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

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.
Parents
  • A very modern conundrum.

    I'd assume at least 3mA of leakage per computer station, perhaps more depending what else is there, printers, network kit, phone chargers etc and realise that a fully functioning 30mA RCD can be in spec and trip at a touch over 15mA. Very soon you have one RCBO per group of 4 or 5 sockets at most.

    RCD sockets do reduce the risk of a one-out everybody-out trip, though testing them every 6 months will be a faff. (like everybody does, right?) But, unless the wiring behind them is in earthed trunking or in SWA or has some similar protection, you  will find that you cannot easily stay within the regs and get away from needing an RCD at the origin anyway. Is the supply to the board 3 phase ? if it is be prepared to split the circuits around the phases,  this helps the earth current cancel to a degree.

    Note that many (but not all) RCD sockets currently have an RCD part that is AC type, rather than the A type that is now becoming de-rigueur in many locations so check before you buy a load. (Devices to BS 7288: 1990 will be of RCD type AC. devices designed to BS 7288: 2016 could be type AC or type A, with the RCD type to be indicated by the manufacturer.)

    Mike

Reply
  • A very modern conundrum.

    I'd assume at least 3mA of leakage per computer station, perhaps more depending what else is there, printers, network kit, phone chargers etc and realise that a fully functioning 30mA RCD can be in spec and trip at a touch over 15mA. Very soon you have one RCBO per group of 4 or 5 sockets at most.

    RCD sockets do reduce the risk of a one-out everybody-out trip, though testing them every 6 months will be a faff. (like everybody does, right?) But, unless the wiring behind them is in earthed trunking or in SWA or has some similar protection, you  will find that you cannot easily stay within the regs and get away from needing an RCD at the origin anyway. Is the supply to the board 3 phase ? if it is be prepared to split the circuits around the phases,  this helps the earth current cancel to a degree.

    Note that many (but not all) RCD sockets currently have an RCD part that is AC type, rather than the A type that is now becoming de-rigueur in many locations so check before you buy a load. (Devices to BS 7288: 1990 will be of RCD type AC. devices designed to BS 7288: 2016 could be type AC or type A, with the RCD type to be indicated by the manufacturer.)

    Mike

Children
  • Devices to BS 7288: 1990 will be of RCD type AC

    The advice is that whilst we might consider the nature of the residual currents these devices (and BS 4293) as Type AC, designers are cautioned there are other differences in the standards that mean the performance (for example EMC) cannot be guaranteed to be the same under all conditions as a Type AC to a later standard.

    devices designed to BS 7288: 2016 could be type AC or type A

    But when you are working to BS 7671:2018+A2:2022 (which you'd perhaps have to if using BS 7288 SRCDs, as they are not included in the list of permitted RCDs in BS 7671:2018, unless you want to write an intended departure), it should be noted that Regulation 531.3.3 contains the following requirement:

    RCD Type AC shall only be used to serve fixed equipment, where it is known that the load current contains no DC components.

    Which pretty much precludes the use of Type AC SRCDs to BS 7288, because, in the general case, the plug may be removed and another appliance (or non-fixed appliance) may be connected through it.