Q on Armoured cable

[ericmark]

There was an article, where it was pointed out some sizes of SWA the earth using the SWA was not good enough, I saved the bit below. 240 mm² is about the largest I have worked with, so using the armour has never been a problem.

Extra from John Peckham article on using the armour of SWA cable.


Table for 70°C Thermoplastic PVC SWA cables.

Conductor CSA	Minimum CSA of SWA to meet 54.7	CSA of armour 2 core	CSA of armour 3 core	CSA of armour 4 core
1.5	3.4	15	16	17
2.5	5.7	17	19	20
4	9.0	21	23	35
6	13.6	24	36	40
10	22.6	41	44	49
16	36.1	46	50	72
25	36.1	60	66	76
35	36.1	66	74	84
50	56.4	74	84	122
70	79.0	84	119	138
95	107.2	122	138	160
120	135.3	(131)	150	220
150	169.2	(144)	211	240
185	208.6	(201)	230	265
240	270.6	(225)	(260)	299
300	338.3	(250)	(289)	(333)
400	403.9	(279)	(319)	(467)

 

[RickH]

The biggest I worked on was 500mm2 Aluminium Conductor and armor wires, hard work jointing that.
Quite good in that the armour wires were in a wave wound pattern, so you could add a service joint without brekaing the armour wires.

 

[JohnW2]

There was an article, where it was pointed out some sizes of SWA the earth using the SWA was not good enough, I saved the bit below. 240 mm² is about the largest I have worked with, so using the armour has never been a problem.

That Table (and associated ones) exists in this forum's wiki (see images below). However, they indicate the adequacy of SWA armour as a CPC per Table 54.7, which is very conservative, really only existing for those who are too lazy, or without the ability, to undertake an adiabatic calculation to determine the actual CPC requirement - since those who do undertake such a calculation will usually find that a CSA appreciably smaller than that in Table 54.7 is adequate.

The first Table below (from our wiki, for 70°C PVC SWA is identical to what you posted, and is really of no relevance to a DIY forum, since, even by Table 54.7, the armour is adequate as a CPC for all cables under 120mm².

However, as the second Table below (again from our wiki) shows that the situation is very different for 90°C XPLE/XLPE SWA, since 'problems' (CPC 'inadequacy' per Table 54.7) start as low as 2-core 10mm² cable, hence of much more potential relevance to DIYers. However, again, adiabatic calculation will usually indicate an appreciably lower CPC CSA requirement than is indicated in Table 54.7.

Kind Regards, John

 

[flameport]

The 90C version is irrelevant to pretty much all DIYer, as they won't be using cables at 90C.
Exceptionally unlikely they would be using any cables at 70C either.

 

[JohnW2]

The 90C version is irrelevant to pretty much all DIYer, as they won't be using cables at 90C. Exceptionally unlikely they would be using any cables at 70C either.

I agree that few people (probably not only DIYers) will use any cable at its maximum conductor temp, but is that relevant? For those who rely on Table 54.7,don't the 'minimum CPC CSA' figures apply regardless of how much (if any) current flows in the cable?

Kind Regards, John

 

[ericmark]

That Table (and associated ones) exists in this forum's wiki

Had not realised, to be frank when the wiki was changed a few years ago, I found it hard to navigate. In my work, rare to work with single phase, so in the main no need to use a core for earth, however when I could I would, although some times a problem locating 5 core in a hurry.

Daft thing is with 5 core there is more SWA, so no need to use a core for earth.

With a long run I would likely look at TT anyway, but the main point is installations need to be designed, all too often people hide behind regulations and tables, the skill of being an electrician is the ability to design. And some where some how one needs to sign to say you have designed the installation.

I had it with LABC inspector, he wanted a third party to do an EICR to show if installation was OK, and son said and what happens if he says it's not OK? The inspector would need to have qualifications which exceeds my fathers, and my dad has a degree, so who will you use that can show my dad is in error? OK tongue in cheek, but this leads to the big question.

What happens when one inspector says another inspector has made an error?

I did worry about this, if I complete an minor works or a installation certificate, and after that some one does and electrical installation condition report, which says I was in error, what then? I was worried about volt drop, and as a result wrote a excel then java script program to work it out, and some guys on here kindly checked it was working as intended, one now has sadly passed away, however the program showed how hard it was to show an error in design calculations, move the loop impedance reading 0.01 Ω and the result changes a huge amount.

So we move from design to the inspection and testing. To show the design is in error, you need to test, press the test button on a loop impedance tester three times, and you will get three different results.

We are now moving into maths,

In mathematics, a norm is a function from a real or complex vector space to the non-negative real numbers that behaves in certain ways like the distance from the origin: it commutes with scaling, obeys a form of the triangle inequality, and is zero only at the origin. In particular, the Euclidean distance of a vector from the origin is a norm, called the Euclidean norm, or 2-norm, which may also be defined as the square root of the inner product of a vector with itself.

if you can understand that, your doing better than me.

We should be given the previous documentation, so we need to prove it was wrong, if we can, then likely it could result in a court case, the cost to replace a cable can be rather high, so if one can claim the designed got it wrong, you can be talking about big money. The reverse is also true, show the EICR is in error, and the author can also be looking at a claim.

 

[JohnW2]

With a long run I would likely look at TT anyway ...

