UPS - Earth - RCD what is required, and testing?

[EricMark]

CAUTION! The UPS must be connected to a grounded AC power outlet with fuse or circuit breaker
protection. DO NOT plug the UPS into an outlet that is not grounded. If you need to de-energize this
equipment, turn off and unplug the UPS.

I read this as meaning an earth rod is required, as the whole idea of a UPS it to give power when the grid is down, and if the grid is down one can't rely on any earth from the DNO be it TN-S or TN-C-S.

As to linking neutral to earth during a power failure, nothing in the spec says it does this, the output is 6 x 13 amp sockets, 3 UPS and 3 direct, and no reference to RCD or any earth fault detection being built into the unit. Manual is available here I wonder if this is normal or not?

I have a far larger unit, 5 kW on 6.4 kWh battery, which does link earth and neutral with a power failure, it has got an earth rod, earth fault detection in the inverter plus RCD type A sockets, and my RCD tester shows they trip at 21 mA and well within 40 mS and a reasonable loop impedance. So I'm alright, Jack, but 780 watt is a far smaller unit, with equipment being plugged directly into the UPS with 13 amp socket. At half load only got 9 minutes, so not a big deal, mine 6 hours and still 37% left running whole house with a power failure only selected sockets, so may never run out if enough sun during the day.

So with the small UPS is lack of earth rod and RCD a real problem, or something we can turn a blind eye to?

 

[davebodger]

CAUTION! The UPS must be connected to a grounded AC power outlet with fuse or circuit breaker
protection. DO NOT plug the UPS into an outlet that is not grounded. If you need to de-energize this
equipment, turn off and unplug the UPS.

I read this as meaning an earth rod is required, as the whole idea of a UPS it to give power when the grid is down, and if the grid is down one can't rely on any earth from the DNO be it TN-S or TN-C-S.

As to linking neutral to earth during a power failure, nothing in the spec says it does this, the output is 6 x 13 amp sockets, 3 UPS and 3 direct, and no reference to RCD or any earth fault detection being built into the unit. Manual is available here I wonder if this is normal or not?

I have a far larger unit, 5 kW on 6.4 kWh battery, which does link earth and neutral with a power failure, it has got an earth rod, earth fault detection in the inverter plus RCD type A sockets, and my RCD tester shows they trip at 21 mA and well within 40 mS and a reasonable loop impedance. So I'm alright, Jack, but 780 watt is a far smaller unit, with equipment being plugged directly into the UPS with 13 amp socket. At half load only got 9 minutes, so not a big deal, mine 6 hours and still 37% left running whole house with a power failure only selected sockets, so may never run out if enough sun during the day.

So with the small UPS is lack of earth rod and RCD a real problem, or something we can turn a blind eye to?

Hi, I have no current quotable qualifications in this, but I have been using APC UPS's in data centers and at home for 30+ years so may know a thing or two. Apologies if I'm telling you stuff you know already.
The smaller UPS relies on the fact it is small so you won't be plugging much into it. Ideally just one item, PC, printer, whatever. The output is likely "galvanically isolated" (or in other words "IT" not "TN" or "TT") and not referenced to earth, so no RCDs because they won't work. The APC ones generally pass through the Line and Neutral when mains is available but break it when the mains supply turns off (via a relay that you can hear click off). This is APC's idea of "line-inetractive" as the inverter is running "in parallel" but not "on-load".
The earth is used to protect you from the conductive part (the metal shell of the UPS). It is generally fed through to the end devices but as I say will not trip RCDs when running on batteries.
Personally I use the 3KVA units for my home network and main PC, all of which is in one room.
If I was you and worried about protection, I would drop the small UPS and add the load to your 5KW unit which sounds like it is better designed and has plenty of spare capacity. If you are worried about run-time then you can always load-shed the smaller PC or add more battery packs (if that's an option).
Be carefull about connecting neutral to earth on the output of a small UPS as if it's not designed for it internally it may just self-destruct or at a minimum blow a fuse. It's not worth experimenting as the result will likely be expensive.
So, to answer your final question, I personally wouldn't worry about adding an earth rod for the small UPS, but make sure it's earthed normally. No RCD is not a real problem because of the output type and limited load. Just follow the manufacturer's instructions.

 

[EricMark]

I have no problem with a single item running on an IT supply. That is what a shaver socket does, after all. And thinking about it, no need for an earth rod, as all within the equipotential bonded area.

The problem arises when there are multi output from the same inverter. But even then looking at two items having a fault, one line 1 to earth and the other line 2 to earth to cause a danger, clearly the more items on the same inverter the higher the risk.

To link one line to earth to form a neutral after the RCD could trip the consumer unit's RCD, all well and good with my 5 kW inverter (part of the solar installation) as it is before any RCD. After the CU RCD either it would need to be always on inverter supply, with no grid tie, or only one outlet. Or if multi outputs all on independent windings from the step-up transformer.

In my own house, I would not worry, I test the equipment and inspect, and unlikely to have two faults at the same time. It is when out of my control that I worry, could I really sign an EICR and pass an installation if there is a UPS supplying multi items? If not part of EICR then would need inspection and testing of in service electrical equipment or PAT test, could I pass an inverter with multi outlets.

As yet not happened, but better to work out what to do before it happens, to return after passing something and saying sorry I should not have passed that, is somewhat unprofessional.