Advice on RCD Protection for commercial office sockets (earth leakage)

[fordselec]

Hi guys,

I've got a job where I'm installing dado trunking on the floor of an office for sockets and data. Some info on job - In total I will be installing 36 double sockets to supply 40 workstations. Majority of the workstations will be a laptop with dual monitors and phone setup. Some of the workstations will have desktop PCs or the smaller NUC style PCs. Some of the sockets allowed are to be installed between the desk locations due the customer wanting some redundancy and flexibility in moving desks. The rows of desks come out vertically from the wall the dado trunking will be located on and site plan to run extension leads from the sockets to supply all equipment. They've requested this as their current setup with floor boxes has caused them a lot of issues and they want flexibility to move the desks forwards/backwards with cable management installed on the back of desks.

Where I'm looking for advice is the best way to provide RCD protection to all of these sockets without having fault issues due to earth leakage.

I've seen varied information on how many PCs should be connected to one RCD circuit from 3-4 to 8 PCs per circuit. I plan to install a new board and run radials for the sockets however I'm not sure how many workstations I should be wiring for one circuit due to earth leakage and it seems like it could require a lot of circuits needed just to stop nuisance tripping.

Is there a better way to go about this or a standard for larger office installations like this ?

The other option I was considering is installing RCD sockets instead of RCBOs however I'm not sure if this is a common way of doing an installation like this with so many sockets or if it would be considered ‘rough’. Using RCD sockets would allow for two rings to be wired for the whole floor which offset the extra socket costs compared to running lots of radials and rcbos/bigger board. Also the added benefit of not tripping out the full circuit if a fault occurs, not as big of a deal as they mostly use laptops but still helpful. Would using RCD sockets like this be considered good practice?

I was also planning to install high integrity earthing by linking out the last socket of each radial in the dado with a 4mm CPC, from what I can tell this is the correct way of achieving it.

Any thoughts or advice would be much appreciated.

 

[i=p/u]

16A rcbo Circuit for every 2 sockets = 20 circuits

High integrity earthing isn't required anymore, I'm nearly certain??

Stickers at board for earth leakage over 10mA isit??

I'm sure someone else will be along with better info.

Can rcds be omitted as its not domestic with risk assessment and computer sockets used which only accept certain type of plug???

Do you need ups??
Separate cleaner sockets?

 

[telectrix]

4 circuits, 10 sockets per RCBO.

 

[bill01803]

High integrity earthing isn't required anymore, I'm nearly certain??

It is still in the blue book, I haven't bought the Brown one yet!
Someone got one?
543.7.2

 

[littlespark]

if they’re running extension leads after the sockets, how many devices off each lead?

Already we see suggestions of anything between 2 and 10 sockets per RCBO, but it’s the number of devices in total that will be the deciding factor.

I suppose laptops would be better than separate desktop and a monitor

 

[timhoward]

So much depends on the age of the PC's as some more recent ones have external power supplies anyway that are class 2.

Can rcds be omitted as its not domestic with risk assessment and computer sockets used which only accept certain type of plug???

Check Amendment 2 risk assessment requirements. From memory you have to be careful who does the RA now. If children / disabled persons are present then an RCD is needed anyway, and it's unlikely they would be able to state they will never employ anyone with disabilities. So I'm not sure this one will fly.

High integrity earthing isn't required anymore, I'm nearly certain??

High integrity protective conductor connections are still very much in the regs.
The confusion might be that for socket circuits you don't actually need to do anything if it's wired as an RFC.
But if wired as a radial you can wire the CPC as a loop or provide a bigger cpc. (The 3rd radial option is sharing a larger CPC between circuits but this makes future testing such a pain I'd avoid that at all costs.)
No changes in amendment 2.

Now RCD sockets are in amendment 2 again, and considering the lack of productivity if an entire floor goes out, there's something to be said for fewer circuits with RCD sockets.
Blue sky thinking - I wonder if a higher current 6mm radial circuit with a 13 amp RCD spur in each area might be worth considering, it would cost less.

I look after some buildings that have at least 25 PCs on a 30ma RCBO if that helps at all.

 

[plugsandsparks]

Its funny (cynical hat on) how where there has been a substantial improvement in both load characteristics and earth leakage, that the electrical regs do not down grade themselves to reduce cost and complexity.

The IT industry have done a great job in making sure older equipment is quickly obsolete / un-supportable so the office of today bears not comparison to 10 - 20 years ago.

The last thing i would be concerned about in a new office environment is the earth leakage and routing such leakage down a dedicated earth connection and more concerned about separation of ccts for fault resilience and ease of maintenance and testing.

I agree with Tel in so far as you would not put the whole thing on one ring as above reasons, so maybe you would put it on two rings, if you are considering two rings may as well put it on 4 radials, B20 RCBO type A.

You will not get a max demand and you wont be able to control it anyway. Its down to experience of the environment.

 

[pc1966]

I don't have new regs book yet but as far as I know the RA option now only applies to skilled & instructed persons so that is going to rule out avoiding RCD for an office. For a data centre you might be fine with that.

I suspect most modern PC stuff is only about 0.5mA, probably less for class II, so putting 9 double sockets on a 20A circuit is fine. If in trunking you can make it radials but with a looped high-integrity earth easier then full RFC style.

