Supplementary bonding

[hermetic]

Extraneous actually means, "of external origin" and having just checked in the BRB 411.3.1.2 is quite clear on this.

"In each installation main protective bonding conductors complying with chapter 54 shall connect to the main earthing terminal extraneous-conductive-parts including the following;
1) Water installation pipes
2)gas installation pipes
3)other installation pipework and ducting
4) Central heating and air conditioning systems
5)exposed metallic structural parts of the building.

 

[hermetic]

"The whole idea of bonding is to reduce the possibility of a PD between conductive parts and extraneous conductive parts."



Exactly, that is why you bond the conductive parts to the extraneous conductive parts and connect all to the MET! then there is no possibility whatever of a PD arising.

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[Geoffsd]

"The whole idea of bonding is to reduce the possibility of a PD between conductive parts and extraneous conductive parts."

No, it's between exposed conductive parts and extraneous conductive parts.

Other isolated conductive parts - door handle, spoon, even an isolated metal bath - should NOT be connected to the bonding.

Exactly, that is why you bond the conductive parts to the extraneous conductive parts and connect all to the MET! then there is no possibility whatever of a PD arising.

There is no danger of a PD between an extraneous or exposed conductive part and ​an isolated conductive part (door handle, spoon or metal bath).

If these items are mistakenly earthed then there is a danger.

The extraneous and exposed parts are only bonded because they are already earthed.
It is the lesser of two evils.
It would be better if all metal parts could be isolated but they can't be

 

[hermetic]

If these items are mistakenly earthed then there is a danger.

Explain this "danger"
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[D Skelton]

Once within the location metallic pipes are no longer likely to introduce an external potential

No, you're quite right, it won't be an external potential ie. 0V, but high resistance joints will introduce a potential difference none the less, the degree of which will be governed by the amount of resistance in the PVC joint.

The key part of this is the term EXTRANEOUS....there is a tendency to believe that all metallic pipes are extraneous,in general the only extraneous pipes are those coming into the location from outside the location.

And the taps, although not effectively in this situation, are still connected to earth, therefore they are extraneous conductive parts.

 

[Geoffsd]

Well, to the extreme -

Hover above metal bath holding a live conductor - no effect (bird on a wire)

Descend into isolated bath (no connection to earth) - still no effect.

Descend into earthed bath (connected to MET) - 230V through body, hand to feet.

 

[HappyHippyDad]

If these items are mistakenly earthed then there is a danger.

Explain this "danger"
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I think I read somewhere its called 'electric shock' :smiley2:

Couldn't resist, sorry!!

 

[HappyHippyDad]

I dont entirely agree....extraneous means 'liable to introduce a potential,usually earth'. Supplementary bonding is only required in special locations as defined in Bs7671. If metallic pipes coming in to a special location are supplementary bonded at the point of entry,the hazard has been dealt with. Once within the location metallic pipes are no longer likely to introduce an external potential,therefore there is no requirement for effective continuity for pipework within the location.
The key part of this is the term EXTRANEOUS....there is a tendency to believe that all metallic pipes are extraneous,in general the only extraneous pipes are those coming into the location from outside the location.

So if the sink has copper pipes leading away from it but then joined to plastic pipework whilst still in the special location (and that plastic pipework continues until out of the special location) then the copper pipes in the special location are not classed as extraneous conductive parts?

 

[Geoffsd]

So if the sink has copper pipes leading away from it but then joined to plastic pipework whilst still in the special location (and that plastic pipework continues until out of the special location) then the copper pipes in the special location are not classed as extraneous conductive parts?

Correct.

Pies could be metal further than that - as long as they have no connection to earth, through other pipes or boiler cpc etc.

 

[D Skelton]

Not neccessarily, the amount of plastic separating the copper pipes will determine whether or not it is an extraneous conductive part. if there is a resistance of less than 23000 ohms between them then they need to be bonded.

 

[hermetic]

I am not going to labour this point, when we get into hovering over a bath holding a live conductor we have descended into unreality. 411.3.1.2 is quite clear and unequivocal
""In each installation main protective bonding conductors complying with chapter 54 shall connect to the main earthing terminal extraneous-conductive-parts including the following;
1) Water installation pipes
2)gas installation pipes
3)other installation pipework and ducting
4) Central heating and air conditioning systems
5)exposed metallic structural parts of the building.


