Electrical extension length limit

[John D v2.0]

Its been a while since I did any PAT, but you could get round the max length rule by fitting an RCD, not sure if it still applies

You could get around it for the earth fault situation, but you'd hit the next limit for the short circuit which the rcd wouldn't help with

 

[ericmark]

You could get around it for the earth fault situation, but you'd hit the next limit for the short circuit which the rcd wouldn't help with

But with a 3 amp fuse short circuit is rather long 16.4Ω loop impedance in my old BS7671:2008 don't know if changed? So 500 meters of 1.5 mm² unfortunately it only shows 3 and 13 amp loop impedance, not a clue on 1, 2, 5, 7, and 10 amp loop impedance figures.

 

[ajohn]

But with a 3 amp fuse short circuit is rather long 16.4Ω loop impedance in my old BS7671:2008 don't know if changed? So 500 meters of 1.5 mm² unfortunately it only shows 3 and 13 amp loop impedance, not a clue on 1, 2, 5, 7, and 10 amp loop impedance figures.

You have to include the supply resistance as well. That can vary. The current site guide is more or less the same. Whole load of cable lengths that can be used without calculation subject to set max figures for that. Then there are allowable volts drops for lights and same for power.

Next comes cut out time from the short circuit protection. The quick trip time on an MCD is 5 times it's current rating. Higher on some types. Fuses - check against the curves for them.

Then heating I^2 x R so while 16R4 limits to 14amps or so that's still 3kw. The sums used for that aspect are interesting.

 

[JohnW2]

But with a 3 amp fuse short circuit is rather long 16.4Ω loop impedance in my old BS7671:2008 don't know if changed?

In BS7671:2018, with the adjustment for Cmin=0.95, the maximum Zs figures are 22.0Ω for 3A BS1362 fuses and 3.64Ω for 13A ones. That is, of course, the maximum Zs for the entire fault path, including the Ze and the fixed wiring of the installation, as well as the extension cable itself. In BS7671:2008, before the application of Cmin, the figures were 23.2Ω and 3.83Ω for 3A and 13A fuses respectively, so I'm not sure where your 16.4Ω figure came from.

So 500 meters of 1.5 mm² unfortunately it only shows 3 and 13 amp loop impedance, not a clue on 1, 2, 5, 7, and 10 amp loop impedance figures.

In theory, you should have more than a 'clue', from the fusing factors implied by the figures they give for 3A and 13A fuses, but the figures in BS7671 seem to be crazy! The max Zs figures equate to minimum PFCs of 9.93A for the 3A fuse and 60.0 A for the 3A fuse. The implied 'fusing factor' of 3.3 for the 3A fuse is 'surprisingly high', but the implied fusing factor of 4.6 for the 13A one seems to be hard to believe/understand! ... and, as above, those 'difficult to understand' figures were the same in BS7671:2008, other than for the application of Cmin.

Kind Regards, John

 

[JohnW2]

This again!! ...

 

[ericmark]

I used 1.44Ω as the setting for sockets in my calculations, clearly that will vary depending how close to centre or ends of a ring final, and for a radial could be higher, as you say type B MCB is magnetic trip time of 3 to 5 times the thermal overload current marked on the device, type C 5 to 10 times, and type D 10 to 20 times.

So yes a radial on a 16 amp type B would give very different results to on a 32 amp type D, and clearly an extension lead can be moved, however we see reduced low voltage leads fed from a transformer with a 13 amp input which since voltage to earth is only 55 volt, means 13x230/50 = 59.8 amp can flow to earth without rupturing the fuse, clearly a high fire risk, but done on a regular basis without fires resulting, so I would say in real terms more down to what volt drop the equipment can run with rather than safety limits.

 

[JohnW2]

... however we see reduced low voltage leads fed from a transformer with a 13 amp input which since voltage to earth is only 55 volt, means 13x230/50 = 59.8 amp can flow to earth without rupturing the fuse, clearly a high fire risk ...

As I've just written, if one were to believe the figures in BS7671 (which, frankly, I don't), they are assuming that it takes 60A to (rapidly) rupture a 13A fuse, in which case your calculation would be come 60x230/50 = 276A !

Kind Regards, John

 

[SUNRAY]

I used 1.44Ω as the setting for sockets in my calculations, clearly that will vary depending how close to centre or ends of a ring final, and for a radial could be higher, as you say type B MCB is magnetic trip time of 3 to 5 times the thermal overload current marked on the device, type C 5 to 10 times, and type D 10 to 20 times.

So yes a radial on a 16 amp type B would give very different results to on a 32 amp type D, and clearly an extension lead can be moved, however we see reduced low voltage leads fed from a transformer with a 13 amp input which since voltage to earth is only 55 volt, means 13x230/50 = 59.8 amp can flow to earth without rupturing the fuse, clearly a high fire risk, but done on a regular basis without fires resulting, so I would say in real terms more down to what volt drop the equipment can run with rather than safety limits.

Apologies to OP for the significant deviation but I'm finding this QI. But now another deviation...

