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

[HappyHippyDad]

We all know that if something is extraneous then it needs to be bonded. However, if we have some water pipes in a bathroom for example that test at 0.2Ω to earth (MET) would you be happy with that? Let's assume it has RCD protection. I realise some say that if the extraneous tests at <1667Ω then the RCD will trip before the potential rises to 50V, however I don't feel very comfortable with this as the RCD is for 'additional' protection.

GN3 states that if the resistance to earth of the extraneous is <0.05Ω then we can assume the extraneous is bonded. So, my question is 'would you feel it necessary to take a separate bond to the pipes in question in the above example as they are > 0.05Ω? If you are happy with this then what resistance would you feel is a maximum before you feel the extraneous needs bonding?

With Zs figures there are very clear limits. The only limit I can see in these sort of examples is 0.05Ω.

 

[telectrix]

the 0.05 value is between a bonding conductor and the pipe it's connected to. you'd expect a higher value from MET to other pipework due to resistance of bonding conductor and copper pipe.

 

[Pete999]

the 0.05 value is between a bonding conductor and the pipe it's connected to. you'd expect a higher value from MET to other pipework due to resistance of bonding conductor and copper pipe.

Hi Tel I'm a bit confused with your answer, not saying your wrong, wouldn't dare, but is the 0.05 ohm limit the reading from the end of the bonding conductor and the pipe near to where the bonding clip is? seem to recall Chris Kirchers description on his video says just that.https://www.youtube.com/watch?v=rnvNfI__kYU

 

[Hellmooth]

I think that's what Tel was saying?

 

[telectrix]

always confusion with this. all i can say is can ayone post the reg. ( in BS7671, not additional publications, which are , by definition, guides). that quotes this 0.05 value for a resistance from MET to a water or gas bond.

 

[Risteard]

As tel says the Guidance Note is merely pointing out that such a negligible reading suggests that the connection e.g. BS951 clamp is making an effective connection to the extraneous conductive part. There is no maximum reading for the bonding conductor, nor is the guidance (which does not nor has it ever) form part of the Wiring Regulations.

 

[Pete999]

I think that's what Tel was saying?

I took it as Tel was saying from the clip to the pipe, at least that's how it came over to me, if I got it wrong then the eggs on my face

 

[Deleted member 9648]

In a bathroom supplementary bonding is not required as long as disconnection times are met....all circuits within the bathroom are RCD protected, and extraneous conductive parts within the location are effectively connected to the main bonding. For the purpose of effectively connected to main bonding a continuity reading of <1667 ohms is considered satisfactory.
If these conditions are not met then supplementary bonding is required.
In a domestic supplementary bonding is only likely to be required in bath and shower rooms. Nowhere else.

Good enough for me.

 

[telectrix]

also, if the piework to MET reads > 22k on IR test, it can be assumed to be non-extraneous. just thought i'd chuck that in.

 

[HappyHippyDad]

In a bathroom supplementary bonding is not required as long as disconnection times are met....all circuits within the bathroom are RCD protected, and extraneous conductive parts within the location are effectively connected to the main bonding. For the purpose of effectively connected to main bonding a continuity reading of <1667 ohms is considered satisfactory.
If these conditions are not met then supplementary bonding is required.
In a domestic supplementary bonding is only likely to be required in bath and shower rooms. Nowhere else.

Good enough for me.

This is what is written and used by many I believe. I can't quite see how we use a figure of 200Ω for a TT, yet we are happy to use the 1667Ω in this case. They are both designed to make sure the RCD operates so that the potential is <50V but with a figure of 1667Ω there is little room for error, in fact none! Is this 1667Ω stable? At such a high value its doubtful that it has any thoughout or rather intended route to the MET so perhaps this resistance is purely due to its location to the general mass of earth... In which case its not stable.

I'd value your thoughts on this Wirepuller and also if you would be happy if you actually did come across a value of 1667Ω between the extraneous and the MET? (Am assuming RCD present)

 

[Deleted member 9648]

To be honest I've not been overly concerned with supplementary bonding in bathrooms since the introduction of RCD protection to all circuits within the location. Main bonding is far more important as far as I'm concerned. Most people just assume if RCD protection is up to scratch then SB can be ignored. Not the case and I do measure continuity, more often than not if continuity readings are >1667 ohms then it can be established that pipework is not extraneous anyway. Just because there is a copper pipe in a bathroom does not mean it is extraneous, often wetpants use plastic under floors and just rise up into the bathroom in copper.
Complying with the regulations is as far as I see the need to go.

 

[Midwest]

This is what is written and used by many I believe. I can't quite see how we use a figure of 200Ω for a TT, yet we are happy to use the 1667Ω in this case. They are both designed to make sure the RCD operates so that the potential is <50V but with a figure of 1667Ω there is little room for error, in fact none! Is this 1667Ω stable? At such a high value its doubtful that it has any thoughout or rather intended route to the MET so perhaps this resistance is purely due to its location to the general mass of earth... In which case its not stable.

This other vid from Chris Kitcher; https://www.youtube.com/watch?v=v38PfIVy2rI talks about the reasoning for those values. Its mentioned at 2.35. Dunno, if that answers your question?

 

[sam400]

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

 

[Richard Burns]

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

 

[i=p/u]

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

 

[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