The most basic problem with DIY work

[mfarrow]

If BS 7671 would apply, why would you feel the need for the rated current of the cable to meet that of the fuse?

 

[JohnW2]

If BS 7671 would apply, why would you feel the need for the rated current of the cable to meet that of the fuse?

Well, as you know, the primary principle of BS7671 is that In ≤ Ib ≤ Iz - in particular, in this case, (and assuming that the cable is adequate for the load - i.e. Ib ≤ Iz) that the In of the OPD protecting a cable must be no greater than the CCC of the cable being protected.

As I've said, AFAIAA one can only escape that requirement if one can argue that the nature of the load is such that an 'overload' situation cannot arise (i..e. load current can never exceed CCC of cable), in which case the requirement for overload (but not fault) protection of the cable may be 'waived'. However, as I also said, in the case of a fridge-freezer, I personally would not be confident that the cable could not be overloaded (in context, by a current between 6A and 13A) should the compressor motor 'jam', would you?

Is there something wrong with my reasoning?

Kind Regards, John

 

[securespark]

I see! As a matter of interest, what was the manufacturers' perspective - was it they who cited BS1363 as their 'excuse'?

I wonder what the underlying issue is. Do you think the manufacturer's feel that a 13A fuse is actually required in order to cope with the inrush current of the compressor, or are they just blindly following some 'rules' (such as BS1363)? Do the MIs require a 13A fuse? If not, did you consider offering to change it to a 5A one and the passing the item?

I'm a bit uncertain as to the scope of various regulations and as to exactly what regulations/rules/whatever apply to PATesting. Does BS7671 not apply to such appliances as this? If it did, the situation you were faced with was clearly non-compliant with BS7671 (unless you would be comfortable in saying that a fridge-freezer cannot create an overload {but I wonder what current the compressor motor would draw if it jammed?}), hence difficult to see how/why it should pass a PATest.

This whole situation seems to be an awful mess!

They did quote 1363, yes. I can't see inrush being a problem, because I've seen fridges run successfully on 3A fuses before now. I did not get to see the MI's unfortunately.

Bit of grey area regarding 7671 and the appliance: I don't think it does apply to the appliance, but to my mind, it does apply to the flex. Maybe I'm being a bit strict here: I suppose you could argue that the flex is actually part of the appliance.

As you say, it is messy!

 

[JohnW2]

They did quote 1363, yes. I can't see inrush being a problem, because I've seen fridges run successfully on 3A fuses before now. I did not get to see the MI's unfortunately.

I agree with you about inrush. It's a bit of a pity that you didn't see the MIs because, if it didn't actually specify a 13A fuse, as I said, maybe you could have offered to change it for a 5A one and then 'pass' the appliance!

Bit of grey area regarding 7671 and the appliance: I don't think it does apply to the appliance, but to my mind, it does apply to the flex. Maybe I'm being a bit strict here: I suppose you could argue that the flex is actually part of the appliance.

As you'll have seen from my exchanges with BAS, BS7671 seems to say that it would apply, at least to the flex/plug, if there were not a 'product standard' covering such issues - but I don't know if there is such a standard or, if there is, whether it addresses this issue.

Kind Regards, John

 

[securespark]

I'm going to ring the IET and see what they say...

 

[SimonH2]

... (unless you would be comfortable in saying that a fridge-freezer cannot create an overload {but I wonder what current the compressor motor would draw if it jammed?}), ...

They typically have a thermal overload built in - so if it fails to start, it will trip.

We have an old freezer we keep for when our modern one packs in and needs defrosting for a few days. It has a failed thermostat so it gets used with an electronic one I made many years ago - but that has a habit of rapid switching under some conditions ("a bit twitchy"). If that happens, the compressor in the old freezer will fail to start as the differential pressure won't have bled down, and will trip after a few seconds. It will reset itself after a couple of minutes.

 

[JohnW2]

... (unless you would be comfortable in saying that a fridge-freezer cannot create an overload {but I wonder what current the compressor motor would draw if it jammed?}), ...

They typically have a thermal overload built in - so if it fails to start, it will trip.

Sure, but that's rather different from saying that the appliance 'cannot' create an overload - the thermal cutout may not exist, or may not work and, in any event, an excessive ('overload') current will presumably usually flow for some period of time before it operates. In any event, the fridge freezer is just an example; other appliance may not (or may not be guaranteed to) have internal protection.

