Redundant copper pipe as conduit?

[EFLImpudence]

I am sorry to keep on

Years ago someone did have this issue but the worst thing was the L touched the metal pipe but it wasn't actually earthed back to the main earthing terminal.

Why would it be?
Do you mean bonded - but if it was not earthed it should not be bonded?

However it was his mistake as he assumed the pipe formed part of the protective earth but it didn't

Why would it?

and instead of bringing an earth wire to the MET he just linked the CPC to the cable.

How do you link a CPC to a cable?

 

[Jurassicspark]

I am sorry to keep on

It's alright mate. I am free all day got a day off from work today.

Why would it be?
Do you mean bonded - but if it was not earthed it should not be bonded?

Earthed. As you would know, bonding it would be unnecessary as no one will be touching the pipes under the floor under a fault condition.

Why would it?

The pipe may be capped but still connected to the main earthing terminal.

How do you link a CPC to a cable?

Dang old age. I meant the cables CPC to the pipe. Don't ask me why. I might still have a picture of it on my old computer.

 

[EFLImpudence]

The pipe may be capped but still connected to the main earthing terminal.

Why?

 

[Jurassicspark]

Why?

If it was extraneous it should have been bonded. Someone's replace the old pipes upstream to plastic and doesn't bother removing the earth wire connected to it. Later the pipes don't carry anything and the earth clamp remains there.

Or you hear the phrase "It's connected somehow back to the MET" by a gas/oil boiler.

 

[JohnW2]

Earthed. As you would know, bonding it would be unnecessary as no one will be touching the pipes under the floor under a fault condition.

Both earthing and bonding are relevant to reduction of the risk of electric shock - something which obviously is not relevant if the pipe is under the floor (other than to anyone who lifts the floorboards).

The pipe may be capped but still connected to the main earthing terminal.

That's certainly not impossible - but 'so what?'

 

[ebee]

RCDs are very good but they are not magical, they do not mitigate all other safety concerns although some folks tend to act as if they do and take additional risks they might not otherwise do,
If we start with the premise that there is an expectation that they (30mA or less) might save around 95% of the population that would not be saved otherwise and another expectation that there might be as much as a 7% failure rate then taking both of those figure we start to see how they might only be useful for additional protection only unless we have no other choice such as in a TT installation.
My working on a TT installation is very rare but whenever I do try to use two (or more) in tandem - the figures above might lead us to consider an overall failure to save rate of over 11% whereas two in tandem might give us a hope that we could possibly reduce that risk up to one and a third percent at best (optimistic? yes, I think so).
However, you would probably only have one of them at 30mA or less rating and instant and the other one three times as insensitive and also delayed so at absolute best one might give personal protection as the normally assumed figures might indicate and the other one might (just might) give some protection in some events if a fault occurs prior to us touching it anyway. One way to limit the chance of "Stiction" might be to have the two RCDs in differing locations therefore differing environments and the test button frequent operation could help a little too . I still recommend 3 month max intervals for that one reason if nothing else. A sobering thought but with TT our choices are limited.

 

[plugwash]

I don't know where your 7% figure came from, I did some quick searching on RCD failure rates, and the numbers seemed to vary massively.

But I've got the impression that there is a big difference between failing to meet specifications and failing so badly that it would not provide effective "fault protection" on a typical TT installation. While an earth fault on a TT will not produce the massive currents it will produce on a TN, it should, in most cases, still produce multiple amps of earth fault current.

And lets not forget that OCPDs are almost never tested in the field. So it's difficult to compare their reliability to RCDs.

 

[JohnW2]

If we start with the premise that there is an expectation that they (30mA or less) might save around 95% of the population that would not be saved otherwise and another expectation that there might be as much as a 7% failure rate then ...

Who knows how many people might be 'saved'? In terms of 'direct life saving' ('personal protection'), despite years of canvassing people, I've only come across a couple of instances of people surviving an electric shock associated with an RCD tripping, and in neither of those cases am I convinced that all the current resulting in the trip went through the body of the victim.

Far more likely as 'life saving' is the clearing of faults before anyone had a chance to get a shock (and, as implied by plugwash {and, to some extent, yourself}, an RCD with an effective IΔn very much greater than 30 mA would be adequate for thst). We'll never really know how often that happens but, again, there are so few domestic electrocutions that there is not much scope for anything to reduce them appreciably.

One problem with your 'premise', even if it is true, is thst it somewhat overlooks the fact that there are so few people who "would not be saved otherwise". In that 'otherwise' scenario, something like 99.996% of the population will not die by domestic electrocution in any year, and even of the tiny handful thst do, the circumstances in at least some cases will probably be such thst no device could have prevented their death.

