Computer room ring or radial

[AdieB]

Got a job in a school. They require power for their computer suite. There will be approximately 20 PC's The question is due to earth leakage should I install 3 off 2.5mm ring circuits cabled in singles all of 2.5mm with the sockets having twin earth connections and the earths at the DB going to seperate terminals. Or 3 off 4mm radials with the sockets having twin earth connections and there being a second earth cable providing a ring circuit for the earths the earths at the DB going to seperate terminals.
What do you think?

AdieB

 

[Jimmy Boy]

The regulations as stated in 7671 give the very narrow circumstances in which those circuits that require RCD protection may be ommitted from protection, the OSG which is nothing more the a guide based on 7671 gives clear advice on documentation you should obtain and the reasons for said ommissins , it's advice not the regs,follow it or not it's a choice for the individual OSG Green page 27 section 3.6.2.2
ATB J

 

[BigBreakfast]

Having wired many computer rooms I tend to go for 20a radials rcd protected with high integrity earth and, as Engineer and others have stated, I limit the number of stations per circuit due to earth leakage.

 

[malcolmsanford]

That is my problem with the OSG. People do not look upon it as such and so take it that you are going against either a Statutory regulation or the BS 7671 regulation if the guide recommends or in some case don't

I sometimes think that they have things so wrong, the OSG was always a good guide IMO, but it is now becoming a Janet and John publication a "One size fits all" thing.

I have yet to put an RCD on any kind of IT installation in an office. I may rethink that in this case for a school, but I would have serious thoughts about it. Not only by increasing the CSA of the cable to allow for the Protective Conductor Currents on the CPC, the larger cable will ensure that the protection device will trip definitely at 0.4secs as recommended, but in all likelihood below 0.1secs as most MCB manufacturers claim

 

[Jimmy Boy]

Or you could wire them in accordance with 543.7.101 / 2 lol
ATB J

 

[malcolmsanford]

That is true mate in the case like this for servers, and I do use the blue 16/32 amp BS 60309-2 for connection over here.

 

[Engineer54]

That is my problem with the OSG. People do not look upon it as such and so take it that you are going against either a Statutory regulation or the BS 7671 regulation if the guide recommends or in some case don't

I sometimes think that they have things so wrong, the OSG was always a good guide IMO, but it is now becoming a Janet and John publication a "One size fits all" thing.

I have yet to put an RCD on any kind of IT installation in an office. I may rethink that in this case for a school, but I would have serious thoughts about it. Not only by increasing the CSA of the cable to allow for the Protective Conductor Currents on the CPC, the larger cable will ensure that the protection device will trip definitely at 0.4secs as recommended, but in all likelihood below 0.1secs as most MCB manufacturers claim

I will go with you on this, if were talking about a standard every day office that has a small IT set-up, but where you have a dedicated IT area or building, where they have brought in a created so-called ''clean earth'' and or where the IT supply is through an isolation TX, it is unlikely that the normal ADS protection is going to be sufficient to disconnect the final circuits in the specified times. As that clean earth is going to be a TT system, so unless that TT system has been designed and installed to give a sub 1 ohm value your going to require RCD protection anyway....

 

[malcolmsanford]

Yes Col we have recently done a Data Centre over here and had to provide both a functional and protective earth. The functional earth was recommended at 0.5ohms for mircowave equipment and I think at the last count we were nearing the earths core lol. In the end we had to install around the building a network of meshes/grids to get it that low, but still only manged just over an ohm and that was pretty stable over a 2 month period. As no moisture but plenty of ferrous metals in the sand.

In the end they had to fit the mircowave units through a TX, but the functional earth is superb for noise on the servers and equipment and this was a Tier 3 centre so quite good fun

 

[Engineer54]

Yes Col we have recently done a Data Centre over here and had to provide both a functional and protective earth. The functional earth was recommended at 0.5ohms for mircowave equipment and I think at the last count we were nearing the earths core lol. In the end we had to install around the building a network of meshes/grids to get it that low, but still only manged just over an ohm and that was pretty stable over a 2 month period. As no moisture but plenty of ferrous metals in the sand.

