Discuss suplementary bonding again in the Electrical Forum area at ElectriciansForums.net

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thekingiam

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just trying to get my head around this as never tested before, bathroom pipes all circuits rcd protected no power except lights, connected one probe to landing cpc in socket and the other to sink pipe tested insulation resistance on 500 volts 1 meg ohm meter reading 0000.00 X underneath 1 volt checked continuity at 20 ohm and got 1.3 ohm.
am i doing this correctly and if so as it's not above 0.2 M ohm do i take it that i need suplementary bonding?
 

ackbarthestar

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Measure between simultaneously accessible metal parts, such as radiators, taps, pipework and cpcs of circuits within the bathroom.

the values you are looking for are <0.05 ohms or >22k ohms

The formula for the latter is Rx > Uo/IAN -Rb, where Uo = 230V, IAN = 10mA, Rb = 1k ohm
 
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thekingiam

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would i need to do this as only crcuit is lighting ,protected by rcd and out of reach?
 

ackbarthestar

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Start with 701.415.2 then if there is still doubt then look at 415.2.2.
From your results in the OP, there is not enough IR to be isolated and too much resistance to be effective enough to keep the touch voltages to a minimum
If there is RCD protection, MPB and the bathroom lighting circuit complies with 41.1 then SBCs may still be required.
The chances of a fault caused from the lighting circuit is probably slight but there are other circuits in the property that may transfer a fault voltage onto the pipework in the bathroom location.
Not that it has ever happened on my watch.....
 
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thekingiam

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ok retested low ohm meter got 0.47 ohm so
0.47 x 50 / 0.03 = 783.33 well below 16670
so no sup bonding required, am i correct anyone?
 

rich.250

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With supplementary bonding you are checking to see that the touch voltage between simultaneously touchable parts will not exceed 50v under fault conditions.
If all ccts are 30mA protected you are looking at 50/0.03 is 1667 ohms. Between all pipe work and the MET.
Above this but below 23k ohms will require supplementary bonding.


This formula proves 'effectively connected to the MET'
Obviously if you have a non Rcd cct you need to replace the 0.03 with the fault current required to operate the OCPD.

Hope this makes sense!!
 
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thekingiam

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where does the 23k ohms come from? is it just a number given or is there a formula that has a relevants?
 

rich.250

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It is based on the current not to be exceeded flowing through the body.
Taken as 10mA.
The formula is Uo/ 10mA - 1000
(1000 is the resistance of the body)
This can be ignored for these purposes.

So 230/.01 = 23000 or 23k ohms.

Some use 30mA but IMO 10mA is much much safer.
 
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Geoffsd

ok retested low ohm meter got 0.47 ohm so
0.47 x 50 / 0.03 = 783.33 well below 16670
so no sup bonding required, am i correct anyone?
No, it's not done like that.

The 0.47 must be less than or equal to 50 / 0.03 (1667) which obviously it is.

This is why as long as the conditions of 715.412 (iv),(v),(vi) are met then supplementary bonding is not required.


Also, above 23kΩ between part and MET is considered isolated and even main bonding is not required.
 
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thekingiam

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cheers geoffsd, been troubled for some time over this so as 0.47 is lower than 1667 no suplementary bonding needed.
 

ackbarthestar

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where does the 23k ohms come from? is it just a number given or is there a formula that has a relevants?
Not to too pedantic....

23k, or more precisely 22k ohms, comes from GN5 and GN8.

Rx >= Uo/IAN - Rb

where U0 = 230V, IAN = 30mA and Rb = average body resistance of 95% of population.
 
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Geoffsd


Yes, but it seems to be splitting hairs for dubious reasons.

Are we really likely to get a reading between 22 and 23 k
Ω?

If it were 22.1kΩ would you test the customer to make sure they don't have a low resistance?
 

ackbarthestar

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I can't see what's dubious about what it states in the guidance notes.

It does seem a little odd though, if the body resistance is reduced from a dry resistance of approximately 1k ohm to a wet resistance of something in the order of 100 ohms which might be the case in a bathroom location.

Having said that it wouldn't be in the Guidance notes if there was a probability of shock.
 
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