Buying large amp bidirectional RCD and RCBO

Energy Storage

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Batpred

(@batpred)

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Joined: 10 months ago

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Topic starter 28/10/2025 3:44 pm

I recently bought a 8k hybrid inverter.

The vendor confirmed that in order to connect it to the consumer unit, cables and protection devices needed to be rated x1.25 the maximum current that the inverter would receive or send. These are typically MCBs (preferably use RCBOs that also enable RCD function).

The following diagram shows what is involved.

In my case, two cables were planned to be buried in a wall and so they would need RCD protection (which is preferably achieved using RCBOs):

- one is used to supply power to backup loads

- the second is used to provide grid power to the inverter. It is also used to provide power to the house and/or grid, when there is production on site. So this requires devices that are not damaged when subjected to bi-directional flows.

In SE England, the shops that you can go and buy them include these that are open extended hours:

- screwfix.com

- toolstation.com

- tlc-direct.com

Additionally there are specialised shops like CEF, that are only open during working hours, so you may prefer delivery in 2/3 days.

The manufacturer advises for 100ma RCDs to be used. But in case your local electricity grid is not rock-steady (like in my case), the inverter cleans the power before using it. This often results in frequent triggering of the RCD. The manufacturer may recommend 300ma or higher RCDs. If PV is produced, this can also result in the need to use less sensitive (higher ma) devices.

In my case, the amp rating of devices required was 62.5 Amp for the grid and 45.5 for the backup. But given these inverters are available in 10Kw+ sizes, higher capacity may be needed.

Finding suitable devices proved time consuming as you find the protection level originally expected needs to be adjusted. Additionally, in some cases, RCBOs of the required rating were not available and so RCDs would have to be combined with MCBs.

Over to @transparent and others to add to my summary description and identify suitable options and example shops.

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Transparent

(@transparent)

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Posts: 2659

28/10/2025 5:27 pm

There won't be many RCBOs with a 100mA trip rating because that's stipulated for fire protection, rather than human safety.

However, Proteus do manufacture a 63A rated Type-A RCBO here in the UK.

CEF are the usual suppliers for Proteus trips, but other professional electrical outlets may order them if you ask specifically.
This isn't the sort of item you'd expect to source through a retail chain such as Toolstation (Travis Perkins Group), Screwfix and B&Q (Kingfisher Group).

Depending on the required cable length and the method of burial in the wall, I would expect the required mains cable size to be 10mm² or 16mm²

For current capacity above 63A you'd need to use a separate RCD (Type-A) and an MCB selected according to your cable size.

The RCD would need to be able to carry at least the same current as the MCB, and 125A would be fine.

The RCD and MCB will be 3-phase of course.
An inverter requiring that much current wouldn't be permitted by the DNO on a single-phase supply.

The other major manufacturer of such high-current RCDs and RCBOs is Chint.

They manufacture in China.
Their products are available here in UK,
or at reduced price by ordering direct from the Chint Electrical store on the AliExpress platform.

Delivery from China is typically 8-10 days at the moment.

Save energy... recycle electrons!

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