Dangerous Henley Block!?

[JohnW2]

Do you need a regulation to tell you what is good practise? The grey is a poorer quality of insulation, intended for mechanical protection. Were that not so, the whole of the cables plastic covering could be made of the same colour as the insulation.

I really don't think that the colour of the PVC has anything to do with how good an insulator it is. Whether the sheath has better, worse or the same insulation properties as compared with the inner insulation, I feel sure that it could be made the same colour as the inner insulation if they wanted to. I would imagine that the main reason for different colours is partially aesthetic, but mainly because it makes it much easier to see 'what one is cutting' when one is stripping it.

We hear a lot about the outer sheath of insulated+sheathed cables being 'poorer quality insulation' than the inner insulation, but i wonder what that really means and, indeed, how true it actually is. Any sort of PVC has such a high dielectric strength that the thicknesses we use for insulating conductors represent considerable 'overkill' - so I suspect that the main reason for 'insulated and sleeved' is probably in case the inner insulation is damaged (or, conceivably' {and despite testing} 'congenitally defective').

I wonder if the truth is perhaps that the sheathing material has similar electric insulating properties to the inner insulation but superior mechanical properties (since it's being used primarily for mechanical protection).

Kind Regards, John

 

[SUNRAY]

I really don't think that the colour of the PVC has anything to do with how good an insulator it is. Whether the sheath has better, worse or the same insulation properties as compared with the inner insulation, I feel sure that it could be made the same colour as the inner insulation if they wanted to. I would imagine that the main reason for different colours is partially aesthetic, but mainly because it makes it much easier to see 'what one is cutting' when one is stripping it.

We hear a lot about the outer sheath of insulated+sheathed cables being 'poorer quality insulation' than the inner insulation, but i wonder what that really means and, indeed, how true it actually is. Any sort of PVC has such a high dielectric strength that the thicknesses we use for insulating conductors represent considerable 'overkill' - so I suspect that the main reason for 'insulated and sleeved' is probably in case the inner insulation is damaged (or, conceivably' {and despite testing} 'congenitally defective').

I wonder if the truth is perhaps that the sheathing material has similar electric insulating properties to the inner insulation but superior mechanical properties (since it's being used primarily for mechanical protection).

Kind Regards, John

In the olden days it was always called double insulated. I easily remember twin mains flexx being a pair of wires twisted together, red/black, clear [seem to be heading back that way], black, white, in fact many colours or figure of eight. I still have a very old length of what was clear fig8 , originally used as one of our mains extn leads for public address work. Our Goblin vacuum cleaner had grey twisted singles.
Sure I remember when tails automatically had exposed insulation at their ends [my home installation does] but new work now seems to be to a better standard and identified with coloured cable ties or printed tape.

 

[JohnW2]

In the olden days it was always called double insulated.

It was and, as we see here, many people still do call it that - but they get 'told off' ('corrected') for doing so.

However, I really don't understand this. There is nothing about the phrase "double insulated" which says, or even implies, that both layers of insulation have to have the same degree of insulating properties - so, even if you are right in saying that the outer PVC layer of an 'insulated and sheathed' cable is 'poorer quality insulation', that doesn't alter the fact that it is "double insulated".

No-one would suggest that the two layers of insulation in a "double insulated" Class II item have to have identical insulating properties, would they?

I easily remember twin mains flexx being a pair of wires twisted together, red/black, clear [seem to be heading back that way], black, white, in fact many colours or figure of eight. I still have a very old length of what was clear fig8 , originally used as one of our mains extn leads for public address work. Our Goblin vacuum cleaner had grey twisted singles.

Indeed - I can easily remember all those things, too (and undoubtedly still have some examples!). However, I'm not sure what point you are making - none of those things was in any sense 'double insulated', and even the 'single insulation' was often fairly iffy!

Kind Regards, John

 

[RF Lighting]

Dangerous Henley Block

• eveares
• 4 Oct 2020

Regards: Elliott.

It looks like the insulation was probably stretched as it was stripped from the cable, and over time has crept back to its proper length.

