Replacement DB to Submain RCD advice

[fatboyslim]

Morning guys,

I would like some advice, currently at my workplace, I am replacing a DB and existing wiring in a warehouse. One of the ways feeds a sub-db to a small office. The sub-db is an old wylex rewireable db with 3 of the 4 ways used (Sockets,sockets,lights). My company does not want to replace the sub-db as there are plans of removing the offices in the future.

My plan is to supply the office DB with 6mm SWA and a B40 MCB from the DB I am replacing. Also, test all outgoing circuits of the office DB to ensure they are in a safe condition. Finally, I would note on the EIC about connecting to the existing board and the client is aware of the DB not meeting current regs.

My question is should I be installing a 40A RCBO instead of an MCB at the DB feeding the office sub-db, at least the sockets and lights would have RCD protection from the new DB that i'm installing? Or am I overthinking this?

Thanks in advance.

 

[Flanders]

I would say you are overthinking it. If your not altering any part of the office installation, then leave it alone ust test the circuits fed from your new DB. Does the office DB have a up to date EICR?
Technically you should not use an mcb to feed a sub main as there will be no selectivity between the supply mcb and the fuses in the wylex board. Saying that I see it all the time, what you need to look at, is will it matter if you loose all power to the wylex board due to fault on one of the final circuits fed from it.

 

[fatboyslim]

I would say you are overthinking it. If your not altering any part of the office installation, then leave it alone ust test the circuits fed from your new DB. Does the office DB have a up to date EICR?
Technically you should not use an mcb to feed a sub main as there will be no selectivity between the supply mcb and the fuses in the wylex board. Saying that I see it all the time, what you need to look at, is will it matter if you loose all power to the wylex board due to fault on one of the final circuit

Thanks for the reply.
Not altering any part of the office installation just supply new cable to the office DB.
Yes the office has a recent EICR (6 Months ago) All circuits passed Zs requirements.
Office DB is made up of 5A,15A and 30A circuits so I think 30A might trip me up on selectivity between 40A MCB and office DB. However, the office is basically redundant, which is why my company is not forthcoming about replacing office DB and bringing wiring up to current regs.
So loosing all power is not a big deal.

 

[pc1966]

Unless those sockets are likely to power outdoor stuff, etc, I would not be too bothered. Though using an RCBO is easy and allows that aspect to be ticked off (assuming its not rammed full of leaky IT stuff).

As @Flanders says you get poor selectivity with a MCB up-stream. It can be OK for overload if you have 1.6:1 or more ratio, but once you have a fault above the instant trip point of the upstream MCB they will generally both fire. How important that is kind of depends on what it is used for and so likelihood of a fault and implications of whole sub-DB going off.

MCCB often are you choice here if budget allows, or a switched-fuse for single circuit so long as you have enough of a margin with down-stream MCB/RCBO. If you look in the better catalogues, such as the Hager commercial distribution one, they have tables near the back of selectivity limits for MCB/MCB and fuse/MCB cascades, etc.

 

[Flanders]

Thanks for the reply.
Not altering any part of the office installation just supply new cable to the office DB.
Yes the office has a recent EICR (6 Months ago) All circuits passed Zs requirements.
Office DB is made up of 5A,15A and 30A circuits so I think 30A might trip me up on selectivity between 40A MCB and office DB. However, the office is basically redundant, which is why my company is not forthcoming about replacing office DB and bringing wiring up to current regs.
So loosing all power is not a big deal.

Ok so In this case I would not worry about selectivity. As you have a current EICR for the wylex DB and is not part of your alterations then I would just test up the wylex board. Compare zs at DB with EICR to confirm all final circuits from the wylex DB will disconnect under fault conditions ,job done