Why are main protective bonding conductors sized as they are

[Pretty Mouth]

Then Only the people who write chapter 54 could possibly give the answer?

I have a feeling you might be right

 

[Marvo]

Aren't CPC's and bonding conductors sized around allowing a maximum of 50 volts elevation under worst case fault conditions? 5o volts being the arbitrary figure that would limit earth leakage to 30mA through anyone in contact with something earthed or bonded.

 

[Pretty Mouth]

Aren't CPC's and bonding conductors sized around allowing a maximum of 50 volts elevation under worst case fault conditions? 5o volts being the arbitrary figure that would limit earth leakage to 30mA through anyone in contact with something earthed or bonded.

By this, do you mean when disconnection times can't be met in the event of a fault, the potential difference between exposed conductive and/or extraneous parts mustn't rise above 50V? Or am I getting mixed up?

 

[pc1966]

There has to be another reason for the larger minimum CSA required for bonding conductors other than durability.

Other than the PME fault current issue, the factor I would say is important is the consequence of failure.

If your main earth bond fails then everything in the system is at risk, not some sub-section, and the could result in significant accumulated leakage currents even without a fault leading to phase current flowing. If your main RCD on a TT system is 100mA or 300mA, both of which are perfectly good if your circuits have instant 30mA RCD protection, you could see a risky leakage between the system earth and true earth without tripping.

I really have no idea about the drafting of the regs, but that would be my worry.

Edit to add: Also what if you have an SPD that might divert a few kA on a surge? Adiabatic might need consideration for that.
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I think a 25kA lightning pulse (the "direct" 10μs/350μs waveform) would require 4mm, so not a likely reason and I doubt most UK sites would ever see that.