As for the 'cost-effectiveness' of residual-current devices (which requires attributing a monetary value to a human life), that seems to be 'getting worse'. Whilst in the days of RCCBs it was possible to say (as was done very recently in this forum) that "it only costs £20 to fit an RCD 'and save a life' ", the increasing popularity of all-RCBO CUs, coupled with the current very substantial difference in cost of MCBs and RCBOs, means that the financial cost of having residual current protection is considerably increasing.

The clock obviously will never be turned back, so I'm essentially talking to a brick wall but when we next come across (as surely we will) of some further new-fangled device for 'saving lives', we should perhaps give more ('up front') thought to the question of how many more lives might be saved if the (probably enormous) cost of deploying the new-fangled technology were instead spent on some other 'life saving' purpose.

Kind Regards, John

 

[Jurassicspark]

Who knows how many people might be 'saved'? In terms of 'direct life saving' ('personal protection'), despite years of canvassing people, I've only come across a couple of instances of people surviving an electric shock associated with an RCD tripping, and in neither of those cases am I convinced that all the current resulting in the trip went through the body of the victim.

What about the time when someone cuts through the lawnmower cable in the garden??

 

[JohnW2]

What about the time when someone cuts through the lawnmower cable in the garden??

I confess that I've done that several times, but with a corded hedge cutter and what has happened every time is that an MCB has operated and/or the plug fuse has blown. These items of garden equipment invariably have 2-core cables, so cutting through the cable, per se, even with something metallic, cannot result in a residual current device tripping

 

[Jurassicspark]

I confess that I've done that several times, but with a corded hedge cutter and what has happened every time is that an MCB has operated and/or the plug fuse has blown.

Yes only if the neutral or even the earth wire (if it's not class II) manages to splice off and result in a short circuit.

A lot of times I've seen the live wire has only been spliced, leaving the situation dangerous with the mud at mains potentional. Before RCD days, this story used to pop up a lot with ELCB's.

RCD's are your friend here.

 

[JohnW2]

A lot of times I've seen the live wire has only been spliced, ....

I've no experience of mowers doing it, but every time I've hopped through the cable of a hedge trimmer, it's chooped the entire cable as if it were molten butted in a fraction of a blink of an eyelid.

leaving the situation dangerous with the mud at mains potentional.

That would not, in itself, be a danger unless a person in contact with that 'live mud' was able to simultaneously touch some mud which was not live.

RCD's are your friend here.

In a few situations, I agree. However, the situation would be much 'safer' if corded garden electrical tools/equipment were required to have 3-core cables (with the third core connected to earth).

 

[robinbanks]

its better protection than nothing

Wasn't it around the 14th edition that we ceased recommending putting cables in metal conduit in a domestic setting ?

Anyways; I think you should not do what you propose because you then have to design for adequate protection in the event of a fault such as someone driving a screw right the way through the copper pipe (easy; it's very soft), and through the live conductor only, effectively connecting the entire pipe up to live. Plus, cutting pipes leaves sharp ends, and you may accidentally skin the wire down to live core as you pull it through

Don't bother; it's not how we do things these days - don't deviate from the norm people expect. You'd be better off cutting the old pipes out and weighing them in, and using the holes left behind through the walls to run your cable, so long as they're helpful/in the right place with regards to safe zone rules.

If you're concerned about protecting cables, as well as having the relevant disconnect devices at the supply end, drill new holes through joists/clip cables so they're at least 50mm away from any surface a person could drive a screw through; it's typical modern installation behaviour where space is available even if RCD protection is present, to position a cable out of potential harm's way

 

[Jurassicspark]

That would not, in itself, be a danger unless a person in contact with that 'live mud' was able to simultaneously touch some mud which was not live.

A bare live wire can cause the earth to have a voltage gradient as you move away from the line. If your feet bridge a gap of the gradient the current will move up one leg and down the other as your body has less resistance than the ground between your feet. In real life, a lot of things will prevent this but just thinking about someone without any shoes on.

That's why they taught us to shuffle away if you ever find yourself close to a pylon or aen energized wire touching the ground.

 

[JohnW2]

A bare live wire can cause the earth to have a voltage gradient as you move away from the line.

Indeed.

If your feet bridge a gap of the gradient the current will move up one leg and down the other as your body has less resistance than the ground between your feet. ....

Again, indeed - but as you go on to say ....

In real life, a lot of things will prevent this but just thinking about someone without any shoes on.

Indeed yet again - in particular, shoes. Not many people mow lawns or cut hedges in bare feet and/or soaking wet conditions

It's also pretty difficult (unlikely?) that the chopped end of an L conductor will make reasonable electrical contact with the ground.

In any event, as I'm said, every time I've done it an OPD has instantly disconnected the supply. It's not easy to see how a metal blade can cut both L and N conductors of a 2-core cable without that happening. However,as I've aid, insistance on 3-core cable (with the third core earthed) would presumably be 'even safer' - but that is not currently a requirement.