Main earth bonding

[Cabbyfan]

We had some electrical contractors doing doing some work in our building today so I went and asked them their opinion. They said on all the domestic jobs they do they take a 10mm min main earth bond to both gas and electric if the internal pipework it metal even if the incomer is plastic.

They also said they would also take an earth back to the earth block from the boiler pipework, 6mm minimum.

Good enough for me.

 

[JohnW2]

We had some electrical contractors doing doing some work in our building today so I went and asked them their opinion. They said on all the domestic jobs they do they take a 10mm min main earth bond to both gas and electric if the internal pipework it metal even if the incomer is plastic. They also said they would also take an earth back to the earth block from the boiler pipework, 6mm minimum. Good enough for me.

Maybe good enough for you, but wrong (well, certainly not required by regulations).

If it were me, I would be inclined to ask them what (if they have a clue!) is their understanding of the purpose of Main (Protective or Equipotential) Bonding!

Kind Regards, John

 

[EFLImpudence]

Ask them if they charge for this unnecessary work.

 

[EFLImpudence]

We had some electrical contractors doing doing some work in our building today so I went and asked them their opinion.

Yes, they will be more acceptable if they say what you want to hear.

They said on all the domestic jobs they do they take a 10mm min main earth bond to both gas and electric if the internal pipework it metal even if the incomer is plastic.

Not necessary.

They also said they would also take an earth back to the earth block from the boiler pipework, 6mm minimum.

Pointless.

Good enough for me.

Whatever.

 

[westie101]

So if any of you came to my house would you be able to tell if any of the copper water or gas pipes were in contact with earth via the structure of the house and the routing of the pipes?

Would you be prepared to accept the responsibility (prosecution or being sued) of not bonding in those circumstances, bearing in mind that in the event of a neutral fault there is a risk of differing potential between metal work that is connected to the supply earth & the metal connected to the gas & water systems which gives an increased risk of death or injury?

Or would you as with most answers on here actually play safe & bond the gas & water pipework?

TNCS supply, plastic water & gas services.

I certainly would seriously contemplate disconnecting the supply to any property found without bonding

 

[JohnW2]

So if any of you came to my house would you be able to tell if any of the copper water or gas pipes were in contact with earth via the structure of the house and the routing of the pipes?

One could usually get a jolly good idea by inspection. If one wanted to be ultra-certain, one could (after de-energising the installation) disconnect the earth connection to the MET and then measure resistance from pipework to earth.

There's nothing magic about water and gas pipework. If one had concerns such as you have voiced (that exposed conductive materials might have a connection to true earth through the structure of the building, then one would have to bond (or measure to exclude the need by measurement) almost every bit of 'fixed' or semi-fixed metal in the building.

Would you be prepared to accept the responsibility (prosecution or being sued) of not bonding in those circumstances, bearing in mind that in the event of a neutral fault there is a risk of differing potential between metal work that is connected to the supply earth & the metal connected to the gas & water systems which gives an increased risk of death or injury?

Yes, provided I had satisfied myself that bonding was not required.

TNCS supply, plastic water & gas services. ... I certainly would seriously contemplate disconnecting the supply to any property found without bonding

Hmmmm. As above, why stop with water and gas pipes?? - what about the metal bath, the metal window frames, the metal sink, work surfaces and splashbacks in the kitchen, the metal gas cooker, the metal rods holding the shower head etc. etc. - would you also contemplate disconnecting the supply if they were not bonded?

Kind Regards, John

 

[EFLImpudence]

They said on all the domestic jobs they do they take a 10mm min main earth bond to both gas and electric if the internal pipework it metal even if the incomer is plastic.
They also said they would also take an earth back to the earth block from the boiler pipework, 6mm minimum.

No mention was made of being unsure of the installation method; it is a blanket statement.

Are there other regulations you feel are incorrect and to be ignored.

Is it your involvement with DNOs that makes you used to over charging?

 

[bernardgreen]

what about the metal bath, the metal window frames, the metal sink, work surfaces and splashbacks in the kitchen, the metal gas cooker, the metal rods holding the shower head etc. etc. - would you also contemplate disconnecting the supply if they were not bonded?

[1] Modern house with dry walls above a damp proof course

[2] Old stone building with no damp proof course

[3] Cottage on hillside with earth retained by wall of the building.

[2] and [3] can have significant conductive paths through the walls to metal objects inside the equipotential zone. So much so that TT may be the only safe type of supply if one accepts the possibility of the network Neutral going open circuit.

