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from the Electrical Engineering department...
High neutral to ground voltage
Computers in manufacturing; also hardware discussion. topic
Posted by T.N.Venkatesh on 3 September, 2007 - 3:36 pm
I have been encountering high neutral to earth voltage in the UPS output. Would like to understand the cause and remedy

T.N.Venkatesh
tn.venkatesh@siemens.com


Posted by Bob Peterson on 4 September, 2007 - 5:54 pm
What do you mean by high?

What is the ups output supposed to be? US, or some foreign standard voltage?

How far from the UPS is this "jhigh" voltage measured? Is the UPS an SDS?


Posted by joydip on 4 September, 2007 - 6:13 pm
The following things have to be cross-checked.

1. Input and output neutral & earth of UPS is common or not.
2. Whether common input neutral and earth are good. We can find out by checking the same at the input of UPS if input and output earth is common. while checking keep the UPS and load "On".
3. By checking the same you will find the root cause. If neutral is found to be floating then 1:1 isolation transformer has to be put and secondary neutral of transfomer has to be grounded. Note the grounding has to be dedicated.

joydip
jdb_2000 @ rediffmail. com


Posted by v.b.sawant on 24 November, 2007 - 6:17 pm
I am facing the same neutral to earth voltage @3to8 V. Checked earthing & found ok. UPS vendor suggesting to earth UPS output neutral and put in grid earthing . whether this is a correct solution.

Also some times i observed neutral to earth voltage @ 40 - 50 v.


Posted by Prakash on 27 November, 2007 - 1:01 am
Hi,

Only sharing my experience.
We had also same experience in our 250kVa UPS. We connect the ground to the neutral of UPS output then problem solved.

This may help you.

Prakash


Posted by jojo on 27 November, 2007 - 11:03 pm
I would recommend you check also the live to earth voltage, together with the neutral to earth voltage. If you have a high neutral to earth voltage, and a low live to earth voltage, this means that the output of your UPS is floating, i.e. most probably you have an isolating transformer on the output of the UPS, and there is no connection between the output terminals and the earth. Obviously the difference between the two readings should be you live to neutral voltage.

Whether you want to connect the neutral terminal to earth, or not, depends on the protection systems you have on the output of the UPS. In addition it depends also on how you want to operate your loads (i.e. grounded system or floating system). Before you connect your neutral to earth directly check what earth fault currents the system can tolerate.

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