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

 

[topquark]

What about printers/scanners etc as well? I'd opt for the ring depending on how you're running it around the room (dado trunking?).

 

[steveberry11]

As this job is in a school have you not been provided with a design or spec?

 

[kingeri]

The last computer room I did I used rings using the 'leapfrog' method, as there were three quite linear cable runs. I used dado trunking and provided a high integrity earth. It works fine. Don't overthink it!

 

[Jimmy Boy]

As with all circuit designs how much is it going to pull ? I agree it needs splitting but 4mm or a 3 x 2.5 rings all at 32A ? is it pulling that much ?.

As for high integrity earthing arrangements you will need to refer to the BGB page 165,543.7.1 through to 543.7.2.101 and decide how you want to deal with the install, the Green OSG page 77 outlines solutions.

ATB J

 

[Moses]

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

What is the length of the RFC circuits?

at 500W a PC, 20 Pc are about 10KW,

32A is equivalent to 7.4KW at 230V

I would personally install 2 x RFC circuits @ 2.5mm leap frog equally divided between 10+10 computers, if RFC length is suitable for Volt Drop. and install 2 x High Integrety Earth ring back to seperate terminals at the CU.

 

[Engineer54]

I take it that these circuits supplying these computers/printers/scanners etc are going to have RCD protection?? If so, then it's the leakage currents that's going to determine how many computers or whatever are going to be supplied by each circuit. Generally i will only supply 5 to 6 workstations per circuit to cater for natural earth leakage currents. So it's not just a matter of load calculation when dealing with IT installations.....

 

[topquark]

A standard personal computer won't use anywhere near 500W either. Unless it's a rack/tower system with an awfull lot of power hungry hard disks.

 

[topquark]

Just done a quick test on my power hungry twin disked tower and monitor combined. It's drawing 180W at steady state (after initial boot up) for both monitor and PC. Leakage is around 0.5mA. As Engineer54 has said, it's the leakage that you should use to use to drive the circuit splitting decisions, as that will usually reach tripping threashold before exceeding supply of current on a circuit.

 

[Tonkatoy27]

If the length/volt drop is short/ok, one 32amp ring without 30mA protection would be fine. Just make sure there is a labelled, 30mA protected socket on hand for the cleaners to plug their vacs into.

I'd probably install 2 rings however to allow for future expansion when the class size gets bigger.

There is no need for RCD protection on IT only circuits. Mark it as a deviation on the EIC.

 

[One_of_them]

I done a computer suite couple year ago. 10 machines and I ran 2 ring finals on RCBO as advised above I wouldn't do more than 5-6 on an RCD/RCBO. Still working no bother and touch wood aint tripped yet

 

[Knobhead]

There is no need for RCD protection on IT only circuits. Mark it as a deviation on the EIC.

The school may disagree.

 

[Jimmy Boy]

If you are going to omit RCD protection on any circuit where it would normally be needed, you will need to satisfy yourself that you are complying fully with the exceptions under 411.3.3 , the OSG defines this under 3.6.2.2,in brief you must have traceable confirmation from the client that they agree to the ommissions ,the guidance is quite clear,without this prior consent RCD protection should not be ommitted , so if the school asks for maximum protection you will need to RCD the circuits.
ATB J

 

[Jimmy Boy]

If the length/volt drop is short/ok, one 32amp ring without 30mA protection would be fine. Just make sure there is a labelled, 30mA protected socket on hand for the cleaners to plug their vacs into.

I'd probably install 2 rings however to allow for future expansion when the class size gets bigger.

There is no need for RCD protection on IT only circuits. Mark it as a deviation on the EIC.

Please explain why complying with the regs is a 'Deviation' from 7671 ?
Not a pop BTW just explain .

ATB J..

 

[malcolmsanford]

Not being funny jimmy but where does this come from?

traceable confirmation from the client that they agree to the omissions

Unless there have been major changes to the EAWR-1998, or the Regs have introduced another amendment.

What would constitute traceable conformation, a letter from the customer saying that no one apart from the school teachers in this case, will un-plug a piece of equipment?

Well I can safely say as sure as eggs are eggs there is no-one in today's world of trying to find someone to blame for someones own stupidity if they were hurt at work, by receiving an electric shock, and a manager/whatever gave this traceable confirmation in their name, law suits would be flying everywhere, it is just not going to happen

So we may as well just RCD everything in sight ...................