The manufacturer today confirmed the following: 

A “N-E” bond is created on the UPS backup output when in backup mode. 

Voltages of 130V between N-E and L-E are normal, due to capacitances in the UPS output.

That doesn't make sense to me - any capacitive coupling between N and PE should be shunted out by the N-PE link (if it's correctly being applied).

   - Andy.

I thought that too

The manufacturer came back today with this drawing: 

The UPS output is a true UPS, not the same as a standard generator which requires a hard/mechanical neutral to earth bond. This capacitor arrangement is switched in only when in UPS mode. 

The voltages between L-E and N-E are because of these capacitors, roughly half of the L-N voltage.

The UPS is regulated and approved to various UPS standards.

Looking at this also, RCDs would also function normally in UPS mode. 

Well that's not compatible with their statement "A “N-E” bond is created on the UPS backup output when in backup mode. ".

The UPS/Generator comparison for N-PE links isn't helpful - it's more a matter of size rather than type - large UPSs will have N-PE links and small generators (typically <3kVA) often run without them.

It's all down to the chosen method of providing protection against electric shock - you can have electrical separation where shocks (to Earth) are prevented by ensuring that the system's connection to Earth is so poor that a dangerous shock current can't flow (i.e. no N-PE) link, or you can have an N-PE link and have automatic disconnection of supply (ADS) to mitigate against shocks. (Or there are other methods such as double/reinforced insulation or SELV/PELV).

The complication here is that BS 7671 only recognises separation for supplying single items of current using equipment in normal circumstances - so you can't use it for supplying multiple items in a fixed installation. And the Electricity Safety Quality and Continuity regulations (law) demands that your system complies with BS 7671 if you have embedded generation.

   - Andy.

Hi, and thanks.

Yes, I think the language barrier is somewhat prohibitive. I asked if there was a N-E link, and they responded with a yes but clearly meant there was some “form”’of link. 

During normal operation, the UPS output of the inverter operates the same way as the non-UPS side. If you measure L-N, L-E the voltages are 230V. If you measure N-E the voltage is 0V.

It is only when the UPS is in output mode, that the voltages between N-E and L-E are at 130V, because of how the inverter operates. 

The supplier has confirmed that MCBs and RCDs would operate normally during backup mode, therefore providing protection against electric shock. 

How does this operation contravene BS7671 specifically? 

vantech said:

The UPS output is a true UPS, not the same as a standard generator which requires a hard/mechanical neutral to earth bond. This capacitor arrangement is switched in only when in UPS mode. 

The voltages between L-E and N-E are because of these capacitors, roughly half of the L-N voltage.

The UPS is regulated and approved to various UPS standards.

Looking at this also, RCDs would also function normally in UPS mode. 

Just because the UPS is provided to UPS standards, doesn't mean it's safe to continue to use it in the UK for fixed installations, or equipment interconnected with other equipment in the installation, in the absence of the grid power.

The issue with the public supply, is that you can't rely on the distributor's N-E connection for alternative sources of power - Regulation 551.4.3.2.1.

The installation downstream of the UPS must meet BS 7671 requirements for protection against electric shock under all intended conditions of operation.

I discussed this below actually after some further feedback from the manufacturer. 

If the UPS output is protected by double pole RCD(s), this I believe covers the requirements set out in BS7671. It disconnects the live and neutral conductors if a fault appears.  

Double pole isolation. 

Short circuit protection (additionally provided by the RCD as well as the MCB). A short circuit between live and neutral would operate the MCB. A short circuit between L or N and E would operate the MCB and the RCD. 

Protection against electric shock by means of the RCD (a human touching the L or N making contact with earth (either a conducive part of true earth). 

Additionally, the system has a customer owned earth rod connected to the MET. 

What are your views? If the link that’s made between E&N by the UPS via means of capacitors, allows protective devices to operate in the correct manner, this would then provide the same level of protection as a direct N-E bond by a contactor/change over switch. 

I don’t think I am missing anything and this is what the manufacturer is suggesting. 

I discussed this below actually after some further feedback from the manufacturer. 

If the UPS output is protected by double pole RCD(s), this I believe covers the requirements set out in BS7671. It disconnects the live and neutral conductors if a fault appears.  

Double pole isolation. 

Short circuit protection (additionally provided by the RCD as well as the MCB). A short circuit between live and neutral would operate the MCB. A short circuit between L or N and E would operate the MCB and the RCD. 

Protection against electric shock by means of the RCD (a human touching the L or N making contact with earth (either a conducive part of true earth). 

Additionally, the system has a customer owned earth rod connected to the MET. 

What are your views? If the link that’s made between E&N by the UPS via means of capacitors, allows protective devices to operate in the correct manner, this would then provide the same level of protection as a direct N-E bond by a contactor/change over switch. 

I don’t think I am missing anything and this is what the manufacturer is suggesting.