Also that Henley block is connected up back to front. The rear part is supposed to be for the live tails and the front removable portion for the live.

It certainly is dangerous and needs to be repaired ASAP. It should be a fairly simple job for a competent electrician.

You’re a bit too far from me otherwise I’d have happily called in and sorted this for you, whether that means re-terminating the existing, or replacing the tails and ditching the redundant Henley all together.

 

[Lectrician]

My 2013 vintage MK twin RCD 12 way CU is not in the best of shape

2013 Vintage?

 

[SUNRAY]

Indeed - I can easily remember all those things, too (and undoubtedly still have some examples!). However, I'm not sure what point you are making - none of those things was in any sense 'double insulated', and even the 'single insulation' was often fairly iffy!

Kind Regards, John

The point I failed miserably to make was tails have always been double insulated since being PVC whereas other parts of the system didn't use to be.

We often see comments on here about the inner blue and brown layer of insulation, or red and black, showing outside of an enclosure being wrong and needs to be corrected. All of a sudden Harry and Andy seem to be saying the correct thing to do is expose the inner layer of insulation outside of an enclosure...
And this is on the most hazardous part of the installation, being only protected by a BS88 fuse without any form of isolation available to the consumer. I would have thought we should all be advocating being more vigilant to ensure a higher degree of protection/safety, not a lower level.

 

[Harry Bloomfield]

We often see comments on here about the inner blue and brown layer of insulation, or red and black, showing outside of an enclosure being wrong and needs to be corrected. All of a sudden Harry and Andy seem to be saying the correct thing to do is expose the inner layer of insulation outside of an enclosure...

No, I did not. The grey mechanical protection layer should always enter an enclosure. Obviously Henley blocks and meters leave little room to do that effectively, but the grey can certainly be trimmed back a few mm. The insulation layer should always cover the conductive core, up to the terminal, unlike the example in the photos. I would consider leaving the grey layer in place to the terminal shoddy and lazy workmanship.

The grey mechanical protection layer should never be exposed to voltages.

Besides, how do those who leave the grey in place manage when terminating in the back of a multi-way switch? The grey takes up a lot of unnecessary space and makes cables difficult to bend inside an enclosure.

 

[AndyPRK]

I wasn't saying you should be able to see the colour from the outside.
However I would expect to be able to see the colour with the cover removed, and be able to glance and see polarity is correct.

The way both insulation parts have been cut at the same point, means that wasn't the way it was done

 

[JohnD]

Do you need a regulation to tell you what is good practise?

What makes you think it is good practice to cut away the protective sheath?

 

[JohnW2]

The rear part is supposed to be for the live tails and the front removable portion for the live.

I think that probably would benefit from a little editing but, one that's been done, I agree - although I presume it's not really an issue so long as the front cover is intact.

Kind Regards, John

 

[JohnW2]

I wasn't saying you should be able to see the colour from the outside. ... However I would expect to be able to see the colour with the cover removed, and be able to glance and see polarity is correct. The way both insulation parts have been cut at the same point, means that wasn't the way it was done

I agree. In fact, I'm not even sure that it is compliant with the the inner insulation not visible (once cover removed). BS7671 requires conductors to be identified (the usual way being by colour) and I'm not convinced that one is complying with that regulation if the inner insulation is not visible (unless the sheath has an identifying colour).

The advice/practice as regards cables entering accessories/enclosures in general is that the sheath should be removed from just after where it enters the enclosure, not the least to allow the conductors to be dressed more satisfactorily.

Kind Regards, John

 

[JohnW2]

The point I failed miserably to make was tails have always been double insulated since being PVC whereas other parts of the system didn't use to be.

"Double insulated, eh? - you've 'done it' yourself I can't say that I paid enough attention to tails in the pre-PVC days, but I'll take your word for it!

We often see comments on here about the inner blue and brown layer of insulation, or red and black, showing outside of an enclosure being wrong and needs to be corrected. ...

Indeed.

... All of a sudden Harry and Andy seem to be saying the correct thing to do is expose the inner layer of insulation outside of an enclosure...

