Will It Hurt My PLC?

No, it won't. Earth conductors are there to provide personnel protection to extraneous metal parts so that, in the event of a fault within the equipment that risks making the metalwork live, the circuit protective device will operate. There should not be, under normal operation, any connection between the phase and neutral conductors and earth within the appliance.

21 V earth to neutral voltage is VERY HIGH. Pls. check the earthing cable and neutral wiring. There is clear indication of problems likely to come up, including damage to your connected equipment also.

Thanks and regards

Ashok Toshniwal, Bangalore, India

Hi,

I would check out the UPS details, sounds like the inverter maybe be a floating design. The PLC and other equipment wont have a problem if the inverter is maintaining a 'clean' voltage. The power supply in the PLC generally have good tolerances and output nice filtered voltage to the PLC components.

With the 40 watt lamp, were you able to measure any current, and did the voltage difference remain at 21V?

Regards

Trevor.

First, your PLC and most other electronic devices run on DC power. So the synthetic AC power output from your UPS feeds the AC input of the DC power supply that the PLC uses.

Second, most UPS devices do not output a sinusoidal voltage. The output may look more like a square wave than a sine wave, but if your DC power supply is designed well enough, then everything should be OK.

Third, a voltage of more than 3 or 4 volts between neutral and earth ground indicates the possible presence of harmonic distortion. That comes from the non-sinusoidal output. You can place ferrite beads on your power line to reduce the higher frequency components of your simulated AC power from the UPS. But the potential to cause problems with the PLC depends on how well the DC power supply handles harmonic distortion. Since it probably has a step-down transformer, you should be sufficiently safe. It will either work, or it will be very confused and act strangely.

What do you mean by "o/p"?

Output

Hi gentelmens, thanks for your reply .There is no current flow when iam connecting load to 21v between earth and nutral.

Current flow will depend on the resistance connected between earth and neutral. DO NOT SHORT EARTH & NEUTRAL, YOU MIGHT SEE SPARK OR EVEN EXPLOSION.