Wiring for a light/lantern on a brick pillar

[JohnW2]

I do understand the risk.Is it more theoretical though?

Well, it's a real enough risk IF there is a fault in the supply neutral at a point which results in a high neutral potential (relative to earth) in the premises. It's obviously not a common occurrence, but nor is it anything like unknown.

To put it into some perspective, exporting a PME earth is probably analagous to omitting the main protective bonding to the installation. In both cases, a serious hazard only really arises in the case of a neutral fault as mentioned above.

Many of the regs, guidelines and 'good practices' relate to 'theoretical' risks which are, in fact, very small risks. If you removed all the bonding from all the electrical installations in the UK, the resulting number of serious injuries and deaths would probably be extremely small - but we live in an era/society which seemingly doesn't think that such risks are 'acceptable'.

Kind Regards, John.

 

[studentspark]

Thanks
Which backs up the point if it is so potentially dangerous, why no definitive guidance like PMB, which would warrant a code 1 on a PIR.

Its the outside lighting I am thinking about.I never considered that as exporting the earth.

 

[JohnW2]

Thanks Which backs up the point if it is so potentially dangerous, why no definitive guidance like PMB, which would warrant a code 1 on a PIR. Its the outside lighting I am thinking about.I never considered that as exporting the earth.

I'm certainly not a person who has ever suggested that everything about BS7671 is sensible - so I guess you'd have to ask the IET about that! As I said, unless someone can convince me otherwise, I would regard exporting a PME 'earth' outdoors as equivalent to having no MPB on a PME installation - which, as you say, would be regarded as a major safety issue.

A lot of modern outside lighting is probably Class II, therefore not really a hazard, although I'm sure there are a lot of 'earthed' light fittings out there.

Kind Regards, John.

 

[RF Lighting]

There's nothing wrong with exporting a TN-C-S earth.

Can you cite one single example of anyone being injured as a result of this sort of installation?

Making a perfectly good supply into a TT is madness. RCDs can and do fail. This leaves all your metalwork at mains potential above true ground.

Again it's a theoretical risk, but I'd have thought it far more likely than a failed CNE conductor

 

[JohnW2]

There's nothing wrong with exporting a TN-C-S earth.

What I would agree is that its no more wrong than having a TN-C-S installation without Main Protective Bonding. In either case, the primary danger exists only in the very unlikely event that someone simultaneously touches an exposed-conductive-part and an extraneous-conductive-part (including wet soil etc. in the case of an exported supply) at the very time when there is a supply neutral fault resulting in a seriously elevated neutral/CPC potential or (for a very brief period prior to disconnection) whilst there is a L-E fault in the installation.. Seemingly an extremely unlikely eventuality, but one which sappears to makes the regs treat the absence of MPB very seriously.

Can you cite one single example of anyone being injured as a result of this sort of installation?

Nope. Analogously, can you cite one single example of anyone being injured as a result of absent Main Protectve Bonding on a TN-C-S installation? As I said, both situations only become seriously hazardous in the same very unlikely scenarios.

Kind Regards, John.

 

[Stoday]

Studentspark is spot on. No need for the old fogies worries. Well not according to the IET's Wiring Matters artical.

Go to the IET site (click here)

Find Wiring Matters issue 16, August 2005 and download the article on supplies to detached outbuildings.

 

[JohnW2]

Studentspark is spot on. No need for the old fogies worries. Well not according to the IET's Wiring Matters artical.
Go to the IET site (click here)Find Wiring Matters issue 16, August 2005 and download the article on supplies to detached outbuildings.

It's not "old fogies' worries' but rather a call for consistency of logic. I agree that the risks we've been discussing are extremely small, such that I probably wouldn't personally regard as being, per se, worth worrying about. However, they are effectively exactly the same risks as exist if one has a TN-C-S installation without any Main Protective Bonding - something which seems to be almost universally regarded as 'unacceptably dangerous'. Why the dramatic inconsistency between this and what you are suggesting?

The relevant part of the IET article is:

Where the garage contains an extraneous-conductive-part such as a metal water pipe Among the options open to the installation designer are to make the installation in the garage part of a TT system or to provide main equipotential bonding to the extraneous-conductive-part in the garage.

In the contexts I've been discussing, the relevant 'extraneous-conductive-parts' (parts liable to provide a path to true earth), when present, are unusual ones - e.g. a wet floor sitting on soil or, in some sitiuations, the soil/ground surrounding the building in the vicinity of the door. If such exist, the IET article offers just two options - to isolate the outbuilding from the PME earth and use a local TT system OR to main bond the e-c-ps to the main installation's MET. With the sort of 'unusual e-c-ps', I'm talking about, the latter approach (main bonding) is clearly not possible.

...the important thing being that the EIT article does effectively acknowledge the hazard, and offers those two solutions - a local islolated TT system OR (if practical) main bonding of any potential path to true earth.

Kind Regards, John.

 

[Stoday]

In the contexts I've been discussing, the relevant 'extraneous-conductive-parts' (parts liable to provide a path to true earth), when present, are unusual ones - e.g. a wet floor sitting on soil or, in some sitiuations, the soil/ground surrounding the building in the vicinity of the door. If such exist, the IET article offers just two options - to isolate the outbuilding from the PME earth and use a local TT system OR to main bond the e-c-ps to the main installation's MET. With the sort of 'unusual e-c-ps', I'm talking about, the latter approach (main bonding) is clearly not possible.

"Unusual"? Don't you mean "daft"?

If you can't bond, you can't comply with 415.2. No matter which option you choose, you've still got to bond ecps. Of course there's a constraint on being too daft in 415.2.2, but that applies to both options too.

 

[JohnW2]

In the contexts I've been discussing, the relevant 'extraneous-conductive-parts' (parts liable to provide a path to true earth), when present, are unusual ones - e.g. a wet floor sitting on soil or, in some sitiuations, the soil/ground surrounding the building in the vicinity of the door. If such exist, the IET article offers just two options - to isolate the outbuilding from the PME earth and use a local TT system OR to main bond the e-c-ps to the main installation's MET. With the sort of 'unusual e-c-ps', I'm talking about, the latter approach (main bonding) is clearly not possible.

"Unusual"? Don't you mean "daft"?

I obviolusly wouldn't say so. An e-c-p is defined as "a conductor liable to introduce a potential, usually earth potential" - and I would hope we can agree that the spirit/intent of this is that it relates to the posibility that a person will come into contact with that potential (usually true earth potential) whilst simultaneously touching some other potential (primarily the CPC potential of the installation). To my mind, a possibly wet surface in continuity with true earth on which one could be standing (possibly with bare feet) whilst touching an exposed-conductive-part certainly comes within the scope of that.

If you can't bond, you can't comply with 415.2. No matter which option you choose, you've still got to bond ecps.

Exactly. Whilst it is not possible to effectively bond a lawn or wet floor (conductive to true earth potential) to a TN_C_S 'earth', it is possible to achieve local equipotential bonding with a local TT system. If the CPCs (hence exposed-conductive-parts) of the outbuilding are bonded to the outbuildings earth rod, that minimises the risk of an appreciable PDs existing between them.

Unless one is prepared (which some may well be, but the regs certainly aren't) to accept the extremely small risk of having an unbonded electrical installation.what we are talking about in the context of an outuilding (or, indeed, any 'outdoor electrics', such as Class I light fittings) is really no different from the situation within a property which has no main protective bonding - and I'm finding it hard to understand that you appear to perceive a difference.

Kind Regards, John.