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Isolating the system for a planned power cut

Solar Photovoltaic (PV)

Last Post by Transparent 3 hours ago

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TRBob

(@trbob)

Estimable Member Member

Joined: 3 years ago

Posts: 50

Topic starter 20/08/2025 2:07 pm

There is an upcoming planned power cut, see the attached letter that I got from NPG. It will be for about 28 hours whilst they do some work to our local network and we will be put on a generator.

It says to isolate generation equipment (a heat pump is an electrical generator?!?) from the mains supply, but I'm pretty sure that my inverter will do this anyway when it loses mains power. Plus my PV isolators will not physically close when under load, I can only turn them off at night.

Would turning the trip off in the consumer unit be enough?

Mitsubishi Ecodan 8.5kWh
4.4kW Solar PV
5.2 kWh Battery Storage
1983 build, 300mm loft insulation, cavity wall insulation (beads)

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AgentGeorge

(@agentgeorge)

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Joined: 6 months ago

Posts: 23

20/08/2025 2:18 pm

Thats how my PV works, it says on the CU to isolate the array in the loft if doing any work on the array, or Id guess doing that at night would be safe 🙂

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Transparent

(@transparent)

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Joined: 4 years ago

Posts: 2455

20/08/2025 4:38 pm

A great many ground-mounted local substation transformers are equipped with connection points for a generator.
I've inserted an arrow in this photo, which is pointing to the connection point for Phase-1 (brown)

To it's left are the connection sockets for Phase-2 (black) and Phase-3 (grey).

Yes, your G98-approved inverter will automatically cease operating once it detects a power-outage.
But they're telling you to physically isolate it so that it's not exporting whilst the generator is in operation.

You should have more than one point of isolation.
These are usually a rotary switch,
and the 32A MCB in your consumer unit.

If your Consumer Unit has an RCBO on the supply to the inverter, then now is a good moment to check if it is bi-directional.
An RCBO will trip either because of over-current, or due to "earth leakage".
Ie it saves your life if you happen to touch part of the system which has become 'live'.

That's a new regulatory requirement which came into force last year.
The usual uni-directional RCBOs have been found to fail when the inverter is exporting back to the grid.
The small solonoid coil which causes it to trip can burn-out when left energised.

If you don't know which type of trip you have, then please post a photo here.

This is a safety issue, and it's important to get it right!

Returning to the matter of your planned power-outage,
there is likely to be a surge-current, and short-term voltage 'brown-out' when the temporary generator is switched on/off,
and again when mains is resupplied from the grid.

Depending on what devices your neighbours have left switched on, the surge might damage electronics.
That's why they want everyone to turn OFF high-current appliances with heating coils.

If your landline and broadband are supplied via copper cable, then the surge will also cause fluctuation on the earth wire in your house.

That's bad news for computers and Smart TVs connected directly to your broadband hub.
In my experience, the communication interface will manifest an intermittent fault as a result of that earth fluctuation.

Intermittent faults are extremely hard to diagnose!

It's best to remove any copper connection leads from your telephone apparatus and router before NPG start switching over to their generator.

A laptop running from its battery, and connecting via WiFi will be OK.
There's no copper cable to be affected by a surge.

This post was modified 3 hours ago 3 times by Transparent

Save energy... recycle electrons!

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