In the end they had to fit the mircowave units through a TX, but the functional earth is superb for noise on the servers and equipment and this was a Tier 3 centre so quite good fun

TT earthing for these types of installations can always be a pain in the arse in the Middle East. To my mind it's well worth the time, effort and cost to drill the rod locations and fill with a conductive mortar such as Magnetite or even Bentonite... In the Middle East, (well always really on my projects. lol!!) 2 X coupled 3m X 3/4'' rods per rod position. Never needed more than 8 such electrode positions to gain sub 1 ohm values in the Riyadh area. Less in the coastal Cities, but then that's to be expected...lol!!!

One of the main problems with such earth systems is the location, in respect to other earthing systems, such as building Ufer earthing, and other structural steelwork (Towers and the like) that has been earthed to other systems...

You mention no moisture in the ground, ...whenever we made/installed ground fields in the Middle East we often also provided a simple irrigation system, that was just enough to keep the area moist. The system can also be controlled by the inclusion of probes, ...used those on the KFMC project!! Not that they were needed at that time as the water table was pretty dammed high in those days, to the point of having to pump it away in some areas of the site!! lol!!!

 

[topquark]

For a school, I'd want to see RCD. If there's any concern over loss of data on power fail then a UPS should be incorporated into the design.

 

[Tonkatoy27]

The school may disagree.

The schools I do work in, (one secondary, 3 primary under 2 different education bodies) never supply a spec or design for IT suites and accept that RCD protection is not a requirement for dedicated IT socket outlets. Indeed, the first time I came across IT circuits tripping RCD's was at the secondary school on a circuit installed by the previous sparky, when I questioned the need for RCD's with ELECSA (I like to run most things that I'm not sure about by them as it feels more like I'm getting my money's worth) they said that there was no need as long as you include the high integrity earth.
At the time ELECSA also confirmed that the non RCD protection should be noted in the deviation box. (Perhaps, 'contradiction to regulations' would be a better name.

411.3.3 states that RCD protection is required by socket outlets not exceeding 20A in general use. Personally, I wouldn't class dedicated IT circuits as 'general use' but like a lot of reg, it's down you your own personal interpretation.
The OSG bit about traceable confirmation is interesting and I haven't seen that before. I'll specify it in emails in the future so I've got a reply and confirmation on record.

I took over from a number of different companies who have previously wired IT suites in these schools and none of them fitted RCD protection to sockets. Also none of the local sparkies in our social circle usually fit them. I guess it's a personal preference, RCD's are safer but fitting them and the extra circuits involved will add a great deal to the cost of the job. (possibly uneccassarily)

Anyway, I'm much more worried about the amount (100's) of indoor and outdoor circuits which are 40-50 years old with only MCB or even rewirable protection.

I'm glad to be having this online conversation as we never stop learning.

 

[Jimmy Boy]

It is indeed a very worthwhile discussion I suppose another option to protect against a socket being used for general purposes would be to fit the non standard earth pin socket outlets, expensive and means cutting off moulded plug tops (ouch) and of course the corresponding plug tops not to be fitted to ext leads.

K1246WHI

J

 

[davetherave1979]

we are pricing a school tomorow for 30 computers in trunking I will be wiring separet ring mains feed 10 computers off of each ring but confused on the need to rcd protect still what if the caretake drills through the cable or a cleaner comes in uses a socket to polish the floor and cuts the cable or a trades person comes to do work in the room and gets a shock

 

[Silly Sausage]

we are pricing a school tomorow for 30 computers in trunking I will be wiring separet ring mains feed 10 computers off of each ring but confused on the need to rcd protect still what if the caretake drills through the cable or a cleaner comes in uses a socket to polish the floor and cuts the cable or a trades person comes to do work in the room and gets a shock

Dave, I think if you're that concerned about it, RCD each cct.
What's the worse that's going to happen, 10 kids might lose a bit of work.
Unlike a bank, say, where there could be financial loss involved.

 

[davetherave1979]

Total agree better be safe then sorry but when I go and price tommorow the company in going to be working for will want as cheep as possiable and not want to pay for rcds

 

[Jimmy Boy]

Dave I think all the important points have been been covered, if your going to use the clauses in the regs to omit RCD protection explain this to the clients and explain the risks get their agreement in writing to the design of the installation , its possible once explained they may well go with RCD protection as they may feel liable if it was ommitted , most computers have UPS which means no loss of data, if there is any chance that the sockets may be used by ordinary persons your laying yourself open to a hit unless you get all this in writing from the client's .Cover your back mate.!
ATB J