Thoughts on earth bonding on lights please

[stillp]

Right, I know have BS6004, which I selected because it includes 6242Y, which seems to be the most common type for which the question of DI or I & S occurs. Indeed there are no insulation resistance tests for the sheath, although there is a test on the completed cable, as follows:
Procedure
Immerse the sample in water at a temperature of (20 ±5) °C for a period of not
less than 24 h. Ensure that the ends of the cable protrude above the water by a
distance sufficient to prevent excessive surface leakage when the test voltage is
applied between the conductor and the water.
Take:
a) each conductor in turn;
b) all other conductors, which are connected together and also connected to
the water.
Gradually apply a test voltage of 2 000 V between a) and b) and maintain at full
r.m.s. value for 15 min.
Repeat the test, but applying the voltage between all conductors connected
together and the water.
In both cases earth the circuit protective conductor if present but do not include
it in the conductors to be tested. While the sample is still immersed, disconnect
the circuit protective conductor from the water and apply a voltage of
1 000 V a.c. for 5 min between this and the water.

The tests for insulation resistance are (of course!) in a reference standard, which I have asked for, so I might be able to add some further clarity. In addition, I've discovered some editorial errors in BS6004, so will ask for a Corrigendum to be issued.

 

[JohnW2]

I can't find BS EN 50363-3 in the Big Green Book...

Nor can I find it in the Big Yellow Book - but all that proves is that it is not referenced/cited in the book. There is no doubt that it exists. The current version is:

BS EN 50363-3:2005+A1:2011 Insulating, sheathing and covering materials for low voltage energy cables. PVC insulating compounds

You can have a copy for £94 if you wish (half that if you were a BSI member)!

Kind Regards, John

 

[JohnW2]

Right, I know have BS6004, which I selected because it includes 6242Y, which seems to be the most common type for which the question of DI or I & S occurs.

Many thanks, but I have to disagree with you premise. At least as far as I am concerned, the "question of DI or I &S" virtually always arises in relation to 6181Y. Many/most people, here and elsewhere (as well as many/most suppliers) seem to refer to that as "DI", but I have rarely, if ever, heard anyone talking about T+E as being "DI".

Indeed there are no insulation resistance tests for the sheath, although there is a test on the completed cable, as follows:

Fair enough - and, as you presumably want to imply, a cable could pass that test solely by virtue of the properties of the inner insulation, even if the sheatrh were highly conductive. However, this seems to take us back to BS EN 50363-3, which appears to be a Standard specifically relating to materials for use for sleeving.

The tests for insulation resistance are (of course!) in a reference standard, which I have asked for, so I might be able to add some further clarity.

Thanks.

Kind Regards, John

 

[stillp]

BS EN 50363-3, which appears to be a Standard specifically relating to materials for use for sleeving

No, everything but!
"Insulating, sheathing and covering materials for low voltage energy cables. PVC insulating compounds"

 

[JohnW2]

....However, this seems to take us back to BS EN 50363-3, which appears to be a Standard specifically relating to materials for use for sleeving.

BS EN 50363-3 may cost £94, but only consists of 10 pages in total, the first 5 of which are title pages/contents etc. There is in fact only one page of text and two Tables - all of which appears in the 'free preview' on the BSI website, other than 'Table 2', which apparently details the required tests (and, by implication, occupies 4 pages). Not a very good £94's worth, IMO.

However, it appears that it should be read "in conjunction with" EN 50363-C - which maybe has a little more to say!

Kind Regards, John

 

[JohnW2]

BS EN 50363-3, which appears to be a Standard specifically relating to materials for use for sleeving ['sheathing']

No, everything but! "Insulating, sheathing and covering materials for low voltage energy cables. PVC insulating compounds"

Apologies for the semi-phonetic typo - collaboration between my brain and fingers sometimes does that

Let you find something else wrong with what I wrote, when I wrote "specifically", I was referring to the fact that the Standard explicitly includes sheathing, not that it is exclusively about sheathing!

Kind Regards, John

 

[stillp]

it should be read "in conjunction with" EN 50363-C

I believe that is BS EN 50363-0, the box around it in the preview makes the 0 look like a C.

 

[JohnW2]

I believe that is BS EN 50363-0, the box around it in the preview makes the 0 look like a C.

Ah, on reflection, that makes sense. Thanks. Whatever, I hope that (if it could be seen) it might contain more than the one page of text and 2 Tables which constitute the £94's worth of 50363-3!

Kind Regards, John

 

[stillp]

I have to disagree with you premise. At least as far as I am concerned, the "question of DI or I &S" virtually always arises in relation to 6181Y.

6181Y is also specified by BS6004. However, BS EN 50363-3 doesn't add much clarity, since the tests in Table 2 thereof are mechanical and environmental, the tests for insulating properties being in 50363-0; or maybe not. I'll have to look into this next time I'm in Chiswick.

Looking again at the Voltage withstand test that I quoted above, you'll notice that it involves applying 2kV for 15 minutes between each conductor and all the others, and between all conductors and the water. If a cpc is present, then 1kV is applied between it and the water for 5 minutes. This implies that the sheath has some insulating properties, but much less than the insulation of the cores.

 

[JohnW2]

6181Y is also specified by BS6004. However, BS EN 50363-3 doesn't add much clarity, since the tests in Table 2 thereof are mechanical and environmental, the tests for insulating properties being in 50363-0; or maybe not. I'll have to look into this next time I'm in Chiswick.

Thanks.

Looking again at the Voltage withstand test that I quoted above, you'll notice that it involves applying 2kV for 15 minutes between each conductor and all the others, and between all conductors and the water. If a cpc is present, then 1kV is applied between it and the water for 5 minutes. This implies that the sheath has some insulating properties, but much less than the insulation of the cores.

