Bonding Help

[Hamez]

Would I be right in thinking that you just test the resistance between the pipe work and main earth terminal? Then check to see if the value is above the figure given in the regs?


Thanks

 

[telectrix]

you diss all bonds from MET, then using a wander lead, test on IR range (500V) between MET and the suspect pipework. if the reading is above 22k ohms ( 0.02M), then the pipework is not extraneous and does not require bonding.

 

[Midwest]

you diss all bonds from MET, then using a wander lead, test on IR range (500V) between MET and the suspect pipework. if the reading is above 22k ohms ( 0.02M), then the pipework is not extraneous and does not require bonding.

Hate to disagree with you, but I was told here recently, that IR testing was ill-advised method for checking extraneous, in that the scale is meg ohms and a meter trying to read a value from 0.01 - 0.02 meg ohms is pretty imprecise?

PS, why do we have to disconnect all bonds for test?

 

[telectrix]

to eliminate parallel paths.

 

[Midwest]

to eliminate parallel paths.

Wot about me first question?

And why worry about parallel paths, if you trying to establish if something is extraneous or not? And I am genuinely trying to learn. I'm thinking pipework in bathrooms for example.

 

[Richard Burns]

From my point of view testing for bonding is attempting to see if the conductive part is extraneous, i.e. is it conductively connected to true earth and therefore can introduce an earth potential into the installations equipotential zone.

If you test from the conductive part to the installation MET all you are checking if the conductive part is connected to the installation earthing.
Now the installation earthing should be (I hope) connected to true earth so it would also check this but it would also catch all interconnections on the installation.
Testing to the disconnected earthing conductor should test for true earth and also catch any interconnections to other installations on the same supply. However this is just my interpretation.

Testing with an insulation resistance tester is merely a convenient method of testing a high resistance, in general the actual accuracy of the reading will not be critical as it will either be low (0- hundreds of Ω) or practically infinite over hundreds of MΩ.
It would only be where the value was on the limit that it becomes relevant.
You could also test the resistance with an autoranging multimeter that would give a more precise figure but the accuracy would be similarly out and it would not test resistance at the voltages that may be expected in the case of a fault.

 

[Midwest]

From my point of view testing for bonding is attempting to see if the conductive part is extraneous, i.e. is it conductively connected to true earth and therefore can introduce an earth potential into the installations equipotential zone.

If you test from the conductive part to the installation MET all you are checking if the conductive part is connected to the installation earthing.
Now the installation earthing should be (I hope) connected to true earth so it would also check this but it would also catch all interconnections on the installation.
Testing to the disconnected earthing conductor should test for true earth and also catch any interconnections to other installations on the same supply. However this is just my interpretation.

Testing with an insulation resistance tester is merely a convenient method of testing a high resistance, in general the actual accuracy of the reading will not be critical as it will either be low (0- hundreds of Ω) or practically infinite over hundreds of MΩ.
It would only be where the value was on the limit that it becomes relevant.
You could also test the resistance with an autoranging multimeter that would give a more precise figure but the accuracy would be similarly out and it would not test resistance at the voltages that may be expected in the case of a fault.

I truly told myself, not to reply to anymore posts way past MY bedtime, but sorry Richard, you've lost me there. I'll have another look in the morning.

 

[ian.settle1]

There is no requirement to bond the short section of visible copper if the rest of the house has been piped in plastic. However it is good practice to provide a bonding connection anyway for future use ie. if the pipes do ever get changed for copper.

In the immortal words of John McEnroe

"You cannot be serious?"

 

[Midwest]

From my point of view testing for bonding is attempting to see if the conductive part is extraneous, i.e. is it conductively connected to true earth and therefore can introduce an earth potential into the installations equipotential zone.

If you test from the conductive part to the installation MET all you are checking if the conductive part is connected to the installation earthing.
Now the installation earthing should be (I hope) connected to true earth so it would also check this but it would also catch all interconnections on the installation.
Testing to the disconnected earthing conductor should test for true earth and also catch any interconnections to other installations on the same supply. However this is just my interpretation.

Testing with an insulation resistance tester is merely a convenient method of testing a high resistance, in general the actual accuracy of the reading will not be critical as it will either be low (0- hundreds of Ω) or practically infinite over hundreds of MΩ.
It would only be where the value was on the limit that it becomes relevant.
You could also test the resistance with an autoranging multimeter that would give a more precise figure but the accuracy would be similarly out and it would not test resistance at the voltages that may be expected in the case of a fault.

