Last Friday I did a UPS test at work (there are 5 small units at one site in various places; as its an old building this makes more sense than having one big one as there is no place to put it). The site is a healthcare facility for frail old people and is in a rural area and its HV/LV trafo (11 000 V / 230/400V ) is shared with a quarter of the village.
Added to which the supply is TN-C-S (neutral and protective earth are combined at the UK Power Networks service head), with 3-phase to the laundry and kitchen where there are heavy appliances that have strong power demands.
All the IT kit is the other side of the building on a single phase. Power quality is not good here – we are lucky to get 230V; at times of high demand the voltage is only just within the UK tolerances (216.2V minimum).
Any imbalance on the other two phases causes the power to sag to the point the UPS alarms go off (which is why they are there). Interference from the neutral (which can happen for many reasons) also contaminates the protective earth when used as a functional earth by IT/telecoms equipment (which it always is in Europe); it was enough to crash the network on some computers and cause issues with USB devices (the metal shield of UPS connector on a desktop always gets connected to protective earth)
One UPS a cheap China model which is only there to keep the wireless router going for about 50 minutes (so it and the telephone exchange the other side should keep going long enough for the staff to warn any senior management the place is off supply).
Unusually its mains lead is fixed rather than on an IEC socket – the other end is a UK plug. It is under a desk in a confined space When I unplugged it; I accidentally brushed my hand against the two exposed pins; and got a strong shock from that! Thankfully it was only around 115V and not much current (or I would have spent the weekend in the hospital or not be here at all).
I did some research into this and learned it is called “backfeed voltage” and arises due to flaws in the UPS design.
The obvious solution is to fit a relay in the UPS operated by the DC from the accus which disconnects the mains inlet (this is the “click” noise a UPS often makes when switching over) but this component is either defective or absent in this particular unit. (FFS these relays aren’t even that expensive!)
Kit like this (which might just be tolerable in 100/115V countries) is not supposed to be sold 230V countries but some must slip through.
So be careful if buying a lower cost UPS……