Indeed, but I'm not so sure about "much less", since I'm not convinced that the duration of exposure will make much difference to such a test. If one ignores the different durations, then if a test of "insulation + sheath" is at 2kV, and a test of "sheath only" is at 1kV, that would seem to suggest that the insulating properties of the "insulation" and the "sheath" are expected (maybe even 'required'?!) to be much the same as one another, wouldn't it??

Kind Regards, John

 

[stillp]

I'm not convinced that the duration of exposure will make much difference to such a test

Oh, it does.

a test of "insulation + sheath" is at 2kV, and a test of "sheath only" is at 1kV, that would seem to suggest that the insulating properties of the "insulation" and the "sheath" are expected (maybe even 'required'?!) to be much the same as one another, wouldn't it?

I think you're misreading the test procedure.

 

[JohnW2]

Oh, it does.

I'm very ready to be educated. However, if a material couldn't withstand the test voltage for 15 minutes, but could withstand it for 5 minutes (which, in itself, I would find a bit surprising), I'm not sure how I would interpret that in terms of in-service use, when the duration of exposure could be anything. I would have thought that if it failed the test after any period of exposure, that would count as 'a fail'!

I think you're misreading the test procedure.

I'm not reading (or misreading) the test procedure, per se, but what you wrote about it. If one removes the bit about testing between (insulated) cores (which is obviously different, because of two layers of insulation), what you wrote becomes "...you'll notice that it involves applying 2kV for 15 minutes between ..... all conductors and the water. If a cpc is present, then 1kV is applied between it and the water for 5 minutes." As I said, if one ignores (whether one should or not) the different durations of exposure, that means that one used double the voltage to test (conductor to water) insulation + sheath that one uses to test (conductor to water) just the sheath - which to me suggests an assumption that the insulation has roughly the same insulating properties as the insulation. If that reasoning is wrong, could you perhaps explain why?

Kind Regards, John

 

[Risteard]

if a test of "insulation + sheath" is at 2kV, and a test of "sheath only" is at 1kV, that would seem to suggest that the insulating properties of the "insulation" and the "sheath" are expected (maybe even 'required'?!) to be much the same as one another, wouldn't it??

No it wouldn't - as you have insulation on all live conductors so a test between these benefits from insulation on both/all of these. Also, the time factor cannot be ignored as insulation will break down the longer the voltage is applied. (After all insulation degrades with use.)

It is a very clear indicator that whilst the sheath might be made of insulating material (which I stated from the outset) it cannot be considered insulation. Its properties are not the same and its purpose is merely (or perhaps primarily would be a better word) to protect the insulation from mechanical insult.

 

[JohnW2]

No it wouldn't - as you have insulation on all live conductors so a test between these benefits from insulation on both/all of these.

As I clarified in my subsequent post, I was talking about the conductor-to-water tests, not between-conductors tests (when, as you say, there would be two lots of insulation involved). The BS6004 conductor-to-water tests are done separately for each conductor, and also with all conductors joined for the test - but, either way, there is just one layer of insulation and one layer of sleeving between each conductor and the water.

In any event, even if I had been talking about conductor-to-conductor tests, my point would still remain - that if it were deemed necessary/appropriate to use double the voltage for a conductor-to conductor test (voltage across two layers of insulation) as compared with the voltage used for a CPC-water test (voltage across one layer of sheathing), that would again suggest that (at least in terms of voltage) that the insulating properties of the 'insulation' and sheath were similar.

Also, the time factor cannot be ignored as insulation will break down the longer the voltage is applied. (After all insulation degrades with use.)

As I said to stillp, I'm very prepared to be educated about this. Degradation of insulation does, of course, happen, in the very long-term, even in the absence of any voltage, but that's obviously a very different thing.

If, as implied by stillp's way of thinking, an applied voltage could be withstood satisfactorily for 5 minutes but would result in breakdown of the insulation if it continued to be applied for a further 10 minutes, I would be rather surprised - but maybe that's how it is?!

Whatever, if it would have failed the test if the test had required the test voltage for 15 mins, but actually 'passed' because the test only required 5 mins exposure, I would not want to read very much into that 'pass'!

Whether it is 'required' or not (we wait to hear), it still sounds to me that, per my intuition, the sheathing probably provides a similar degree of insulation to that provided by the 'insulation' - or, at the very least, a 'substantial' degree of additional insulation.

Kind Regards, John

 

[JohnW2]

6181Y is also specified by BS6004. However, BS EN 50363-3 doesn't add much clarity, since the tests in Table 2 thereof are mechanical and environmental, the tests for insulating properties being in 50363-0; or maybe not. I'll have to look into this next time I'm in Chiswick.

I'm come across another Standard which might be worth looking at....

Having looked around many 6181Ys, it seems that (if they say anything) most say that the insulation is "PVC Type TI1" to BS EN 50363-3 or BS 6004. However, those which say anything about the sheathing say that it is "PVC Type 6" to BS 7655-4.2 - so maybe that latter would be worth looking at.

I've also been looking at PVC in general. It would seem that there is relatively little variation in either dielectric strength and resistivity across the whole range of PVCs - it's really just the mechanical/environmental properties which vary appreciably. Hence (as always, regardless of what may, or may not, be 'required'), it looks as if, even if one wanted to, it would probably be difficult or impossible to manufacture a PVC sheath which did not have fairly similar 'insulating properties' to those of PVC 'insulation'.

The fact that the BS6004 test requires the sheath alone to be able to withstand 1,000V for 5 minutes inclines me to regard it as 'insulation', but that's just me!

Kind Regards, John