Sorry Sajeev, it's probably due to a poor earthing system and Ohm's Law, I should imagine. It could even be due to too many spurs (instead of ring mains circuits). That is, loads effectively in series, rather than parallel. But, to be specific, where are the potentials you mention actually being measured ... at the outlet where the unit is located (after all, that's what matters in this case)?

I once encountered an arrogant fool (who, by the way, boasted of a Masters Degree in Electrical Engineering from a University in Cairo) who insisted that the x-ray unit we were planning to install at his hospital did not need an earth. He said (quite rightly, up to a point) "the machine will work very well without an earth"! As may be imagined, he was trying to avoid the expense of adding an earthing system.

The next time I visited, sure enough, there was the earth cable, "neatly" tacked to the wall with staples. My enquiries regarding where the cable actually ran to just drew blank stares. In the end, I just walked away.

PS: that same genius had wired the general lighting in the operating theatres via the isolated power supply. Naturally, whenever the lights were on, the LIM was already at "half-cock". No wonder, then, that it was "always alarming" as soon a couple of bits of kit were plugged in. Nothing I could do (including, I recall, sketching a diagram in the dust on the floor) could convince this absolute idiot of the error of his ways.

mmmm...in the same way, an administrator who was a doctor insisted me to install about 20 ERBE ICC ESU`s with out proper grounding system. The result....machines installed with an installation report and less than a month service calls started pouring in ( actually about 5 machines failed due to fault in the RF generators obviously due to floating ground ) which forced the administrator to go for a proper grounding system morel: feel it to believe it hahahah

One thing I can understand is that your area / your hospital draws lot more inductance load that leads to fluctuation (harmonics) of N-G voltage. Try to arrange a separate earth pit exclusively for the ultrasound scanner. Its gonna solve ur problem for ever.Upto 5V is acceptable practically.And zero V theoretically.

Hai sajeev,

some time you can find non medical equipment (kettle/table fan) connected to the UPS-IPS system. In this case you can get more than 5 voltage on the N-G. check the UPS system fitted with the insulation monitoring device in accordance with IEC 61557-8. check any of the L/N/G wires coiled on the path. Check the copper ERB is still there after electrical installation.

Measurements shall also be made between the earth point on all mains sockets in the scanning room and the ERB. If any devices are plugged into sockets outside the scanning room and then used within the scanning room, these should be supplied from the same phase as any sockets within the scanning room, and the touch voltage on the earths of these sockets also measured. The touch voltage should be less than 10 mV,AC or DC. If a voltage greater than 10 mV is found, the measurement should berepeated using an IEC filter. If the touch voltage is still above 10 mV, the source of the voltage should be investigated.
Once it has been established that there are no significant touch voltages present, the bonding resistance should be measured. A battery operated four-wire milliohmmeter should be used, in order that the meter can be kept at a safe distance from the midline of the field during the measurement. The meter shall have a resolution better than 10 milliohms, and be capable of performing the measurement at a current greater than
100 mA.
The resistance between the ERB and all accessible conductive surfaces of installed equipment should be less than 100 milliohms. The resistance between the earth point of all mains sockets and the ERB should also be less than 100 milliohms.
Any portable devices shall be plugged directly into a conveniently located hardwired socket.