I had an instance recently, where I discovered some metal pipework in a bathroom may have been regarded as extraneous, and would of required bonding;

http://www.electriciansforums.co.uk...-conductive-parts-bathroom-2.html#post1185200

However, when the copper cold service (plastic incoming) and copper hot service were bonded and I re-tested between the bathroom lighting cpc and pipes, the readings revealed the said pipe work did not require bonding.

Am I confusing extraneous pipework in bathrooms and another test to see if incoming extraneous metal work into a property requires bonding, i.e. the former, test between known earth and pipe and latter, test between disconnected earthing conductor and pipe?

 

[Midwest]

I've been researching the internet, and the majority of methods do NOT seem to suggest having to disconnect the EC to carry out test for extraneous parts. The only test where there is concern for parallel paths, is regards confirming continuity of bonding conductors.

All other tests refer to testing extraneous parts with a 'known earth', i.e. MET or local cpc. As regards incoming metal service pipes, I see that no test is suggested, as BS 7671 requires these to be bonded, come what may.

 

[Richard Burns]

As I say this just my view of things.

For an item that is not entrenched into the installation such as your 300mm section of copper pipe in a bathroom then testing against a known earth should demonstrate whether or not it is extraneous.
Admittedly I am not sure how you got those readings there must be some sort of other connection in place somewhere, especially as once you had provided limited supplementary bonding the readings changed.

Where there is a main service, say, entering in plastic, changing to metal then potentially going underground again and then around the house; to test against a known earth in the house would be likely to give an unreliable reading due to cross connections of services and cpcs.
To test against a known earth that was not part of the installation should give a valid reading of connection to earth.

Since all earthed points in the installation should be connected together at the MET (and other points around the installation) then removing the main earth to test against (with the installation off) would readily provide an earth that was not connected to the installation. An earth rod outside the property would also provide an earth reference but would be less likely to be available.

It may well be that the supply earth is connected to the next house which has main bonding to the water supply and so this would give you another incorrect reading but only if the service between the houses was conductive in which case you would need to bond anyway.

Generally I think the published guidance will work for most cases, however in a few cases I think it will give a false impression of zero potential, you could then test for this false reading against an earth reference outside of the installation.

 

[Midwest]

As I say this just my view of things.

For an item that is not entrenched into the installation such as your 300mm section of copper pipe in a bathroom then testing against a known earth should demonstrate whether or not it is extraneous.
Admittedly I am not sure how you got those readings there must be some sort of other connection in place somewhere, especially as once you had provided limited supplementary bonding the readings changed.

​Yes those readings are curious, I suspect push fit plastic under the floor in the bathroom, but can't understand how connection to the cold water service did not improve the readings, but cross bonding to the hot water service did. The owner is going to refurbish the bathroom, so I might find out then!

Where there is a main service, say, entering in plastic, changing to metal then potentially going underground again and then around the house; to test against a known earth in the house would be likely to give an unreliable reading due to cross connections of services and cpcs.
To test against a known earth that was not part of the installation should give a valid reading of connection to earth.

But then we are back to the question of deciding whether the pipe is extraneous or not, such like in the bathroom scenario above?

Since all earthed points in the installation should be connected together at the MET (and other points around the installation) then removing the main earth to test against (with the installation off) would readily provide an earth that was not connected to the installation. An earth rod outside the property would also provide an earth reference but would be less likely to be available.

It may well be that the supply earth is connected to the next house which has main bonding to the water supply and so this would give you another incorrect reading but only if the service between the houses was conductive in which case you would need to bond anyway.

I can see your point there.

Generally I think the published guidance will work for most cases, however in a few cases I think it will give a false impression of zero potential, you could then test for this false reading against an earth reference outside of the installation.

Thanks Richard for your advice.

 

[Deleted member 26818]

Generally an installation where the incoming services are in plastic, will not require bonding.
Having a short length of copper pipe inside the dwelling would be no different to having a metal filing cabinet.
It's not extraneous and is unlikely to introduce a difference in potential.

 

[Midwest]

Generally an installation where the incoming services are in plastic, will not require bonding.
Having a short length of copper pipe inside the dwelling would be no different to having a metal filing cabinet.
It's not extraneous and is unlikely to introduce a difference in potential.

Don't think anyone has suggested that to the OP, might be wrong.