Am I missing the obvious??

[ADS]

Hi,

This has been confusing me for a while so maybe someone can explain.

It's to do with computers and the power circuits supplying them.

Often on the forum it's stated - 'don't RCD a circuit supplying computers as they leak current to earth'

This bit I get - functional earths etc.

What I don't get is this:

The way I understand it is that the power lead enters the rear of your

computer and the first thing it comes into contact with is a step-down

transformer - right or wrong??

If this is right, then the primary windings will be completely seperate from the

secondary windings.

We all know that RCD's monitor the balance of current in the line and neutral

conductors - not the actual leakage to earth.

So surely any fuctional earth leakage on the secondary side of the

transformer (computer circuits) will not be registered by an RCD monitoring

the supply circuit on the primary side, even if they're using a common earth.

Or have I missed the boat completely??? ?:|

Cheers

 

[Lurch]

Boat missed. The leakage is on the filtering on the mains side of the power supply usually.

 

[ADS]

Boat missed. The leakage is on the filtering on the mains side of the power supply usually.

I don't fully understand that -

I must admit I'm not that clued up on the inner workings of computers.

I'll have to take mine apart sometime and have a lookbad day explode ha ha

 

[Lurch]

Depends on the power supply but most switch mode power supplies (as found on PC's amongst other things) will have some form of filtering, which can be as simple as a couple of capacitors across the incoming live cables and earth, which then leak to earth. Some SMPSU's have a more complex filters and other bits added as well.

 

[ADS]

I take it the leakage per PC is only tiny, then, and only becomes

an issue if you're wiring a circuit to feed a load of them - like

an office?

 

[Lurch]

Yes.

 

[septiclecky]

It's not only PC's that have a leakage current but all electronic equipment that needs a regulated DC supply which have filter circuits.

As these filter circuits use the earth as a return path it is a big problem with RCD protected circuits.

It gets on my nerves when I get called to an office because the RCD has tripped because they have 10-12 PC's plus printers on 1 RCD protected circuit and reply that IT said it was OKX(

 

[PC Electrics]

Without being 'clued up' on the inner workings of PCs, try thinking of it this way:

All of the current used by any appliance must arrive at the device via the live line of the supply.

As is usual with any equipment, that current returns along the neutral.

However, a PC contains surge protection which uses capacitors and other elements to "dump" surges and interferance to earth. Thus a small proportion of the supply current returns to the supply down the earth path.

I believe the current to earth of a typical PC is 1 - 3mA. On this basis it would seem sensible to keep the number of PCs on one 30mA RCD below 10.

 

[SteveH37]

just been reading through my guide to the 17th edition and came across this section, the author suggested using an average of 1mA as a guide when designing an I.T installations circuit.