Black cable for 110v site extension leads?

[ericmark]

On the building of Heathrow Terminal 5 we found some lighting sets with 110 volt sockets were in fact from a 110 - 0 - 110 volt generator and we had to blank them off. It does happen from time to time also the other way around where a 55-0-55 transformer is used on a control circuit which only has fuses on one live wire the other live being a common return unfused which should have been neutral.

Other than shaver socket I have only ever worked on one IT supply. It was part of a Robin tunnel boring machine control and lighting were from a delta wound transformer oddly the supply (660 volt) was star wound which makes me think it was a step up transformer being used to step down.

At Sizewell 'B' we had a problem finding extension plugged into extension plugged into extension most with four sockets and as a result overloading and tripping out. The three phase transformers had two three phase sockets so we started making up three phase spiders this mean three sockets one on each phase but also a 4 pin plug so it could only be plugged direct into transformer it could not be daisy chained. As the job progressed we needed 110 volt power in areas where there was no 415 volt power so we started using 25kVA transformers and 32A three phase extension boxes each with 9 sockets three 16A three phase breakers and it worked well. It is the only site I have seen 3 phase 110 volt used like this. We also had very large vacuum cleaners with 3 x 10A motors these also used the 16A three phase sockets.

With the three phase the cores were Brown, Black, and Black with Green/Yellow earth it was before we started using grey as a phase colour.

It was working on the 63 - 0 - 63 volt supply that made me think about core colours before that I had not really thought about the fact there should not be a blue core in a 110 volt cable.

The larger site transformer 4kVA and above always have the MCB on the 110 volt side as well as 400 volt side but up to 3kVA it's common to have a little red reset button on input only. My maths is 3000/55 = 54.54 amp line to earth which is well over what a 1.5mm flex can carry.

Personally with RCD protection I don't think there is a need for 110 volt any more 230 volt is safer as RCD protected and has much less fire risk then 110 volt.

 

[JohnW2]

Though I also fully agree it is entirely down to assumptions about the supply, and I can't dictate 'any' of that in my extension lead (except hopefully the connectors would suggest use at 110v). So perhaps any attempt to colour the conductors appropriately could always be thwarted. ... I find such things a bit annoying TBH; I'm not a qualified electrician but I try to be informed and do things properly, yet there are so many grey areas in the various standards that as a non-professional its hard sometimes, even on things that are common and should be straight forward.

It's an interesting discussion but, as I wrote recently, I really don't think that the colours of insulation within the cable of any made-up extension lead are actually of any practical importance.

Kind Regards, John

 

[KevDIY]

Thanks chaps!

Ericmark, interesting stuff! It seems inherently there have to be 'some' safety advantages of voltages being lower, and I believe UK sites do have a much better record in electrocutions compared to European 240v ones. But yes, none of the stuff I've read includes any figures from electrically caused fires, so I don't know how it works out in the wider picture - it could be either way for all I know.

Many of the smaller portable transformers seem to have thermal trips these days, but they are hardly fast acting and I take your point, none that I've used have had anything specifically protecting the 110v output (combined with their intermittent rating they seem pretty dodgy things, now I think about it). Maybe I'll upgrade my extension lead plans to use 2.5mm even for shorter 16A ones... perhaps then there would be some chance of the 240v fuse/trip going in many fault situations, before the 110v lead gave out.

John, yes that makes sense. As long as they are wired correctly then its probably not important. I suppose even if someone opened the connector for some reason its probably better that I stick to the colours that are expected/common rather than trying to do it 'right' but unconventionally.

Cheers
Kev

 

[JohnW2]

John, yes that makes sense. As long as they are wired correctly then its probably not important. I suppose even if someone opened the connector for some reason its probably better that I stick to the colours that are expected/common rather than trying to do it 'right' but unconventionally.

Quite so. I have any number of 'extension cables' of one sort or another and, with only a few exceptions, I haven't a clue as to what colour insulation is in the cable, and never will know (and "don't care"). If I ever do have to 'open one up', I would obviously take note of 'what was connected where' - and all that matters is that conductors are connected to corresponding places at the two ends.

Kind Regards, John

 

[KevDIY]

I used to work with a Portable Appliance Tester, and remember him saying what a surprising number of brand-new extension leads were made with live and neutral crossed over at one end. But I'll bet there would also have been some with the live/neutral colours reversed at both ends; he would not normally have detected that due to them being electrically consistent and most likely, as you say, he wouldn't have especially cared either; no impact on any normal users who shouldn't be opening them up in any case.

