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giri
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Join Date: May 2009
Location: Some where on Earth !!!.
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UPS Floating Neutral

11/12/2009 1:06 PM

We are having a 110v UPS supply for DCS panels. Why it is not necessary to earth the neutral at the UPS panel as one can measure a voltage of near about 50v neutral to earth & phase to earth? Isn't it harmful on earth fault due to floating neutral? What will happen if phase to earth fault occurs?

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PWSlack
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United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#1

Re: UPS Floating Neutral

11/13/2009 3:06 AM

If it's 50V from "neutral" to earth and also 50V from "phase" to earth then one possibility is that the mid-point of the output transformer is already connected to earth inside the unit. Connecting either the neutral or the phase to earth under these circumstances will cause the circuit protective device to operate.

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capblanc
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Join Date: Jul 2008
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#2

Re: UPS Floating Neutral

11/13/2009 6:46 PM

See the "Floating Earth" thread in today's postings.

A UPS is a combination charger and inverter. Most switch the input voltage straight through when mains supply is available. Since in many countries the plug can be inserted either way it to the outlet it would be impossible to ground the output side as that may be connected through to the live in the plug.

You probably read 50V because of EMI caps on the output of the unit. This can be proven by connecting a low power bulb between on leg and the case ground when running from batteries and I think you will find that it will have pulled that leg to the same potential as the case earth.

Regards

Chas

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