On 05/12/2021 10:40, Fredxx wrote:
> On 04/12/2021 20:47, Mike Humphrey wrote:
>> On Sat, 04 Dec 2021 16:43:56 +0000, ARW wrote:
>>> On 04/12/2021 09:43, Theo wrote:
>>>> It would be possible to put the changeover downstream of the CU, so
>>>> you're only feeding a specific circuit rather than all of them.
>>> How? I cannot visualise this at all. Enlighten me please.
>> That's quite straightforward. Disconnect the circuit from its circuit
>> breaker, and wire it instead to the output of the changeover switch. Wire
>> one input of the changeover switch back to the breaker in the CU. Wire
>> the other input to the generator.
>> When the power goes off, move the switch over. That circuit is now
>> connected to the generator (and not the mains). Everything else is still
>> connected to the mains (only) and so is dead until power is restored.
>> There's no cross-connection as the mains input of the switch is isolated
>> from the generator input.
>> Perfectly regs compliant as long as you remember:
>> - provide a local earth rod
> Is this still necessary where the incomer is TN* variety? As long as the
> earth is experted to the generator.
With TN-C-S and a loss of supply, it is possible to lose the combined
Neutral and Earth as well (imagine a tree bringing down a LV overhead
wire). That could leave your main earth terminal floating.
(typically they use PME with multiple earth connections along the route
to try an ensure this never happens)
Likewise a TN-S setup could get cut and you lose the separate earth
connection to the substation. (although depending on where the cut is,
the cable sheath alone might make a reasonable TT earth if it's an old
metal clad cable)
>> - RCD on the generator circuit (you're now running TT earthing)
> Most Consumer units will be wholly RCD protected, or even RCBO. Does the
> generator need another?
Not necessarily... unless you need to protect the main feed from the
generator against earth faults.
>> - MCB suitable for the circuit on the generator feed (or an RBCO to also
>> address the point above)
>> - the required warning label about multiple supplies.
>> You could also go for an in-between option - split the supply to two CUs.
>> One is fed direct from the mains, the other via the changeover switch.
>> This allows you to have lights on a separate circuit to the boiler etc
>> while still having both fed from the generator when needed.
> I might at least suggest a visual means of knowing power had been restored.
>  In the case of a known neutral fault then obviously this won't work.
> and the earth will need to be isolated from the network.
Earth connections should not generally be "switchable". If going the
separate CU route then making that whole CU TT might be an option (if
possible - it can be tricky when you introduce main bonds to water
supplies etc if they are in metal pipes - you might just find you have
got a defacto TN-C-S earth again via someone else's TN-C-S neutral earth
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