RCBO mixed with MCBs

[peka1977]

I have an outdoor CU with an RCD main switch and 4 x MCBs.
Two of the MCBs control in-ground lights and the two remaining MCBs control an electric gate and a set of smart switches.

After many years of solid operation the RCD has started to trip all the time.
It looks like the light circuits have developed an earth/neutral fault and completely disconnecting them (live and neutral) has allowed the two remaining circuits to operate normally.

There are 25 lights in total, and whilst I check them all, I would love to have a system where the gate and smart switches are not continually powered off.

How can I get the light circuits to trip individually without disrupting the other circuits?
Can I just exchange their respective MCBs for RCBOs?
Unfortunately there is not enough room in the CU to add an RCD just for the light circuits.
Thanks

 

[Murdoch]

You would have to remove the main switch RCD and replace the mcbs with RCBOs

where is this circuit fed from and is it rcd protected ?

 

[peka1977]

You would have to remove the main switch RCD and replace the mcbs with RCBOs

where is this circuit fed from and is it rcd protected ?

Hi, the four circuits have MCBs and all of them are connected to an RCD main switch.
So you're saying replace all the MCBs and main switch with RCBOs?

 

[binky]

Hi, the four circuits have MCBs and all of them are connected to an RCD main switch.
So you're saying replace all the MCBs and main switch with RCBOs?

no 4 RCBOs and a main switch.

 

[peka1977]

no 4 RCBOs and a main switch.

Gotcha
Just seen an RCBO CU on Screwfix.
I could try that.
Basically each RCBO has a neutral in and out and an earth in addition to the live in/out.
Is the isolator switch anything, or just and on off switch?

 

[binky]

to isolate - switch off in laypersons terms.

 

[peka1977]

to isolate - switch off in laypersons terms.

Thanks Binky.
I noticed that some RCBOs have the functional earth wire and some don't.
Since the circuits are connected to the earth bar, what's the point of the FE wire?

 

[binky]

Thanks Binky.
I noticed that some RCBOs have the functional earth wire and some don't.
Since the circuits are connected to the earth bar, what's the point of the FE wire?

part of the workings of the RCBO, most don't have it anymore.

 

[peka1977]

part of the workings of the RCBO, most don't have it anymore.

Many thanks, this was helpful to me

 

[Fleeting]

Can you show a pic of the consumer unit. Advising to change things may not be possible with your existing unit.

 

[Mike P]

If you have an RCD circuit feeding the outside consumer unit, it may continue tripping that. The question is, is the feed from a MCB or RCD?

 

[peka1977]

If you have an RCD circuit feeding the outside consumer unit, it may continue tripping that. The question is, is the feed from a MCB or RCD?

The CU feeding the outside CU comes from the garage, and that is an MCB with an RCD main switch.
The feeding CU never trips.
I will upload photos of the replacement RCBO unit I got

 

[Mike P]

The CU feeding the outside CU comes from the garage, and that is an MCB with an RCD main switch.
The feeding CU never trips.
I will upload photos of the replacement RCBO unit I got

The fact that the garage RCD is not tripping is a concern. It would be worth doing a ramp test on it and check the earth from garage to outside board.

 

[SPECIAL LOCATION]

The CU feeding the outside CU comes from the garage, and that is an MCB with an RCD main switch.
The feeding CU never trips.
I will upload photos of the replacement RCBO unit I got

Having multiple RCD's connected at various positions along the same circuit supplying the same final load(s) does not present any electrical safety issues...

But they can introduce an inconvenience as the RCD nearest a fault may not be the first to operate.... Basically it is a bit of random pot-luck about which one operates first or if multiple RCD's all operate together!

The technical requirements state that a 30ma RCD must not operate at 15ma..
But it must operate at 30ma or higher, within less than 300mseconds (just under a third of a second).

So earth fault currents anywhere from 16ma to 29ma+ can operate a standard (BS61009/BS EN 62423) RCD/RCBO.
and two identical spec RCD's / RCBO's can operate at different speeds for the same earth leakage current.

 

[Mike P]

Having multiple RCD's connected at various positions along the same circuit supplying the same final load(s) does not present any electrical safety issues...

But they can introduce an inconvenience as the RCD nearest a fault may not be the first to operate.... Basically it is a bit of random pot-luck about which one operates first or if multiple RCD's all operate together!

The technical requirements state that a 30ma RCD must not operate at 15ma..
But it must operate at 30ma or higher, within less than 300mseconds (just under a third of a second).

So earth fault currents anywhere from 16ma to 29ma+ can operate a standard (BS61009/BS EN 62423) RCD/RCBO.
and two identical spec RCD's / RCBO's can operate at different speeds for the same earth leakage current.

I was just suggesting that the Garage RCD needs checking to confirm it functions correctly.