Main proective bonding

[piz]

Can anyone please give me the calculation for working out the size of the main bonding conductor on a tn system.
and the water and gas please
I need to understand if a fault flows to earth if the cable can take the current
Hope this makes sense

main fuse bs1361 100amp
Cheers

 

[ringer]

Not sure if this what you need, but...

BRB 543.1.3

S = √(I^2t)/k (that is root of I squared times t then divide by k - couldn't find how to enter squared symbol)

S is nominal cross section area of protective conductor
k is obtained from tables 54.2 to 54.6 depending on installation method
t is operating time in seconds

or, much simpler method by selection rather than calculation, 543.1.4 states that if the CPC is same material as line conductor, then the minimum cross section area of CPC is same size as line conductor. This applies for line conductor 16mm or less. For line conductor 16 - 35 use 16mm CPC. Above that min CPC size is half line conductor.

 

[jpmiff]

if I remember correctly its the "adiabatic equation"

 

[Amp David]

The adiabatic equation isn't for working out the size of main bonding conductors. The sizes are given in the BRB somewhere, not sure which section/page though.

Adiabatic is for CPCs i'm sure

 

[WayneL]

Hi,

Amp David is correct.

Bonding is sized from table in regs.

Bonding conductors don't carry fault current anywhere, by the way.

 

[Darkwood]

You say for a TN system but regs differ between TNS and TNCS, although for the small supply you are concerned with the results may seam the same.

 

[ezzzekiel]

as stated values are taken from tables (typically 10mm domestic) adiabatic can not be used to size main bonds

 

[jpmiff]

no problem, thanks

 

[Sparky Ninja]

Adiabatic can be used to verify size of earthing conductor.

Regulation Group 544.1 covers the sizing of the main protective bonding conductors.​

These regulations require that the conductor used for connecting the consumer earth terminal to the extraneous-conductive-parts should be at least half of the value of the earthing conductor, and should be at least 6 mm2 for a TT or TN-S supply. ​

For a TN-C-S system where PME (Protective Multiple Earthing) conditions apply then the minimum figures in Table 54.8 apply and the protective bonding conductor should be at least 10 mm2.
This relationship between sizes of equipotential bonding conductor and TNC systems work with similar consideration as to why TNC systems are not to be installed in Agricultural premises or exhibitions (sections 705 & 711). The objective being to achieve as low a touch voltage (U) as practicable, while taking into consideration the pfc that will flow on a TNC system.
​

 

[piz]

Thanks for your help