I have to say that is not how I interpreted what they were saying, and Andy has now confirmed that my interpretation was correct. Andy was saying (and I thought that Harry was also saying) that the inner insulation should be visible within the enclosure - and, as I've just written, I'm not convinced that the situation is compliant with BS7671 if it is not visible. After all, what would be the point in having a regulation requiring a conductor be identified if that identification were not required to be visible?!

Kind Regards, John

 

[Harry Bloomfield]

"Double insulated, eh? - you've 'done it' yourself I can't say that I paid enough attention to tails in the pre-PVC days, but I'll take your word for it!

Pre-PVC, the direct precursor for PVC was rubber with a woven cotton waxed outer for mechanical protection. Again it was good practice to trim the cotton back, clear of the terminal. A poor way to do it, without risk of damaging the rubber by ringing it with a blade, was to simply push the cotton back from the terminal.

Another method with a less tidy result, was to pull the cotton back, tease a hole through the weave where you wanted to trim the cotton back to, then thread the inner rubber and copper through the hole. You then simply snip off the cotton. I use a similar method for terminating a coax on a PCB, where the screen is a woven copper screen and I need to solder to a PCB, except unlike the snipping the cotton, I need to use the screen.

 

[SUNRAY]

No, I did not. The grey mechanical protection layer should always enter an enclosure. Obviously Henley blocks and meters leave little room to do that effectively, but the grey can certainly be trimmed back a few mm. The insulation layer should always cover the conductive core, up to the terminal, unlike the example in the photos. I would consider leaving the grey layer in place to the terminal shoddy and lazy workmanship.

The grey mechanical protection layer should never be exposed to voltages.

Besides, how do those who leave the grey in place manage when terminating in the back of a multi-way switch? The grey takes up a lot of unnecessary space and makes cables difficult to bend inside an enclosure.

I wasn't saying you should be able to see the colour from the outside.
However I would expect to be able to see the colour with the cover removed, and be able to glance and see polarity is correct.

The way both insulation parts have been cut at the same point, means that wasn't the way it was done

My apologies, I misinderstood the post. However I think many people would be hard pressed to view the inner layer of insulation when it doesn't show inside the device and even harder to strip the outer layer such that the inner shows knowing how thin some of the splitter blocks walls are.


I've delayed replying until I heard back from a meter fitter, he says the policy of habitually having the inner exposed outside any enclosure changed more than 10 years ago and their current instruction is to trim both layers to the same length at the meter and anywhere else with the same sort of clearance. He even used the Henley block [unprompted] as an example where the L will have the red or brown exposed into the rear terminals but the N will have both layers trimmed to the same length as it's not reliable stripping without shrinking back in the 3mm of the enclosure wall on some makes. My own block has maybe 8mm of space between the brass bar and the side of the tray so plenty of space but these single blocks don't: https://cpc.farnell.com/eterna/cb100sp/100a-single-pole-connector-block/dp/LA04540?st=henley block ,nothing of the wire shows inside and the walls are <3mm thick.
They used to use brown & blue cable ties but now rely on L & N marker tape to eliminate the number of accident reports due to sharp ends left on the ties.

 

[SUNRAY]

Pre-PVC, the direct precursor for PVC was rubber with a woven cotton waxed outer for mechanical protection. Again it was good practice to trim the cotton back, clear of the terminal. A poor way to do it, without risk of damaging the rubber by ringing it with a blade, was to simply push the cotton back from the terminal.

Another method with a less tidy result, was to pull the cotton back, tease a hole through the weave where you wanted to trim the cotton back to, then thread the inner rubber and copper through the hole. You then simply snip off the cotton. I use a similar method for terminating a coax on a PCB, where the screen is a woven copper screen and I need to solder to a PCB, except unlike the snipping the cotton, I need to use the screen.

I'm very familiar with the tecnique for coax but I can't imagine doing it with rubber /cotton.
The tails at my parents house 7/0.044" for power, 7/0.029 for lights and something thicker cutout to meter were beautifully finished with a waxed cotton lapping right up to the end of the rubber inner. they were installed in 1951 and only replaced a few years ago at meter change.