NE bonding is a vexing thing, and the clear cut answer for one situation is equally strongly the ‘never do’ for another.

That said, I'll put my head over the parapet, and wait to be shot at…

So in the UK the ECSQR require that N and E are not bonded within a consumer installation. 

Diverted currents are raised as the reason, but many other places with 230V and PME/ MEN like installations (Eire, Oz, NZ, SA for a start) have no such restriction, only a requirement to size conductors for the likely currents. They do not seem to be overly beset by extra deaths or fires because of this.

Of course a private substation, or a large UPS are not really consuming equipment, more supply equipment, and a bond is expected and indeed required for correct operation of the ADS though it may be some distance from the transformer, depending how things are wired. Indeed no NE bond at all is very bad on a substation type transformer as the LV side can float up to a dangerous fraction of the HV via the capacitance between adjacent windings, and must be solidly earthed.

It is common to have multiple NE bonds on one site if for example your installation is large enough to have two or more HV to LV transformers, but the N and E would normally be interlinked in such a cross-section of conductor that a diverted current equal to the full load would not be an overheating issue.

A UPS is more like having a genset on-site, that perhaps only supports a fraction of the main load, and there one may need to switch in an additional NE link at handover to UPS, especially if one of the conditions of the genset/ups operating is total loss of supply, including severing of the mains earthing. Personally I am not a fan of the idea of switched links, and would rather have  solid ones and suffer a line that cannot be RCD protected. (the RCD seeing the other NE bond as a fault.)

A really small UPS, say a plug in one supplying perhaps one desk, is not such an issue if it runs as an IT (fully floating no earth at all), indeed if you pull out the plug, this is unavoidable.

I think these 3 cases, solid bond, switched bond, no bond, cover all situations between them, for large medium and small set-ups. On the boundary, roll the dice.

Mike.

NE bonding is a vexing thing, and the clear cut answer for one situation is equally strongly the ‘never do’ for another.

That said, I'll put my head over the parapet, and wait to be shot at…

So in the UK the ECSQR require that N and E are not bonded within a consumer installation. 

Diverted currents are raised as the reason, but many other places with 230V and PME/ MEN like installations (Eire, Oz, NZ, SA for a start) have no such restriction, only a requirement to size conductors for the likely currents. They do not seem to be overly beset by extra deaths or fires because of this.

Of course a private substation, or a large UPS are not really consuming equipment, more supply equipment, and a bond is expected and indeed required for correct operation of the ADS though it may be some distance from the transformer, depending how things are wired. Indeed no NE bond at all is very bad on a substation type transformer as the LV side can float up to a dangerous fraction of the HV via the capacitance between adjacent windings, and must be solidly earthed.

It is common to have multiple NE bonds on one site if for example your installation is large enough to have two or more HV to LV transformers, but the N and E would normally be interlinked in such a cross-section of conductor that a diverted current equal to the full load would not be an overheating issue.

A UPS is more like having a genset on-site, that perhaps only supports a fraction of the main load, and there one may need to switch in an additional NE link at handover to UPS, especially if one of the conditions of the genset/ups operating is total loss of supply, including severing of the mains earthing. Personally I am not a fan of the idea of switched links, and would rather have  solid ones and suffer a line that cannot be RCD protected. (the RCD seeing the other NE bond as a fault.)

A really small UPS, say a plug in one supplying perhaps one desk, is not such an issue if it runs as an IT (fully floating no earth at all), indeed if you pull out the plug, this is unavoidable.

I think these 3 cases, solid bond, switched bond, no bond, cover all situations between them, for large medium and small set-ups. On the boundary, roll the dice.

Mike.