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?
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.
Alistair Riddell - BOFH
Microsoft - because god hates us
Agreed - unless local circumstances & history indictate otherwise.
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
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.
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
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.
On 17 Jan 2004 11:30:40 -0800, firstname.lastname@example.org (N. Thornton) wrote:
If you feed the PCs off the protected side of a UPS then you
automatically get surge protection - and it helps a lot with
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.
Sending email to my published email address isn't
guaranteed to reach me.
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.
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
Sending email to my published email address isn't
guaranteed to reach me.
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.
Having problems understanding usenet? Or do you simply need help but
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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.
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
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.
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.
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