We have Work station 220 Volt sockets showing 3.5 Volts between Ground to Neutral. I want to know what is the limit? If this is more what is the remedial action required. Is there any International standard for this.

Please reply to my email address also.
Regards
H.V.Seshadri
[email protected]

 

Generally speaking the Neutral to Ground voltage as measured at the load for a single phase circuit is a function of the load current and the impedance of the neutral wire. The US National Electrical Code (NEC - NFPA 70) in Section 210.19A.1 Informational Note 4 limits the voltage drop in a branch circuit to 3% (5% total for feeder and branch circuit) for a reasonable efficiency of operation. Based on this the N-G voltage limit on a single phase 120 VAC circuit is 3.6 VAC and for a single phase 220 VAC circuit 6.6 VAC (equal to the allowable voltage drop on the hot wire). This does not necessarily mean your particular equipment will function at these voltage drop limits. The solution is shorter wires or bigger wires.

If you have a three phase circuit, excessive N-G voltage can mean excessive third harmonic current flowing in the neutral.

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP
Any information is provided on Caveat Emptor basis.

 

In the case of computer work stations, any measurable ground to neutral voltage is an indication that of the need for an isolation transformer. Often, local APC (battery backup) will suffice.

Am familiar with cases where Gnd-Neutral offsets between outlets led to equipment damage when the two pieces of equipment were subsequently interconnected. Specifically, a monitor connected to one circuit, with a ground offset of 25 vac, was temporarily connected to the workstation in the same room. This led to Motherboard failure of the original and the repaired system.

A good idea to sort out the actual wiring/grounding deficiencies of your system, to avoid major cost/headaches later.

Good luck

 

In the early days of microprocessor based equipment, this equipment was more susceptible to N-G voltage but improved designs have eliminated much if not all of this. In was common to have low N-G voltage specifications on equipment (e.g. 0.5 v). This specification, if it exists, should always be reviewed when purchasing equipment. Here is an article written a few years back on the subject of N-G voltage, which also discusses issues common mode disturbances that can appear on the N-G voltage that may be of some interest.

http://ecmweb.com/power_quality/electric_clearing_neutraltoground_voltage/

I would be interested in some more information about of how a 25 VAC ground offset was achieved in the following statement:

"Specifically, a monitor connected to one circuit, with a ground offset of 25 vac, was temporarily connected to the workstation in the same room. This led to Motherboard failure of the original and the repaired system."

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

Seshadri... see the following control.com topic:

http://control.com/thread/1281070067

Regards, Phil Corso

 

Wiring in the room was not designed for computer use and being used temporarily.

Work station and primary monitor fed from one circuit. A second monitor was later installed from a socket in the same cubicle and presumed to be from the same circuit. The second circuit exhibited a neutral/ground potential different from the first circuit. Back fed 25vac (measured) due to stray pick up on a ground wire that was later shown to have lost continuity, through the video port (Din cable not USB) and into the mother board. You might think that the second monitor would have achieved ground connection through the workstation case ground, and would have caused not problem, but that is not what happened.

How stray voltage via the the video port made its way to the mother board dc rails and cause board failure on an workstation properly grounded and protected from surges is beyond me, unless it was a electrical transient due to storms in the area that managed to get into the second monitor circuit.

Learned what we all know, check your grounds, especially where computer equipment is involved.