that the capacitive “bond” if you like between N and E does not meet the requirements of BS7671 to claim to be a TN-S setup

A capacitive "bond" is definitley not a TN-S arrangement - where the N is directly connected to Earth. It could be IT (which might be either isolated from Earth or connected to Earth via an impedance).

It might be good to establish the values of these "capacitors" - often they're just incidental from the proximity of conductors or small values intended for noise suppression and small values won't be capable of passing significant currents.

IT systems from supplies (such as inverters) that can only provide very modest fault currents can be "interesting" from a shock protection point of view. RCDs won't trip on first faults unless the capacitance is reasonably large. Tripping on second faults (with the second fault from a different live conductor from the first) may not be detectable by RCD either, so normally relies on overcurrent devices for disconnection - but if your source can't provide enough ommph to flow fuses or open circuit breakers in a short time, then that isn't going to happen either. The inverter itself might spot the overload and shutdown, or just produce a reduced voltage, but then you're relying on details of the inverter's behaviour to be sure of safety from shock (and ensure that that behaviour is reliable).

   - Andy.

Thanks, one would like to think this has been detailed and thought through by the manufacturer.

This is a large brand, and the products are sold globally and in the UK, they are approved to G98, G100, etc. 

I am thinking maybe I am over thinking this. The manufacturer has confirmed this is normal behaviour. 

I have asked for other specifics and await their response. 

Thanks. 

vantech said:

No, I am not a manufacturer or an importer. 

Apologies, had to ask the question for a number of reasons.

vantech said:

I have this installed on a property. Just looking for clarification as to why the system operates this way, because it’s function isn’t outlined in any guidance. The manufacturer seems to suggest this is normal. I am just trying to understand how they are getting to that. 

The arrangement is acceptable in some other countries, but that doesn't mean it's fully suited to the UK.

vantech said:

So you are saying that the output from the inverter is in fact an IT arrangement,

Probably ... but the manufacturer will need to confirm that for you, i.e. if the N to PE link is broken in the supply, is an IT arrangement formed?

vantech said:

and that the capacitive “bond” if you like between N and E does not meet the requirements of BS7671 to claim to be a TN-S setup?

Definitely, but that doesn't mean BS 7671 prohibits an IT earthing arrangement. What I am saying is that the industry guidance, for lots of different reasons, doesn't recommend an IT earthing arrangement particularly in domestic settings.

vantech said:

If this setup is complimented by an earth rod owned by the consumer, can the N-PE link then be relied upon? 

No, the electrode is required to maintain a connection with Earth for the PE of the system, to prevent things like static charging that might occur if the system were to float ... but also to help with detection and clearance of faults in the IT system.

vantech said:

There may be a preference to not use IT systems, but does this actually make the system non-compliant? 

No, but there are reasons of safety why its use should be limited, and I think AJewsbury has already covered in his recent post.

vantech said:

Is it possible the regulations really focus on “break before make” style backup systems, rather than systems that are designed to ensure continuous power? 

The Regulations (BS 7671) permits UPS to be used. However, that doesn't mean their earthing arrangements can be ignored by the installer (or manufacturer's installation instructions). This may not be a problem in countries such as Germany, where the installation is likely to have something known as a foundation earth electrode already, and where the N-E link is solidly made at the consumer's premises - but that doesn't mean German installation practices always translate for use in the UK.

Sorry I didn’t see the rest of your response.

I am contacting the manufacturer again for clarification. I find it hard to believe a global brand like this could miss something so important.

Could it just be possible that all of the required protection is built into the inverter itself, perhaps monitoring devices and overload/short circuit protection, that deliver the same level of protection as let’s say, the manual method of doing things with switchable links? 

As far as I can see, the only issue is the fact the N-E link is not connected directly when in UPS mode. The rest of the solution is absolutely compliant and tested in accordance with G98, G100 and all other relevant standards. 

I feel annoyed as I was told the output formed a TN-S output and it appears this isn’t the case. 

vantech said:

As far as I can see, the only issue is the fact the N-E link is not connected directly when in UPS mode. The rest of the solution is absolutely compliant and tested in accordance with G98, G100 and all other relevant standards. 

Don't forget the consumer earth electrode if you haven't got that already?

vantech said:

I feel annoyed as I was told the output formed a TN-S output and it appears this isn’t the case. 

I think I would be also.

You may think they have thought about this, but there are a great many smaller UPS and gensets that are indeed floating output, and this is IT, not TN-S whatever they say, only a solid metal link from N-E makes it TN-S. The assumption often that the loads will either be double insulated, or one item or logical item (a free standing desk of equipment perhaps) that is class 1.
The capacitors are often set more by considerations of meeting EMC specifications, rather than thoughts of how downstream ADS may work - in small systems it often does not need to, as the inverter just dies under overload.

Note that the UK is quite unusual in prohibiting in law an NE connection in a consumer installation connected to the public network. There are some cases when it would be easier if it was worded in a way that allowed it under limited specific conditions (like if the earth cross-section was the same as the neutral). I have had a few conversations over the years with folk from places where it is allowed who are surprised at the extra complexity it can introduce.
(consider this NZ/AUS example of how to wire an extension - where would we put an RCD ? )

Mike

mapj1 said:

Note that the UK is quite unusual in prohibiting in law an NE connection in a consumer installation connected to the public network. There are some cases when it would be easier if it was worded in a way that allowed it under limited specific conditions (like if the earth cross-section was the same as the neutral). I have had a few conversations over the years with folk from places where it is allowed who are surprised at the extra complexity it can introduce.
(consider this NZ/AUS example of how to wire an extension - where would we put an RCD ? )

The situation that can arise with multiple N-E connections, if the installation is not properly arranged, is the unwanted operation of RCDs in single-phase systems where the neutral is one of the poles of supply. Not an issue with "split-phase" or "centre-tapped transformer" single-phase systems though ... but in those arrangements the UPS would again be floating.