Single Pole Isolation.

Does anyone actually use single pole isolation main switches in single phase TN-S or TN-C-S installations, (or just line switching in  three phase installations) where main bonding is correctly installed?

Think of the cost saving? 461.2.

Z.

  • 462.1.201 [...] A main switch intended for operation by ordinary persons, e.g. of a household or similar installation, shall interrupt
    both live conductors of a single-phase supply.

  • Just as an aside, I've never understood why the N must be switched only for singe phase installations - if you're worried about dangerous voltages on the N (e.g. due to PEN fault), wouldn't the same rationale apply to a TP household?

  • On a single phase installation there is a small but real chance that an upstream open neutral will result in a dangerous voltage on the neutral, hence the need to isolate the neutral for safe working.

    On a three phase system, exactly the same could happen, but isolating the neutral on three phase carries its own risks. If the 4 pole switch opens the neutral just BEFORE the phases, or closes the phase connections just before the neutral, then gross over voltages could destroy single phase loads. For this reason, not isolating the neutral has historically been acceptable on three phase.

    These days special 4 pole switches are available, that by design will always open the neutral last and close it first. I expect that neutral isolation via such switches will become a requirement in future on three phase systems.

  • You need a special switch if you want to switch 3 P and N - one where the neutral contact is set back so it closes early and breaks late, relative to the 3 phases.

    If you do not do this then during opening and closing, unless loads are in perfect balance so you could cut neutral and walk away with it,  some loads  gets over and undervoltage when the neutral breaks first and there is a tug of war between the 3 load impedances to determine the new centre voltage.

    4pole switches for 3 phase use have a clearly marked N terminal.

    You may not care on a resistive or motor load that can tolerate up to 400V for a fraction of a second, but things like electronically ballasted lights do not like it at all, and do not work again after the briefest flash of 400v.  Sadly in a load imbalance of lights versus heating or cooking, lights generally come off worst, being the higher impedance.

    In a single phase case the balance of risk is different, as even if neutral is not switched offset, the load only sees 230V or nothing.

    Mike.

  • I think that you would be hard pressed to find a SP main switch, but perhaps you are thinking of older installations and EICRs? In any event, main switches are cheap as chips, so not much of a cost saving.

    We may need SPSN main switches, but we seem to be happy with SP switching for the circuits.

  • So with a three phase installation, the use of this 4 pole switch may cause problems if the N opens fractionally before the three lines upon turning off.

    https://www.ebay.co.uk/itm/173003722235?epid=28010512496&hash=item2847d34dfb:g:zeYAAOSwj99aHWFm

    Z.

  • Potentially, but we don't know what happens inside the switch.

  • Surely this is a misnomer, single pole switching does not mean isolation.

    Jaymack