Interesting summerhouse dilema

[mikhailfaradayski]

Have you checked the IR of the feed to the outbuildings?

Finding low IR on a buried cable over that distance would not be a surprise.

Couple that with any leakage current from appliances may result in >30mA to earth.

 

[flameport]

What I need to clarify is it the voltdrop which is causing the trip or an earthing issue?

There is no way for anyone here to know.
Coule be the cable is damaged
Could be the high capacitance of the ridiculously long and undersized cable
Could be a faulty RCD
Could be wired incorrectly
Could be faulty appliances
Could be high leakage currents in the house and the summerhouse mess just adds enough to trip the circuit
Could be any combination of the above, or other things not mentioned.

Having an RCD in the house for this circuit is wrong, should be in the shed/summerhouse.
2 RCDs on the same circuit is wrong.
Adding an earth rod won't fix the problem.
4mm SWA connected to a 40A MCB is wrong

The electrician you had in was right - to install this circuit properly using the correct size cable and other materials will be ridiculously expensive and completely OTT.
Even if using 10mm 2-core, the cable alone will cost hundreds of pounds, and when rolled on a spool will weigh over 100kg - far too heavy for a single person to lift.

 

[Spark123]

Landie - when you applied for Building Regulations approval, how did you say you planned to comply with Part P?

I do not think building regs are required for a summer house, as it is classed as a temporary structure(presuming timber construction & no masonary involved)

Part P still applies as you are adding to a system which shares it's supply with a domestic dwelling. This work is notifiable too.

 

[LANDIE]

I tried the 2kw heater in the furthest shed from the house and this worked ok. So you are saying that the fitment of a RCD at the summerhouse alone is the correct means of curcuit protection? So what if I now run a independent 6 or 10mm SWA from the house via a 30a MCB to my new 6mm to the summer house?

 

[flameport]

So you are saying that the fitment of a RCD at the summerhouse alone is the correct means of curcuit protection?

That would be PART of the required protection.

I tried the 2kw heater in the furthest shed from the house and this worked ok.

Something working is very different from it being safe or installed correctly.

Apart from the fact that 30A MCBs no longer exist, 30A will be no good even with 10mm due to the cable length.
On voltage drop alone, for a 32A circuit over that distance you would need at least 50mm cable.

160m of 10mm cable will be too long anyway as the disconnection time won't be met for a 32A type B circuit breaker even if you have a TNCS supply.

In summary -
Attempting to provide power to this summerhouse is a complete waste of time, money and effort.
You clearly don't have the required knowledge to design or install this circuit correctly, so if you insist on going ahead with it, please employ a competent electrician to design and install it for you.