In my research for this I also came across a review for a new 110v lead that had them crossed over at one end, too. TBH I wouldn't have expected that to be disastrous in practice for a 55-0-55v supply with no neutral, but the chap had still been put through the ringer because of it. Apparently it meant the lead failed the PAT on a large site, which kicked off an enquiry about why sub-standard equipment was being brought on. I guess the colours were immaterial, it was the electrical inconsistency that was found and what mattered.

Cheers
Kev

 

[Risteard]

Personally with RCD protection I don't think there is a need for 110 volt any more 230 volt is safer as RCD protected and has much less fire risk then 110 volt.

I definitely wouldn't agree (apart from the fire risk point) as RCDs have a relatively high rate of failure (I believe around 7%), and even if functioning correctly would only save around 95% of the population 95% of the time.

There are no recorded fatalities from electrocution on a site arising out of the use of a reduced low voltage system. After all a fault will only generate an exposure on metalwork of around 30V.

 

[JohnW2]

Personally with RCD protection I don't think there is a need for 110 volt any more 230 volt is safer as RCD protected and has much less fire risk then 110 volt.

I definitely wouldn't agree (apart from the fire risk point) as RCDs have a relatively high rate of failure (I believe around 7%), and even if functioning correctly would only save around 95% of the population 95% of the time.

I couldn't agree more, and it always rather worries me when I see people assuming/implying that RCDs are a panacea. As you say, even when RCDs function correctly, they aren't such a panacea (and never could be for L-N or 'L1-L2' shocks). Indeed, as we've observed in the past, it's very hard to find reports or anecdotes of any cases in which an RCD may have saved a life (i.e. cases in which a shock, which may or may not otherwise have been fatal, has caused an RCD to operate).

Furthermore, as you say, there is this belief that in-service RCD failure rate may be quite high (I can never remember, without looking it up, whether it's 7% or 1 in 7, but I think 7%!), albeit that derives from limited, imperfect and increasingly old data. However, if the in-service failure rate is even remotely as high as that, this again eats into the 'panacea' notion. It also makes me wonder a bit when I see people advising that it is "totally unnecessary" to install a RCD socket, RCD FCU or RCD sub-CU etc. if the primary circuit is already RCD-protected - if in-service failure rate were anything like as high as 7%, then (assuming that one believes that RCD protection, per se, is worthwhile) there would be quite a lot to be said for 'belt and braces'!

Kind Regards, John

 

[KevDIY]

I have electrocuted myself in the past whilst somehow managing not to provide sufficient path to earth for the RCD to trip, so I can testify that in some situations they are not a full solution. But 'usually' I've managed to trip them so they are certainly of benefit.

Hmm, I've often wondered why site transformers are centre-tapped to earth rather than more safely floating, but probably thats the reason. Without some reference to earth an RCD wouldn't trip if a body bridged the 110v between L1 and L2, but with a reference to earth theres at least some chance that enough current would also leak to earth and trip it. And the centre tap only creates 55v to earth anyway, so its not too terrible a compromise. Thats if I've got my head around how a 240v RCD would react through a transformer anyway...

Cheers
Kev

 

[JohnW2]

I have electrocuted myself in the past whilst somehow managing not to provide sufficient path to earth for the RCD to trip, so I can testify that in some situations they are not a full solution. But 'usually' I've managed to trip them so they are certainly of benefit.

Someone will probably point out to you that 'electrocute' strictly implies a fatal outcome Are you saying that you have experienced electric shocks which have caused an RCD to trip?

Hmm, I've often wondered why site transformers are centre-tapped to earth rather than more safely floating, but probably thats the reason. Without some reference to earth an RCD wouldn't trip if a body bridged the 110v between L1 and L2, but with a reference to earth theres at least some chance that enough current would also leak to earth and trip it.

No device could protect a person against an L1/L2 (or L/N) shock - since the situation is electrical indistinguishable from a genuine load. If the tranny is supplying only one item of equipment, floating may well be the safest, but things change if there there is more than one tool, particularly if Class 1.

Thats if I've got my head around how a 240v RCD would react through a transformer anyway...

An RCD on the 240V side of the transformer would know nothing about any leakage to earth on the 110V side. Any RCD protection would have to be on the load side of the tranny.

