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Neutral and Earth voltage with battery storage


I have a battery storage system which is grid tied. This has been G98 registered. The battery has a grid tie and a UPS backup circuit.

The system works as expected, however when in UPS backup mode, I have noticed something that I need to clarify.

When in normal grid tied mode, the following voltages are recorded on the UPS output: 

L-N = 230V 

L-E = 230V 

N-E = 0V 

When in UPS mode, the following: 

L-N = 230V

L-E = 130V

N-E = 130V 

The manufacturer confirmed that when in UPS mode, a N-E link is made automatically and disconnected automatically. I have contacted them but being in China, they are slow to respond.

What I noticed, is that if you switch off loads on the UPS output, the voltage between L&E slowly increases to around 200V. I haven’t as yet tested to see if the same is apparent with N&E. 

Any ideas? This is an inverter so I am unsure if this sort of reading/phenomenon is normal? 


  • Those voltages suggest to me that the N-PE link isn't being closed in island/UPS mode. Floating outputs aren't unknown in small ordinary UPS designs but as you've confirmed that the should be a N-PE link in place, it sounds like a fault to me.

       - Andy.

  • "Backup" mode (term used in BS HD 60364-8-2 is "island mode") is not "UPS" unless the product is certified to BS EN 62040 series.

    A comprehensive switching arrangement is required for the system to operate in island mode. BS 7671 Regulation 551. states that in this mode, the distributor's means of earthing cannot be used, so all lines and neutral of the supply must be physically isolated, and a new neutral to earth connection switched in.

    It sounds to me, that the island mode power arrangement may not have the required switching, and simply to add a neutral to earth reference may contravene the ESQCR as well as BS 7671.

    The IET Code of Practice for Electrical Energy Storage Systems covers the topic of "floating" (unearthed) backup power outlets in Appendix F (page 157 in the 2nd Edition), and states:

    It is not recommended that such a socket-outlet is provided with a means of connection to an electrical
    installation or a permanently installed item of equipment.

    The Code of Practice then goes on to provide reasons for this.

  • It would seem to me, that in order to provide an earthed neutral in island mode, WITHOUT use of the suppliers means of earthing, that this unit requires an independent means of earthing. Such as a length of green/yellow cable to an earth rod. Is this installed ? I suspect not.

    I would expect FOUR incoming wires, phase, neutral, and earth, probably combined in a three core cable, like any other class one appliance, AND an additional earth wire to the earth rod. The two earth connections should be permanently connected to each other within the equipment.

    In normal or grid connected mode, I would expect that the outgoing neutral, to the loads is connected to the incoming neutral. The incoming neutral is earthed elsewhere by the DNO, and in grid connected mode NO ADDITIONAL  connection between neutral and earth should be made as this is contrary to UK regulations.

    In island mode the incoming neutral and phase conductors should be disconnected by a relay or other suitable means. The earth wire might remain connected but is not to be used as the means of earthing, or it might become disconnected. The outgoing neutral, now isolated from the incoming neutral, should now be automatically earthed by connection to the earth rod. 

    In grid connected mode, this means that the unit has TWO earth connections, one via the earth core core in the mains supply cable, and a second one via the independent earth wire to the earth rod. This is fine,and  no different from a water heater earthed via the green/yellow core in the supply cable and again earthed via metallic water pipes, or say a central heating pump.

    In short, multiple connections to earth are fine and often occur fortuitously. The neutral conductor must be earthed, but ONLY ONCE either by the DNO, or in island mode by the customer, NEVER BOTH at the same time.

  • Hi, 

    Here is a data sheet for the inverter/battery which is certified to IEC 62040-1 and IEC 62477-1 (and the relevant G98 certification).

    It appears the output is a true UPS. 

    A customer owned earth rod has been attached to the MET, to remove the reliance on the DNO supplied earth.

    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.

    So they have clarified the findings are normal? It sort of makes sense, as this L-E voltage increases as loads are switched off. I am yet to test to see if the N-E voltages decrease when loads are removed. 


  • 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? 

  • 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.