comply to Regs to install a MCB for the Sub Main to the 1st Floor DB

[cliffed]

Replacing DB’s with all singing all dancing RCBO’s
Problem’s we have now…Existing installation consisted of DB Grd Floor.. DB 1st Floor…Grd Floor DB supplies 1st Floor DB…16mm T&E installed IN fabric of building.
DB 1st Floor supplies Remote Garage Block 16mm T&E
With the Sub Main to DB 1st Floor Rcbo protected, also the Garage being Rcbo protected…there is no Selectively between Rcd devices
Is it possible to comply to Regs to install a MCB for the Sub Main to the 1st Floor DB

 

[pc1966]

Tricky. If you have good reason to know the route of the sub-main is not hidden less than 50mm of a wall surface, or if it has suitable protection against being accidentally nailed, etc, then no problem. But often that is impossible to determine if you can't see the route.

 

[cliffed]

Tricky. If you have good reason to know the route of the sub-main is not hidden less than 50mm of a wall surface, or if it has suitable protection against being accidentally nailed, etc, then no problem. But often that is impossible to determine if you can't see the route.

Glad you could understand my writing, it’s difficult to put into correct wording… thanks …

 

[Welchyboy1]

Yes this is a problem i have encountered before and there is no way to totally comply with the regs one way or the other

What did you deceide to do ?

 

[Rockingit]

Pretty sure we've debated this in here before and concluded it's a busted flush either which way. There seemed to be an option in electronic RCD style devices with intelligent monitoring and self-resetting functions........at a cost!

 

[7029 dave]

Yeah this situation is a problem, take most of these sub mains are being fed via 60A switch fuse.
BTW I was told off for using the (old) term sub mains on here, its called distribution circuit. lol (still cant get used to this new terminology)

 

[pc1966]

Yes, in practice you have three competing things to consider:

• What the regs demand
• What the overall risks are
• What the customer is willing to pay/tolerate in decorating

Regs say 30mA RCD for any shallow cables without armour, etc, which after installation we can't tell. Having said that, BPG #4 only codes this risk as C3.

But if you try that on a sub-main you will likely get serious nuisance tripping that poses are real risk of injury from loss of lighting on stairs, etc, to spoiled food in freezers, etc. For cables that have been around for decades without a drilling incident that seems a backward step in safety. An auto-re-closer might help, but only if the trip threshold is occasionally hit as if you add a few leaky electronic things you might find a 30mA RCD won't stay closed at all.

To fix it (i.e. allow no RCD additional protection) means running new sub-mains in SWA or similar, and for many installations that is simply not happening due to cost, permission to run through other's property, and resulting redecorating costs.

It is a bit like the "per-1966 lights without CPC" question on CU replacement that comes up from time to time, but at least there the BPG #1 section 8 has industry-approved guidance about how to address related work that is non-compliant in the least-worst way, and that you could fall back on if challenged on your actions.

 

[Dan]

Renamed thread title to comply to Regs to install a MCB for the Sub Main to the 1st Floor DB. Please be more creative. It attracts more replies if the thread is well named.

 

[buzzlightyear]

I'm sure this a few years ago, a 16mm feed in a fabric of a building, and was told to put in a lower fuse with a 60amp to a 40amp in switch fuse.

 

[Rockingit]

I'm sure this a few years ago, a 16mm feed in a fabric of a building, and was told to put in a lower fuse with a 60amp to a 40amp in switch fuse.

Except that does nothing in real terms, and can even be argued as making it potentially worse due to an increased disconnection time.

 

[Welchyboy1]

When I had this issue i used an upfront a 100mA TD RCD and dp RCBOs on final circuits

I know its not considered additional protection but its the best i could come up with at the time

 

[cliffed]

Thanks for all comments still debating how to solve this, i personally would install MCB, having an idea
the cable is installed under floorboards & drilled through joists preventing any penetration by nails. Saying that the mighty Few will decide the outcome.