Bonding

[rich.250]

Can someone explain to me where the above 23k ohms doesnt require bonding comes from??
I was told it as an apprentice and just followed it.....
Is it ohms law? Meaning its for supplies up to 100amps??
Thanks

 

[telectrix]

23k ohms relates to 10mA flowing through you in the event of touching a live part and the metalwork that is deemed to be not extraneous if above 23k to MET.

 

[buzzlightyear]

hi bros new member on loading 3 phase sub mains board from 3phase main 16x 16 on board 50 mtrs form main
16mm 4 core would 60amp mcb would do

 

[MarkieSparkie]

The metallic part can be assumed not to be an extraneous-conductive-part if the following condition is met:
Rcp> (Uo/Ib) - Ztl
where,
Rcp is the resistance between the conductive part and the MET in ohms
Uo is the nominal voltage to Earth in volts
Ib is the value of current through the body in amperes that should not be exceeded. (The value may be taken as 30 mA for a disconnection time of up to 0.4 s, as given in DD IEC/TS 60479)
Ztl is the impedance of the human body in ohms. The value suggested in DD IEC/TS 60479 is 1000 ohms where Uo is 230 V (50 Hz) under dry or wet conditions.
Taking Ib as 30 mA and Ztl as 1000 ohms (as suggested above where the disconnection times in the installation are 0.4 s or less and Uo is 230 V), the limiting value of Rcp is given by:

Rcp > (230/0.03) - 1000
Rcp > 6667 Ohms (This is often seen rounded up to 7k Ohms in NICEIC documents)


Thus, if Rcp exceeds 6,667 ohms, the pipe may be considered not to be an extraneous-conductive-part, such that main bonding of the pipe is not required.


Traditionally most electricians prefer to use a 10mA value of current through the body (the Let-go limit) that should not be exceeded, which would change the outcome of the formula to 22k Ohms. To make things more simple the impedance of the human body can be ignored, giving a conservative 23k Ohms, this is the recommended value advised in Guidance Note 8.

 

[MarkieSparkie]

hi bros new member on loading 3 phase sub mains board from 3phase main 16x 16 on board 50 mtrs form main
16mm 4 core would 60amp mcb would do

Err! What is the relevance of this to the thread?

 

[buzzlightyear]

none bro !

 

[ackbarthestar]

I think you'll find this information in GN5 and GN8

the formula given to check that there is no more than 10mA that might flow under a fault condition. It is a test to prove that there is sufficient insulation between simultaneous conductive parts not to apply supplementary bonding

It is specifically applied to locations where there is a greater possibility of shock, such as bathrooms, Swimming pools, Hot tubs at the bottom of the garden,

The formula for calculating the value is:

Rx = Uo / I - Rb




Where Rx = minimum value of resistance between exposed and extraneous parts
Uo = Nominal supply voltage
I = maximum fault current (10mA)
Rb – Human body resistance taken to be 1k

That would be 22k Ohms
The 23k Ohms assumes that you have a skin resistance of around 1k Ohms which is applicable to about 95% of the population. Goodness knows what happens to the other few percent who are likely to be wilting buds with hot soft sweaty hands !??!

 

[ackbarthestar]

Apologies to sparkiemike, I started writing the last post before you posted

Edit :That would be Markiesparkie

 

[rich.250]

Thanks for the responses guys. That makes sense now.
Will look at the guidance notes in the morn.
Is it odd this wasn't even covered in my 2391??