120VAC 1ph UPS Floating Neutral 1

[cherryg]

I have a 80kVA UPS (Chloride Apodys) with 120VAC, 1 phase, 60Hz output. Its a double redundant system with isolation transformers on the output side. The measured voltage at one of the consumer end is: L-N : 115VAC, N-G: 85V, L-G = 115VAC. This is a floating neutral system. The O&M Manual for the UPS reveals that neutral link to earth is available but left for the end user to be connected.
Now the queries:
a. Is a floating neutral system required for the DCS and PLC power supply ?
b. What in these power supplies causes the neutral to float so far from the ideal 0V?
c. If any other consumer cannot tolerate the N-G voltage, would separate isolation transformers solve the problem?
What sort of problem will be faced by non-DCS and non-PLC consumers?

 

[FacEngrPE]

Except for some special cases floating neutrals are undesirable, even in assocation with PLC and DCS systems.
The presence of the isolation transformer allows you to set up a "separately derived neutral" downstream of the isolation transformer, in accordance with the requirements of the electric code.

 

[che12345]

Dear Mr. cherryg
Unless you have special reasons, otherwise earth the neutral at the output of the isolating transformer. With the Neutral EARTHED, the readings shall be L-N=115Vac, N-G=0V, L-G =115Vac.
Che Kuan Yau (Singapore)

 

[Kiribanda]

120V/2 wire UPS distribution is desirable from protection point of view.
Because to trip UPS distribution circuits a fault should happen between the two conductors.
But if the distribution is 120V single phase with neutral grounded, then
there may be nuisance trips even for a small ground fault on the line conductor.
Therefore, if you are looking for a stabilized uninterrupted 120V UPS
distribution then I support isolated neutral similar to an isolated 125 DC system.
But the UPS should provide an efficient GF detection scheme to alarm only.

 

[cherryg]

Thanks all for your inputs. In this particular plant, all UPSs have floating neutral in the outgoing 120VAC 1ph circuits with no intentional earth connection. Thus, several kms of UPS distribution network is operating with floating neutral. Now, possibly, in old DCS / PLC, the voltage power supply uses voltage divider which means the neutral is already earthed at the DCS / PLC end. But I am not sure about that. Maybe that is the reason for the neutral floating at 85V to earth. SInce this is a legacy system, changing from floating to earthed will not be an easy one. After all, nobody wants an operating system to change overnight, with millions at stake. As far as safety is concerned, since the entire system is well insulated, it can be considered like the 125VDC ungrounded system.

 

[FreddyNurk]

90V to earth for outputs of plugpacks or similar domestic supply equipment (i.e. 9V / 12V equipment with barrel connectors) isn't uncommon when measured with a high impedance meter but disappear when a test light or similar is hooked up to it.

Not necessarily the case here as most of the examples of that are a 240V supply rather than 120V but worth mentioning regardless.

che12345, can you advise why one would use an isolation transformer if the neutral on the load side is to be grounded regardless?

EDMS Australia

 

[waross]

In a simple bridge rectifier, AC to DC circuit, The DC must be floated when the AC side has the neutral grounded to avoid shorting the line side to ground during the negative half cycles.
Most UPS units will have internal isolation but there may be exceptions that require an isolation transformer on the output if both line side and load side are to be grounded.

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Ohm's law
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It's the LAW!