.

However it is more than an academic concern so I see your reason for raising it.

 

[sparkticus]

PS, you may not declare yourself as a qualified electrician but you seem to know your stuff. Is a rose not a rose by any other name?

 

[kai]

To cite the example I mentioned in that other thread, what would people think (indeed, how would they deal with this in a PIR) if they found a dedicated circuit, say to an immersion heater, wired completely or partially (maybe just the final flex) in 1.5mm² cable, but protected with (for whatever reason!) a 32A or 40A MCB? Would you regard this as OK so long as the circuit's fault protection checked out satisfactorily?

Did you check to see whether there was a 13amp DP switched fuse-connection unit protecting the 1.5.mm flex etc??

 

[JohnW2]

Did you check to see whether there was a 13amp DP switched fuse-connection unit protecting the 1.5.mm flex etc??

It was a hypothetical example - but the whole point of the example is that there is no FCU and the only thing protecting the cabe is a 32A or 40A MCB - yet seemingly probably compliant with the regs, even if I would not let such a design anywhere near my home!

Kind Regards, John.

 

[kai]

Code Two.

I'll mark it with a Code Two, being in need of correction, it sounds risky to me, the OCPD being far too high for the intended load, alternatively, where the design permits, I'll fit a suitable rated fuse-connection unit in order to fuse down the supply to match the intended design current of the equipment.
This can often be done safely where a disused instant shower circuit wired in 6mm cable off a B32 breaker, is used to power a new shower pump only, for a megaflow fed shower.

 

[JohnW2]

Ah yes, I see your point. Though in reality I have not seen many faults that produce a sustained increased load current (above the design current) across L=N or L1 - L2 - L3 without the situation rapidly evolving to a current which trips the protective devices. It is not impossible but is generally accounted for in the design of the majority of modern heating systems and motors etc.
However it is more than an academic concern so I see your reason for raising it.

Yes, I agree it's a pretty unlikley scenario, even though not impossible.

The main thing which interested me about the suggestion is that, whatever the regs might say, I think it really flies in the face of what most people would regards as 'good practice' - i.e. I don't think many electricians would be inclined to have an MCB with an In considerably higher than the Iz of the cable, regardless of what was on the end of the cable.

Kind Regards, John.

 

[JohnW2]

Code Two. I'll mark it with a Code Two, being in need of correction ....

I very much sympathise with your feelings but what do you say about the argument I've presented above (and it wasn't me who started this!) that the design is technically compliant?

You can't really 'mark something as Code 2' if it complies with the regs, can you - even if, like me, you don't really like it?

Kind Regards, John.

 

[JohnW2]

PS, you may not declare yourself as a qualified electrician but you seem to know your stuff. Is a rose not a rose by any other name?

That may sometimes be a contentious issue around here, but I think it's probably true to say that I am not a 'typical DIYer'

Kind Regards, John.

 

[ban-all-sheds]

Since you're new to the forum, I should preface my comments by explaining to you that I'm not a qualified or practising electrician....

People like you get everywhere....

 

[JohnW2]

Since you're new to the forum, I should preface my comments by explaining to you that I'm not a qualified or practising electrician....

People like you get everywhere....

It probably takes one to know one

I've been wondering where you were. I'm sure I don't have to tell you that, without knowing it, you were the person who 'started' this thread!!

Kind Regards, John.