You might also want to put in a handful of sockets on a 32A RFC or whatever cleely labled (yellow/black test or whatever) for cleaners or tradesmen to use so they don't trip the IT user's supply and then someone is left searching for the DB key to reset it.

Another thing you see occasionally in some labs and offices are the RCDs per circuit set in one of these so easier to reset:

RCD/MCB housing - Marshall-Tufflex

Up to 4 modules can be accomodated. Note: RCDs are not supplied by Marshall-Tufflex.

www.marshall-tufflex.com

 

[Lister1987]

Schneider go a great RCD guide and has figures for leakage based on load, BSI wait stupid money for the same information.

Earth Fault Protection: How to design efficient earth fault protection with Residual Current Devices (RCD) | Schneider Electric

Earth Fault Protection: How to design efficient earth fault protection with Residual Current Devices (RCD)

www.se.com

 

[fordselec]

It is still in the blue book, I haven't bought the Brown one yet!
Someone got one?
543.7.2

Its in the latest OSG wgich is why I was installing it, no mention of omitting it.

 

[fordselec]

So much depends on the age of the PC's as some more recent ones have external power supplies anyway that are class 2.

Check Amendment 2 risk assessment requirements. From memory you have to be careful who does the RA now. If children / disabled persons are present then an RCD is needed anyway, and it's unlikely they would be able to state they will never employ anyone with disabilities. So I'm not sure this one will fly.

High integrity protective conductor connections are still very much in the regs.
The confusion might be that for socket circuits you don't actually need to do anything if it's wired as an RFC.
But if wired as a radial you can wire the CPC as a loop or provide a bigger cpc. (The 3rd radial option is sharing a larger CPC between circuits but this makes future testing such a pain I'd avoid that at all costs.)
No changes in amendment 2.

Now RCD sockets are in amendment 2 again, and considering the lack of productivity if an entire floor goes out, there's something to be said for fewer circuits with RCD sockets.
Blue sky thinking - I wonder if a higher current 6mm radial circuit with a 13 amp RCD spur in each area might be worth considering, it would cost less.

I look after some buildings that have at least 25 PCs on a 30ma RCBO if that helps at all.

Yes im going to go to site and confirm how much is class 2. Am I right in saying class 2 devices do not leak any current to earth as they are not connected to it?

I do like that idea with RCD spurs. Seems like a happy medium with the added benefit of the rcd being able to be mounted on the DADO for local resetting. Would also make it easy to expand and add more sockets/spurs in the trunking if they require more devices in the future that could cause tripping.

Nothing has changed in the regs regarding rcd protection on cables has it, T and E in trunking and clipped direct doesn't require rcd protection ?

Thanks for the reply

 

[fordselec]

Schneider go a great RCD guide and has figures for leakage based on load, BSI wait stupid money for the same information.

Earth Fault Protection: How to design efficient earth fault protection with Residual Current Devices (RCD) | Schneider Electric

Earth Fault Protection: How to design efficient earth fault protection with Residual Current Devices (RCD)

www.se.com

View attachment 98503

Thats a great bit of info , thanks.

 

[timhoward]

Yes im going to go to site and confirm how much is class 2. Am I right in saying class 2 devices do not leak any current to earth as they are not connected to it?

Correct

I do like that idea with RCD spurs. Seems like a happy medium with the added benefit of the rcd being able to be mounted on the DADO for local resetting. Would also make it easy to expand and add more sockets/spurs in the trunking if they require more devices in the future that could cause tripping.

Since I had that idea, I've had one concern:

I have a dim and distant red flag memory that BS7288 devices (RCD sockets and RCD spurs) have something in the BS7288 standard that says the upstream circuit requires additional protection. I don't know if this is an old wives tale and I don't have a copy of the standard, but some chaps over on the IET forum occasionally talk about it.
I quote from a 3rd party "BS 7288 stipulates that SRCDs provide supplementary protection, but that additional protection must be provided upstream". To stress, that is NOT a quote from a standard, but from a bloke with a beard.
It might be worth you getting a definitive answer to this before you plough ahead.

Assuming the above is wrong (which I would hope to be the case otherwise there is little point in the products at all!) as you've already mentioned the distribution circuit would need to be installed either surface mounted, mechanically protected, or >50mm deep to avoid requiring RCD protection itself. Clipped direct in trunking would be ok.

(An alternative is maybe just using 'proper' RCBO enclosures instead)

 

[timhoward]

Here's some light reading:

The application of BS 7288 RCDs - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

The application of BS 7288 SRCDs has cropped up from time to time, so what is their application since BS 7671:2018? 531.3.6 specifies that RCDs for additional protection

tinyurl.com

 

[pc1966]

I have a dim and distant red flag memory that BS7288 devices (RCD sockets and RCD spurs) have something in the BS7288 standard that says the upstream circuit requires additional protection. I don't know if this is an old wives tale and I don't have a copy of the standard, but some chaps over on the IET forum occasionally talk about it.

I think that is something that was reverse-ferreted in AM2, but don't have a copy yet.

(An alternative is maybe just using 'proper' RCBO enclosures instead)

I would do that, as you could then have 20A C-curve MCBs in the DB for sets of sockets on 2.5mm so a more generous margin than a 13A fuse in case folks plug in a bit more than laptops (e.g. testing servers or just some plonker with a kettle, etc).