Definition of high protective conductor currents

Hi Andy,

I’m curious about your heat pump install setup. Would using SWA cable eliminate the need for RCDs in your case? Also, does your heat pumps have a dedicated earth connection point, similar to solar PV inverters, where you could attach a 10mm copper conductor to handle high protective conductor currents? 

Have you seen the Heat Pump Association updated guidance from March 2025? Lot of useful information on RCDs, although doesn’t mention isolation transformers as an alternative to manage leakage.

To save others from spending time  looking for it that guidance is on-line here HPA-RCD-Guidance-March-2025.

Mike.

I’m curious about your heat pump install setup. Would using SWA cable eliminate the need for RCDs in your case? Also, does your heat pumps have a dedicated earth connection point, similar to solar PV inverters, where you could attach a 10mm copper conductor to handle high protective conductor currents? 

In my setup, it's TN so Zs will be adequate, the supply HP cables won't be concealed in a wall or under a floor, or run underground, won't run through a bathroom, aren't supplying sockets or domestic lighting, and the HP although outside weighing in at over 100kg and being bolted down onto a concrete foundation (via large rubber feet) I would suggest wont count as "mobile". To my reckoning therefore I don't need an RCD (30mA or otherwise) from a BS 7671 perspective, regardless of the type of wiring system. If the cables were concealed or run underground I reckon SWA or similar might be needed.

Manufacturer's instructions are confused - the printed manual that came with the HP only asks for an RCD where the sit requires one, and diagrams show overcurrent protective devices, but no RCDs. Other (later?) on-line documentation, says "all" their HPs need a B-HP RCD, I've got an open enquiry with the manufacturer to try and clarify that point.

In my case I'm opting for 4mm² BS 8436 cable (partly because I rather like shielded cables, mostly because I have some left on a roll from previously) - run in flexible conduit outside to protect the sheath from UV, I've not spotted a dedicated stud (just a PE terminal on the main PCB) - but suspect I will be able to improvise something easily enough. I'm hoping to go down the 543.7.1.203 (ii) route - i.e. single 4mm with the BS 8436 cable providing equivalent protection to flexible conduit,

  - Andy,

I’m curious about your heat pump install setup. Would using SWA cable eliminate the need for RCDs in your case? Also, does your heat pumps have a dedicated earth connection point, similar to solar PV inverters, where you could attach a 10mm copper conductor to handle high protective conductor currents? 

In my setup, it's TN so Zs will be adequate, the supply HP cables won't be concealed in a wall or under a floor, or run underground, won't run through a bathroom, aren't supplying sockets or domestic lighting, and the HP although outside weighing in at over 100kg and being bolted down onto a concrete foundation (via large rubber feet) I would suggest wont count as "mobile". To my reckoning therefore I don't need an RCD (30mA or otherwise) from a BS 7671 perspective, regardless of the type of wiring system. If the cables were concealed or run underground I reckon SWA or similar might be needed.

Manufacturer's instructions are confused - the printed manual that came with the HP only asks for an RCD where the sit requires one, and diagrams show overcurrent protective devices, but no RCDs. Other (later?) on-line documentation, says "all" their HPs need a B-HP RCD, I've got an open enquiry with the manufacturer to try and clarify that point.

In my case I'm opting for 4mm² BS 8436 cable (partly because I rather like shielded cables, mostly because I have some left on a roll from previously) - run in flexible conduit outside to protect the sheath from UV, I've not spotted a dedicated stud (just a PE terminal on the main PCB) - but suspect I will be able to improvise something easily enough. I'm hoping to go down the 543.7.1.203 (ii) route - i.e. single 4mm with the BS 8436 cable providing equivalent protection to flexible conduit,

  - Andy,

Are you planning to use a separate consumer unit fed independently via Henley blocks ?

Are you planning to use a separate consumer unit fed independently via Henley blocks ?

Nope, not initially at least (longer term I might go for a 2nd CU in the house so that the PV could be removed from the main house CU where the RCBO isn't strictly speaking of the correct directionality and overload protection doesn't meet the letter of the current regs, and give me a spare way or two for anything else that might come along in the future). At present I have a HBC fuse DB at the intake in the cellar (feeing the house CU on the ground floor and another in the detached garaged) - that has a spare way, so I was going to pop a 16A fuse in that and take the HP supply from there.

   - Andy.