Cross Bonding

[Scott F]

R<50v/Ia

How do you arrive at the 0.05 figure as Ia being the operating current in amps (like scots spark said would generally be 100 Amps so 50/100 =0.5
I have the regs and GN3 so I know the figure is 0.05
So how does one arrive at the figure?
Cheers Scott

 

[Gravid]

Is it that the 0.05 is what you are hoping to get below if you were to measure the resistance between two pipes, say hot and cold water pipes for example. If its above this then you would have to bond them together. think thats what GN3 is saying on p 35.

This equation, are you supposed to use the cut out or your RCD rating?
If you have a 60A cut out does this mean its got to be below 0.8? (50/60)
and if you do the equation with a 30ma RCD it comes out at 1667 !

To be honest really struggling to get my head around all this !!

 

[scotsparky]

Gravid the 1667 figure look familiar???? it should its the max ZS for 30ma rcds up to 230v (see page 50brb.)
the resistance measured is between the pipe and the MET at the D.B subtracting the long lead and test leads.

Use the RCD tripping currrent when using the formula. As you have the resistance and the In of the protective devics if you transpose your formula to rxIn <50 then you are ok as any voltage present in the metalwork during fault conditions is less than 50v (touch voltage) remember 120v dc


0.05 is normally used it goes on the side of safety as with the tripping times. If you are above 0.05 then either do the maths and prove if it need bonding ok put in supplimental bonding so its under 0.05. ()for all the time it takes i would bond it)

 

[Gravid]

Yeah it did look familiar, TT systems sprang to mind.

 

[scotsparky]

All based on the same theory. do you understand it now?

 

[Gravid]

Yes mate i've got it now, just had to go back to the basics, i understand your making sure the touch voltage in the event of a fault doesn't go above 50v.

 

[scotsparky]

thats the chappy!