RCDs on Ring Mains Supporting Computer Equipment

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

Reply to
thescullster
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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.

Reply to
The Natural Philosopher

Thanks NP

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

Phil

Reply to
thescullster

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

Reply to
The Natural Philosopher

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

Reply to
Capitol

That would be more about surge than leakage though.

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.

Reply to
Andy Burns

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

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.

They are certainly compatible - up to a certain point.

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)

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.

Reply to
John Rumm

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