TT Submains time delayed

Clinchy · Jul 14, 2022

Hello,

I've been asked to install a supply for a TP air source heat pump to heat a swimming pool.

The pool room CU is feed from a barn with a TP supply and luckily the SWA feeding the pool room CU is a 4 core.

The barn is going to be converted in the near future into 2 holiday lets so I'm going to upgrade the ancient board that's in there.

My question is can I cover the couple of SWA submains by a 30 or 100ma time delayed RCD and have it conform? I have hardly touched any TT systems as we don't have many around where I am.

Many Thanks.

Electro-tech · Jul 14, 2022

Time delayed 100ma S type are usually a main switch with a split load board of maybe 2 30 ma RCDs to give you discrimination. What is the type of supply at the origin. Is the ground condition suitable to drive in a Earth Electrode, may be test the soil resistance first if you can The issue I think you may have is the inrush currents of the heat pump unless it has a soft start built in. Rcd’s don’t usually like high start up currents. The heat source pump Would need to be protected by a 30 mA Rcd .
There is bound to be other guys replying once the pubs turn out .

Clinchy · Jul 15, 2022

At the source I have a 3 phase TT incomer supplying a 3 phase board. There's 3 sub mains from that board to the pool house, tennis court and garden room.

My plan is to upgrade the 3 phase board in the barn using a 4 pole 100ma time delayed RCD as the main switch to cover the 2 SWA sub mains and then 30ma RCBO the barn finals. Then have a 1 way three phase board in the pool house for the pump.

Electro-tech · Jul 15, 2022

Clinchy said:
At the source I have a 3 phase TT incomer supplying a 3 phase board. There's 3 sub mains from that board to the pool house, tennis court and garden room.

My plan is to upgrade the 3 phase board in the barn using a 4 pole 100ma time delayed RCD as the main switch to cover the 2 SWA sub mains and then 30ma RCBO the barn finals. Then have a 1 way three phase board in the pool house for the pump.

Your Plan sounds okay to me as long as you get a low enough Earth Electrode resistance. I think you may need to check with the Pump Manufacturer as to what 3-phase RCBO or RCD they may specify. C type etc. They will know the expected inrush current.

pc1966 · Jul 15, 2022

Both 100mA and 30mA RCDs should have rod impedances less than 200 ohms (for evidence of stability of earthing, not the 500ohm/1.67k that 50V/In gives) and that is normally OK to meet with a single rod.

But check any existing rods for both soundness of connections and their Ra.