UPS and earth?

[CarbonPepper]

UK. Domestic computer setup with uninterruptible power supply - APC brand devices BK650EI and BK350EI.

The manufacturer offers almost no information on earth and I presume that protected appliances are on their own independent circuit. Modest testing (ordinary multimeter) suggests there is no earth continuity between a computer metal case and supply earth.

I'm wondering if this is by design, or there's a fault?

Additionally, how safe is a scenario where a separate earth is made between a PC case, and say a central heating pipe? Often done for anti-static precautions. e.g. a running PC hot-swapping a SATA drive, or, to try and solve an audio buzz.

Many thanks for advice and sorry for any terminology errors.

 

[pc1966]

As a general rule if it has a metal case then it should be earthed. There are some cases when that is not true, but for a PC that has a built-in power supply (not laptop with "brick" on the cable) I would expect it to be earthed.

But there could be many places when that fails, and typically you find it is power cables & connectors that are most commonly the fault!

I would not advise testing anything live with a multimeter as there are so many things that can go wrong with possibly fatal results, but if you are testing things without them being plugged in it is fairly safe.

First thing to do it check between the PC power supply inlet (assuming they typical "kettle" IEC connector) and see in the middle/longest pin is connected to the PC case.

Then fit its power cable, and check again from the earth pin on the 13A plug back to the PC case. Try flexing the cable in case you have any intermittent faults.

Do the same for the UPS, again being careful as they can be live & dangerous with no input (their job after all!) and verify that your feed from UPS plug back to PC is OK.

If that all looks OK you might have a faulty socket or fixed wiring (i.e. cable from consumer unit / fuse box to sockets) and really you would be best to get a professional in to sort that. You can get socket testers but they are not very good:

• If a socket tester tells you there is a fault, you definitely need to get it investigated and fixed.
• If a socket tester gives you "all clear" it might be really wrong, as they are poor at determining if the earth is good or not.

Some testers are much better than others, for example:

TIS Socket Tester With Loop & RCD Check

A pocket sized checker for indicating loop impedance, socket polarity and RCD 30mA are all ok. Indication is by LED's and buzzer. An ideal unit for initial "checking out" of a 13A socket and will advise if the circuit is ok, needs attention, or needs urgent attention. An ideal product for...

www.toolstation.com

But obviously not going to match the capabilities of an electricians multi-function tester costing £500-1k or so (and the sparky who know how to use it).

 

[CarbonPepper]

Turns out, power supply in this particular PC has been mounted with sound dampening. I didn't spot it before as it's pretty wedged into a desk cabinet. I've added a Fly-lead. Might even cure speaker buzz too.

I still wonder though about any addition of a separate earth. If supply drops, the battery kicks in, and there's a fault downstream of the inverter side of the UPS. the whole shebang is now isolated from main circuit protection. Just curiosity.



I recognise and welcome your cautionary notes ? Did IEEE about 30 years ago (but I'm not in the trade), I confident it's only me who's to blame if I forget safety.

 

[pc1966]

Ah, good to know a definite reason has been found!

Most small UPS disconnect from the icoming supply when they go on to battery, but the chassis/earth is still there. For big systems you can have all sorts of problems with that floating arrangement and some have local transformers so they can maintain isolation and supply referencing to Earth at the same time. Bit it costs!

 

[Matt2507]

The earthing arrangement when a UPS is on battery is a bit of a grey area in the industry. As it has been previously advised most smaller UPS models have built in back feed protection that isolates the input supply on a mains failure. This also includes the neutral. This means while on battery the UPS output is floating with respect to earth. This means should you have a live earth or a neutral earth fault nothing will happen in fact the UPS won't even react to it because that earth conductor no longer has a fault path back to the UPS neutral output. What we do to solve this usually is place an isolation transformer on the output of the UPS and create a new earth neutral reference point. It is a big no no connecting the neutral to earth on the UPS output thought as this can cause current to flow down the earth conductor instead of the neutral.

 

[CarbonPepper]

This means while on battery the UPS output is floating with respect to earth. This means should you have a live earth or a neutral earth fault nothing will happen

This was one of the grey areas I was thinking about. If say, a PC technician is working on the PC and has attached an anti-static lead from the PC case to a nearby radiator, does the floating in respect to earth become a different problem?

 

[Matt2507]

This was one of the grey areas I was thinking about. If say, a PC technician is working on the PC and has attached an anti-static lead from the PC case to a nearby radiator, does the floating in respect to earth become a different problem?

It won't really change anything by doing that to be honest. The point where the earth and the neutral is bonded back at source is no longer connected to the UPS output neutral. You can imagine what sort of headache this is to the people in the industry designing the infrastructure. Again even though an isolation transformer will sort the neutral earth bond issues we then have an issue where the RCD that should protect you won't work correctly as the UPS inverter will not provide a high enough fault current to trip it. Its not really an issue though if you think about it. In order to get a shock off say a PC case where you have a live earth short it would need a return path back to source. This is only achievable through the neutral. This is assuming the cable run from the UPS to PC is relatively short. Longer runs can cause a higher impedance which actually can cause current to flow.

 

[pc1966]

Also those anti-static leads typically have 1Mohm resistance as a safety feature: low enough to discharge static, high enough that you won't get any significant shock at 230V.