Cable Colours

[EFLImpudence]

Don't know what's gone wrong but -

JohnW2 wrote:
You mean by the fuse(s) in the plug(s)? If so, then change the socket (on a 1.5mm² dedicated circuit protected by a B50) to an immersian heater.

I did answer that.

Quote:
Is that not because such spurs are specifically 'dispensated'? If one relied on the regs in general, wouldn't the length of an unfused spur be limited to 3m?

Yes to dispensation and 433.1.103 - if the CCC of the cable is not less than 20A and ...the load current in any part of the circuit is unlikely to exceed for long periods the CCC of the cable.

Quote:
and as for 'especially these days', haven't we been moving in the direction of much smaller lighting loads?

Not with downlights.

Quote:
Possibly/probably. I did qualify my statement by saying that one would need an acceptable Zs. With a small circuit and 1.5mm² cable, I would have thought that it could well be possible, though.

A lot of things are possible but it can only be three metres anyway.

Quote:
There is, indeed - so, as above, how does that fit in with 2.5mm² spurs?

You have already answered that.

Quote:
Yes, but what started all this was the discussion about a situation in which there was no fuse in the plug, hence a thin flex from plug to appliance would not be benefitting from adequate overload protection from anything.

I don't see what you are getting at.
As long as it can cope with fault current until the opd operates it complies.
It doesn't need overload protection because it can't be overloaded so it could be said that it does have adequate overload protection.

I have had adequate protection against ultra violet the last couple of days - none.

 

[ban-all-sheds]

Yes I have heard of them and have installed such to single sockets where the maximum load that could be taken from the socket is 13 amp and thus less than the safe continuous current that 2.5 mm² can carry.

So when you did that did you think that you were contravening the Wiring Regulations?

Or did you think that there was a regulation which allows a circuit to have cable rated at lower amperage than the amperage at which the circuit protection operates?

 

[JohnW2]

Don't know what's gone wrong ...

The discussion has,indeed, become very confused/confusing, probably largely due to me - but I think also because we are jumping backwards and forwards been two related, but different, concepts.

Is that not because such spurs are specifically 'dispensated'? If one relied on the regs in general, wouldn't the length of an unfused spur be limited to 3m?

Yes to dispensation and 433.1.103 - if the CCC of the cable is not less than 20A and ...the load current in any part of the circuit is unlikely to exceed for long periods the CCC of the cable.

Exactly - so the allowance of unfused 2.5mm spurs (of any length) is a special case (permitted by virtue of 433.1.103), rather than a general example of the regs allowing CCC<In (which is what I think many will have asumed BAS to be implying when he questionned Bernard).

A lot of things are possible but it can only be three metres anyway.

True, but the combination of 433.2.2 and 434.2.1 is contorted and potentially confusing in the extreme. In particular, 433.2.2(i) appears totally redundant. At first sight, it appears to be an alternative to 433.2.2(ii) (which contains a '3m rule' and requirements regarding 'installation manner'), but by requiring one to satisfy 434, it invokes 434.2.1 which brings us right back to a 3m limit and the same requirements regarding 'installation manner'!

Yes, but what started all this was the discussion about a situation in which there was no fuse in the plug, hence a thin flex from plug to appliance would not be benefitting from adequate overload protection from anything.

I don't see what you are getting at. As long as it can cope with fault current until the opd operates it complies. It doesn't need overload protection because it can't be overloaded so it could be said that it does have adequate overload protection.

It's all back to what I said early in the thread. If we assume that malfunctions never occur which result in alow, but not negligable, L-N or L-E impedance 'fault', then you're right ("can't be overloaded"). However, a s I said before, even if rare, such occurrences certainly aren't impossible.

Kind Regards, John

 

[bernardgreen]

So when you did that did you think that you were contravening the Wiring Regulations?

Common sense told me that the current would be limited by the fuse in the plug. An electrician told me that it was safe in the event of a short circuit in the socket as the 2.5 mm would survive until the fuse in the fuse board blew and would not create a hazard of burning PVC cable. Obviously the situation at the short circut would be a hazard due to localised heating if the short became resistive before the fuse blew.

Or did you think that there was a regulation which allows a circuit to have cable rated at lower amperage than the amperage at which the circuit protection operates?

It appears that the regulations allow for that. Safety then depending on the down stream limit of 13 amps.

I assume the 3 metre rule is to deter people adding more sockets to the spur and thus having a down stream limit ( X times 13 amps ) greater than the safe capacity of 2.5 mm

 

[ban-all-sheds]

Common sense told me that the current would be limited by the fuse in the plug.

