Mystery spur..any ideas?

[JohnW2]

It would be half a job if they stopped selling those 10A rated extension leads, where did that stupidity start from?

Very good question - you tell me! I've seen "6A" ones as well.

Kind Regards, John

 

[EFLImpudence]

I suppose one can buy all sorts of things which are unsuitable when wrongly used.

 

[JohnW2]

Yes assuming the zs is low enough. The situation under discussion is where the zs to the end of the lead is too high for the 13a fuse in the plug. Then a fault (to earth) would not be cleared in time. I suppose that's more about the shock risk than a fire ....

That's what I said/implied. However ....

... All very unlikely, but it doesn't give eebads so wouldn't comply with the regs if it were fixed wiring.

I don't think the arithmetic supports the feasibility of the scenario you are postulating ... to get 0.4s disconnection time (i.e. assuming TN installation), a 13A BS1362 fuse requires a current of about 36A. With a Cmin of 0.95, that equates to a max Zs at the end of the extension lead of about 6.07Ω. Assuming the extension lead is fed from the 'highest Zs socket' on a circuit protected by a B32, which has just a low enough Zs (i.e. 1.37Ω), that means that the maximum (R1+R2) at the end of the extension cable (max for EEBADS) would be about 4.70Ω (6.07Ω - 1.37Ω) - which, if the extension cable were 1.25mm² flex, would equate to a maximum permissible extension lead length (max for EEBADS) of about 127 metres - longer than any extension lead I can recall having encountered!

KInd Regards, John

 

[John D v2.0]

extension lead length (max for EEBADS) of about 127 metres

so I can't plug my fan heater in on the moon after all
Maybe that's why standard plug fuses are limited in comparison with your average socket circuit

 

[JohnW2]

I suppose one can buy all sorts of things which are unsuitable when wrongly used.

Indeed, but I think it's probably reasonable to consider probable modes of 'wrong use' and, where possible, take steps to minimise the risks.

I would have thought that it is pretty predictable that if one sells an extension lead with a "13A plug" on one end and one or more "13A sockets" at the other end, that a good few people will, sooner or later, try to use it for load(s) approaching (or even exceeding) 13A. I therefore don't think it unreasonable that one should only sell such with a cable capable of safely carrying a sustained 13A current.

Kind Regards, John

 

[EFLImpudence]

According to Table 41.2,

the maximum Zs for a 13A 1362 is 2.3Ω.

 

[JohnW2]

Maybe that's why standard plug fuses are limited in comparison with your average socket circuit

I suspect that 'limitation' probably resulted from the fact that it was not intended, nor considered appropriate, that BS1363 plugs/sockets should be used to connect loads greater than 3kW ... it's probably not just a coincidence that the plugs, sockets and fuses were designed for a max current of 13A.

Kind Regards, John

 

[JohnW2]

According to Table 41.2, .... the maximum Zs for a 13A 1362 is 2.3Ω.

So it does (I've never noticed 1362's in that table before!). However ...

... so, as I said, around 36A to get disconnection in 0.4 secs ... and. as I stated before, (230 x 0.95) / 36 = 6.07Ω ... so I wonder where the Table 41.2 figure came from?

Hang on .... Ah, I was looking at the wrong curve! .. In the above I used the minimum current required for 0.4s disconnection. However, it could be as high as 90A ....

... that changes the calculation to (230 x 0.95) / 90 = 2.42Ω, not a million miles from the Table 41.2 figure.

So, using the Table 41.2 figure (2.3Ω), we get the max (R1+R2) of the extension lead as 0.93Ω (2.3 = 1.37), which equates to a maximum extension length (1.25mm² cable) of only about 25 metres, which is quite a credible length for an extension reel.

... so, JohnD was right, and my apologies for having looked at the wrong curve on the graph (and for not knowing that there was a readdily available max Zs in Table 41.2, anyway!)

Kind Regards, John

 

[John D v2.0]

I was too lazy to work it out so there was little chance I'd make an error there.
So with a marginal 20a radial rather then a 32a ring you couldn't even use a 25m extension!

 

[JohnW2]

So with a marginal 20a radial rather then a 32a ring you couldn't even use a 25m extension!

The Table 41.2 max Zs of 2.3 for a 13A BS1362 implies that, at (0.95 x 230V), one would require a current of 95A to guarantee disconnection in 0.4sec. That is so close to the current required to guarantee disconnection by a B32 in 0.4s (i.e. 100A) that one could not have hardly any length of extension lead if the Zs at the supplying socket was already 'on the borderline' (unlikley, see below) and there were a requirement (which there isn't) to satisfy the disconnection requirements at the end of the extension.

As for the likelihood of your scenario arising, a 2.5mm² radial protected by a B20 would have to be about 93m long for the socket at the end to have a borderline Zs - not impossible, but very unlikely in a domestic setting. Indeed, if one is concerned about such things, a 2.5mm² radial would have to be no longer than about 24.5m if one wanted the voltage drop with a 20A load applied at the end to be within the 5% guideline. For those concerned about VD, that circuit length limit is one of the downsides of a 20A radial.

Apologies again by having confused things by looking at the wrong curve!

Kind Regards, John

 

[SUNRAY]

I do Public Address systems and other associated works

Very good question - you tell me! I've seen "6A" ones as well.

Kind Regards, John

I have leads right down to 0.5mm² to limit the weight and size but their use is carefully controlled.

, if the extension cable were 1.25mm² flex, would equate to a maximum permissible extension lead length (max for EEBADS) of about 127 metres - longer than any extension lead I can recall having encountered!

KInd Regards, John

I have 100m leads but in 2.5mm² & 6mm² but using things that long are not a walk in the park [actually that's the sort of place they would be used and a lot of walking is involved with long cable runs] always have to do the tests and calcs to work out what size MCB's to use and sometimes we're down to 4A for a long 6mm² run.

Then I see several domestic caddies used to power a tea urn, or worse, a tea urn and a kettle as the urn takes so long. The public generally have no clue when it comes to mains cables.

 

[JohnW2]

I have 100m leads but in 2.5mm² & 6mm² but using things that long are not a walk in the park ... always have to do the tests and calcs to work out what size MCB's to use and sometimes we're down to 4A for a long 6mm² run.

Fair enough, but don't forget that my "127m" calculation was 'incorrect' (for a worst-case-scenario calculation) - it should have been nearer to 25m (per my subsequent post).

Then I see several domestic caddies used to power a tea urn, or worse, a tea urn and a kettle as the urn takes so long. The public generally have no clue when it comes to mains cables.

Indeed, but the manufacturers are not blameless, since they give no guidance/warnings with long extension leads. I have a couple of 50m reels ones, and neither says anything about their use, other than that they should be fully unwound if used to their 'full capacity'.

In my particular case, it's not so much of an issue, since OPD-mediated EEBADS is not theoretically relevant to my (TT) installation and, in any event, my use of the long extension reels is nearly always to power Class II equipment.

It's also worth remembering that the calculations we do relate to very much 'worst case scenarios' which will rarely be experienced in practice ... for a start, feeding an extension from a 'marginal Zs' socket is an extreme scenario and, secondly, (when I look at the right curve!) we look at the worst-case current needed for the BS1362 fuse to result in disconnection in ≤0.4s. I suspect that, in the case of a 13A BS1362, the required current is probably much nearer to the bottom that the top of the ~36A to ~90A range - in which case my initial ('incorrect') calculation which led to the 127m figure may usually not be far from the truth. ... and also don't forget that I was assuming a 1.25mm² extension lead - if higher CSA things would be very different, and probably not a concern.

Kind Regards, John