Gas pipe earth???

[JohnW2]

One "special situation"... The possibility of the CPC derived from the Neutral in my cottage coming iinto contact with the CPC of the TT system in the ajacent shop was considered by the DNO engineers who installed my new supply as a serious hazard as high currents could flow in the CPCs as a result of neutral bounces.

As we discussed when you first mentioned this, I never really understood this concern, since any currents between the two sets of CPCs would be limited to what the shop's TT electrode could sink - i.e. not very much. As far as I can make out, it would only be an issue at all if the CPCs were joined by something much smaller in CSA than the CPCs in either installation (small enough for 5A or less to be a hazard) - and, if there were such a very thin connection, then that would presumably melt very quickly, essentially acting as a fuse.

A coming together of the two sets of CPCs simultaneously with a supply neutral fault (I would have thought an extremely improbable combination of events) would, of course, bring the same hazrads resulting from the neutral fault to the shop as existed in your cottage - which may have been a concern, but that's a different matter.

Kind Regards, John

 

[EFLImpudence]

Your 'No' is inappropriate, since that is precisely what I meant by a "true (and compliant) TT installation".

Fair enough but I was still considering your initial post referring to no connections between any of the electrical installation and pipework by immersions, boilers etc.

 

[EFLImpudence]

if it didn't need bonded but was would that be dangerous? This whole earth/bonding thing has me intrigued!

If a metal part is bonded unnecessarily - wrongly.

Firstly,
the fact that it does not need bonding means that it is not an "extraneous conductive part which is not part of the electrical installation" and therefore is totally isolated from any source of earth and electrical activity.

Also,
as it is not an "exposed conductive part of (the electrical) equipment which can be touched and which may become live under fault conditions" it therefore should not be earthed.


I think the metal bath supplied by plastic pipes is the best example of an isolated metal part but a door handle or spoon would do.

Imagine -
You are leaning on the bath (or spoon) with one hand you touch a faulty electrical item such as a worn cable with the other hand.

With isolated (correctly unbonded) bath - current will travel through you to the floor while you will be relatively poorly earthed through the longest resistance path of the body to whatever material of which the floor is made. All this resistance will limit the current. You still get a shock but limited.

With earthed (wrongly bonded) bath - same scenario but this time the current travels from one hand across the heart to the othehand and straight to earth via metal bath and metal wire with very little resistance allowing much greater current causing much greater shock and, most dangerously, across the heart.

 

[JohnW2]

Your 'No' is inappropriate, since that is precisely what I meant by a "true (and compliant) TT installation".

Fair enough but I was still considering your initial post referring to no connections between any of the electrical installation and pipework by immersions, boilers etc.

The "true (and compliant) TT installation" to which I was referring is one in which the only connection to earth is via the TT electrode, with no use of (let alone 'reliance upon') extraneous-c-ps for a connection to earth. In the absence of any extraneous-c-ps (as in my neighbouring villages), that is obviously satisfied (give or take the 'quibbles'!), even if there are (as there usually will be) incidental connections between the electrical installation and pipework.

Kind Regards, John

 

[JohnW2]

if it didn't need bonded but was would that be dangerous? This whole earth/bonding thing has me intrigued!

If a metal part is bonded unnecessarily - wrongly......

Whoops, I didn't read dylanlennon's post carefully enough - hence, as you and he will have seen, I answered the wrong question - about the danger of not bonding when one should bond. Apologies.

As for the actual question, I agree essentially with what EFLI has written. The only thing I would add is that the classic examples of metal baths with plastic plumbing, door knobs and spoons are very clear-cut, since there is no way that they can possibly be connected to true earth. Bonding is therefore not required and, as EFLI has said, unnecessarily connecting metal things to earth merely increases the chances that one will get a shock by touching that earthed metal at the same time as touching something live.

However, with things like the gas pipework/cannisters and the garage door/frame, one has to make a risk assessment/judgement - since they possibly could, under certain circuimstances, represent significant paths to earth (e.g. if a lot of wetness/water was around). One then has to make an assessment/judgment as to whether or not the risk of not bonding them when one probably should (e.g. when the pipe/gas cannister or door is swimming in water) is greater than the risk of bonded them when one probably doesn't need to (e.g. when everything is bone dry). Electricians ought to be experienced about making such a judgement.

