continuity

[scott 123]

Cables in con unit suggest bonding to gas and water.but I can't find connection..is there a way i can I test for it

 

[Richard Burns]

Would you ever use the 0.05Ω suggestion Richard in GN3 to confirm a bonding connection if the bonding clamps are unable to be found?

GN3 only suggests that the resistance of a main bonding conductor should be in the order of 0.05Ω, however this is patently incorrect as I have measured bonding conductors reading 0.00Ω on my meter!
If you say that 0.00Ω is in the order of 0.05Ω then so is 0.10Ω so what is the point in them saying 0.05Ω.
In a domestic property it is likely that the resistance of a bonding conductor is <<0.05Ω because you cannot get the physical length required, in an industrial setting then it is likely that the sheer mass of parallel paths will reduce the resistance measurement to similarly <0.05Ω, in a commercial setting you may get bonding long enough to exceed 0.05Ω without parallel paths but it would still be firmly connected to the MET with a low resistance path.
When measuring the resistance of the bonding conductor you are really checking it is connected not that it has a specific resistance, if you get 0.3Ω then there is probably something wrong.
As said in other posts GN3 also says that the bonding clamp to extraneous conductive part resistance should be of the order of 0.05Ω; so which is it? the total resistance or just the joint, you cannot have both!
I am veering away from BS7671 and just putting my opinion here but it does seem sensible to me.

 

[stevethesparks]

Not with surety.
All you can say is that the connections are not available for testing (however they should be available for testing, but are often boxed in)
Even if you test continuity from the MET to the nearest point of entry of the metallic service you will very likely get a low reading as there will be interconnections from boilers, etc that will give a low reading so it cannot be proved there is bonding present without being able to test the end to end continuity of the bonding cable.
However if you test continuity and you get a high reading then you can almost certainly say there is no bonding or it not connected.

Whats wrong with powering down and disconnecting the bonding conductors from the MET, then measuring to the gas / water pipe where accessible?

 

[Leesparkykent]

Whats wrong with powering down and disconnecting the bonding conductors from the MET, then measuring to the gas / water pipe where accessible?

it would be unlikely that they were the only path. Supplementary bonding, immersion heater, boiler connection, etc

 

[Richard Burns]

If you do that with an unidentified conductor that you do not know where it is connected you may be making a mistake, in all likelihood you are testing a 10mm² green and yellow cable that is probably bonding, but not with surety as I say, it may be you are testing the actual water bond cable and getting continuity to gas via the boiler and so when you test this cable you get a low resistance to the gas pipe, great gas is bonded, no it is maybe connected by the FCU cpc for the boiler?
If you can see where it starts and stops then you have more surety.
If you test another cable and get lower resistance that might be the right cable, who can be sure.
You may be able to make a reasonable assessment but is it right?

 

[stevethesparks]

Will still give a much better clue than just connecting to the MET

 

[Richard Burns]

Will still give a much better clue than just connecting to the MET

True....

 

[Leesparkykent]

For main equipotential bonding, there is no single fixed value of resistance above which the conductor would be deemed unsuitable. Each measured value (if it's measurable) should be compared with the relevant value for a particular conductor length and size.

 

[sparkzalot]

Surely can disconnect every cpc that could cause a parallel path to earth (boiler, shower, imm. heater etc) and then individually test each bonding cable to gas / water via long lead??