Meter Tails

[JohnW2]

Well if we take the fusing factor of a 100A cut-out fuse as 1.47, it is apparent that tails will need to carry at least 147A for a period without failing. The same will apply to any cable as we don't want cables failing before fuses/MCB's operate on load.

Indeed, but that's almost identical to the fact that the BS7671 tabulated CCC's apparently take on board (i.e.include a margin for) the fact that a Type B MCB can carry 1.45 their In for up to 1 hour - so that can't explain the difference.

Kind Regards, John.

 

[westie101]

This link is interesting: -

http://thecablediretoryblog.com/200...-–-whatever-happened-to-the-factor-of-safety/

as it makes reference to the

Maximum sustained conductor temperature

which is totally different to the Maximum permissible conductor temperature.

We rely on the fact that equipment can run hotter for short periods (we have them defined) and can use those higher ratings. In a domestic situation, except where storage heating is installed, the chances of a load sustaining at the BS7671 limit is very rare and unusual, so if a peak occurs for short time that is acceptable to us.

Just as BS7671 does not recommend using the overload capability of a protective device, I think that the rating tables also limit it for the cables. On a DNO network we do not have such rigid constraints and as engineers we can use some discretion.

 

[JohnW2]

...as it makes reference to the

Maximum sustained conductor temperature

which is totally different to the Maximum permissible conductor temperature.
We rely on the fact that equipment can run hotter for short periods (we have them defined) and can use those higher ratings.

As before, so seemingly does BS7671. For reasons I explained, I think it's implicit that BS7671 is happy for cables to carry 1.45 times their tabulated CCCs for 1 hour. Are your regs/CoPs more generous than that?

Kind Regards, John.

 

[westie101]

I'd have to check the percentage overloads permitted.
But I get the feeling that BS7671 does not suggest that this should be designed into an installation for normal usage.
e.g. to permit a smaller size of cable/protective device to be used to avoid replacing a circuit or allow 2 x 13A loads on a double socket supplied by a 20A radial.

We can design it in!

 

[JohnW2]

I'd have to check the percentage overloads permitted. But I get the feeling that BS7671 does not suggest that this should be designed into an installation for normal usage.

That's sort-of true. For 'normal usage', BS7671 does not allow CCC to be less than the 'design current', even though that means (and BS7671's CCCs presumable take into account) that currents of up to 1.45 times greater may flow for up to 1 hour in 'overload' conditions.

However, I say 'sort-of true', because BS7671 also accepts the concept of diversity, so what is 'normal usage' becomes a bit of a grey area. With a cooker circuit, for example, BS7671 allows one to size the cable on the basis of current demand after application of diversity - which presumably implies an acceptance that sometimes in 'normal usage' (everyday meaning) currents in that theoretical 'overload' territory will actually be drawn.

Kind Regards, John.

 

[xr4x4]

How long have 100a cuts out fuses been around..?

How long have 25mm2 tails been around and fitted..?

Not the same time i bet.

So at some point 16mm2 was deemed ok for a 100A cut out at some point in the past.

 

[westie101]

As with the main earth where 16mm copper is being installed as standard! A lot of older services are 16mm (equivalent) so 10mm copper is quite adequate

 

[JohnW2]

How long have 100a cuts out fuses been around..? How long have 25mm2 tails been around and fitted..? Not the same time i bet. So at some point 16mm2 was deemed ok for a 100A cut out at some point in the past.

Quite - but, although BS7671 requires 25mm² for 100A fuse (and that's what I have on my side of the meter), at least some DNOs (mine, for example) are clearly still happy to leave 16mm² tails in use with 100A fuses. I know for certain that my 100A fuses have been there for at least 25 years, probably a lot longer, and despite 3 meter changes during that period (the last less than 10 years ago), they've left the 16mm² tails on their side of the meter. However, as I wrote earlier, I've never found them even perceptibly warm to the touch.

Kind Regards, John.

 

[ban-all-sheds]

That's sort-of true. For 'normal usage', BS7671 does not allow CCC to be less than the 'design current', even though that means (and BS7671's CCCs presumable take into account) that currents of up to 1.45 times greater may flow for up to 1 hour in 'overload' conditions.

However, I say 'sort-of true', because BS7671 also accepts the concept of diversity, so what is 'normal usage' becomes a bit of a grey area. With a cooker circuit, for example, BS7671 allows one to size the cable on the basis of current demand after application of diversity - which presumably implies an acceptance that sometimes in 'normal usage' (everyday meaning) currents in that theoretical 'overload' territory will actually be drawn.

533.2.1

 

[JohnW2]

However, I say 'sort-of true', because BS7671 also accepts the concept of diversity, so what is 'normal usage' becomes a bit of a grey area. With a cooker circuit, for example, BS7671 allows one to size the cable on the basis of current demand after application of diversity - which presumably implies an acceptance that sometimes in 'normal usage' (everyday meaning) currents in that theoretical 'overload' territory will actually be drawn.

533.2.1

I don't think that 533.2.1 alters anything I said. I presume you're referring to the second paragraph ("In certain cases..."). All that's saying is that, in certain cases, it may be necessary to take peak current demand into account to avoid unintentional OPD operation. If anything, I would say that underlines my point that if application of diversity successfully avoids 'unintentional operation' occurring 'during normal usage' (everyday sense), then it's OK (in the eyes of BS7671) to have 'normal usage' (everyday sense) sometimes resulting in the load current exceeding In of the OPD.

Kind Regards, John.

 

[kai]

As far as the DNO are concerned, cable protection is not that important, as their main service feed cables are rated at around 100 amps, but are fused at anything up to 800 amps - the cable are intended to self-destruct in the event of a short-circuit or major overload. Perhaps Westie or someone else can correct me if I am wrong here.

 

[JohnW2]

As far as the DNO are concerned, cable protection is not that important, as their main service feed cables are rated at around 100 amps, but are fused at anything up to 800 amps - the cable are intended to self-destruct in the event of a short-circuit or major overload. Perhaps Westie or someone else can correct me if I am wrong here.

Well, that might possibly be their philosophy as far as distribution cables are concerned - but once one is past their cutout in a property, the subsequent cable (which is what we're talking about), is protected by the cutout fuse. ... and, even if they are not too concerned about their cables 'self-destructing' underground or up in the air, I would have thought that they (or their lawyers and/or insurers) would not be too keen on that happening inside a customer's premises, particular given that 'meter cupboards' have been known to be full of combustible materials

Kind Regards, John.

 

[westie101]

There is a distinct difference between a short circuit fault and a prolonged overload!
(Oh the ratings are up to 150A 35mm aluminium and 630A dependant upon transformer size)

Yes in the case of the former we would sacrifice a smaller service cable. (though it is not unknown for a fault on one to operate a 400A fuse) and they generally burn to an open circuit.
As regards a fault at the service position downstream of the cut-out, don't forget we have to ensure minimum loop impedances to guarantee the operation of our fuse.

For an overload there are a number of elements that could fail, but I have never seen melted cables except in the vicinity of poor contacts where the heat was concentrated.

Recently dealt with one where some 16mm tails were carrying over 100A (protected by 160A fuses) on a regular basis, yes there was some heat at times but no where near 70degC

 

[westie101]

I can't help but wonder how many installations have failed an inspection for 16mm tails where the incoming service is 16mm or below. (the same for 10mm earths)

 

[JohnW2]

I can't help but wonder how many installations have failed an inspection for 16mm tails where the incoming service is 16mm or below. (the same for 10mm earths)

Good question. Those wot do the inspecting can presumably tell us - although, of course, 'failing an inspection' is not that easy to define!

Kind Regards, John.