Whilst we're on the subject of bonding...

[sam400]

If you have an RCD in place then the fault current only needs to be 30mA before disconnection takes place, at this current the 1667Ω resistance will only generate a 50V touch voltage which is the deemed safe level.

V= I * R = 0.03A * 1666.6666Ω = 50V

Thanks for the reply, yea that's a transposition of the R>50/Ia formula, but how does an RCD supplied live conductor come into contact with the extraneous conductive part that we are testing lets say a pipe?

I don't see how this relates to touch voltages, I thought that you would want any extraneous conductive parts to rise to the same potential as an exposed conductive part during single fault conditions so that a dangerous potential doesn't exist between the two - until ADS removed the fault.

 

[sparksburnout]

Ok here's another scenario, a property has 2 incoming water pipes. One is the usual one under the sink and the other is in what once was a garage but is now a utility room with a washing machine and sink. Before the garage was converted it had its own supply, there is not a pipe under the (garage) utility room floor. I guess it branches in the garden somewhere. The "second" pipe just feeds the washer and sink, there are no sockets near either, washer is on an FCU. Main pipe is bonded along with gas pipe as per usual. Reading from MET to second pipe is 0.02 ohms. I am assuming that is down pipe to branch u/g then back via bonding on first pipe. Should second pipe be bonded to MET?

 

[Richard Burns]

Thanks for the reply, yea that's a transposition of the R>50/Ia formula, but how does an RCD supplied live conductor come into contact with the extraneous conductive part that we are testing lets say a pipe?

I don't see how this relates to touch voltages, I thought that you would want any extraneous conductive parts to rise to the same potential as an exposed conductive part during single fault conditions so that a dangerous potential doesn't exist between the two - until ADS removed the fault.

I think they are going for the worst case scenario where the fault current does flow through the supplementary bonding.
Unlikely in the extreme. It would also mean that the local resistance would not be too high to prevent disconnection if one device fails.
I did write out a long answer but got too rambling!

Ok here's another scenario, a property has 2 incoming water pipes. One is the usual one under the sink and the other is in what once was a garage but is now a utility room with a washing machine and sink. Before the garage was converted it had its own supply, there is not a pipe under the (garage) utility room floor. I guess it branches in the garden somewhere. The "second" pipe just feeds the washer and sink, there are no sockets near either, washer is on an FCU. Main pipe is bonded along with gas pipe as per usual. Reading from MET to second pipe is 0.02 ohms. I am assuming that is down pipe to branch u/g then back via bonding on first pipe. Should second pipe be bonded to MET?

I think the assumption is not permitted and that according to the regulations the water pipe in the garage should be bonded.
If it is using the underground metal supply pipes to provide the low resistance then these might be changed to plastic and the pipe in the garage would then be providing its own earth reference and need bonding. If it was using the internal supply pipes then continuity might be assured but what if something changes?
Admittedly the most likely thing to change would be the removal of the bonding conductor by unknowing persons and so the point becomes moot.

 

[DPG]

main bonding/supplementary is useless nowadays as all piped in plastic and rcds on scene.....

Always? Daz

 

[sam400]

I think they are going for the worst case scenario where the fault current does flow through the supplementary bonding.
Unlikely in the extreme. It would also mean that the local resistance would not be too high to prevent disconnection if one device fails.
I did write out a long answer but got too rambling!

There might not be any supplementary bonding? Sorry still don't understand it?

cheers

 

[i=p/u]

Always? Daz

well anything i seem todo, mostly houses these days forn moi daz

 

[i=p/u]

#rcdschangeeverythingandanything.com

 

[bigspark17]

On installs with plastic pipe always run 10mm and ask plumber to install small copper pipe after stop tap and earth to met. Main bonding connected to MET check!!

