I despair, I really do
Some basic theory for a shock received when touching a faulty appliance.
In an installation with Protective “equipotential” bonding
Ut = If * R2 volts
If the bonding is removed
Ut = If * (R2 + Se) volts
Where (for simple small installations):
Ut = the touch voltage due to the steady state fault current;
If = the steady state fault current;
R2 = the resistance of the circuit-protective-conductor(s) (cpc) within the installation – so for this example - R2 would typically include the resistance of the cpc in the final circuit conductor and the resistance of the cpc in appliance flex;
Se = the impedance (here treated simply as a resistance) of the supply earth path e.g:
1) PEN conductor in TN-C-S;
2) “Cable sheath” or similar in TN-S;
3) Earth electrode resistances of the installation electrode and the supply transformer electrode in TT.
So removing the main protective bonding conductors increases the touch voltage from:
Ut = If * R2 volts to Ut = If * R2 + If * Se volts
So this gives the mind blowing conclusion that, for the situation discussed here, main bonding removes an effect that occurs outside of the installation – good init!
Now there is more – Ut is not limited to 50 volts by this method of protection.
The only way you can guarantee Ut <= 50 volts for the whole duration of the fault (in an installation with totally effective bonding) is to arrange that the resistance of R2 is around 1/4 that of R1 (the line conductor resistance). This means you should use a 10mm2 cpc with a 2.5 mm2 line conductor.
If you use PVCTWE cables you should note the following:-
Under the conditions described above, and assuming 240 volts for Uo (the system voltage between line and earth):
1.0 / 1.0 gives Ut = 120 volts
2.5 / 1.5 gives Ut = 150 volts
4.0 / 1.5 gives Ut = 175 volts
Ever wondered why our EU buddies don’t like PVCTWE – any cable where line and cpc are the same size gives Ut = 120 volts for Uo = 240 volts.
Now don’t tell me you use this cable in your home
The sizing of these protective bonding conductors is not related directly to the above. It has more to do with faults occurring on the supply network than those occurring within an installation. It is generally related to mechanical strength and, for TN-C-S, the ability to handle network faults, in particular a broken supply neutral – its resistance is not generally a factor.
When I was a boy main bonds were generally 2.5mm2 [D13 14th Edition] in small installations and, in terms of reducing touch voltage, they worked just as well as the larger ones used now.
For the OP
Supply network conditions have changed since I was a boy
- so you should install the bonding conductors sized as recommended in BS 7671 for the type of earthing system you have. Many people just use the worst case conditions (TN-C-S) and install a minimum of 10 mm2 Protective Bonding Conductors and a 16 mm2 Earthing Conductor.
A water pipe is no longer considered to be a reliable connection to earth as most services eventually get upgraded to plastic pipework.
Now there is far more to this subject than given above so if you seek understanding you will need to do a lot of reading.