Low voltage cables through flexible plastic pipe/hose?

[plugwash]

The question that is what exactly does the UPS do when switching to battery mode, I see three possibilities.

1. The output remains referenced to one pole of the input. This carries the risk that a fault on the output of the UPS could potentially cause the input to become live. It also carries the risk that in a lost neutral scenario the output could end up with one pole at mains voltage, and the other at twice mains voltage.
2. The output is disconnected from all poles of the input and starts to float
3. The output is disconnected from all poles of the input and is re-referenced to mains earth.

 

[JohnW2]

Line interactive type, so the output is not isolated from the input. A fault which would trip an RCD will still do so if it occurs on the output of the UPS.

That presumably depends upon how the "interactive type" is implemented in a particular UPS product, doesn't it?

My understanding (perhaps wrong!) is that the 'voltage adjusting' transformer through which power is transferred during
'normal' operation (i.e. in the absence of mains power failure) in a "line interactive type" UPS can be an auto-transformer, a two-winding (i.e. 'isolating') 'buck/boost' transformer or a 'ferroresonant' transformer - and I think what you have written would only be the case with the first of those three (although I have no idea which of the types of transformer is most commonly used) ?

Is my understanding incorrect ?

 

[JohnW2]

1. The output remains referenced to one pole of the input. This carries the risk that a fault on the output of the UPS could potentially cause the input to become live. It also carries the risk that in a lost neutral scenario the output could end up with one pole at mains voltage, and the other at twice mains voltage.
2. The output is disconnected from all poles of the input and starts to float
3. The output is disconnected from all poles of the input and is re-referenced to mains earth.

In a TN-C-S 'lost neutral scenario', your (1) and (3) would presumably be essentially the same (with same consequences) since the 'mains earth' to which the UPS had been "re-referenced" would be connected to the installation's neutral ?

 

[plugwash]

On a TN-C-S system you have two different types of "Lost neutrals".

A "Lost PEN", where the combined neutral and earth conductor breaks. In that scenario your whole earthing system becomes live and the details of equipment inside the installation matter little.

A regular lost neutral, after the split point, which behaves the same as a lost neutral on any other earthing system.

 

[flameport]

That presumably depends upon how the "interactive type" is implemented in a particular UPS product, doesn't it?

It does, but the most common buck-boost transformer arrangement will not provide an isolated output.
Neither would an autotransformer, although that would be an unlikely choice as it would be significantly larger and more expensive.

A ferroresonant transformer would be an isolated output but that will be unlikely as the cost would make it far too expensive for a consumer level UPS, and for larger systems double conversion (online) would be a better option for reasons of cost, size and efficiency.

 

[JohnW2]

On a TN-C-S system you have two different types of "Lost neutrals".

There are indeed.

A "Lost PEN", where the combined neutral and earth conductor breaks.

Quite so - and since that what everyone so often goes on about, and so many seem to 'fear' (and many just refer to as "lost neutral"), I assumed (seemingly incorrectly!) that's what you were talking about when you wrote "a lost neutral scenario"

In that scenario your whole earthing system becomes live and the details of equipment inside the installation matter little.

Indeed - provided that the building was properly constituted as an equipotential zone.

A regular lost neutral, after the split point, which behaves the same as a lost neutral on any other earthing system.

Fair enough - so you were just talking about a break in the neutral connection between CU/DB and the UPS? I'm trying to think of the undesirable consequences of the UPS having and output with "one pole at mains voltage, and the other at twice mains voltage."...

... as far as the connected equipment itself was concerned, it would presumably only 'see' the L-N potential difference, which should be 'normal'. Off the top of my head, the only issues I can think of would be the working voltage of any L-E filter capacitors on the power input to the equipment - or, I suppose, the 'voltage withstanding capability' of insulation separating live parts within the equipment from earthed metal. Anything else?