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Bonding conductor sizes

Former Community Member
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?
  • Because of heavy diverted N currents. With TN-C-S supplies, a broken PEN conductor somewhere in the street could involve the N current from the part of the street beyond the break being diverted via bonding along e.g.  a metal water pipe and into a house on the other side of the break then, via further bonding back to the unbroken part of the PEN.

  • wallywombat:

    Because of heavy diverted N currents. With TN-C-S supplies, a broken PEN conductor somewhere in the street could involve the N current from the part of the street beyond the break being diverted via bonding along e.g.  a metal water pipe and into a house on the other side of the break then, via further bonding back to the unbroken part of the PEN.




    Which is also a good reason to bond in the first place!

  • Note that main bonding sizes only relate to the supply N for PME systems (or potentially PME systems) - for other systems (TT, true TN-S etc) they're related to the earthing conductor size & a simple minimum (6mm²)  to cater for physical robustness & possible currents imported through extraneous-conductive-parts.


    It's not only during broken CNE conditions that bonding conductors on PME systems may have to carry significant diverted N currents - bonding conductors (together with extraneous-conductive-parts running between installations) often form a parallel path to the supplier's CNE so will carry a proportion of the N current even under normal conditions. Looking at the sizes involved (in table 54.8) you might even get the impression that they're sized on the assumption that they'll be carrying only about half the installation's N current - which might suggest they're only really sized for normal conditions - rather than broken CNE events when a bonding conductor could easily be pressed into carrying the entire installation's N current (possibly more if several installations share the same extraneous-conductive-parts).


       - Andy.
  • 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.


  • Yes, the requirements for sizing of main protective bonding conductors seems to be a rather blunt instrument. A TT installation with a 2.5 earthing conductor sized using the adiabatic would require a 6mm MPB. The same installation with an earthing conductor selected on the basis of table 54.1 where the conductor has no protection against mechanical damage or corrosion and therefore needs to be 25mm2 would have a 16mm2 MPB. On the other hand, where the risk of diverted neutral currents exist on a similar installation with a PME terminal, the MPB can be as little as 10mm2.