Earth leakage and computers

[divadoll_1]

In 7671 there is mention that no more than 10 computers can be connected in a system as the possible earth leakage of 3mA per pc would put the RCD out if it were a 30mA
HELP!!!
Does this apply to the screens as the old CRT type monitors were no doubt less efficient than the new TFT screens or does the earth leakage come from somewhere else?
I know that this is a failsafe rule of thumb sort of rule, but rules is rules, so how do I get around this problem?
My BF is setting up his computer business in an office at the bottom of the garden (Damned big shed!!) and I think we have overcome the volt drop problems, but earth leakage may mean that in times of glut when he has more than 10 pcs on the go at any one time, he may have some nuisance tripping....
what can we do ????

 

[Steve]

Have the run of cable to the shed NOT RCD protected (it doesnt need to be)

Then in the shed, have 2 circuits with RBCOs (these are mini RCDs built into an MCB)

But your electrician should know this anyway.

 

[ZenStalinist]

AFAIK the 3mA per computer allowance is because the switch mode power supplies that computers use can have that amount of earth leakage.
I'd have thought that the leakage from the monitor would be negligable in comparison.

 

[divadoll_1]

This is part of my NVQ so was trying to find out if there was any other way than putting in RCBOs. We have an existing CU unit being moved anyhow and was hoping to use this for the office as we will need about 4-5 ways and it is a 7 being replaced by a 12 in the house.
Also...as a side line...is it more expensive to have an additional supply to the house or to have the existing uprated?

 

[divadoll_1]

Thanks Zen...at least I know where it is coming from now

 

[Steve]

RCBOs are a much better solution than blanket RCD protection.

A new supply would cost in the region of thousands. If you're thinking about monitoring energy costs for this business unit, fit a private meter in the shed in order for charging the business for electricity useage. This way, a major cost of the business is accountable for.

http://www.tlc-direct.co.uk/Products/RDCRED60.html

 

[Spark123]

Take a look at section 607 of the regs.

 

[divadoll_1]

Had already covered the metering angle Crafty I know just how much my leccy bill has gone up since he moved in, and am still paying for the bills I got when he worked in the loft before I evicted his business
Now due to the landlord of his business park going bust he is now homeless again and we are trying to get this sorted as quick as possible. I have already said that the rent he paid the landlord there should just about cover it

 

[Lectrician]

Dont RCD the sockets for the computers - Having a risk assessment in place that these will not be used for outdoor use is enough.

You could fit the MK sockets with T shaped earth pin so the computer sockets can only be used for computers, but if you swap the computers regularly (is he repairing?), then this is not good.

You also need to comply to sec 607 - you need high integrity earthing - that would also include the submain - make sure the submain has atleast a 10mm earth core. Fit crabtree sockets, or any other make that has dual earth terminals on the rear - don't confuse these with clean earth sockets which also have two earth terminals - one for the clean earth, and for the protecive earth. Run a ring, or two radials, and join the end of the radials with a link of 4mm earth. Earths must go in a terminal on their own - not shared, hence two terminals. Also at CU.

Hope thats clear - typing quickly - busy!

 

[divadoll_1]

Lectrician...yes he is repairing so they will be swapping and changing regularly. There will only be the rare occasion where more than 5 or 6 pc's are on, but this does happen.
Due to the limited space the larger equipment like the printer and photocopier may be hard wired in on fused spurs like a cooker, as they will never be moved. I am fully conscious of all that power being used in a wooden structure and want to use the belt and braces principle to avoid the inevitable bonfire effect

 

[JohnD]

I don't think the use of non-standard sockets is a prob. The equipment will almost certainly have "kettle" connectors on it. So he can keep a few dedicated flexible cords with connectors at one end and non-standard plugs at the other.

RCBOs are great! Or if running a business he could buy some RCD protected sockets, then none of then will carry the load and leakage of more than 1 or 2 machines.

I am in favour of RCD protection for workbench sockets when you will sooner or later be poking about inside the equipment that's plugged in.

 

[TicklyT]

I am just wondering what effect installing a UPS would have on any earth leakage.
Are the input and output effectively isolated from each other, so any earth leakage on the output side does not appear as a current differential on the input side?

 

[bernardgreen]

I am just wondering what effect installing a UPS would have on any earth leakage.
Are the input and output effectively isolated from each other, so any earth leakage on the output side does not appear as a current differential on the input side?

The isolation depends on the type of "UPS" you are talking about.

UPS ( Un-interuptable Power Supply ) used to mean a system running as an inverter from batteries constantly charged by the mains. If the mains failed the power output was un-affected. If the inverter failed they fell back on to mains with a glitch on the output.

SPS ( Stand By Power ) used to feed the mains through with the inverter ready to take over within a few cycles of the mains failing. These produced a glitch in their output when the mains failed until the inverter came fully up to load.

UPS were isolated when running on the inverter and most of the better quality ones did have isolation when running on the mains input.

One option to consider is a simply 230 to 230 isolation transformer for the work bench if you are testing PC's A PC with a defective power supply would then NOT trip the RCD protected supply to the workshop.

 

[Adam_151]

As far as I know, we call them all UPS now

SPS is Offline UPS
Proper UPS is online UPS
And there is a catorgory inbetween called a line interactive UPS, that runs the inverter in parallel with the load all the time, but with the mains taking 90% of the load until the supply fails

 

[plugwash]

if by computer buisness you mean computer repair buisness then i would not want the repair bench on the same RCD as everything else.

assuming the board you plan to put in the shed is a split load i'd suggest RCD sockets for the repair bench area and a single rcd for all other circuits that need to be rcd protected. That way if a machine with a dodgy PSU is brought in for repair it will only trip out the socket it is plugged into.

if the buisness isn't involved in repair and the computers are in good condition with newish power supplies i don't belive having them all on one RCD will be a problem.