electronic earth 1

[jain1616]

We need to achieve N-G voltage of less than 0.250volts for a sensitive electronic equipment. This equipment is fed from phase to neutral of a three phase UPS with dedicated clean earth pits (resistance 1 ohm).

1) The UPS has imbalance capabilities with an output isolation transformer which is star connected. Can we achieve the desired values since the neutral current is very high because of imbalace in loads.

2) Will a higher N-G damage electronic cards when signal cables are plugged-in or out when the system is operational assuming shields to be grounded.

 

[alehman]

1) Many UPS's can be configured as "separately derived", in which the neutral of the output transformer is grounded at the UPS. This would get the N-G bond as close to the load as possible, minimizing V N-G. You may need to oversize the neutral conductors as well to reduce voltage drop. 0.25V may be difficult to achieve unless your loads can be balanced or circuit lengths are kept very short.

2) It would be hard to answer this. It is possible, but it depends on the equipment. Typical modern office computer network equipment includes electrical isolation for the cabling so it's not as much of a problem as in the past.

 

[jain1616]

Thnaks for your reply Alehman.

The neutral of the output transformer in this case is grounded and neutral is oversized to the same size as phase conductors. However it may be difficult to minimize neutral current because of imbalance in load.

How about a single phase dedicated UPS in this case with single phase isolation transformer with grounded neutral. Neutral to earth current will only flow incase of fault so N-G voltage will be earth pit resistance multiplied by a negligible value.

Also why should N-G voltage be significant for signal cables with grounded shield. I think static or body resistance is more important here than neutral.

 

[alehman]

N-G voltage results from voltage drop in the neutral conductor between the source (where the neutral is grounded) and the load. This is proportion to the load current in the neutral (not neutral to earth current). To minimize N-G voltage, you must minimize this voltage drop which means you must:

1. minimize the neutral current
2. minimize the length of the circuit
3. minimize the impedance of the wiring

1. To minimize the neutral current, you should balance the load as well as possible. This can only happen with a 3-phase system. A single phase system is essentially 100% unbalanced meaning all load current returns on the neutral, and N-G voltage is maximum. Of course if your load can't be balanced, it may not make much difference.

2. It may be possible to reduce the effective length of the circuit by installing a transformer or other form of isolation, very close to the load. A small single-phase UPS may serve the same purpose.

3. Minimizing wiring impedance can help. You may consider increasing the neutral to 200 to 300% of the phase conductor size.

I would be warry of any new electronic equipment which is this restrictive. It tells me the mfg. is not confident of their design. I would inquire thoroughly as to the reason for this requirement so you can better understand what is required.