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Topic Title: Computer room help
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Created On: 28 October 2013 05:21 PM
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 28 October 2013 05:21 PM
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fraserd

Posts: 106
Joined: 25 July 2008

A bit of a vague description so I apologise was a last minute look at a job. Basically 4 storey building in a school, floor in question has a 3phase db fed from the mains on the ground floor via a 4c 50mm swa then a single phase db fed from this from a 40a mcb via a 16mm swa
This single phase db has a rcd 30ma main switch and it basically keeps tripping out, I need to do some more investigating but from what I've been told because of 5 computer circuits the total earth leakage is taking the main rcd out, now one of the lads said that an electrician had been before to look at it and said that it needed an earth pulling in back to the main incomer and that there was no high integrity earthing and so needed rewiring. Now I was thinking of changing the board to individual rcbos which will solve the problem if it is due to the excess overall leakage but where do I stand on the high integrity earthing if it's not present can I not do the board change without sorting this first also anyone highlight me on why a seperate earth would require pulling back to the main bar assuming all efli readings are good.
 28 October 2013 05:47 PM
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AJJewsbury

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from what I've been told because of 5 computer circuits the total earth leakage is taking the main rcd out

Plausible - especially if they're older machines and what's on each circuit. 30mA devices can validly trip above 15mA.

said that it needed an earth pulling in back to the main incomer and that there was no high integrity earthing and so needed rewiring.

protective conductors of 10mm2 or larger are deemed high integrity so I don't see a need for going back to the incomer (unless it's for reasons beyond BS 7671) - so it sounds like the submains are already OK. You probably only have to worry about the final circuits. If you can arrange things so each final circuit leaks <10mA each, then no high integrity earthing is needed at that level.

why a seperate earth would require pulling back to the main bar assuming all efli readings are good.

High integrity earthing is all to do with reliability of the connection, rather than it's resistance (Zs) - smaller conductors get duplicated, or mechanically protected, larger ones OK as they are.

- Andy.
 28 October 2013 06:08 PM
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fraserd

Posts: 106
Joined: 25 July 2008

Spot on. Thanks a lot for the reply, just nice to get things clarified.
 28 October 2013 08:39 PM
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cookers

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I agree with Andy's approach, probably no need for any high integrity earthing.

Just a bit dubious of the "diagnosis" , and what these 5 "computer" circuits power up. If this is the cause it would take quite a lot of rather ancient PC's to cause the trouble.
 29 October 2013 09:39 AM
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AJJewsbury

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I'd have a think about the 40A MCB feeding the DB too - not only are you unlikely to discriminate with the downstream MCBs/RCBOs, but a switch-on surge might well trip it out - e.g. when power returns after a power cut (I've found an small office worth of PCs easily trips out a 32A B-type on switch-on). If they got things racked with sequential start PDUs (or similar setup) then the switch-on surge will be mitigated, but otherwise there's a potential problem.
- Andy.
 29 October 2013 11:12 AM
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Delbot321

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It sounds like this is the old chestnut of people keep adding more and more to an existing DB and circuits without every taking into consideration that this is actually a change of use. 30 years ago classrooms had a couple of sockets for vacuums or over head projectors (when I was a lad) but now they expect to have 30+ computers (or laptops) and the network infra-structure behind it. They budget to buy all the IT equipment and have a few extra sockets. This then happens in another classroom and another and then they discover they need some air conditioning to stop the equipment overheating. At no point did they ever consider that the mains distribution around the school is not suitable for the extra load. Then when it keeps tripping due to overload it's the electricians fault for not designing it correctly.

Whilst this may not be the primary reason for your involvement in this job its worth pointing out to them that at some point they need to invest in upgrading their sub mains and the like. You tell them I can fix it (or at least keep it going) for now and it will be good for a while and take the glory (and the pay check) for fixing it but will also stop you getting the blame when in 6 months time they have more problems due to the fact that they've added yet more computers etc. leaving you in the ideal position for getting the upgrade job.
 30 October 2013 05:28 PM
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HarryJMacdonald

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Many offices leave out the RCBO on circuits for computers - especially considering the potential loss from an unnecessary power cut. If you label these sockets "for computers only" and supply other socketss with RCBO protection for vacuum cleaners, kettles, phone chargers and so on then you should be OK.
 30 October 2013 05:49 PM
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AJJewsbury

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Many offices leave out the RCBO on circuits for computers - especially considering the potential loss from an unnecessary power cut. If you label these sockets "for computers only" and supply other socketss with RCBO protection for vacuum cleaners, kettles, phone chargers and so on then you should be OK.

But in a school...?
 30 October 2013 10:32 PM
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HarryJMacdonald

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Originally posted by: AJJewsbury

[But in a school...?


