Main bondage

[ultinator]

Does main bonding in every circumstance always need to be under 0.05. main bonding in a house where the main water was moved has 10mm cable but resistance of 0.25 ohms!!>>

 

[Silly Sausage]

The gas pipe is connected to the green wire,
The water pipe is connected to the other green wire,
The green wire is connected to the MET,
The MET is connected to the big green wire..............

 

[KAS1]

I think if we could replace all the wood in all the houses with copper and everything connected together problem solved, mind you how would we be able to sleeve everything..........such a shame wood has not got very low resistance........... then again if we were on the moon we'd have to moon everything lol

 

[Des 56]

I think if we could replace all the wood in all the houses with copper and everything connected together problem solved, mind you how would we be able to sleeve everything..........such a shame wood has not got very low resistance........... then again if we were on the moon we'd have to moon everything lol

And if we were on Titan,we would have to Titan the pipes

 

[Silly Sausage]

I think if we could replace all the wood in all the houses with copper and everything connected together problem solved, mind you how would we be able to sleeve everything..........such a shame wood has not got very low resistance........... then again if we were on the moon we'd have to moon everything lol

That's where the Yanks have got us beat, they Ground everything.

 

[Gavin A]

well thats not quite right is it ?
a minimum sized bond conductor is specified , but no upper resistance value needs to be complied with for that conductor if its the correct csa.

ok, but when testing the 10mm2 bonding conductor of unknown length in a domestic setting, it'd be sensible to use some sort of ball park figure for what might be expected to be a reasonable maximum figure in that setting - say 0.05 ohms or 27m of cable rather than just doing the test but saying ah but there's no figure specified in the regs and the cable could be infinitely long, so it doesn't matter what figure you get.

which presumably is where the OSG figure comes from.

 

[RmBo_0]

I think if we could replace all the wood in all the houses with copper and everything connected together problem solved, mind you how would we be able to sleeve everything..........such a shame wood has not got very low resistance........... then again if we were on the moon we'd have to moon everything lol

Faraday cages - homes of the future

 

[Gavin A]

Does main bonding in every circumstance always need to be under 0.05. main bonding in a house where the main water was moved has 10mm cable but resistance of 0.25 ohms!!>>

Sod it, I'm going to have a go at answering this.

The resistance reading for a 10mm2 main bonding cable in a domestic setting should hardly ever be above 0.05ohms, and only if there is good reason to think the cable might be longer than 27m should a reading higher than this be potentially deemed acceptable without needing further investigation.

If the resistance reading is greater than 0.05 ohms with no reason to suspect a cable length longer than 27m, then this would indicate that there is a problem with the cable that means it is unlikely to be 10mm2 for its entire length, and / or has a high resistance joint in the cable, and this should be investigated and rectified, or if that's not possible then a judgement would need to be made about whether the reading is sufficiently high to justify running a new unbroken 10mm2 cable to ensure compliance.

So only in a very limited number of cases should the resistance reading of the 10mm2 cable be higher than 0.05 ohms in a domestic setting, and only in a massive mansion would it be expected to get anywhere close to 0.25 ohms.

or rule of thumb - it should usually be lower than 0.05 ohms in a domestic setting, anything higher usually indicates a problem that should really be sorted out.

That's basically how I see it in the absence of any hard and fast rule from BS7671. Have we been arguing semantics here, or does anyone have a significantly different understanding of the situation?

 

[shanky887614]

in responce to question, why not 1mm2

if i remember right, doesnt the conducter have to be a minimum off half the incoming tail size (id have to check notes first, forgive me but havent done really any domestic and even that was small alterations)

e.g. 25mm tails

16mm earth.

 

[Guitarist]

Gavin, I think you've summed up what several of us have "summed up" already, but good job.

 

[telectrix]

in responce to question, why not 1mm2

if i remember right, doesnt the conducter have to be a minimum off half the incoming tail size (id have to check notes first, forgive me but havent done really any domestic and even that was small alterations)

e.g. 25mm tails

16mm earth.

bonding conductor needs to be min. of 50% of the size of the main earthing conductor, with a min. of 6mm., except with PME< then it's min. 10mm.

 

[Guest111]

OK can we get this thread back on track please? the title was main bondage but it seems to have drifted into earthing.

 

[ezzzekiel]

Ok , maybe the value is used to ensure all bonding has a resistance of similar value otherwise you could have a diffence in potential.

Checked all books today and as already mentioned it is only refered too as a value between extraneous parts

 

[Geoffsd]

Checked all books today and as already mentioned it is only refered too as a value between extraneous parts

Isn't it (0.05) used wherever a value of negligible resistance or negligible difference in resistance is required?

Testing between joined metal parts for continuity, difference between R1+R2 at various locations on a ring, and of course main bonding continuity between extraneous parts and MET.

It IS a value regarded as negligible.

 

[ezzzekiel]

Besides which its unlikely the bond would be 27m anyway so would expect less than 0.05 anyway

 

[Silly Sausage]

I reckon this magical 0.05 figure comes from taking account of instrument error from trying to get a reading from the arse end of its scale.