Kind Regards, John

 

[KevDIY]

Ah, yes I did survive, good point

I was saying I've experienced shocks which haven't caused the RCD to trip, due to me being sufficiently isolated from earth at the time. I don't believe I've ever felt anything when I've actually tripped an RCD, though.

Yes I realise a purely L1/L2 shock would never trip an RCD, and that would always be the case if the supply was floating, even if the person was holding an earthed copper pipe or something at the time. What I was thinking is that because site supplies are now referenced to earth then a similar type of accident would be far less likely to 'be' purely L1/L2 as at least some current would follow the path to earth in many cases. So by referencing it to earth, the chance of a 55v shock to earth has been created by losing the floating nature of it, but on the other hand it would now be possible for an RCD to be tripped if something happened.

But yes on reflection, you're right - said RCD would have to be on the 110v side to work, so without one it would probably be better for the 110v supply to stay floating.... unlike all the 3kva site transformers I've seen which are not floating and yet have no 110v RCD... the worst of both worlds it would seem

Cheers
Kev

 

[Risteard]

have no 110v RCD... the worst of both worlds it would seem

Not really, because as said only around half of the voltage will appear on exposed conductive parts in a fault (half of either 55V for a single phase system or 63.5V for a three-phase system) which still should not pose a shock risk. So disconnection is simply for thermal reasons.

 

[JohnW2]

I was saying I've experienced shocks which haven't caused the RCD to trip, due to me being sufficiently isolated from earth at the time.

Yes, I realised that was you first point, but ...

I don't believe I've ever felt anything when I've actually tripped an RCD, though.

... this is the bit which interests me. Are you saying that (like virtually all of us) you have never tripped an RCD by a current flowing through your body (you would most certainly 'feel something' if enough current flowed through you to trip a 30mA RCD!!)? When you say "when I've actually tripped an RCD" are you perhaps referring to the situation (we've all been there!) when you have tripped an RCD by letting N and E touch on a circuit with only single-pole 'isolation' (i.e. with no current flowing through you)?

Kind Regards, John

 

[KevDIY]

have no 110v RCD... the worst of both worlds it would seem

Not really, because as said only around half of the voltage will appear on exposed conductive parts in a fault (half of either 55V for a single phase system or 63.5V for a three-phase system) which still should not pose a shock risk. So disconnection is simply for thermal reasons.

hmm, I think thats gone over my head. Although perhaps not, and I'm just thinking of worst-case situations in which a person could experience the maximum 110v, such as a connector coming off and exposing both L1 and L2 or something like that.

Cheers
Kev

 

[JohnW2]

Not really, because as said only around half of the voltage will appear on exposed conductive parts in a fault (half of either 55V for a single phase system or 63.5V for a three-phase system) which still should not pose a shock risk. ....

Do I take it that you didn't really intend to type the words I've emboldened?

Kind Regards, John

 

[KevDIY]

I don't believe I've ever felt anything when I've actually tripped an RCD, though.

... this is the bit which interests me. Are you saying that (like virtually all of us) you have never tripped an RCD by a current flowing through your body (you would most certainly 'feel something' if enough current flowed through you to trip a 30mA RCD!!)? When you say "when I've actually tripped an RCD" are you perhaps referring to the situation (we've all been there!) when you have tripped an RCD by letting N and E touch on a circuit with only single-pole 'isolation' (i.e. with no current flowing through you)?

Kind Regards, John

Interesting, John. I certainly recall working in a house where when I touched the neutral it tripped the RCD - without having moved any wiring or anything, so I'm pretty sure I wasn't bridging it to earth except through me. It sticks in my mind because having reset the RCD I stupidly did exactly the same a second time!

It hadn't occurred to me that I should have felt a shock. Possibly with it being the neutral the potential may have only barely been high enough to cause 30mA through my forearm - I was lying in a low loft space leaning on my elbows, so I suppose it wouldn't have to have gone through my whole body or have needed the potential to do so. It would still have been 30mA through my arm though

Perhaps the other times it could be as you say and I've touch conductors together rather than to me. If so then I've been more successful than I thought at not touching them, but of course I'm certainly always trying not to, so its possible.

EDIT: I'm almost tempted to try touching a live just to compare, but that way lies 'asking for it', so instead I'll just try to take special note if/when I trip anything again by accident..

Cheers
Kev