That raises an interesting question for someone as concerned about 'what regulations actually say' as you appear to be. Do you believe that what is at the far end of a feed to an outhouse count as part of the electrical installation of the house? If so what do you have to say about the regulatory requirement to runs a (I presume 'adequate') CPC to "every point" in an installation.

I did worry about this, if I complete an minor works or a installation certificate, and after that some one does and electrical installation condition report, which says I was in error, what then? I was worried about volt drop, and as a result wrote a excel then java script program ...

You worry too much With very few exceptions there is no regulatory requirement in relation to voltage drop, so estimating it incorrectly is hardly a capital offence!

We are now moving into maths, .... if you can understand that, your doing better than me.

I can understand it, but only because my education has been very different from yours, but I don't see it's relevance to this discussion!

We should be given the previous documentation, so we need to prove it was wrong, if we can, then likely it could result in a court case, ....

I'm afraid that I cannot really share your apparent great interest in hypothetical 'court cases' in relation to electrical matters. I can't help but wonder how many such cases there have been in, say, the past 50 years?

Kind Regards, John

 

[SUNRAY]

That raises an interesting question for someone as concerned about 'what regulations actually say' as you appear to be. Do you believe that what is at the far end of a feed to an outhouse count as part of the electrical installation of the house? If so what do you have to say about the regulatory requirement to runs a (I presume 'adequate') CPC to "every point" in an installation.

Well there will be just as soon as your local earth is installed...

For safety The last thing we want to do import a potentially hazardous and unwanted earth into a location.

Next question?

 

[flameport]

For those who rely on Table 54.7,don't the 'minimum CPC CSA' figures apply regardless of how much (if any) current flows in the cable?

That table assumes that the initial temperature of the conductors is 70C or 90C, which would be for a cable loaded to the maximum permitted by it's installation method for an extended time.
In reality, very few circuits are designed that way.

Current in the cable will directly determine the conductor temperature.
A 90C rated cable used at a lower current than it's maximum so that the conductor temperature only reached 70C would have the same minimum CPC requirement as a 70C cable used at it's maximum current rating,

 

[JohnW2]

That table assumes that the initial temperature of the conductors is 70C or 90C, ...

[which, as you go on to say, would be a very rare situation in practice] Whatever the Table assumes (it says nothing about any assumptions, regarding initial temps or anything else) the fact remains that it is all that is available, in any situation, to enable those who do not wish to undertake adiabatic calculations to determine the required CPC size.

Hence, certainly when CPC and line conductor are the same material, I see no alternative (for those who do not wish to calculate) but to go with what the table says, no matter how low the 'initial conductor temp' in their circuit.

Even when the materials are different (e.g. SWA) the only sense in which initial temp comes into the picture is because the values of 'k' for the two materials, as given in Tables 43.1 and 524-52.6, are only given for an 'initial temp' corresponding to the cable being 'fully loaded' (to its 'CCC'). However, the only effect those k-values have on Table 54.7 is that the minimum required CPC CSA is multiplied by the ratios of the k-values for the two materials. I don't even know to what extent those k-values vary with temp for a given material, but I rather suspect that that ratio (of two for different materials) will probably not vary dramatically over a wide range of 'initial temps'.

Current in the cable will directly determine the conductor temperature. A 90C rated cable used at a lower current than it's maximum so that the conductor temperature only reached 70C would have the same minimum CPC requirement as a 70C cable used at it's maximum current rating,

That's certainly what one would expect, but I don't know how anyone armed with only BS7671 would be able to work out what CPC size would be required for any real-world situation, and I certainly don't know whether what they ended up with (by whatever process) would be "BS7671-compliant".

However, it gets worse. Even those who are not lazy, and therefore undertake adiabatic calculations, if they rely on BS7671 the only values of 'k' available to them are those in Tables 43.1 and 524-52.6 which, as above, relate to an 'initial temp' equal to the maximum permitted temp during normal use - which is very rarely a real-world situation.

I'm not denying that it would correspond with common sense, but are you suggesting that, on the grounds that the 'initial temp' in your circuit/cable will be lower (than seemingly 'assumed' by BS7671), you can use a CPC with a CSA less than the 'minimum' obtained from Table 54.7, or by adiabatic calculation (per 543.1.3), and still be compliant with BS7671? I have to say that I personally see nothing in BS7671which suggests that such is the case. ... and, if you were going to take that approach how would you determine how much smaller tha the 'BS7671 minimum" CSA would be adequate?

Kind Regards, John

 

[flameport]

With only BS7671, it's impossible, as it only contains a few values for k, and the calculation suggested is massively simplified.

The rest of it is in BS7454, which is another example of paying £££ for very little. 8 pages in this case.

 

[JohnW2]

With only BS7671, it's impossible, as it only contains a few values for k, and the calculation suggested is massively simplified.

That was exactly my point - isn't that pretty ridiculous? It's all very well saying ....

The rest of it is in BS7454, which is another example of paying £££ for very little. 8 pages in this case.

(which, indeed, is an alternative to the 'BS7671 calculation' specified in BS7671) .
... but would you like to hazard a guess as to what proportion of practising electricians have ever seen BS 7454, let alone undertaken calculations as described in it? Are most electricians just 'assuming' that their CPCs are adequate (and compliant with BS7671), or what?

Kind Regards, John