In other words, when looking at the 2.5mm² spur cable you were quite happy that the characteristics of the downstream load could be considered when deciding it it was at risk from overload.

But you seem unhappy to do the same with an appliance flex.

It appears that the regulations allow for that. Safety then depending on the down stream limit of 13 amps.

It appears?

Do you mean that all this time, if someone had asked you, you would have said that common sense made it OK, but if push came to shove you would admit that it contravened the regulations?

I assume the 3 metre rule is to deter people adding more sockets to the spur and thus having a down stream limit ( X times 13 amps ) greater than the safe capacity of 2.5 mm

You don't have a very good grasp of the regulations, do you.

People don't need to be "deterred" by limiting the cable length, multiple sockets are not allowed, and that's all there is to it. Anyway - 3m wouldn't be any deterrent to having multiple sockets.

 

[bernardgreen]

In other words, when looking at the 2.5mm² spur cable you were quite happy that the characteristics of the downstream load could be considered when deciding it it was at risk from overload.

The charactoristics of the FUSE as when fusing down using an FCU

But you seem unhappy to do the same with an appliance flex.

I am unhappy with non UK plugs that cannot limit the current to be less than the safe capacity of the flex they are connected to.

but if push came to shove you would admit that it contravened the regulations?

If it contravened ( I think ) I would say so. That said there have been times when common sense and experience can make the installation safer than strict obeyance of the regulation in force at the time. (*)

People don't need to be "deterred" by limiting the cable length, multiple sockets are not allowed,.

But some people do not follow regulations. If several sockets were put on the spur it is un-likely but not impossible that a load of more than 3 kw could be taken in that small area.

(*) a meter installer once refused to install a meter because the new supply had two RCDs It seems that the regulations at the time did not accept two RCDs Explaining that overall site safety would be compromised if all power was lost led to the meter being installed the next day, not that the meter installer was happy that his boss told him to do it.

 

[ban-all-sheds]

The charactoristics of the FUSE as when fusing down using an FCU

It doesn't matter what it is that's downstream - it's the fact that the nature of what it is limits the current which matters.

You have a 2.5mm² cable protected by a 32A breaker, but that's OK because the nature of the load it is supplying means that there will not be an overload.

Same with a flex supplying an appliance. One flex supplies one appliance - it can't suddenly start supplying two appliances. A flex fixed into the back of a CD player can't suddenly start supplying a kettle.

I am unhappy with non UK plugs that cannot limit the current to be less than the safe capacity of the flex they are connected to.

They don't need to - see above - the loads they supply determine how much current flows in the flex.

 

[bernardgreen]

it's the fact that the nature of what it is limits the current which matters

Have you ever done fault analysis ? If so was it predictive or post incident. The nature of the the device will almost certainly limit the current in normal operation, but when the ab-normal occurs the current may be several times higher than normal.

Some items have internal fuses. Suggests the manufacturer is aware that overloads can occur. How they justify the cost of adding a fuse can be either protecting the item from fire ( the items is already suspect / damaged otherwise there would be no overload ) or protecting the user from burning flex if there is no fusing down to the capacity of the flex.

If we extend your ideas about the nature of the load limiting the current then there would be no need for fuses on the incoming supply as the nature of the average house will limit the current it takes from the supply network.

 

[ban-all-sheds]

So basically you think that the entire world with the exception of ourselves, Cyprus and a few other ex-colonial-influenced places have got it wrong, you think that they have insufficiently safe electrical installations and you think that they cannot see the advantage of plug fuses because they are incapable of doing fault analysis.

Fair enough.

Your last comment, BTW, shows that you still don't get the idea of fault protection.

 

[bernardgreen]

So you are saying we fault protect everything except the cable from plug to appliance. And to ensure excessive currents do not turn that cable into a fire lighter we make it capable of carrying the current that the protective device in the consumer unit will let through.

" Fault " in your mind seems to be only a dead short with extremely high current that will "instantly" operate the protective device. You need to include serious overload currents as "faults" because a fault ( albeit in an appliance ) may not be a dead short that "instantly" trips the MCD's magnetic trip but passes enough current to over heat the flex.

One common "fault" is the use of a low current IEC ( aka kettle ) lead intended for a lap top as the lead for a kettle. Without the fuse in the plug that can reasult in a melted lead.

 

[EFLImpudence]

So you are saying we fault protect everything except the cable from plug to appliance.

No. It has to comply with fault protection conditions.
That's the whole point'

And to ensure excessive currents do not turn that cable into a fire lighter we make it capable of carrying the current that the protective device in the consumer unit will let through.

(Not sure if you meant the 'And'.)
No. The amount of current the load will draw

" Fault " in your mind seems to be only a dead short with extremely high current that will "instantly" operate the protective device.

