Thanks.

If circuits are protected by RCBOs (double pole) does this then provide the required protection, or indeed a single, double pole RCD that covers all circuits? 

The RCBO will operate normally during grid mode, with its RCD and MCB functions operating as expected. 

The RCBO will offer 3 levels of protection during UPS mode (as would a main RCD): 

Double pole isolation 

Protection against electric shock (any leakage between L or N and E would trip the RCD circuit). Assume a 30mA rating. 

Protection against short circuit (any leakage from L or N to Earth would trip the RCD). 

I don’t think I am missing anything here? 

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. 

The UPS inverters are missing from the diagram you showed, although I assume it's connected L-N only.

That being the case, on first fault, if PE is disconnected from N, a TN system will be formed if there is no clear fault path. The RCD will clear on the second fault.

Is there an up-front RCD at the UPS output itself, or are you relying on downstream RCBO's?

You would need to check with the manufacturer that the requirements for IT Systems are met, i guess.

Hi,

The backup circuit I believe should ideally be comprised of the following: 

1x 100mA RCD (as a main switch - double pole) 

“X” 30mA RCBOs (circuit protection)

This means, that the main RCD provides additional protection and shuts down all circuits when a fault appears anywhere on the UPS output (assuming the fault is large - much like a TT system), including inside of the DB. Individual RCBOs protect each circuit.

This also means there is selectivity between the main RCD switch and each individual circuit.

This setup in my view works for both grid and UPS mode.

We then have overload protection, short circuit protection L-E & N-E (RCBO & RCD), shock protection (RCBO and RCD). 

vantech said:

1x 100mA RCD (as a main switch - double pole)

No selectivity - would have to be S-type.

vantech said:

We then have overload protection, short circuit protection L-E & N-E (RCBO & RCD), shock protection (RCBO and RCD). 

It would be necessary to treat the system in island mode as an IT system as the N to PE connection cannot be relied on (in the UK, BS 7671 Regulation 551.5.4.3.2.1)?

Regardless, we would still need a consumer's earth electrode for the IT system, as IT systems still require a means of earthing if PE is distributed to Class I appliances, 3-pin socket-outlets etc.


The following should also be noted:

• The IET Code of Practice for Electrical Energy Storage Systems does not recommend IT systems are used for island mode applications.
• The MCS MIS 3012 The Battery Standard (Installation) states a firm preference for TN-S earthing arrangement to be formed in island mode, and has a requirements that limits the use of IT systems in island mode to systems under maintenance by competent persons, and even then requires automatic disconnection of first fault (i.e. usually IMD not RCD approach) unless the installation is under  constant supervision of a responsible person.

Hi,

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

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

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

If the setup with RCDs (noted about S type) provides adequate protection on all fronts, surely this is then complaint with regulation? 

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

vantech said:

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

No.

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? 

Just because a product can provide continuous power, doesn't mean they are always suitable for use in systems with interconnected components, unless the installation is suited to those components.

Is it possible that product standards for products designed to provide continuous power are not always aligned with installation conditions and interconnected products made to other standards?

vantech said:

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

There is a difference between UPS products supplying a single, stand-alone product, and those supplying an installation - and there are differences between the operational conditions in commercial/industrial installations, to those in domestic installations.

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 distributor's means of earthing cannot be relied upon. The simple reason being, that the distributor may be working on that link (TN-S, TN-C-S or TT)  or the link may be connected by a cable in the network that is broken or being worked on (TN-S)

vantech said:

If the setup with RCDs (noted about S type) provides adequate protection on all fronts, surely this is then complaint with regulation? 

At the most basic level, yes - ignoring other factors that I have already put forward.

By any chance ... are you working for a manufacturer or importer of such a system, and wanting to find some way through the standards and guidance?

Are you, but any chance, working for a manufacturer or importer of such a system, and are looking for a route through the UK standards and industry guidance?

No, I am not a manufacturer or an importer. 

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.