UPS 2

[vennivivi]

Hi,

I need your help. I have got a 1.0 k VA 230 V 50 Hz ups which is powered through a 400 V/ 230 V isolating transformer. The idea behind using the isolating transformer is to get rid of the supply neutral which had found in the past to be quite unstable.So long as the supply is solidly connected to the UPS , the voltage between the neutral and the earthing connection at the output of the ups is around o.5 to 1,2 V. However, when the input plug to the UPS is solidly disconnected, the voltage between the neutral and the earthing point at the output of the ups rose to 115 V. Is this normal ? I believe that the ups is using the transformer neutral point as its reference. I am being pushed by the sales guy of an equipment which is powered through this ups.

Thanks for comments.

Grundig

 

[BrianG]

Hi Grundig,

Are you saying that the "Lo" (neutral) output of the UPS is NOT bonded to ground? If so then, if I understand your circuit description correctly, removing the input side neutral will allow the whole UPS to "float".

UPS that I have used do not specifically tie the Lo output to ground to avoid loops; the "ground" connection terminal is often just to safety-bond for any exposed metalwork and may not necessarily connect to the UPS circuit.

I would say that the installation should ground the UPS output neutral; the input isolating transformer (secondary) neutral reference should go to the same point.

 

[DanEE]

quoted from above... "I would say that the installation should ground the UPS output neutral; the input isolating transformer (secondary) neutral reference should go to the same point."

I would agree.. If there is a transformer outboard, or you are using a transformer based UPS (e.g. a FERRUPS type UPS), then "XO" (ground/neutral bonding) must be derived (connected together at the UPS) and connected to building ground...

 

[ESOXmaniac]

Ok- Here's the setup from someone who has been in the UPS industry for +26 years -> Your UPS is "not" separately derived. Meaning it use's the input supply neutral as an earthing reference for the output. When you unplug the supply, the output now is floating. Because of the stray distributed capacitance between both the neutral and phase conductors inside of the UPS with respect to chassis earth you see a voltage between them and chassis that is 1/2 of the output voltage, i.e., 115VAC. This is NORMAL!

The UPS vendor will likely tell you that unplugging the UPS is not intended as an normal operating mode, i.e., the UPS is intended to operate with the supply cord connected. It's also an inappropriate way to test the UPS, i.e., you should turn off the supply circuit breaker.

Will this likely damage any connected load equipment? IMHO - No, not for most UL Listed or CE marked equipment, i.e., because the testing/certification of the equipment requires the phase & neutral to be reversed. This is no different than reversing the connection on a European Shuko plug or a US 2-prong 120V plug by turning it 180 degrees.

Al

 

[ausgen]

I agree with ESOXmaniac. I have loads of rackmount UPS on site and am forever telling the field crews not to discharge test them by pulling the IEC input connector for exactly those reasons. If you check the equipment manuals, there should be a note along these lines buried somewhere.

Further to this though, I know that our Eaton (Powerware)UPSs disconnect both active and neutral whenever the unit is on battery to prevent an internal fault providing a backfeed from inverter output via the static bypass feed to the exposed input pins. Not sure if this is just an Aus requirement(?).