Kind Regards, John

 

[EFLImpudence]

He asked for both ways.

 

[JohnW2]

He asked for both ways.

Ah well, he got my whole answer then, in two installments - sounds as if I need to open another bottle, to improve my attention!

Kind Regards, John

 

[dylanlennon]

JohnW2 & EFLImpudence thanks for your help & for putting things into laymans terms for me! I can see why this could be a tough decision wheather to bond or not. I was just talking to my brother online & he said he can do a test when he gets home to see, an insulation resistor test or something like that I think he said!

 

[dylanlennon]

This is probably another stupid question but...won't the steel have an earth at it already as the bit that the flex goes into for the electric garage door, sits on top of the steel frames, joined by nuts & bolts?

 

[EFLImpudence]

Quite possibly - so it may need bonding even if it were otherwise isolated.

 

[dylanlennon]

Quite possibly - so it may need bonding even if it were otherwise isolated.

I'm even more confused now.

 

[EFLImpudence]

I am probably being over-cautious thinking some bits may be connected to the earth and others may not.

Brother will test.

 

[JohnW2]

This is probably another stupid question but...won't the steel have an earth at it already as the bit that the flex goes into for the electric garage door, sits on top of the steel frames, joined by nuts & bolts?

I think you're probably getting confused by two different 'earths' we are talking about - (1) the 'earth' of your electrical installation (i.e. the 'earth conductors' in the wiring of your house and garage) and (2) 'true earth' - which in this context means the ground/soil on which the garage is built (and which the metal of the garage door frame may be in some degree of electrical continuity with, particularly under wet conditions).

For (1) to be connected to the metal door frame is fine (and doesn't create the need for bonding) - that's no different from any other metal appliance, tool, switch or socket in your house or garage. The potential danger which, if it exists, requires bonding [of (1) to (2)] arises if any of the garage metalwork (like the door frame) is connected to (2).

Is that clearer?

Kind Regards, John

 

[bernardgreen]

I think the metal bath supplied by plastic pipes is the best example of an isolated metal part but a door handle or spoon would do.

Provided the water in the pipes is also non conductive then the impedance of the current paths from bath to ground will be high enough to keep the shock current below a lethal amount. At least it should be in a dry, non humid bath room. I agree that earthing or bonding the bath will increase the shock current in the event that someone manages to touch a live conductor and the bath at the same time so that is one reason to keep the bath isolated.

But on the other hand if an electrical fault occurs that creates a path from Live to the bath then anyone touching the bath while touching something that is earthed or bonded will get a shock.

Which is more likely to occur in the bathroom, bearing in mind that not all bathrooms are the same.

Extend it further, metal electric kettles should not be earthed in case someone is holding the kettle and then touches an item that is Live. More likely to happen in a kitchen than a bathroom as portable appliances are not ( normally ) used in bathrooms. The concern about bathrooms maybe because there is more water there, no that can't be the reason because we are told that water is not conductive, well not if it is in a plastic pipe to a metal bath. ( have yet to see plastic insulating bath taps )

 

[bernardgreen]

I never really understood this concern, since any currents between the two sets of CPCs would be limited to what the shop's TT electrode could sink - i.e. not very much.

As long as it remains a TT supply there is no significant risk of a CPC acting as a fuse. Even so the idea of a "fuse" several feet long glowing white hot before melting under the floor boards is not one I can allow.

A coming together of the two sets of CPCs simultaneously

would be permanent via the common metallic water supply if the isolating plastic section had not been added. Even that presents a minor concern with possible electrolysis of water into gas.

Sadly the increase of metal theft from substations means the number of disconnected neutral faults is increasing. Unbalanced loading on the network also means neutrals are often a few volts above ground. If the shop, which is fed from a different street cable did convert to PME then the CPC to CPC link would become a link between two different network neutrals and would then carry some of the network load when the two network neutrals were at different potentials.