I did some subbing for a controls company who specialised in water treatment, we had a green 'cabin' in the workshop, this sort of thing:

Their own electrician, as opposed to controls wireman, had wired in a 5KVA 110V isolating transformer feeding 4 16A yellow sockets each with their own 16A MCB in a seperate ('110V') CU. It was powered by a C20 MCB in the 230V CU.
I instantly spotted he'd used SP MCB's for the 110v and queeried it with him. I was told to mind my own ******* business and go back to my 'little toy wires'. Bearing in mind I was only temping there and hoping to continue I dropped it until the following day when the the Contracts Manager came to the workshop and pointed it out to him. He advised it was normal and done by the qualified electrician so it must be OK. After that I spoke to MD who asked what the problem is, but he didn't understand.

I got a long (20m sort of length) piece of 1.5mm singles from the scrap bin and poked it in the 'E' & '-' holes of a yellow socket and waited. The MD switched it off when things started smoking and called the Contracts Manager for a demo. Then they called the electrician and asked him to come in, He wasn't happy as it was Friday and already finished for the day.

We all sat in meeting room to discuss the installation where electrician again insisted it was OK and a number of sketches were made. We then went to the W/S to demonstrate the problem and he was instructed to put it right which included rewiring and replacing heat damaged plastic trunking.

Following week they started a program to go round the previous installations to change to DP MCB's. My name was **** amongst the site staff but managers seemed to be OK with it.

It meant I had a fair bit of work there while they completed that. And I learnt a lot about water treatment plant includung equiping a 1500 edit LITRE acid storage tank in its bund with all of the dosing kit, plastic welded pipe etc on top of the controls panel and wiring that I was expecting to do and the pump skids that went with it... Oh and I did the electrical bits too which didn't go down well with the electrician who thought he had the company sewn up.

 

[SUNRAY]

My next job following the 'problems':

 

[ericmark]

Very nice, and yes it has caught out many an electrician treating 110 volt same as 230 volt, the big melt I remember the control 110 volt transformer was renewed, the original was 0-110 but the replacement had an internal link centre tap to earth and no one even realised it was centre tapped until after the burn out.

 

[ajohn]

Maybe having numbers mixed up so wont quote them, the 2 BS numbers on these things is covered in this

http://www.farnell.com/datasheets/311614.pdf

So the 5amp may be a bit different to the 3 and 13. Other makes will be very similar to te Busman going on different fuses I have looked at.

E=IR so 16.4*3 amps = 49.2v to much of a drop to meet regs

1amp, 16.4/230v = 7.1% too much for lighting from memory.

 

[JohnW2]

E=IR so 16.4*3 amps = 49.2v to much of a drop to meet regs
1amp, 16.4/230v = 7.1% too much for lighting from memory.

Although the guideline max VDs of 3% for lighting circuits and 5% for everything else are often thrown around, one does not have to follow those guidelines in order to "meet the regs", since all the regs require is:

Where fixed current-using equipment is not the subject of a product standard the voltage at the terminals shall be such as not to impair the safe functioning of that equipment.

Indeed, if one wanted to be pedantic in terms of this thread, even the guidelines would only be relevant if the OP's extension lead were regarded as part of the fixed wiring of the installation that was supplying "fixed current-using equipment".

 

[mattylad]

The problem with having shorter extension leads is that they just get more and daisy chain them all.
So which is worse? A 30m lead or 2 15m leads daisy chained?

 

[ajohn]

If those are the numbers, 3 and 5% I would use those and if a 3amp fuse assume a 3amp supply. In this case that just relates to cable size I'm not inclined to remember things like this. Don't need to often so use a spreadsheet and make sure that comes up with correct results and then just use that.

I have the on site guide and the regs. So far I haven't seen anything in the guide that is incorrect. I have a rather poor opinion of the format of the regs in terms of it being suitable for something intended to be for people to work to. That would cause me to look for qualifiers on volt drop other than the one mentioned for fixed current ... blah.

The regs end at the socket. It is possible to buy extension leads that use 2.5mm cable despite the apparent restriction in the size of the holes available in the plugs.

 

[SUNRAY]

even the guidelines would only be relevant if the OP's extension lead were regarded as part of the fixed wiring of the installation that was supplying "fixed current-using equipment".

This opens up another tangent which has caught many out. I would treat an extension lead as fixed wiring if it is fixed in place or appears to be fixed. If the cable passes through a hole which is too small for the plug or socket to fit through then I would deem it fixed. If everything plugged in were to be excluded from the fixed description we would see installations consisting of a cutout, meter, maybe a fuse and a large socket. Then the rest of the house would be exempt from regs.

The problem with having shorter extension leads is that they just get more and daisy chain them all.
So which is worse? A 30m lead or 2 15m leads daisy chained?

I absolutely state fewer joins is better, particularly using 13A connectors. Restricting the length creates more problems than solved. In my line of work I've often run hundreds of metres of leads, in the early days it was mostly 15A with 20m to 25m lengths. When we changed over to 13A I'd get very nervous during set-up wondering if 6 13A extension leads daisy chained would work first time, now of course I have 4343's and don't have anything like the same issues and additionally have leads unto 100m long.