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Bonding conductor sizes
Former Community Member
What is the reason for bonding conductors being sized according to the size of the supply neutral? I can see how earthing conductors / cpc’s are selected according to adiabatic considerations: making sure they can stand up to a fault. I understand that the bonding is to maintain an equipotential zone but how does the neutral csa and bonding csa relate to this?
To complete the circuit picture, there is an assumption that all metalwork disappearing into the ground is actually bonded to supply neutral at least twice - once at the property being considered, and again somewhere else beyond your control. This makes perfect sense for a metal water main running along a street, where it is presumably bonded to neutral at pretty much every property with a PME supply, so is indeed running up the street wired in parallel to the supply main neutral. (and if the supply neutral went open circuit at some point, you may not even notice until someone disconnected their plumbing - there have been cases of folk working on the water meter or stop taps outdoors getting a shock because of this, and at least one electrocution in the last 20 years or so.)
However if the object being bonded is known for certain not to to be bonded anywhere else - perhaps a bond to the gas or oil pipe to a private tank in a remote farmhouse, then the resistance of the earth, in this sense the muddy terra-firma stuff, not the CPC wiring, will mean that the passage of large numbers of amps is impossible, even under fault conditions. In such a case, the bond cross-section could be reduced to the same as that you might use for any other electrode.
The recommendation of the regs however is that rather then measure Ze, you always assume the diverted neutral condition is possible.
To complete the circuit picture, there is an assumption that all metalwork disappearing into the ground is actually bonded to supply neutral at least twice - once at the property being considered, and again somewhere else beyond your control. This makes perfect sense for a metal water main running along a street, where it is presumably bonded to neutral at pretty much every property with a PME supply, so is indeed running up the street wired in parallel to the supply main neutral. (and if the supply neutral went open circuit at some point, you may not even notice until someone disconnected their plumbing - there have been cases of folk working on the water meter or stop taps outdoors getting a shock because of this, and at least one electrocution in the last 20 years or so.)
However if the object being bonded is known for certain not to to be bonded anywhere else - perhaps a bond to the gas or oil pipe to a private tank in a remote farmhouse, then the resistance of the earth, in this sense the muddy terra-firma stuff, not the CPC wiring, will mean that the passage of large numbers of amps is impossible, even under fault conditions. In such a case, the bond cross-section could be reduced to the same as that you might use for any other electrode.
The recommendation of the regs however is that rather then measure Ze, you always assume the diverted neutral condition is possible.
