Industrial DB’s in a domestic install

[Rich..]

Hi all,

Im after a little advice. Got an install with a house attached to a large double 2 story garage. Garage has a Schneider Acti9 DB in for tools, sockets and lights. Wired to the main house DB with 10mm SWA.

The garage has various tools with VFD’s that will trip a standard RCD. Garage distro has main switch with some MCB’s and some RCBO’s depending on the load.

Im thinking of using another Acti9 board in the house as the main DB. One of the multi service options, such as SEA9AN108MS, meaning all standard house loads come off the Isobar side, fed by a main switch and individual RCBO’s. These boards also have a separate 4 way Din rail module, which i could then put in a Type A SI RCD, then a 63a type C MCB. This will tolerate the VFD’s i am told.

Can anyone see an issue with this setup or using an ‘Industrial’ db in a domestic setting?
Thanks
Rich

 

[westward10]

If you are only replacing the house consumer unit I assume, why does the garage supply need rcd protection.

 

[Rich..]

It has external SWA ran to the detached garage. It seemed better than to do it this way than the other way of non RCD protected supply, And split the board in the garage for non rcd protected load. If an SI RCBO for Acti9’s existed then my life would have been a lot easier.
Rich

 

[westward10]

Why does it require rcd protection.

 

[Rich..]

Unsure of earthing arrangement, havent seen it yet with lockdown going on, just trying to plan how i would do it. If its not TT i would swap out the garage main switch for an SI RCD, but the question would still stand as i would like to keep the two boards the same. Is there an issue against using an industrial aimed bored in a domestic setting?
thanks
Rich

 

[James]

Many large properties have industrial style distribution boards, it is often needed for 3 phase distribution and simply to provide enough ways for all the circuits and distribution boards spread around the property.

Main problem normally is the size and look of the means they can’t blend in to the background, have to be hidden away for aesthetics.

 

[westward10]

Unsure of earthing arrangement, havent seen it yet with lockdown going on, just trying to plan how i would do it. If its not TT i would swap out the garage main switch for an SI RCD, but the question would still stand as i would like to keep the two boards the same. Is there an issue against using an industrial aimed bored in a domestic setting?
thanks
Rich

Difficult to plan without knowing the earthing system. Are you replacing the garage board.

 

[pc1966]

Is this a three-phase setup?

One thing that always bothers me about garages used as workshops is the risk of an fault/accident tripping the supply and loss of lights - not only is the person probably crapping themselves from, say, chopping a cable, but suddenly it is dark and there are lots of hazards around!

So selectivity with upstream protection for the garage DB is important, and it might be worth installing an emergency light in the garage so if the power fails for any reason they are not in complete darkness.

Usually it is hard to get good selectivity between MCBs as the instantaneous magnetic trip times are similar so any fault that peaks above the upstream one probably will trip both. Some MCCB allow the first phase of "instant" trip to have a small time-delay to achieve good selectivity here, but that is getting in to a different league of installation.

If you make the upstream MCB big enough then it will be reasonable, and a 63A C-curve is a good start, but you might find that is not going to disconnect on time for the feed. A quick look in the OSG has a max impedance of 0.27 ohms so you may not achieve that even on an TN-C-S supply!

For comparison a 63A BS88-2 fuse is much easier to meet at 0.62 ohm but even there it might not met it on some TN-S supplies, so really you need to ideally measure Zs at the garage DB to get an idea of what is actually usable for this (as well as evaluating the total load for there).

Schneider has an on-line tool for computing the selectivity limits, both for breaker-breaker and for breaker-fuse arrangements, you might already know of it but here is an attempt to link to it (gibberish style of URL, sadly):
Electrical Calculation Tool - https://hto.power.schneider-electric.com/cbt/app/index.html?code=34a79e11-548d-4e49-8a1e-10f9fa18492b&client_id=cbt#/Dashboard

As already raised, you probably don't need an RCD on the DB feed as it is SWA and the board already has RCBO for (presumably) those circuits needing it, so going for a fused-switch for the sub-main feed might be your best overall route.

If you do need an incomer RCD because it is a TT supply and so you won't clear L-E faults on the SWA otherwise, you might be better with a separate 300mA S-type just for the garage feed fused-switch, and another one on the house DB as an incomer for that side, so a major fault on the garage side is not taking out the house supply.

Even if you do end up feeding the garage from a MCB in the main DB with a TT-style of overall RCD incomer then having a DP isolator for the SWA feed could be handy if you are trying to isolate a N-E fault causing tripping of it as just flicking off the related MCB is not going to help there.

I am sure others on there will have a bit more practical advice though!

 

[Vortigern]

Why not a domestic CU and henley box with separate box for garage?

 

[pc1966]

Just to add - the Schneider tool page does not work in Firefox, sadly it seems to be a crappy Chrome-only design
[automerge]1589014360[/automerge]
Out of curiosity, and to save folk a bit of time, I run the Schneider tool for two cases where the downstream breaker is an iC60H 32A B-curve MCB as you might have for a 13A ring circuit. Results were:

• 63A BS88 fuse (gG characteristic) - selectivity to 1.9kA
• 63A iC60H C-curve MCB - selectivity limit 0.5kA

For now that is just a illustration, as you would need to find out the garage DB load and the related Zs there, but it shows one of the advantage of the humble fused-switch.

The other is fault current limiting, but I doubt that is an issue here!

 

[davesparks]

It has external SWA ran to the detached garage. It seemed better than to do it this way than the other way of non RCD protected supply, And split the board in the garage for non rcd protected load. If an SI RCBO for Acti9’s existed then my life would have been a lot easier.
Rich

Don't put the garage/workshop onto a single RCD, and dont protect the submain with an MCB.
Both are very poor design choices and leave the garage liable to nuisance tripping.
Protect the submain with a fuse and the garage circuits which need RCD protection with RCBO's

How many of the garage circuits need RCD protection that don't already have it?

 

[pc1966]

Unsure of earthing arrangement, havent seen it yet with lockdown going on, just trying to plan how i would do it.

Is the property occupied just now?

Maybe you could get the owner to send some photos of the boards (and current breakers installed) as well as showing the DNO cut-out and meter, etc. That might answer some of the points discussed above.

TT, or not TT, that is the question...