Probably depends on how accessible the sockets are. Hidden under the desk and properly tested the risk should be deemed negligible.
 31 October 2013 03:41 AM
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Jaymack

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Originally posted by: HarryJMacdonald
If you label these sockets "for computers only" and supply other socketss with RCBO protection for vacuum cleaners, kettles, phone chargers and so on then you should be OK.

Or install a different pin configuration.

Regards
 13 November 2013 02:04 PM
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msandoz

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Schools / higher education etc are one property where there is definitely always a caretaker, so Supervised can safely be assumed.

Take the RCBO's off, far too much agro.
 13 November 2013 02:45 PM
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Avatar for OMS.
OMS

Posts: 19764
Joined: 23 March 2004

Originally posted by: msandoz

Schools / higher education etc are one property where there is definitely always a caretaker, so Supervised can safely be assumed.



Take the RCBO's off, far too much agro.


You are kidding, right ?

Regards

OMS

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Failure is always an option
 13 November 2013 03:31 PM
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daveparry1

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Yes OMS, i'm sure he is kidding! A caretaker supervising the usage of electrical equipment, don't make me laugh! (you'd have to find him first)

Dave.
 14 November 2013 08:55 AM
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joepostle

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Caretakers can be a nightmare and some have no building qualifications or sense whatsoever, just simply bods plucked off the street.

Has been a while since spec'ing a school IT suite but the other thing to consider maybe is what the local authority stipulates. I know at one point it was considered 'best practice' to have the red mushroom buttons fitted in IT suites much to the children's amusement (& not the teachers!), the schools could (at the time) decide whether to adopt this method or not. I know along the south coast it was considered best practice by local authorities to only fit 6 doubles per 30a circuit. RCBOs sound like a solution to minimise disruption but obviously costly, most of the schools I have visited don't have any RCD/RCBO protection for their IT suites / admin offices.

If the PCs are relatively new there is a chance they could be double-insulated, most of the space-saving all-in-ones are (except iMacs!).

I agree with the comments Delbot321 has made. It is amazing to see in Schools the number of submains which are still 60a SPN - most of which seem to be increasing to 12+ ways and far beyond 60a. I am amazed that (compared to commerial or industrial sites) most Schools have their own substations (& mainly dedicated) and the size of some of the services 800a and beyond. Schools can be the biggest birds-nests going!
 14 November 2013 10:32 PM
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peteTLM

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I think there is a distinction to be made between sockets and computers that are going to be used by students, where the loss of a bit of work wouldnt be the end of the world, and the sockets used by teachers and admin staff (and indeed a locked computer/server suite) where the loss of power would possibly be critical.

Such critical sockets are not going to have knitting needles poked in them by kids as they will be in locked offices/ rooms that only staff can get to.
Sockets accessable to kids- RCBO's/ RCDs are a definate requirement IMHO.

With a proper design, there is no need to worry about having a moderate number of computers on each circuit. 6 Double sockets per socket is something ive done before as well.

Cost? where and why does this come into it? If it needs it, it gets installed. The school will just have to buy less worthless stuff and put it towards something that provides safety for children.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 15 November 2013 09:17 AM
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joepostle

Posts: 59
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Originally posted by: peteTLM

I think there is a distinction to be made between sockets and computers that are going to be used by students, where the loss of a bit of work wouldnt be the end of the world, and the sockets used by teachers and admin staff (and indeed a locked computer/server suite) where the loss of power would possibly be critical.

Totally agree about distinction. Agree about a bit of work, from my observations most of the children spent most of their 'IT lessons' playing games behind the teachers back (who seem completely oblivious!).

Such critical sockets are not going to have knitting needles poked in them by kids as they will be in locked offices/ rooms that only staff can get to.

Sockets accessable to kids- RCBO's/ RCDs are a definate requirement IMHO.

Agreed, I suppose really I was/should be knocking the local authorities for not stipulating or enforcing this more, is required at home by regulations so why not in a risk area, even if it isn't a wood/metalwork room or science lab. Some wood/metalwork labs I've seen are nowhere near having RCD/MCBs!

With a proper design, there is no need to worry about having a moderate number of computers on each circuit. 6 Double sockets per socket is something ive done before as well.

I know exactly where you're coming from, though does seem strange at facevalue having to build-in such redundancy for in-rush current when most computers use 1amp at the most, thus making an IT suites use cira 30amps...

Cost? where and why does this come into it? If it needs it, it gets installed. The school will just have to buy less worthless stuff and put it towards something that provides safety for children.

Seen so much worthless stuff in Schools, some of which costing £ and unused...
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