There are 2 concerns, as you now have 2 live-ish  poles, relating to single pole breakers and switches only removing one pole so something may be 'dead' and still give a shock.

second, depending on the capacitor values,  the shock cuurrent may be modest or lethal, and yet limited by the capacitors may not operate a fuse or MCB for either a line to earth or 'neutral' to earth fault.

The ADS of normal wiring is not really designed for this split phase operation unless all breakers are DP.  A real contactor between N and E would be a better thing and the expected on larger UPS.

Mike.

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? 

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. 

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. 

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.