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Disconnection of mains

The key issue here is to ensure that there can no NO back-feed from the emergency generator onto the normal supply mains.

If the normal supply wasn’t off, or unreliable; we wouldn’t be trying to make an emergency supply. So, we can assume that the mains are not energised. But safety first, so

  1. Turn all main switch(es) OFF.
  2. Test to confirm mains are de-energised.
  3. Treat the mains as live at all times (unless the supply fuses have been removed); because normal supply could be restored at any time.
  4. Carefully label each active conductor; to ensure correct phase rotation when normal supply has been restored.
  5. Even more carefully label the neutral; to ensure correct polarity when normal supply has been restored.
  6. Disconnect each mains conductor in turn, and cap each with an insulated terminal.

    Insulation tape is not enough

The TB stipulates disconnection of mains from (line side of) main switch. This is correct for cases where the metering is upstream of the main switch as is often the case for domestic installations.

However, we don’t want the energy from the emergency generator to be recorded by the meters; so if the metering is on the load side of the main switch; find a suitable alternative position. For example, the first terminals supplied from the meters. If possible; avoid breaking any seals for metering / load control.

Larger installations may have several main switches, including separate switches for each “safety service” (eg lifts, fire pumps, etc). We need to carefully consider which part(s) of the installation are “essential” enough to require the emergency supply. And it’s important that buildings are not occupied unless there is power for all “safety services”.

The TB also specifically requires disconnection of the mains neutral. This is unusual; and contrary to specific rules that normally apply.

In normal circumstances; the MEN system relies on the integrity of the mains N, in conjunction with the MEN link, to carry fault currents. It also relies on the earthing systems of all the connected installations, acting in parallel, to maintain the distribution N at close to zero volts relative to mass of earth. Strictly speaking, it’s actually a PEN conductor, with the “PE” part indicating that it’s performing a protective earthing function. Which is why AS/NZS 3000 prohibits switches or circuit breakers from operating in it [clause 2.3(b)] unless it’s linked with devices that switches the actives at the same time. It’s also why clause 7.3 prohibits a source selection (changeover) device from operating in the incoming (mains) neutral.

But what’s wrong for normal conditions is actually right for these unusual conditions. When there’s no supply from the grid; it doesn’t much matter if the voltage on the out-of-service distribution PEN floats up a bit. And when we’re not taking supply from grid; the mains (PE)N is not part of the fault current path. So, it does no harm to disconnect it from the installation’s MEN; and doing so means that even if the connection of the emergency generator to the installation is botched; we can’t energise the network via the disused mains.