If one were so inclined, I suppose that one could try the 'cannot create an overload' argument for most fixed-wired domestic loads, particularly if they are on dedicated circuits. However, would you really suggest that the cable of the fixed wiring of such circuits does not need 'adequate' overload protection?

Kind Regards, John

 

[JohnW2]

I'm going to ring the IET and see what they say...

Thanks. If you get something approaching a proper answer, that could be very interesting.

Kind Regards, John

 

[ban-all-sheds]

If one were so inclined, I suppose that one could try the 'cannot create an overload' argument for most fixed-wired domestic loads, particularly if they are on dedicated circuits. However, would you really suggest that the cable of the fixed wiring of such circuits does not need 'adequate' overload protection?

If you ask the IET about that they will say 433.3.1(ii)....

 

[JohnW2]

If one were so inclined, I suppose that one could try the 'cannot create an overload' argument for most fixed-wired domestic loads, particularly if they are on dedicated circuits. However, would you really suggest that the cable of the fixed wiring of such circuits does not need 'adequate' overload protection?

If you ask the IET about that they will say 433.3.1(ii)....

Sure, that's the regulation we're discussing. However, the issue is that it relies upon a judgement that the characteristics of the load are such that the cable is "not likely to carry overload current". That's a pretty subjective judgement, which is why I have generally be asking if people would be 'comfortable' to invoke the reg.

I'm not suggesting that there would be any reason/sense in doing it, but how would you feel (assuming Zs was low enough for adequate fault protection with the MCB concerned) about a dedicated immersion circuit wired in 1.5mm² T&E (clipped direct) which was protected by a B32 MCB(or maybe even B40, if Zs would accommodate it)? Would you be 'comfortable' with that?

Of more practical relevance, how would you feel about the (common, in outhouses etc.) situation of having a 1mm² lighting circuit fed from a 32A ring or radial circuit - but without the traditional 'fusing down' with an FCU? One might well be able to argue that such was compliant, by virtue of 433.3.1(ii), but how many people would actually be comfortable to do, or advise, that?

We obviously don't know how far they thought people would go in invoking 433.3.1(ii), but I would strongly suspect that they did not have in mind the sort of situations I've mentioned above. Maybe the most interesting thing would be to ask those who undertake EICRs whether or not they would 'pass' a simple lighting circuit wired in 1mm² cable which was fed from a 32A sockets circuit without any 'fusing down' - I may be wrong, but I rather suspect that most would not 'pass' it, even if the Zs was low enough for fault protection.

Kind Regards, John

 

[SimonH2]

... (unless you would be comfortable in saying that a fridge-freezer cannot create an overload {but I wonder what current the compressor motor would draw if it jammed?}), ...

They typically have a thermal overload built in - so if it fails to start, it will trip.

Sure, but that's rather different from saying that the appliance 'cannot' create an overload - the thermal cutout may not exist, or may not work and, in any event, an excessive ('overload') current will presumably usually flow for some period of time before it operates. In any event, the fridge freezer is just an example; other appliance may not (or may not be guaranteed to) have internal protection.

Sticking with just the freezer for the moment ...

In the same way that people witter on about adiabatic equations to work out what cable size/type/length and fuse size/type combinations are acceptable for submains - the appliance manufacturer may well have decided that the characteristics of the overload in the compressor may restrict the magnitude*time product to something a smaller cable can safely handle. We all know that you can exceed the "normal" CCC of a cable by a large margin provided that the duration of the overload is restricted - that is in fact a key underlying principal of system design.

So we come down to :
For a fault condition, we've already established (I believe) that the fuse can be larger than the CCC of the cable as the fuse will blow before the cable overheats.
For an overload condition, the manufacturer may well have considered that the load side characteristics may preclude it damaging the cable.

Or the manufacturer may be an idiot

 

[SimonH2]

I'm not suggesting that there would be any reason/sense in doing it, but how would you feel (assuming Zs was low enough for adequate fault protection with the MCB concerned) about a dedicated immersion circuit wired in 1.5mm² T&E (clipped direct) which was protected by a B32 MCB(or maybe even B40, if Zs would accommodate it)? Would you be 'comfortable' with that?

I wouldn't, but that's based on the knowledge that heater elements can fail to an overload condition rather than just a short. Eg, if the element failed with a short circuit from element to sheath around it's midpoint, then the current drawn would be about double. It's conceivable that the fault might not then "burn out" the element as it's water cooled.

