RCBO's on TT system

[Barry Cade]

Excuse my ignorance,can someone explain to me the problems of using RCBO's on a TT system,and whether and why its unsafe

 

[MDJ]

There is no problem, The problem with RCBOs is they are single pole and not the answer to some types of special location like caravan sites, a double pole s type RCD as the main switch in the Distribution Board and then RCBOs for individual circuits is a very good method for TT systems IMO, Tin hat on, E54 will be here in a few hours haha

 

[not clever]

No problem at all on a single phase TT supplied cu, so long as there is a DP main switch also in use on the CU.......have been fine on my assessments up to now.

The argument given by some is:
There is a requirement for DP disconnection for isolation purposes, the main switch covers this nicely.
There is a requirement for all live conductors to be disconnected under fault conditions, the SP rcbo does this fine.
Others will argue (correctly) that the neutral is also a live conductor and as such should be disconnected in the event of a fault, a single pole rcbo does not achieve this.

On a three phase TT supply all three phases & neutral need to disconnect under fault conditions (not got regs to hand so cant give the number)

 

[Bobster]

You'll find some manufacturers state single pole rcbos shouldn't be fitted to TT or IT systems.

So in fitting them your going against manufacturers spec.

I think there are only a couple of companies that produce single module double pole rcbos at the moment, I predict there will be a few more companies selling them in the coming years.

 

[Bobster]

No problem at all on a single phase TT supplied cu, so long as there is a DP main switch also in use on the CU.......have been fine on my assessments up to now.

The argument given by some is:
There is a requirement for DP disconnection for isolation purposes, the main switch covers this nicely.
A main switch will not clear a N>E fault
There is a requirement for all live conductors to be disconnected under fault conditions, the SP rcbo does this fine.
Others will argue (correctly) that the neutral is also a live conductor and as such should be disconnected in the event of a fault, a single pole rcbo does not achieve this.

On a three phase TT supply all three phases & neutral need to disconnect under fault conditions (not got regs to hand so cant give the number)

This is why it is so strongly recommended that a 100mA s type Rcd be used for a main switch.

 

[not clever]

To be fair a main switch wont clear any chuffing fault will it, hence it is there for isolation purposes only.

I think there are only a couple of companies that produce single module double pole rcbos at the moment, I predict there will be a few more companies selling them in the coming years.

I use compact double pole rcbo's these days, they do the job just right.

 

[Barry Cade]

thanks not clever mdj and rob2 and I thought RCBO's were the mutts,it would seem I need to install rcd main switches too or just stick to rcd main switches only,the trouble I've found with them is if you get an N/E fault you lose the whole board,bit of an inconvenience for the customer,and having both main and rcbo seems a bit belt and braces

 

[Barry Cade]

who makes compact rcbo not clever

 

[not clever]

who makes compact rcbo not clever

PM sent with details

 

[Engineer54]

On TT systems, all OCPDs should theoretically be DP, or SP+N. It's also recommended that CU/DB casings should not be of metal construction. Now how far you want to take all this is up to you!!....