Yes.

You need to include serious overload currents as "faults" because a fault ( albeit in an appliance ) may not be a dead short that "instantly" trips the MCD's magnetic trip but passes enough current to over heat the flex.

That would be overload.

Plug a normal load (not a motor) into a socket and see if you can get it to overload the cable.

One common "fault" is the use of a low current IEC ( aka kettle ) lead intended for a lap top as the lead for a kettle. Without the fuse in the plug that can reasult in a melted lead.

If the lead is not capable of handling 8 to 13A then yes. Do they fit?
So it would if left on a hot hob.

One person was injured recently decanting petrol next to the cooker.

 

[JohnW2]

" Fault " in your mind seems to be only a dead short with extremely high current that will "instantly" operate the protective device. You need to include serious overload currents as "faults" because a fault ( albeit in an appliance ) may not be a dead short that "instantly" trips the MCD's magnetic trip but passes enough current to over heat the flex.

Whilst I totally agree with the concepts of what you're say, as for the language I'm afraid that we're essentially stuck with the language/definitions and mindset of BS7671. It's not just in BAS's mind - when BS7671 speaks of 'faults' (and 'fault protection'), it is talking about dead shorts (well, faults of negligible impedence, aka 'bolted shorts'). Any other current which is excessive in relation to the circuit is called an 'overload' - whether due to the connection of abnormally high (but normally functioning) load(s) or else (something which BAS/EFLI etc. {and maybe event the IET} consdier so unlikley that it can be ignored) 'faults' (in the everyday sense) which result in excessive currents but not as high as one gets with a 'dead short' ('BS7671 fault'). Like you, I am not all that comfortable about ignoring that (admittedly low) risk, and I also feel that, so long as not all manufacturers include internal protection in their appliances/equipment, the fuse in a plug can also be useful to provide some protection to the innards of what's on the end of the cable.

Kind Regards, John

 

[bernardgreen]

Hello John

I wonder what amperage of abnormal current is the dividing line between a fault and an over load. I guess the impedance through which the fault current flows determines that.. So what range of impedance is defined as a fault and what range of abnormal impedance in an appliance is defined as a cause of an overload.

EFLI said

Plug a normal load (not a motor) into a socket and see if you can get it to overload the cable.

I have seen a burnt out lead when a 3 amp lead had the 3 amp fuse in the plug replaced by a 13 amp fuse and the lead used to power a kettle.

Yes that was a mistake by the person who wanted a new lead for the kettle. Had the fuse not been changed, then the fuse would have blown before the lead overheated when the normal load of the kettle overloaded the cable. And yes the lead was probably substandard even for its rated current[/quote]

 

[JohnW2]

Hello John, I wonder what amperage of abnormal current is the dividing line between a fault and an over load. I guess the impedance through which the fault current flows determines that.. So what range of impedance is defined as a fault and what range of abnormal impedance in an appliance is defined as a cause of an overload.

It's obviously aribrary but, in practice, I think we can understand what distinction is in their mind. A 'fault', per their mindset, arises when there is direct contact between L and E or L and N conductors. Admittedely, the 'quality' (hence impedance) of that contact may vary a little, but not much. Anything 'less' than direct L/N or L/E contact is regarded as an 'overload'. We know that, for the purpose of calculations and assessment of the adequacy of protective devices, the impedance of the fault is assumed to be zero - so that the fault current is determined solely by supply voltage and Zs.

I have seen a burnt out lead when a 3 amp lead had the 3 amp fuse in the plug replaced by a 13 amp fuse and the lead used to power a kettle.

That's one of the reasons why I'm not happy with this concept that a single load cannot cause overloading of the cable. If it is a 'fixed' (hard-wired') single load or an appliance with factory-fitted cable then, as I've said, provided it was wired with an adequate cable in the first place, I will agree that the probability of a fault (everyday sense) in the load leading to a high (but not 'extremely high') current which overloads the cable supplying it is low (but not zero). However, as you illustrate, once the cable and the appliance become separable, all bets are off, and overloading of the cable most certainly is possible, even without equipment malfunction. Of course, as you again illustrate, having a fuse in the plug is no protection against that if the fuse is of too high a rating for the cable.

Kind Regards, John

 

[ban-all-sheds]

Overload current: An overcurrent occurring in a circuit which is electrically sound.

Fault current: A current resulting from a fault.

Fault: A circuit condition in which current flows through an abnormal or unintended path. This may result from an insulation failure or a bridging of insulation. Conventionally the impedance between live conductors or between live conductors and exposed or extraneous conductive parts at the fault position is considered negligible.