We do not live in houses where it is possible to create a double insulated world where "all metalwork should be isolated" and neither is this situation desirable. In the real world our beloved 17th BRB actually applies to a tiny minority of installations, in the vast majority ( erected under former control) all metal pipework, baths, sinks and structures are bonded and earthed, and the fact that electrically we probably live in the safest country in the world with the highest electrical standards is a reflection of this, and not of opposing opinions of what should and should not be earthed. When I asked for an example of this danger, I expected a real world answer. I am disappointed, but not surprised.

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[oldtimer]

I think you need to look at this with regards to history when metal pipes were used in both internal and external you had what was termed Electrically & Mechanically sound in other words if you earth bonded the incoming pipe weather water or gas the whole pipework installation was earthed and even sometimes you got your earth from the pipework.

So what happened to change this well water and gas supply pipes were being replaced with plastic fine but as this moved on they started to use plastic pipe and fittings within the property then the 15th edition came along and as usall threw total confusion into the mix as there was no clear guidance so they earthed everything and supplementary crossbonding was the norm now at the time people like myself a 14th edition spark was saying this is stupid as the installation is Electrically & Mechanically sound all we should be doing is is ensuring that it is earthed and inform the householder that if they get any plumbing work and they want to fit plastic pipe or fitting then get their earthing checked.

But nope plastic pipe fitting became the norm and then we moved on to RCD control where the regulations said no SB required and again
we are left with a delema because now we can have a spark put SB in then a plumber can come along and fit plastic fitting rendering the job as US then there is another problem I was on a job where the tenant got a tingle from the sink in the utility room from the gas boiler he said well no he got it from the metal sink that had plastic pipe supplies but someone had earth bonded it to the washing machine socket that lost its earth and the washing machine was faulty.

Thing is where do you stop you cannot make the whole house plastic or metal so me thinks we will be discussing this same time next year

 

[Geoffsd]

I am not going to labour this point, when we get into hovering over a bath holding a live conductor we have descended into unreality. 411.3.1.2 is quite clear and unequivocal
""In each installation main protective bonding conductors complying with chapter 54 shall connect to the main earthing terminal extraneous-conductive-parts including the following;
1) Water installation pipes
2)gas installation pipes
3)other installation pipework and ducting
4) Central heating and air conditioning systems
5)exposed metallic structural parts of the building.

The deciding word in the above list is extraneous. If the listed parts are not extraneous bonding is not required and should not be fitted.



It is disappointing that you did not understand my extreme example, so -

turn it around.

You are standing naked and wet in a metal bath and inadvertently touch a live (faulted exposed) part.

Would you rather the bath was isolated (no connection to earth) or wrongly 'bonded' i.e. earthed.

If the bath is extraneous due to metal pipes then we have no choice.

 

[D Skelton]

I think the point of the OP was to reconfirm that supp bonding IS required unless certain requirements are met. Even with an RCD, supp bonding will still be required if you cannot confirm an effective connection between all extraneous conductive parts in a location.

Essentially, a property with all metal pipework and with RCD protection should never need supp bonding, but as soon as the plumbers come in and start shoving PVC fittings all over the joint then you would really need to reconsider the need for it.

 

[Geoffsd]

We do not live in houses where it is possible to create a double insulated world where "all metalwork should be isolated" and neither is this situation desirable.

It would be if it were possible.

In the real world our beloved 17th BRB actually applies to a tiny minority of installations, in the vast majority ( erected under former control) all metal pipework, baths, sinks and structures are bonded and earthed,

Only parts that are themselves earthed need bonding.

and the fact that electrically we probably live in the safest country in the world with the highest electrical standards is a reflection of this, and not of opposing opinions of what should and should not be earthed.

It is not opinion. It is electrical fact.

When I asked for an example of this danger, I expected a real world answer.

Well, now I have given you a 'real world answer'.


You asked why wrongly 'bonding' a part introduced a danger and I have explained.

I am disappointed, but not surprised.

Not as disappointed as I am in your inability to understand.

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