Hi all

Further to previous posts about electrical testing, I was wondering
about the likely issues with RCDs on general office power circuits.

We recently had a couple of supplies added specifically for a server
room and specified no RCDs for this.

However, we are now looking at the general office ring mains which
currently have no RCD protection. In discussion with ARW, I believe I
could argue a case for leaving the office as is, but supplying local
RCDs for potential outside work with extensions etc.


However, I haven't discounted the possibility of RCBOs if they are
available for the board.
My question today is regarding the nature of the power supplies fitted
to computer equipment and whether they are compatible with RCDs at all.
I have been told that the power supplies are "leaky" and will generally
pass current to earth in normal operation.
So how many computer-type devices can be attached to one power circuit
without risking RCD trips?

Thanks

Phil

On 07/06/16 09:50, thescullster wrote:
Hi all

Further to previous posts about electrical testing, I was wondering
about the likely issues with RCDs on general office power circuits.

We recently had a couple of supplies added specifically for a server
room and specified no RCDs for this.

However, we are now looking at the general office ring mains which
currently have no RCD protection. In discussion with ARW, I believe I
could argue a case for leaving the office as is, but supplying local
RCDs for potential outside work with extensions etc.


However, I haven't discounted the possibility of RCBOs if they are
available for the board.
My question today is regarding the nature of the power supplies fitted
to computer equipment and whether they are compatible with RCDs at all.
I have been told that the power supplies are "leaky" and will generally
pass current to earth in normal operation.
So how many computer-type devices can be attached to one power circuit
without risking RCD trips?


about 10-15 IME.

What 'leaks' is the RFI filters that are needed to stop RF going back
into the mains when using SMPSUs. In general there are capacitors from
live to earth and neutral to earth.

These leak a mA or two.

But a worse problem is the presence of all that capacitance if you get a
sharp step change in voltage, like the neighbours arc welder switching
off. That causes a huge surge, and trips the RCD


The sums for calculating what capacitance will draw 1ma on 240V/50Hz are
as follows

240,000=1/(2 * pi *50 * C)

C= 1/24,000,000 * pi

Which comes to around 13nF.

But for step changes, which represent far higher frequencies for a short
time, the current will be much higher.


Thanks

Phil


--
"Anyone who believes that the laws of physics are mere social
conventions is invited to try transgressing those conventions from the
windows of my apartment. (I live on the twenty-first floor.) "

Alan Sokal

On 07/06/2016 10:01, The Natural Philosopher wrote:
On 07/06/16 09:50, thescullster wrote:
Hi all

Further to previous posts about electrical testing, I was wondering
about the likely issues with RCDs on general office power circuits.

We recently had a couple of supplies added specifically for a server
room and specified no RCDs for this.

However, we are now looking at the general office ring mains which
currently have no RCD protection. In discussion with ARW, I believe I
could argue a case for leaving the office as is, but supplying local
RCDs for potential outside work with extensions etc.


However, I haven't discounted the possibility of RCBOs if they are
available for the board.
My question today is regarding the nature of the power supplies fitted
to computer equipment and whether they are compatible with RCDs at all.
I have been told that the power supplies are "leaky" and will generally
pass current to earth in normal operation.
So how many computer-type devices can be attached to one power circuit
without risking RCD trips?


about 10-15 IME.

What 'leaks' is the RFI filters that are needed to stop RF going back
into the mains when using SMPSUs. In general there are capacitors from
live to earth and neutral to earth.

These leak a mA or two.

But a worse problem is the presence of all that capacitance if you get a
sharp step change in voltage, like the neighbours arc welder switching
off. That causes a huge surge, and trips the RCD


The sums for calculating what capacitance will draw 1ma on 240V/50Hz are
as follows

240,000=1/(2 * pi *50 * C)

C= 1/24,000,000 * pi

Which comes to around 13nF.

But for step changes, which represent far higher frequencies for a short
time, the current will be much higher.


Thanks

Phil



Thanks NP

How much relevant E is there in "IME" in your case?

Phil

On 07/06/16 11:40, thescullster wrote:
On 07/06/2016 10:01, The Natural Philosopher wrote:
On 07/06/16 09:50, thescullster wrote:
Hi all

Further to previous posts about electrical testing, I was wondering
about the likely issues with RCDs on general office power circuits.

We recently had a couple of supplies added specifically for a server
room and specified no RCDs for this.

However, we are now looking at the general office ring mains which
currently have no RCD protection. In discussion with ARW, I believe I
could argue a case for leaving the office as is, but supplying local
RCDs for potential outside work with extensions etc.


However, I haven't discounted the possibility of RCBOs if they are
available for the board.
My question today is regarding the nature of the power supplies fitted
to computer equipment and whether they are compatible with RCDs at all.
I have been told that the power supplies are "leaky" and will generally
pass current to earth in normal operation.
So how many computer-type devices can be attached to one power circuit
without risking RCD trips?


about 10-15 IME.

What 'leaks' is the RFI filters that are needed to stop RF going back
into the mains when using SMPSUs. In general there are capacitors from
live to earth and neutral to earth.

These leak a mA or two.

But a worse problem is the presence of all that capacitance if you get a
sharp step change in voltage, like the neighbours arc welder switching
off. That causes a huge surge, and trips the RCD


The sums for calculating what capacitance will draw 1ma on 240V/50Hz are
as follows

240,000=1/(2 * pi *50 * C)

C= 1/24,000,000 * pi

Which comes to around 13nF.

But for step changes, which represent far higher frequencies for a short
time, the current will be much higher.


Thanks

Phil



Thanks NP

How much relevant E is there in "IME" in your case?


Running a company full of IT equipment?

