Is the UPS floating ? (Neutral -earth) not connected or are they connected . If N-E bonded you could fit secondary RCD to the output of the UPS to comply with 411.3.3

gkenyon said:

1. If the plug is removed, equipment will still be powered but there will be no connection of the PE conductor to Earth as well.

I see this too. But if the UPS battery takes over as the new supply source what purpose does the earth connection of the exposed conductive parts to the upstream distribution earth or building MET actually serve? 

I should add that in this particular installation the equipment earths are bonded to a PE terminal within the UPS enclosure so don’t solely rely on the UPS load side earth I.e. the ups could be unplugged and PE connection remain intact). 

So is the system to be classed as a TN system with electrical separation or treated as an IT system using a isolation transformer and then using an insulation monitoring device? I’m going with a TN system with electrical separation as the output of the UPS in battery mode is floating with respect to earth, but the exposed conductive parts of the equipment are still earthed.

it is only TN something if there is a solid CPC to neutral bond while the UPS is operating. It may well be that there is a loosely earthed neutral via some indeterminate stay impedance, but it may not be enough to fire the largest MCB. However, if there is earth fault protection relay or RCD that breaks L and N, then in practice the fact it is neither a truly floating IT nor truly connected TN may not matter.

But it does make it awkward to classify and test, and some faults occurring while on UPS may not trip anything until mains is restored- equally of course you may not need to worry about that..

Mike.

Udtwmc said:

what purpose does the earth connection of the exposed conductive parts to the upstream distribution earth or building MET actually serve? 

Very simple. It provides a means of connecting "first fault" to Earth, so that either "second fault" is detected, or an IMD can operate on "first fault".

Further, regards the statements I made re Electrical Separation vs IT, if there is no Earth connection to PE, it can't be an IT system, which means it's only safe (under all conditions) to supply a single item of equipment, unless the installation is under constant supervision of skilled or instructed persons.

This is the important differentiator between IT and Electrical Separation.

AMK said:

I’m going with a TN system with electrical separation as the output of the UPS in battery mode is floating with respect to earth, but the exposed conductive parts of the equipment are still earthed.

If the exposed-conductive-parts are still Earthed, that would be an IT system, not electrical separation. [And in this case, you can't rely on the distributor's means of earthing as discussed above.]

In electrical separation, there is no connection between PE [exposed-conductive-parts] and the general mass of Earth.

Udtwmc said:

I should add that in this particular installation the equipment earths are bonded to a PE terminal within the UPS enclosure so don’t solely rely on the UPS load side earth I.e. the ups could be unplugged and PE connection remain intact). 

If "floating" (connection with Earth lost but exposed-conductive-parts are connected to a live conductor) that system is not defined in IEC 60364, HD 60364 or BS 7671 at the present time, and potentially not [always fully] compliant. This is because there is a specific fault condition that can lead to exposed-conductive-parts and connected metalwork becoming immediately hazardous-live with respect to Earth. I believe there has been at least one case of death (and prosecution) resulting from similar circumstances.

However, that doesn't mean it's always unsafe .,.. importantly , it can be unsafe under certain specific conditions. For this reason, protection by RCD that disconnects all live conductors, as close as possible to the source (in this case the UPS inverter) is imperative ... but that doesn't necessarily mean 30 mA.

There are odd occasions, such as the supply of a single EV from a single transformer that are cited in BS 7671 as compliant. In other cases, e.g. BS 7909, where the installation is under constant supervision, or specific cases in BS 7430, it may also be acceptable.

Work is ongoing to research this type of system, but in the mean-time, using it outside the use-cases in Section 722 of BS 7671, or BS 7909, or BS 7430, might lead to issues.

It is imperative for BS 7671 compliance in this situation (Regulation 551.4.3.2.1) that, unless the supply is from private transformers (where the installation has its own earthing system) there must be an additional consumer's earth electrode connected to MET.

That system is not defined in IEC 60364, HD 60364 or BS 7671 at the present time. So is the system a hybrid TN/electrical separation? What would you call the current setup ? So, When the UPS is running normally or as a backup, the installation uses the TN system and the protection is (ADS). When the power fails the UPS/battery becomes the new supply source, and the installation switches to the IT system ? So options are use monitoring devices to detect the first fault to earth but this option requires that the UPS output neutral is separated from earth but all the class I devices are connected to earth. Or use electrical separation to ensure that each class I device is supplied by its own individual circuit from the UPS output and that there is no common connection between them. The UPS output neutral is connected to earth but each class I device is not and each device is individually protected?

AMK said:

That system is not defined in IEC 60364, HD 60364 or BS 7671 at the present time. So is the system a hybrid TN/electrical separation? What would you call the current setup ? 

So, provided it's connected to an Earth electrode belonging to the consumer, it's TN-S.

If it's connected to the MET, which is only earthed by an incoming DNO supply, not usually permitted under BS 7671 (and BS 7671 is specified by ESQCR in Great Britain and the NI version in Northern Ireland) if you have switched alternative supplies like a UPS. See Regulation 551.4.3.2.1.

If it's not connected to a means of earthing, again doesn't comply with BS 7671.

Point of discussion: some people think that either 551.4.3.2.1 and/or Regulation 21 of BS 7671, doesn't apply to a UPS because a UPS is not a "switched alternative supply" ... BUT ... that would only be true for a UPS with no bypass switching arrangement.

so treat it like an isolated generator configuration, common on small single phase generators. No connection to the mass of earth or between neutral and protective conductor, an I-S system. with the protective conductor connection to earth then would be classified as an IT system. On the I-S system with a number of class 1 equipment, each item should have its own RCD.

so treat it like an isolated generator configuration, common on small single phase generators. No connection to the mass of earth or between neutral and protective conductor, an I-S system. with the protective conductor connection to earth then would be classified as an IT system. On the I-S system with a number of class 1 equipment, each item should have its own RCD.