Protection for garage/studio off of house main

[Auchlossen]

I have a studio[flat] and garage run off the house supply.

Solar panels on roof of garage, so solar feeds via its control unit into the studio sub-board.

Electrician put in MCB to supply/protect mega ducted underground cable to studio sub-board.

There is a lot of potential load here with water heater, mini induction hob, ring main, lights, etc.

I have been recommended to have RCBOs in the studio/garage circuits to protect and isolate them individually;

tho not for the circuit feeding the solar.

I am concerned at the lack of discrimination between MCB in house and RCBOs in studio.

What is the preferred option for this sort of circuit?

 

 

[Jeremy Harris]

It's fine to have an RCBO on the PV circuit, in fact it's good practice to fit one, rather than just an MCB.  I'm a fan of all-RCBO set ups, double pole ones for all outside stuff if practicable.

[ProDave]

Some makes of inverter trip rcd's and advise against them.

 

You haven't mentioned what size cable feeds the studio, and how it is connected and protected in the house. That is a much more important question.

[Temp]

I think some form of RCD protection either a shared RCD or individual RCBO is mandatory for new circuits on the ground floor.

[Jeremy Harris]

8 hours ago, ProDave said:

Some makes of inverter trip rcd's and advise against them.

 

You haven't mentioned what size cable feeds the studio, and how it is connected and protected in the house. That is a much more important question.

 

That's interesting, Dave, any idea why?

 

The MI's for ours specify that an RCD or RCBO must be fitted, something I only picked up when the installer went to connect it up.  I thought that it would need an MCB, so had provided one ready for it, but the installer pointed out the bit in the MIs and replaced the MCB with an RCBO.

 

 

[ProDave]

Some say Sunny Boy inverters don't like an rcd. But mine has been running for 5 years on an rcd and has never tripped yet.

[Jeremy Harris]

I wonder if it might be related to internal filtering?  Some types of harmonic filter can cause a small current imbalance, I believe, perhaps enough to cause a sensitive RCD to trip.

 

I'd heard the same as you, that PV inverters didn't like RCDs, and that's why I'd fitted an MCB in the PV slot in the CU, and was a bit surprised when the installer insisted on replacing it with an RCBO, which was when I went digging through the MIs.

[ProDave]

I suspect it's that the SB inverters are transformerless, so any leakage on the DC side (poor installation) would directly lead to leakage on the ac side.

[Jeremy Harris]

That makes a lot of sense; I can see how there could be leakage to earth on the DC side, especially given some of the dodgy installations that were pretty commonplace during the big FIT boom, when every man and his dog was out fitting panels.

 

Interesting that the SB is transformerless, I'll have to see if I can dig out how they have done that.  It's the transformer in our inverter that makes the damned thing so heavy, I'm sure.