Earth fault - potential on exposed conductive part

[KeenPensioner]

As can be seen from my profile I am not a qualified electrician but I'm studying in my own time just for the sake of it. I understand the principals of earthing and bonding but in this video by GSH Electrical....

.....at 2:10 minutes in he says exposed conductive parts under earth fault would have "up to 50 V appearing"....can someone explain how he gets 50 V please .....I'd have thought it'd be instantaneously 240 V before ADS

 

[Nigel]

50v is the maximum permitted voltage that can be measured across two exposed conductive parts before a protective device isolates the supply.

 

[Lucien Nunes]

Or, phrased another way, ADS is designed so that any fault that could create a potential of 50V or more between exposed conductive parts, must cause a protective device to operate. The 50V is a designed-in limit.

 

[Wilko]

Just for fun - check out 3:50 where chap states the limit for bonding conductor resistance is 0.05 Ohms. As far as I'm aware this is not so - it's an indicator of a good connection rather than a hard limit (from GN3 p38). So there is no need to exceed the sizing requirements in BS7671. What do you think ?

 

[Ian1981]

Just for fun - check out 3:50 where chap states the limit for bonding conductor resistance is 0.05 Ohms. As far as I'm aware this is not so - it's an indicator of a good connection rather than a hard limit (from GN3 p38). So there is no need to exceed the sizing requirements in BS7671. What do you think ?

You are correct wilko.
It’s a figure that many take to be gospel yet they quote GN3 which has neve actually said this.
In fact the latest GN3 18th edition version actually goes onto say that a 0.05 ohms reading is not from the MET to the pipework or bonding conductor in question

 

[KeenPensioner]

Thank you all for your replies, you pointed me in the right direction and I found this....

"The maximum value is R = 50/Ia. Where Ia is the operating current for the protective device in 5 seconds. Ref. 413 - 02 -28. GN3 recommends less than 0.05 ohms."..

.
...it might not be bang up to date but I understand the principle now.... ....Ia being the pfc?

 

[Wilko]

You are correct wilko.
It’s a figure that many take to be gospel yet they quote GN3 which has neve actually said this.
In fact the latest GN3 18th edition version actually goes onto say that a 0.05 ohms reading is not from the MET to the pipework or bonding conductor in question

Oh dear, this means I will just have to get a new GN3 ...

 

[telectrix]

the 0.05 ohms figure is for the resistance between the bonding cable end and the pipe to which it is connected. i.e. to verify that the bonding clamp is securely connected to the pipework metal. nothing to do with the resistance of the cable.

 

[Ian1981]

Thank you all for your replies, you pointed me in the right direction and I found this....

"The maximum value is R = 50/Ia. Where Ia is the operating current for the protective device in 5 seconds. Ref. 413 - 02 -28. GN3 recommends less than 0.05 ohms."..

.
...it might not be bang up to date but I understand the principle now.... ....Ia being the pfc?

R=50/Ia

For example if you had no rcd protection in a bathroom and you wanted to verify the effectiveness of the supplementary bonding.
Lighting circuit protected by a 6amp type B circuit breaker
Then it would be R=50/30=1.67 ohms.
If it was a type C 6amp mcb then it’s 50/60=0.83 ohms.
That is the maximum resistance between pipework before any fault current in the location between exposed and extraneous parts exceeds 50v.
The 0.05 figure is a red herring.
It’s for measuring between pipework where the clamps are hidden or ‘built in’
A 0.05 ohms reading between pipework verifies that there’s little resistance between said pipework