For the other ones, not so clear cut. In theory, as you hint, acceptable - but I'd come down on the "no" side. We know that a lot of cheap imported light bulbs don't have an internal fuse. And I've personally witnessed a magnetic ballast flouro fail to an overload situation (internal short in the ballast I think).

But for a permanently attached flex on an appliance - I'd accept that on the assumption that the manufacturer knows what they've put in the appliance. I know that's quite an assumption, but if we don't assume some level of competence on the part of manufacturers then we may as well pack up, switch off at the mains, and go back to a non-electric lifestyle

 

[JohnW2]

Sticking with just the freezer for the moment ... - the appliance manufacturer may well have decided that the characteristics of the overload in the compressor may restrict the magnitude*time product to something a smaller cable can safely handle.

That's seems like a slightly back-to-front way to think about it (I presume you mean 'a smaller cable than would be required to be adequately protected, in terms of it's 'CCC', by the fuse in question) ... but, yes, they might have done those calculations and decided that. However, why would they bother? If (as I think most of us are agreed), a fridge-freezer would be fine with a 5A fuse, and if they want to use 0.75mm² flex (undoubtedly adequate for the normal running current), then why don't they simply put a 5A fuse in the plug and thereby avoid all these discussions (and arguments about 'failed' PATests)?

For a fault condition, we've already established (I believe) that the fuse can be larger than the CCC of the cable as the fuse will blow before the cable overheats.

I certainly have no disagreement with that.

For an overload condition, the manufacturer may well have considered that the load side characteristics may preclude it damaging the cable.

Possibly, but see comments above. The problem is that the electrician undertaking an EICR, or the person undertaking a PATest, would need to be acquainted with (and agree with) what the manufacturer had determined before they could really conclude that the overload of the cable was unlikely - and they often/usually won't have that information. Far simpler if 'the fuse is appropriate to the cable CSA', per normal BS7671 principles.

Do you have any views on the scenarios/questions I've just posed to BAS?

Kind Regards, John

 

[JohnW2]

I'm not suggesting that there would be any reason/sense in doing it, but how would you feel (assuming Zs was low enough for adequate fault protection with the MCB concerned) about a dedicated immersion circuit wired in 1.5mm² T&E (clipped direct) which was protected by a B32 MCB(or maybe even B40, if Zs would accommodate it)? Would you be 'comfortable' with that?

I wouldn't, but that's based on the knowledge that heater elements can fail to an overload condition rather than just a short. Eg, if the element failed with a short circuit from element to sheath around it's midpoint, then the current drawn would be about double. It's conceivable that the fault might not then "burn out" the element as it's water cooled.

Exactly, and that's the very sort of reason why I would be uncomfortable to invoke 433.3.1(ii) for almost any load - since it's usually possible to think of some fault mode (even if rare) that could result in an 'overload situation' (e.g. 'between-winding' shorts in motors). If one wants to invoke that reg, it all comes down to one's view of what is "unlikely", which is obvioulsy very subjective. I personally would err in the direction of caution in order to make myself 'comfortable'.

For the other ones, not so clear cut. In theory, as you hint, acceptable - but I'd come down on the "no" side. We know that a lot of cheap imported light bulbs don't have an internal fuse. And I've personally witnessed a magnetic ballast flouro fail to an overload situation (internal short in the ballast I think).

Quite, as above, exactly my thinking - even with lighting, I wouldn't often be comfortable assuming that an overload situation could never arise.

But for a permanently attached flex on an appliance - I'd accept that on the assumption that the manufacturer knows what they've put in the appliance.

At least in the case of the fridge-freezer, no-one is arguing about cable size in relation to the appliance - I'm sure that 0.75mm² is adequate for the appliance under normal circumstances. It's the rating of the fuse which is protecting it that's the issue - and I've addressed this one in my last post to you.

Kind Regards, John

 

[EFLImpudence]

I wouldn't, but that's based on the knowledge that heater elements can fail to an overload condition rather than just a short. Eg, if the element failed with a short circuit from element to sheath around it's midpoint, then the current drawn would be about double. It's conceivable that the fault might not then "burn out" the element as it's water cooled.

When you say knowledge do you mean experience?

If so, would you please explain exactly what happened and how?