Neutral and Earth voltage with battery storage

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:

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,

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). 

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.

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. 

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.

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? 

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.

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? 

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.