External IP65 CU off existing Wylex box?

[starfire]

Hi,

I have a 20A RCBO from my house CU feeding my garden studio via a 2.5mm SWA into another CU in my studio fitted with a 30mA RCD and MCBs.

I would like to have an external FCU on the outside of my studio to which I can wire a Solar inverter (via AC isolator).

As the inverter needs it's own circuit, and I do NOT want to drill through the studio wall to the CU due to various issues with its internal insulation, can a qualified electrician take a feed from the Wylex box already on the outside of the studio (where it's wired to the existing CU) and fit a new IP65 CU like this fitted with a suitable MCB and then onwards to an IP65 FCU ?

Once that new circuit is in then I can wire up my inverter to it.

Diagram with existing setup attached.

Thanks!

 

[flameport]

No. The inverter must have it's own circuit. Connecting it at any point of the existing circuit means it's still sharing the RCD protected circuit from the house.

Possible option is to remove the RCBO at the house and replace with an MCB, and connect the inverter to that part of it. However that depends on whether the SWA circuit from the house requires an RCD or not.

 

[starfire]

Great, thanks for your input, appreciated.

I don't know whether the SWA circuit from the house requires an RCD.

I wouldn't have thought so as I have an RCD in my existing studio CU that the SWA connects to so everything in my studio is going through this RCD.

 

[starfire]

Just to add, what if I had the electrician add an RCBO into my studio CU and we wire the inverter off that, would that be acceptable as effectively it would have it's own RCD ? My house CU only has RCBOs in it.

 

[starfire]

If I was to replace the studio CU with a split load model, with 2 RCDs, and one RCD has a single MCB dedicated to the Inverter that would be OK?

 

[flameport]

No.
The problem is the RCBO in the house.
If you connect an inverter to any part of the circuit with that RCBO in place, you can't connect anything else to it.
Inverters cannot share an RCD or RCBO with any other circuits or devices.

The reason is disconnection time - if a fault occurs, the RCD must disconnect within 40ms.
Inverter shut down time is typically several seconds, which would result in the disconnected circuit remaining energised after the RCD has disconnected the supply.
If other items are connected to that circuit, they also are energised for however long it takes the inverter to disconnect.

 

[starfire]

Thanks for the detailed reply.

I understand about the disconnection time and need for a separate circuit.

Don't understand why the split load CU in my studio wouldnt prevent this problem though.

If the RCD for the inverter circuit trips wouldn't that kill all power sent from the inverter instantly?

If the other RCD for my sockets trips then wouldn't that stop any power sent from the inverter reaching that circuit immediately?

Sorry for probably dumb questions just trying to understand

 

[flameport]

RCBO in the house trips, inverter continues to supply the sockets until it shuts down.

The only options are to get rid of the RCBO in the house, or connect the inverter to it's own circuit somewhere else.

 

[starfire]

If the house RCBO trips, then the inverter can still power the studio sockets as sockets RCD has not tripped? Ok think I understand.

So MCB to replace RCBO in house CU? The electrician did say when he wired my studio if I get nuisance trips may need to replace it.