Earthing voltage


Thread Starter


I have installed the system based on S7 300 plc .

when i measured the earthing volatge between neutral and earth. It is coming 4 v, Is it safe to work with that much earthing voltage?

how can i reduce this earthing voltage since i already have seperate earthing pit for earth connection?

Bob Peterson

It's not unusual to have a small voltage between earth and neutral. It's the voltage on the neutral from where you are measuring it to where the neutral-earth bond is made. You can't get rid of it, and there is no reason to try.
You should not normally be dumping any current through earth ground, so it should stay close to the voltage you would see at the main panel.

Neutral on the other hand is constantly flowing current. You will see I*R drops on the neutral wires leading all the way back to your main panel. This is probably where the potential is coming from.

Try shutting down all of your major power consumers and you should see this voltage drop. If you shut down your entire panel and you are still seeing a drop it is coming from your building's wiring somewhere. The only way to cut it down would be to run fatter neutral wires inside your machine and back to your main panel. The way your power is distributed in your building may be a factor as well. If you are sure you are using correctly rated wires I probably wouldn't worry too much about it.


William \(Bill\) L Mostia Jr PE

If you are measuring at the PLC neutral terminal to the PLC equipment ground (earthing) terminal, the voltage is normally caused by the voltage drop from the neutral "return" current IR (voltage) drop due to wiring impedance. Four (4) volts is high and it nominally should be on the order of 0.5 volts, however, it unto itself may not indicate an unsafe condition but may indicate an underlying condition that is potentially unsafe or at least undesirable. This can be caused by an undersized neutral, too long a neutral, a bad connection, other higher than normal resistance in the neutral circuit, shared neutrals, etc.. Less likely is that the voltage is due to the equipment grounding terminal being at a potential that is different than the "earth" potential that the neutral is actually grounded at (referenced to). In both cases, the neutral conductor path (as well as the equipment grounding conductor path) should be traced back and be inspected for any irregularities (wrong size neutral, bad connections, wrong connections, shared neutrals, neutral being grounded in more than one place, etc).

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

All Information is provided on a Caveat Emptor basis.
Sandeep... a continuous voltage measurement across the EarthING-conductor (or GroundING-conductor) presumes a continuous current in the Earthing or Grounding Plane.

This operation is a violation of the USA's NEC. I strongly suggest you implement the tests suggested by Bill Mostia!

Regards, Phil Corso