Can I use a mcb as main switch? And also RCD protection? Read

[Grapejuice18]

So as the title says can I use a mcb as the main switch. Basically to keep it short I’ve took a 25mm sub main to power an outbuilding. Mains coming in will be Henley blocked and new tails will go into enclosure with b63a mcb which then feeds 25mm swa to outhouse which contains its own c/u which is split load rcd protected and contains its own main switch

Also does the sub cable need to be rcd protected or will this cause problems as the outbuilding is rcd protected by its own c/u.

Will the b63 mcb be sufficient or do I need a main fuse in the enclosure or rcd protection for the swa from the house to outbuilding?

Hope this makes sense. Apologies in advance if it doesn’t.

 

[westward10]

Whether it requires RCD protection will be dictated as to whether the type B 63A device provides fault protection on it's own.

 

[Grapejuice18]

Whether it requires RCD protection will be dictated as to whether the type B 63A device provides fault protection on its own.

The cable is buried below ground and being SWA is highly unlikely to get damaged. I could use a rcbo to protect the cable at the house but I thought it would cause potential nuisance tripping as there would be 2 rcd’s the ones at the outhouse and the one at the mains surely this would be a problem?

 

[westward10]

You are misunderstanding my post, does the 63A device as it stands provide fault protection to comply with ADS.

 

[davesparks]

The cable is buried below ground and being SWA is highly unlikely to get damaged. I could use a rcbo to protect the cable at the house but I thought it would cause potential nuisance tripping as there would be 2 rcd’s the ones at the outhouse and the one at the mains surely this would be a problem?

Being SWA doesn't make it unlikely to be damaged, the likelihood of damage is just the same. Being SWA makes it far less dangerous if it is damaged because it has an earthed armour.

The point being made was that RCD protection may be required for fault protection, this will depend on the earthing system type and earth fault loop impedance of the circuit.

What is the earthing system?

 

[davesparks]

Also does the sub cable need to be rcd protected or will this cause problems as the outbuilding is rcd protected by its own c/u.

What is the earthing system and what is the calculated Zs for the circuit?

Will the b63 mcb be sufficient

Surely you established that before calculating the cable size required?

 

[MJPD29]

Concerning questions. As westward say with the b63 comply with ads? Why would/wouldn't the swa require rcd protection?

 

[Grapejuice18]

What is the earthing system and what is the calculated Zs for the circuit?



Surely you established that before calculating the cable size required?

It’s a TNC-S and the ZS is 0.40 which upon checking GN3 the max ZS for B63 is 0.56 so this is fine. I guess I was lazy and used a quick cable calc app that works out the size for you based on the load and length but after being prompted to get back to the books and the calcs I guess I answered my own question thanks for that lol

 

[westward10]

So what is your calculated Zs.

 

[Grapejuice18]

So what is your calculated Zs.

Going off the max DNO calc for TNC-S of 0.35 and the R1+R2 of 1.87 over the 30m run was 0.05 so the max calculated ZS was 0.40

 

[westward10]

Have you actually measured the Ze.

 

[Grapejuice18]

Have you actually measured the Ze.

I have not maybe if I’d of taken the time do so I wouldn’t be looking back at tables in the GN and regs book at half 9 on a Thursday evening but I have calculated to worse case so can put it to bed now