 

[RF Lighting]

(Which it won't)

 

[westie101]

It's an interesting situation that comes down to the question "Just how sure are you"?

How about the school in Lancashire where, last year, following a neutral fault on a TNCS supply pupils and staff were getting shocks from metal door handles.
I'm sure whoever wired that and followed the book thought that could not happen!

Is it your involvement with DNOs that makes you used to over charging?

Hmm you listen to too many folk
1/ Any charges we do or did make are strictly regulated, are yours?
2/ The charge per hour in most DNOs for a 2 man team is less than most electricians make for a single person
3/ For both regulated and non regulated work we openly encourage customers to do as much of the work as is possible. Do you?

Some years ago I spent a lot of time quoting customers for work on a T&M basis for service alterations (no profit is or was allowed), most were surprised how little the charge was to be after "horror" stories they had been given by so called friends or at work or the pub!

So go on what is your hourly rate to charge out?
Mine as an Engineer is around £65, a jointing team is just below £60

 

[bernardgreen]

(Which it won't)

Is that saying network Neutrals never go open circuit ?

 

[JohnW2]

It's an interesting situation that comes down to the question "Just how sure are you"?

Well, if you measure, you can be totally sure at the the time of measurement - but I accept that things may change.

However, as I said, there is nothing magic about gas and water pipes (when incomers are plastic). On the contrary, such pipes are usually supported by plastic clips designed so that the screw going into the wall or whatever does not come into contact with the pipe, so are probably amongst the least likely of bits of internal metal to develop a path to earth, even if the walls are sodden. Other internal metalwork is more likely to acquire a path to earth under such circumstances. As you go on to say ...

How about the school in Lancashire where, last year, following a neutral fault on a TNCS supply pupils and staff were getting shocks from metal door handles.

... are you seriously suggesting that all metal within a building should be bonded?

As you (and bernard) imply, the potential problem is TN-C-S, even if the supply neutral faults that everyone gets so excited about are, I imagine, incredibly rare. If the 'M' of PME was much greater, things would not be so bad.

Kind Regards, John

 

[westie101]

are you seriously suggesting that all metal within a building should be bonded?

No just illustrating that it isn't as easy as just reading a book and thinking that by following it all will be safe.

As I've found in the real world, when a fault occurs the results are not predictable.
Neutral faults where everything is bonded so in theory a Faraday cage is in operation in the house.
Well not in every case, the odd on sees folk stood on a tiled kitchen floor getting shocks from "earthed" metalwork

 

[bernardgreen]

The network Neutral doesn't have to go open circuit to create a problem. A large unbalance across the phases causing a high current in the Neutral can be problematic. Unlikely that the voltage on the Neutral / CPC will be come dangerously high but the neutral current going via bonds and pipes to ground could be a hazard in it is too high for the bonds and /of pipes to carry without getting hot enough to start a fire.

 

[JohnW2]

what about the metal bath, the metal window frames, the metal sink, work surfaces and splashbacks in the kitchen, the metal gas cooker, the metal rods holding the shower head etc. etc. - would you also contemplate disconnecting the supply if they were not bonded?

[1] Modern house with dry walls above a damp proof course
[2] Old stone building with no damp proof course
[3] Cottage on hillside with earth retained by wall of the building.
[2] and [3] can have significant conductive paths through the walls to metal objects inside the equipotential zone.

How significant? Have you ever actually measured a resistance from a damp wall to earth which was sufficiently low to represent a direct threat to life, even if someone managed to touch it at the same time as an exposed-c-p connected to the installations CPCs (whilst there was a neutral fault on a TN-C-S supply)?

So much so that TT may be the only safe type of supply if one accepts the possibility of the network Neutral going open circuit.

As you imply, TN-C-S is what gets people worried - even though the 'lost neutral' is very rare, and the chances of a person touching the right/wrong two things at the time there was a neutral fault a lot rarer. Unless you're going to contemplate extraordinarily improbable situations (multiple simultaneous faults), TN-S is probably not appreciably worse than TT in terms of the 'risks' you are talking about.

Even with TT, under the worst sort of scenarios you are contemplating, one could get appreciable pds between different things (even different parts of the same wall) within a building with sodden walls/floors. A full-blown L-E fault will result in the TT 'earth' rising to almost line potential. Unless/until an RCD clears the fault, you're therefore likely to have something approaching 230V between any exposed-c-ps and anything attached to your wet walls.

Kind Regards, John