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

Ordinarily, the UPS is floating, no N-E or L-E bond assuming no faults already exist when it kicks in, but which might be the case if the mains RCBO has just tripped. 

I’m certainly looking at fitting an RCD to cover a secondary fault although I would want to avoid nuisance tripping and there are a couple notes under 411.6.3 that are putting me off using one.

I am thinking an IMD that can warn the skilled operators might be more useful which is an option under IT systems …. so I’m also interested to know if the UPS (when in battery mode /floating L-N) is a true IT system and can be treated as such? 

A Isolated UPS (floating) providing true isolation is an inherently safe electrical environment, if a line to earth fault occurs there is no reference to the incoming earth, and therefore no fault current can flow. But do we still need to comply with 411.3.3; 415. Or are you looking at something like Fig 710.2 for medical IT system ?

Udtwmc said:

I’m also interested to know if the UPS (when in battery mode /floating L-N) is a true IT system and can be treated as such? 

Only the manufacturer can answer that.

Udtwmc said:

I’m certainly looking at fitting an RCD to cover a secondary fault although I would want to avoid nuisance tripping and there are a couple notes under 411.6.3 that are putting me off using one.

You'd certainly need to think about protective conductor currents and as such the RCD may operate on first fault.

There are a couple of other problems:

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.

2. If the supply to the premises is TN from a distributor, consider Regulation 551.4.3.2.1. You can't rely on the distributor's means of earthing for such a UPS, because if the distributor is working on their cables, the connection with Earth may be removed.

To fix 1. , you would need a separate earthing connection from the distribution board (or MET if you want to address the risk of electrical maintenance removing the PE connection to the distribution board without powering down the UPS - there are some important issues here for the CDM risk assessment).

To fix 2., you can provide a consumer's earth electrode connected to the MET. An  additional consumer earth electrode is actually recommended for TN systems in any case, see Regulation 411.4.2.

AMK said:

A Isolated UPS (floating) providing true isolation is an inherently safe electrical environment, i

Only for a single item of equipment - in fact, where electrical separation supplies multiple items of equipment, this can only be done where the installation is controlled or under the supervision of skilled or instructed persons (see Regulations 413.1.2, 413.1.3 and Section 418).

It's important to differentiate IT System with ADS (Regulation Group 411.6) from Electrical Separation (Section 413).

It is also important to consider with information technology equipment (ITE), especially Class I, or where combination power supplies have Class I input and Class III-compatible output, that the circuits which lead to protective conductor currents may at some point provide (non-zero impedance) fortuitous earthing of live conductors in the low voltage AC circuit.

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.

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.