Agreed. It's the kind of thing that's far from ideal, but happens rather a lot in the real world and doesn't cause disaster very often. On the positive side most household circuits have quite a lot of margin - e.g. a 1.0mm² lighting cable is probably good for over 6A at 60 degrees C and a 2.5mm² can probably carry 10-13A for ever at 60 - and rather more for shorter periods - whereas the long term average consumption of an non-electrically-heated house is probably below 5A. Loads have typically reduced in recent years (e.g. 5W or 10W LED lamps rather than 50W or 100W halogen or GLS) and appliances generally are more efficient. Likewise gas boilers often run with a flow temperature of 60 degrees or below to keep them in condensing mode (rather than the 70 or 80 degrees that was more usual in the past).

The picture could be rather worse if you had long duration high current loads though, or insist on running the boiler at max flow temperature. Presumably you've not got storage heaters or an immersion along side heating pipes, but EV or a heat pump might be a possibility, or cables serving an electrically heated outbuilding.

the bathroom radiator has a 4mm earth bonding that goes back to the CU

I think someone's rather missed the point there (the idea of supplementary bonding is to tie together exposed- and extraneous-conductive-parts of the location, not reinforce potentials from further afield), but as long as main bonding (if needed) is in place and everything is 30mA RCD protected, it's probably a mute point.

  - Andy.

Agreed. It's the kind of thing that's far from ideal, but happens rather a lot in the real world and doesn't cause disaster very often. On the positive side most household circuits have quite a lot of margin - e.g. a 1.0mm² lighting cable is probably good for over 6A at 60 degrees C and a 2.5mm² can probably carry 10-13A for ever at 60 - and rather more for shorter periods - whereas the long term average consumption of an non-electrically-heated house is probably below 5A. Loads have typically reduced in recent years (e.g. 5W or 10W LED lamps rather than 50W or 100W halogen or GLS) and appliances generally are more efficient. Likewise gas boilers often run with a flow temperature of 60 degrees or below to keep them in condensing mode (rather than the 70 or 80 degrees that was more usual in the past).

The picture could be rather worse if you had long duration high current loads though, or insist on running the boiler at max flow temperature. Presumably you've not got storage heaters or an immersion along side heating pipes, but EV or a heat pump might be a possibility, or cables serving an electrically heated outbuilding.

the bathroom radiator has a 4mm earth bonding that goes back to the CU

I think someone's rather missed the point there (the idea of supplementary bonding is to tie together exposed- and extraneous-conductive-parts of the location, not reinforce potentials from further afield), but as long as main bonding (if needed) is in place and everything is 30mA RCD protected, it's probably a mute point.

  - Andy.

On the supplermentary bonding, they have taken them all back to the CU, not sure why. My guess is because the CU is under the batharoom so it was easier?  

I suspect that originally, there was a length of green and yellow from the CH to the CU and a further one looped between each set of taps and may be the WC back to the CU.

If the bathroom was refitted, the latter would have been discarded, but if the CH was not touched, that bonding could have been left in place.

My question is whether when a CU is updated to include RCD protection, is there any risk associated with leaving part of the supplementary bonding in place as opposed to either leaving or removing all of it?