Sockets for PC's

[Toby Maxwell-Lyte]

Hi,

In our office we currently run our PC's off surge protected trailing leads. I consider this to be a bit of a hazard since in some instances there are about 7 things connected to one socket. This is also a pain since the boss insists in turning off all appliances at the wall which involves crawling under the desks. I suggested to the boss that we install several extra sockets e.g. 12 doubles at desk level to make life easier and safer.

He asked about surge protection, is there some way that an entire ring main can be surge protected? Is it okay to just spur off other sockets? Is there a limit to the number of sockets that can be spurred off existing ones?

At the moment there are old fuses, is it okay to just replace these with MCBs? Are there MCB's that are suitable for computer equipment?

I also want to install proper network sockets to avoid the spider web of ethernet cable under the desks. Is there a problem with running cat5e and power cables in the same trunking?

Cheers Toby

[Alistair Riddell]

You can get DIN rail mounted surge protectors which could be installed in the distribution board or a separate enclosure. Alternatively, a UPS system could be installed to feed an entire ring main.

A lot of surge protection really provides very little benefit at all except to the balance sheet of the manufacturer.

You are permitted to spur no more than one socket (single or double) off an existing socket on the ring. You can spur more than one socket using a fused spur protected by a 13 A fuse.

no limits as such, notwithstanding the above, but there are rules about the maximum floor area which can be served by a ring circuit.

In general it is a bad idea to install a lot of spurs rather than extending an existing ring main or installing a new circuit.

Yes, provided the existing distribution board can accept MCBs.

All MCBs are suitable for computer equipment, but normally type B ones are used as these are more sensitive than type C.

According to the regs it is OK, provided there is no unsheathed mains cable (i.e. you use twin&earth rather than conduit cable). Also a barrier must be provided where there is any unsheathed cable i.e. at any sockets.

However I would not consider it good practice. There are plenty of types of two and three compartment trunking available.

[Andy R]

Consider a dedicated ring main for the computers and peripherals and use the ring main for the data as well. It'll be a lot easier than installing RJ45 sockets all over the place and reduce the amount of cables under the desks.

Rgds

Andy R

[Chris Oates]

you could put it all in this

[Andy Wade]

Agreed - unless local circumstances & history indictate otherwise.

100 m^2.

That's not right. Without physical segregation both cables would have to be insulated for the highest voltage present. Whether the mains wiring is in sheathed cable or 'singles' makes no difference. The relevant regulation is

528-01-02 (qv).

Yes - this is the universally adopted method.

There are two more important things you haven't mentioned:

- in view of the loads to be connected to this circuit (lots of IT equipment) the protective conductor current (aka earth leakage) will be high and the high integrity earthing requirements of BS 7671 Section 607 will apply;

- this is clearly an 'at work' situation and the Electricity at Work regulations will apply, legally requiring the work to be done by a competent person. With all due respect to the OP, it's clear from his questions that he doesn't have the necessary competence to do this electrical work legally; an electrical contracting firm, or qualified electrician should be called in. On completion of the work a Minor Electrical Installation Works Certificate should be issued.

[Alistair Riddell]

Last time I looked at the spec for some Cat5 cable it claimed to be insulated for 250VAC... which is hopefully the highest voltage present.

[Toby Maxwell-Lyte]

Thanks for the info guys. Must admit I wasn't overly confident in carrying it out at work. Didn't fancy the consequences of getting it wrong. Quite happy to do that sort of thing at home though :o)

I shall suggest to the boss that he gets someone in, maybe the landlord will pay for it???

Thanks Toby

[N. Thornton]

good idea

There is no good reason to 'surge protect' PCs, unless youre running off a generator.

You wont be able to spur 4 doubles off one double, you'll need to extend the ring. A plug in multiway socket with a 13A fuse in the plug doesnt require extending the ring.

You may find them prone to tripping with a room of computers. Fuses meet current regs - or do if they meet current regs anyway :) Why do you want to replace them?

Regards, NT

[PoP]

If you feed the PCs off the protected side of a UPS then you automatically get surge protection - and it helps a lot with brown-outs too.

Even if the UPS doesn't have much capacity for keeping the PCs running for more than a few minutes it does at least give you a fighting chance of closing them down cleanly.

PoP

Sending email to my published email address isn't guaranteed to reach me.

[David Hearn]

Well, people at work have been to a client site (US financial company with offices in Brighton) and every time they've been, their laptop PSU's have died - unless they take a surge protected 4-way along with them and plug into that. So far, in about 2 months, we've lost 3 PSUs after visits to this client's site. 2 before using a surge protector, and the 3rd when a different person went down and didn't know about using a surge protector. In over 2 years at the company, we've had only 1 PSU fail that wasn't related to this client site. Bizarre.

D

[PoP]

Sounds to me as though a mains analyser (Dranetz or similar) needs to be put on that site for a week to provide a report on what is happening.

PoP

Sending email to my published email address isn't guaranteed to reach me.

[Steve Firth]

I had exactly that experience at a client's site and the reason for the failures turned out to be that the client had all of the sockets in the room in question wired to an on-line UPS. The UPS was faulty and was introducing random spikes over 400V into the line.

[Christian McArdle]

If you are rewiring, you should ensure that the new circuit uses the new high integrity earthing stuff. If the electricians hired doesn't know what that means, then fire them.

Commercial circuits don't normally use ring mains, but radials. However, using ring circuits without spurs actually simplifies the provision of high integrity earthing, so I wouldn't be surprised if this changes.

Christian.

[Lobster]

Whassat then? Relevant to domestic installations?

David (not an electrician!)

[Christian McArdle]

Not unless you have a room full of computers.

The idea is that circuits which you expect to have a high earth leakage should have two independent paths to earth.

Christian.

[Peter Scott]

Very likely will. I had to replace a trip that was constantly cutting out with two PCs and a printer. It was 10mA. replaced with a 30mA. Now OK but the electrician said you need to split a large group of PCs over several RCBs. I think he said eight to ten max on each one.

[Chris Oates]

with an allowed leakage of 3mA ten is tops

over 3mA & you're not supposed to plug two machines into one twin socket

we're (at work) putting in high integrity earthing because of huge earth currents

often leakage currents can be reduced by replacing the EMI filtered sockets on IT gear with decent ones

[Andrew Gabriel]

Each class I (earthed) IT appliance is allowed to leak 0.75mA max. (Actually, I think it's more complicated than this, but for typical office IT appliances, this figure is good enough.)

If you are intending to protect the circuit with a 30mA RCD, the circuit leakage should not add up to more than 7.5mA (a quarter of the RCD rating), which is a design maximum of 10 class I (earthed) IT appliances per 30mA RCD protected circuit. There is no requirement to RCD protect IT appliances in normal circumstances though.

If you are refering to extra filters in trailing sockets and adaptors, then there's normally no reason to use these at all. People seem to buy them thinking it will stop Windows crashing so often, but since mains borne interference isn't the cause, they don't help. They could however contribute to significant earth leakage.