 

[Midwest]

I have to be honest I don't really understand where 415.2.2 is coming from,
I thought the purpose of supplementary bonding was to keep touch voltages to a minimum, having a resistance of 1667 between an extraneous conductive part and exposed conductive during single fault conditions will surely give rise to a touch voltage approaching the supply voltage?
can someone enlighten me?

thanks

sam

Did you have a look at the vid at#12? The figure 1667 is obtained when the protective device is a 30ma RCD, this figure could be as low as 1.6, if it was B type 6a MCB.

Have a butchers at John Wards vid, it may answer your question, I think; https://www.youtube.com/watch?v=CVxBettQpPU

 

[Midwest]

Sorry, just realised that was a 'Have' and been had!

 

[sam400]

Did you have a look at the vid at#12? The figure 1667 is obtained when the protective device is a 30ma RCD, this figure could be as low as 1.6, if it was B type 6a MCB.

Have a butchers at John Wards vid, it may answer your question, I think; https://www.youtube.com/watch?v=CVxBettQpPU

Thanks for the reply Midwest, I have seen these videos before unfortunately neither of them explain the relationship between the disconnection of a protective device and the resistance between an exposed conductive part and an extraneous conductive part,
Unless Iam missing the obvious! (Quite possible)

The 1.6 ohms comes from R = 50v/30a = 1.667ohms I guess?
How would a live conductor supplied by this 6a MCB come into contact with the Extraneous conductive part (that we have measured) in order to cause this voltage limited disconnection?

 

[Midwest]

Thanks for the reply Midwest, I have seen these videos before unfortunately neither of them explain the relationship between the disconnection of a protective device and the resistance between an exposed conductive part and an extraneous conductive part,
Unless Iam missing the obvious! (Quite possible)

The 1.6 ohms comes from R = 50v/30a = 1.667ohms I guess?
How would a live conductor supplied by this 6a MCB come into contact with the Extraneous conductive part (that we have measured) in order to cause this voltage limited disconnection?

Can't do multi quote, so in answer to your questions.....I think;

1) Bonding, or protective bonding, is an electrical connection maintaining various exposed conductive parts and extraneous conductive parts at substantially the same potential (cited by NIC Snags & Solutions earthing & bonding). If they are not, it could result in electric shock, in the case of a potential fault in an unearthed electrical appliance for example.
2) (1st part) Correct, Ia for overcurrent devices, the corresponding to automatic operation in 5 s (415.2.2). (2nd part) ?? Faulty unearthed appliance connected to water supply (i.e. electric shower), damaged cable placed across metal pipework.

I have a feeling, I've misunderstood your questions?

 

[sam400]

Can't do multi quote, so in answer to your questions.....I think;

1) Bonding, or protective bonding, is an electrical connection maintaining various exposed conductive parts and extraneous conductive parts at substantially the same potential (cited by NIC Snags & Solutions earthing & bonding). If they are not, it could result in electric shock, in the case of a potential fault in an unearthed electrical appliance for example.
2) (1st part) Correct, Ia for overcurrent devices, the corresponding to automatic operation in 5 s (415.2.2). (2nd part) ?? Faulty unearthed appliance connected to water supply (i.e. electric shower), damaged cable placed across metal pipework.

I have a feeling, I've misunderstood your questions?

1) Would you say 1667ohm between extraneous conductive and exposed conductive is at 'substantially the same potential'?

2) So we are testing to see if supplementary bonding is required in case a piece of pipe work becomes live?
historically (16th) supplementary bonding was fitted for the same reason as main protective bonding to reduce the potential between exposed conductive parts and extraneous conductive parts and not to provide a fault path for the disconnection of the protective device?

cheers

sam

 

[Deleted member 9648]

On installs with plastic pipe always run 10mm and ask plumber to install small copper pipe after stop tap and earth to met. Main bonding connected to MET check!!

I do hope you are joking?

 

[bigspark17]

I do hope you are joking?

Absolutely not.. Diferent ph levels and salts in water can make it more or less conductive according to levels.. What is stopping the water board putting conductive pipe joint underground and then this becoming extraneous conductive.. Small copper pipe after stop tap earth to MET no PD between earths in installation, tick jobs a good one!