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Neutral-Earth link is Island mode

David Slate

Hi all, quick questions.

Why the need for a N>E link in island mode?

If the system is TT or the back up part is, and L&N are disconnected by the inverter from the grid, once back up has switched, therefore supplying its own L&N why would that N need to be connected to the TT earth?

Or is the N>E link relay only necessary for systems that don't provide a neutral terminal on the back up side?

We have an Alpha Smile 5 unit. its provides a L&N in back up/UPS mode. Talking with Alpha this back up is effectively an IT network so requires a TT electrode for earth

Thanks in advance!!

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    David Slate said:
    We have an Alpha Smile 5 unit. its provides a L&N in back up/UPS mode. Talking with Alpha this back up is effectively an IT network so requires a TT electrode for earth

    The IET Code of Practice for Electrical Energy Storage Systems recommends against IT system being used in island mode. See Section 9.4.5 and Appendix F, item F1. The MCS Battery Standard (Installation) - MIS 3012 - limits the use of IT earthing arrangement in island mode to systems that are maintained by competent persons, which pretty much precludes many domestic installs.

    The problem with IT systems, is that, if they are used to supply multiple items of equipment, or equipment that has connections to other equipment (e.g. through wired networks, HDMI, etc.), the system can become "accidentally earthed". IT systems must still have a consumer's earth electrode (BS 7671 says you can't rely on the distributor's earthing terminal for this purpose ... you can connect to it, but must have a supplementary earth electrode also). There can also be issues with equipment (or circuits) having high protective conductor current in island mode, and some Class I products are not really suited to it.

    I think the issue being, just because a product is available, and complies with standards (assuming it does), doesn't mean it's always considered safe to supply parts of a fixed installation in the UK.


    I think the issue being, "UPS Mode" is not, strictly, "Island Mode", they are two different things. This is illustrated by the fact that, in island mode, the island mode supplied circuits should be isolated from the grid, yet UPS usually go into "bypass" for large current faults. There is the potential for discharge of energy back into the grid, I guess, using a system like that, which I don't think would fully comply with G99 requirements?

  • 0 in reply to gkenyon

    There was a similar discussion about 3 weeks ago, here: engx.theiet.org/.../neutral-and-earth-voltage-with-battery-storage

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