nuetral to ground voltage 1

[jcraft]

Not sure if this is the right forum but here goes. We have some new circuits ran of a 20 kva UPS system in our office. One of the circuits feeds 4 network data switches. they have burned up 3 data switches in the last 5 months ever since these circuits were put in. We noticed that there is 2.5 volts between the nuetral and ground on this circuit. any thoughts if this could be taking them out or what could cause this voltage to occurr. We measured in the panelboard where they originate and the voltage is zero. Some of the other circuits they ran that are close to the ups( within 50') are around 1.5 volts. theone that the 2.5 volts is on is about 200' away from UPS. Any thoughts. Someone said that this may be normal and harmonics could be playing a role. I don't want to spend a huge amount of time on this if this could be normal and it wouldn't affect the switches.

 

[ruggedscot]

Hmm this is one that you need to watch - You can get round it but to do this requires an isolation transformer between your load and the UPS. Then you can link the neutral to ground and remove this voltage float.

Check to see what is going on with your load and see where the neutral to earth rise is coming from, is it UPS derived or is your load causing it ? Maybe your load is already galvanically isolated from the supply and there is no earth reference and you are floating, If so I cant see where you are buring out your kit, maybe there is some pull through from your kit to other equipment that it fed from a seperate source, giving circulation currents and such.

Be careful about grounding your neutral after the UPS, some UPS units hate this....

rugged

 

[DanEE]

Put an oscilloscope on the circuit and look at the shape of the 2.5volts. (I'm guessing this the RMS value made with a standard AC voltmeter).

Make sure this is measured properly with the oscilloscope if the scope is AC powered, to avoid yet more ground loop noise e.g. standard trick is to make a differential measurement using a two channel scope with inputs added and inverted.. Do NOT connect the scope probe ground leads to anything..

It's possible you may have peak voltage spikes higher than 2.5volts RMS. The waveform may give you a clue as to the source of the noise.

And as posted above, don't add any connections between neutral and ground in an attempt to fix the problem. There should be only one bond between neutral and ground and that's at the XO point.

We just fixed a problem at a customer account where there was an additional connection.. and neutral current was flowing on conduit all over the place and green wire grounds.. i.e. any available parallel path from improper ground/neutal connection back to the UPS.

 

[rbulsara]

I think this could be normal. This is one of the most misunderstood issue and at times used by IT equipment mfrs as an excuse to cover real reasons for a malfunction of the equipment if there is any.

If the voltage is zero between N and G at the source, its all that matters. N should be grounded only at one place that is at the source or the first disconncet after the transformer.

If there is a current (I) in the neutral, and you measure the voltage between N and G near the load end of a relatively long circuit you will and should see a voltage drop equal to I*R, where R is resistance of the neutral conductor. Draw a diagram and do your math, you should reach your own conclusion.

 

[ScottyUK]

Picking up on DanEE's advice, while the 'scope is out have a look at the shape of the supply voltage to your equipment. Some UPS topologies produce an awful excuse for a sinusoid. Good ones do remarkably well. Your 'scope should hopefully show a nice sinusoid without any flat-topping, notches, dropouts, multiple zero crossings, switching hash, etc. If you have anything other than a fairly clean sinusoid, be suspicious of this as a potential cause of your problems. In my experience equipment with active PF-corrected front ends can behave particularly badly with heavily distorted supply voltages. The word 'meltdown' was memorably used to describe the damaged power supply at the time!




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[stevenal]

Low voltage equipment is insulated to 600V or better. 2.5 V is nothing, rbulsara is correct. No point in putting in isolation transformers to cure it. But if there is an isolation transformer already present, it should be bonded.