BS 7671 has some specific requirements for which we need to understand the answer to some questions:

(a) Is the LV supply from a public network or private transformer;

(b) is there a Common Bonding Network in the installation, for example for control or information technology equipment?

(c) Which country is the installation in?

LV supply from a private transformer.

I'm not aware there is a common bond network. so let's go with no.

This is a UK installation (England to be precise)

Udtwmc said:

LV supply from a private transformer.

OK, so TN-S then (assuming this is a PNB installation as discussed in GN 8)?
(I will ignore issues regarding TN-C between transformer and swtichboard,  and whether others believe this complies with Reg 8(4) of ESQCR if it really is TN-C, but if it's a private transformer earthed at the transformer, with no backup generator, then it really ought to be TN-S from the transformer.)

If it is TN-S, main protective bonding sized in accordance with Regulation 544.1.1 is half the CSA of the Earthing conductor, subject to a minimum csa of 6 sq mm and max of 25 sq mm.


BUT

If there really is a private TN-C supply circuit that also serves other buildings or parts of the installation, we are outside the realms of what is covered by BS 7671. My tendency would be to go with PME recommendations 544.1.1 (Table 54.8) for main protective bonding size in this instance, but would caution that it's tricky unless the TN-C distribution circuit has been installed to DNO rules (whilst the distribution network portion of PME is TN-C, PME has additional requirements for earthing over and above those used for TN-C in other countries).

Udtwmc said:

I'm not aware there is a common bond network. so let's go with no.

OK, reason for asking is that with CBN inductance (for functional earthing) is more important, and may need other than 25 sq mm round conductors.

I also thought about earthing of pump control if the pump is VSD, because of harmonic currents in the earthing system (high protective conductor currents). The frequencies involved may also be impacted by the armour of the SWA., and that would be a reason for providing additional copper earthing. BUt because of frequencies, if more than a couple of metres, depending on size of VSDs, round copper might not work.

Udtwmc said:

This is a UK installation (England to be precise)

Reason for asking is that, even if BS 7671 is specified, sometimes local wiring codes have additional requirements to consider.

Now, as to whether the control cabinets require main protective bonding ... they are not extraneous-conductive-parts by definition as they  are part of the electrical installation, so the answer is a clear "NO".

Udtwmc said:

My misunderstanding, i thought protective bonding was to reduce touch voltages to acceptable levels - that being < 50V.

BS 7671 does not limit touch voltages in a fault, unless supplementary protective equipotential bonding is applied (Reg 415.2) - usually only applied where ADS disconnection times can't be achieved, or in special locations (bathrooms, which follows the 50 V AC/120 V DC of 415.2, or medical locations where the voltage is reduced to 25 V AC/60 V DC).

Udtwmc said:

LV supply from a private transformer.

OK, so TN-S then (assuming this is a PNB installation as discussed in GN 8)?
(I will ignore issues regarding TN-C between transformer and swtichboard,  and whether others believe this complies with Reg 8(4) of ESQCR if it really is TN-C, but if it's a private transformer earthed at the transformer, with no backup generator, then it really ought to be TN-S from the transformer.)

If it is TN-S, main protective bonding sized in accordance with Regulation 544.1.1 is half the CSA of the Earthing conductor, subject to a minimum csa of 6 sq mm and max of 25 sq mm.


BUT

If there really is a private TN-C supply circuit that also serves other buildings or parts of the installation, we are outside the realms of what is covered by BS 7671. My tendency would be to go with PME recommendations 544.1.1 (Table 54.8) for main protective bonding size in this instance, but would caution that it's tricky unless the TN-C distribution circuit has been installed to DNO rules (whilst the distribution network portion of PME is TN-C, PME has additional requirements for earthing over and above those used for TN-C in other countries).

Udtwmc said:

I'm not aware there is a common bond network. so let's go with no.

OK, reason for asking is that with CBN inductance (for functional earthing) is more important, and may need other than 25 sq mm round conductors.

I also thought about earthing of pump control if the pump is VSD, because of harmonic currents in the earthing system (high protective conductor currents). The frequencies involved may also be impacted by the armour of the SWA., and that would be a reason for providing additional copper earthing. BUt because of frequencies, if more than a couple of metres, depending on size of VSDs, round copper might not work.

Udtwmc said:

This is a UK installation (England to be precise)

Reason for asking is that, even if BS 7671 is specified, sometimes local wiring codes have additional requirements to consider.

Now, as to whether the control cabinets require main protective bonding ... they are not extraneous-conductive-parts by definition as they  are part of the electrical installation, so the answer is a clear "NO".

Udtwmc said:

My misunderstanding, i thought protective bonding was to reduce touch voltages to acceptable levels - that being < 50V.

BS 7671 does not limit touch voltages in a fault, unless supplementary protective equipotential bonding is applied (Reg 415.2) - usually only applied where ADS disconnection times can't be achieved, or in special locations (bathrooms, which follows the 50 V AC/120 V DC of 415.2, or medical locations where the voltage is reduced to 25 V AC/60 V DC).