Hello everyone plus quickish bonding questions

[Ducksy]

Hello guys and girls been using the info on this great site now for about 6 months never needed to join and ask questions because of the handy search button ^^^

I understand that unless omitted (RCDS, disconnection times achieved, main sup bonding on water, gas, Ect.) bonding must be done across all exposed/extraneous conductive parts inside a room containing a bath or shower.

i also understand that the resistance must be low so that volt drop must not exceed 50v when carrying the operated current of the protected device (r < 50 / Ia) for a 30ma RCD (50 / 0.03 =1667) and if metalwork to MET (person likely to be wet) 50k ohms or higher and 25k ohms (person likely to be dry) does not need bonding.

My question is:

At a existing installation without RCD protection if you where to mesure the resistance between the hot and cold pipes under a bath and found a reading of 0.01 ohms would you assume the pipes have been earth bonded (maybe the bonding is not visible) what min ohm's reading acceptable and how do you work it out???

Another example is you replace a shower like for like on a circuit with RCD protection and find a high ohm's reading between the showers earth terminal and the pipework (maybe a qualified plumber has used a pvc push fit between the metal pipework:lol what would be the min acceptable reading you would expect and how do you work it out.

I really hope these questions make sense. i've got all the books OSG, BS7671, john Whitfields electricians guide, guide to Part P and i must be missing something or being stupid knowing me probably both

Thanks in advance.

Martin

 

[Ducksy]

Hey again peps, i know my answer lies in reg 415.2.2

so

What it means is, to check the effectiveness and compliance of supplementary bonding, total resistance (R) between exposed and extraneous conductive parts must not exceed the calculation of 50/the mA rating of the associated RCD.

I.e, 50/0.03 = 1667ohms.

Where there is no RCD protection then the current causing the fuse/MCB to disconnect in 5s is used instead of the RCD rating.

So just so im getting this straight lets say a 40amp 60898 circuit breaker has a 5 sec disconnection time @ 400amps which 50/400 = 0.125 max ohm's reading from conductive to MET is that correct??? as it would solve my second question above^^^^

Ok after much searching i found this test question and answer

13. When considering if supplementary bonding can be omitted from a location
containing a bath or shower, the maximum resistance of extraneous conductive parts
connected to the Main Earth Terminal is
a) 1.66 (415.2.2) correct answer
b) 0.05
c) 0.5
d) 7.6

So if this is true it means, to make sure bonding is required, measure between said metal work and met and if lower then 1.66ohms its fine no bonding required, can anyone confirm this please.

I know you guys are probably bored of the bonding questions 10 years ago but this one is has got me for months now.

Thanks again guys.

 

[leo79]

Whats the question again?

 

[Ducksy]

Here you go.

At a existing installation without RCD protection if you where to mesure the resistance between the hot and cold pipes under a bath and found a reading of 0.01 ohms would you assume the pipes have been earth bonded (maybe the bonding is not visible) what min ohm's reading acceptable and how do you work it out???

 

[Geoffsd]

Where there is no RCD protection then the current causing the fuse/MCB to disconnect in 5s is used instead of the RCD rating.

So just so im getting this straight lets say a 40amp 60898 circuit breaker has a 5 sec disconnection time @ 400amps which 50/400 = 0.125 max ohm's reading from conductive to MET is that correct??? as it would solve my second question above^^^^

You seem to have it sussed but you have used the wrong number -
A 40A 60898 will trip in 0.1s @ 200A(Ia) See time/current graphs and tables
so 50/200 = 0.25Ω is the maximum between e-c-ps in the location - not back to the MET

13. When considering if supplementary bonding can be omitted from a location
containing a bath or shower, the maximum resistance of extraneous conductive parts
connected to the Main Earth Terminal is
a) 1.66 (415.2.2) correct answer
b) 0.05
c) 0.5
d) 7.6

So if this is true it means, to make sure bonding is required, measure between said metal work and met and if lower then 1.66ohms its fine no bonding required, can anyone confirm this please.

That would be if the lighting circuit mcb (6A 60898) was the maximum within the location.

Also conductive parts with a reading of <23kΩ (or whatever value you consider safe) to the MET should NOT be bonded as they are deemed not extraneous and so isolated.

 

[Ducksy]

Thanks mate i did mean 200A dont know why i said 400A and didn't mean back to the met but to e-c-ps.

What about extraneous to extraneous how would you know if the pipework needed bonding together (as the bond could not be visible due to it being run under floorboards) is there a minimum permissible measurement between extraneous conductive parts???

Thanks for your help its really appreciated.

 

[Geoffsd]

Yes the same, 50/Ia, for the maximum rated mcb of the location.

Measure between all extraneous and exposed c-ps.

If lower than 50/Ia then it does not need bonding - whether because it already is or does not require it because of the pipework runs being really short does not matter.

If a really low reading is obtained it is likely to be bonded out of sight.

 

[Ducksy]

FOUND IT!!!!.......right under my nose in guidance note 3 confirm bonding connections that may be "built in" a test between exposed metalworks and should be looking for a reading of 0.05ohm's or lower.

WOOHOOOOOOOO finally got bonding bag'd.

Thanks all..........mainly Geoff.