At one point, when we had a power cut, we left all the kit on.

When the power came back on it blew the 60A site fuse.

That was around 70PCS.

If also had similar issue with about 5 TVS and 4 PCS and a router and a
wwitch and a PABX...to the point where 15 SMPSUS seems about all a 30mA
RCD can handle.

Apparently today you don't do whole house RCD, you RCD selective circuits.

Anyway I think that if you try and assume 2mA per SMPSU you should be on
the right side of tripping



--
If you tell a lie big enough and keep repeating it, people will
eventually come to believe it. The lie can be maintained only for such
time as the State can shield the people from the political, economic
and/or military consequences of the lie. It thus becomes vitally
important for the State to use all of its powers to repress dissent, for
the truth is the mortal enemy of the lie, and thus by extension, the
truth is the greatest enemy of the State.

Joseph Goebbels

The Natural Philosopher wrote:

At one point, when we had a power cut, we left all the kit on. When
the power came back on it blew the 60A site fuse. That was around
70PCS.

That would be more about surge than leakage though.

Anyway I think that if you try and assume 2mA per SMPSU you should be
on the right side of tripping

Looking at class Y capacitors they do seem to have a 1-2mA leakage for a
10A rated parts, a desktop PC may very briefly pull that sort of current
in start-up surge. So 15 pieces of IT equipment (PCs/printers/copiers)
per circuit should keep you on the safe side of tripping a 30mA RCD.

I'd say most sites I work at (mostly NHS, schools and local govt) don't
have RCDs on "office" circuits.

Andy Burns Wrote in message:
The Natural Philosopher wrote:

At one point, when we had a power cut, we left all the kit on. When
the power came back on it blew the 60A site fuse. That was around
70PCS.

That would be more about surge than leakage though.

Anyway I think that if you try and assume 2mA per SMPSU you should be
on the right side of tripping

Looking at class Y capacitors they do seem to have a 1-2mA leakage for a
10A rated parts, a desktop PC may very briefly pull that sort of current
in start-up surge. So 15 pieces of IT equipment (PCs/printers/copiers)
per circuit should keep you on the safe side of tripping a 30mA RCD.

I'd say most sites I work at (mostly NHS, schools and local govt) don't
have RCDs on "office" circuits.


Many thanks NP and Andy, just the level of info I was looking for.

Phil
--


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

thescullster wrote:
On 07/06/2016 10:01, The Natural Philosopher wrote:
On 07/06/16 09:50, thescullster wrote:
Hi all

Further to previous posts about electrical testing, I was wondering
about the likely issues with RCDs on general office power circuits.

We recently had a couple of supplies added specifically for a server
room and specified no RCDs for this.

However, we are now looking at the general office ring mains which
currently have no RCD protection. In discussion with ARW, I believe I
could argue a case for leaving the office as is, but supplying local
RCDs for potential outside work with extensions etc.


However, I haven't discounted the possibility of RCBOs if they are
available for the board.
My question today is regarding the nature of the power supplies fitted
to computer equipment and whether they are compatible with RCDs at all.
I have been told that the power supplies are "leaky" and will generally
pass current to earth in normal operation.
So how many computer-type devices can be attached to one power circuit
without risking RCD trips?


about 10-15 IME.

What 'leaks' is the RFI filters that are needed to stop RF going back
into the mains when using SMPSUs. In general there are capacitors from
live to earth and neutral to earth.

These leak a mA or two.

But a worse problem is the presence of all that capacitance if you get a
sharp step change in voltage, like the neighbours arc welder switching
off. That causes a huge surge, and trips the RCD


The sums for calculating what capacitance will draw 1ma on 240V/50Hz are
as follows

240,000=1/(2 * pi *50 * C)

C= 1/24,000,000 * pi

Which comes to around 13nF.

But for step changes, which represent far higher frequencies for a short
time, the current will be much higher.


Thanks

Phil



Thanks NP

How much relevant E is there in "IME" in your case?

Phil


For 50Hz, the rule of thumb is about 80mA/uF.

On 07/06/2016 09:50, thescullster wrote:
Hi all

Further to previous posts about electrical testing, I was wondering
about the likely issues with RCDs on general office power circuits.

We recently had a couple of supplies added specifically for a server
room and specified no RCDs for this.

Which is fair enough - its a special installation, and likely the kit
will have UPS protection anyway!

However, we are now looking at the general office ring mains which
currently have no RCD protection. In discussion with ARW, I believe I
could argue a case for leaving the office as is, but supplying local
RCDs for potential outside work with extensions etc.

That would be one option. Although keep in mind in modern offices people
tend to be plugging in lots of phone changers and other kit along with
the IT stuff. So I would be reluctant to lose the RCD protection.

However, I haven't discounted the possibility of RCBOs if they are
available for the board.

My question today is regarding the nature of the power supplies fitted
to computer equipment and whether they are compatible with RCDs at all.

They are certainly compatible - up to a certain point.

I have been told that the power supplies are "leaky" and will generally
pass current to earth in normal operation.

The input filters on the PSUs will pass some current to earth. With lots
of IT kit this creates a potentially higher than normal earth current.
(ans as such the wiring should adopt high integrity earthing practices)

So how many computer-type devices can be attached to one power circuit
without risking RCD trips?

How long is a piece of string? You don't have the massive inrush surges
of CRT monitors these days, so the problem is reduced a little. Typical
input filters may have 100nF of capacitance, as so leak a bit under 1mA
at 240V/50Hz. So 20 devices per circuit ought to be ok. More than that
and you might start getting into a region where the RCD is sensitised
and more likely to trip.

Doing 1 circuit per 10 users, on its own RCBO might be a reasonable
approach.


--
Cheers,

John.

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