Induction Voltage

If it is an induction problem, then it will go away by adding a suitably-rated resistor across the BMS/PLC input terminal and its common rail.

It may be EM pick up voltage in open contacts. By terminating contact point by 1M resistor this must get reduced else their is some leakage current and your relay is bad now..

Drop it across the internal resistance of a meter to earth. If the current drops to 0, it is likely inductance.

Look at the voltage with an oscilloscope. If it is AC it is not induction, it is capacitive coupling between the contacts. If is inductive it will show as a voltage spike and diminish with time (DC). Knowing which it is can allow the proper solution. If it can be read steady state with a digital meter on AC volts it is not induction. An induction spike tries to go to infinity, limited only by leakage current. A capacitive coupling gives a steady state AC. The other possibility is leakage current through poor insulation or electronic apparatus that can look like capacitive coupling but is extremely variable. One way I did low voltage tests for this was comparing digital meter readings with analog meter readings. The voltage varied greatly between the two meters.

Inductive voltage spikes are very short lived, usually in a fraction of a second, and tend to blow up anything attached because they are very high in voltage. Warning, they can be lethal.

AC doesn't cause induction? Guess there is no need for substation ground mats...et cetera.

AC doesn't cause induction? AC doesn't cause induction???

Wait, let me quote you here: "If it is AC it is not induction"

Don't tell the transformers that!!! What have you been smoking???

Cuba Pete

He is looking at a transient voltage or a low level voltage. Induced voltages can be steady state but normally require an iron core as in a transformer to do so. He is looking for a voltage that is either transient or is leakage. A voltage from a transformer is meant to deliver power and cannot be easily loaded down. The induced voltages just between wires are very easily loaded, even with a small incandescent bulb. Stay on the topic and don't get obsessed with something you don't understand. Really! And do you really thing the ground mats in substations are for induced voltages? If all we had to deal with in substations are steady state induced voltages, it would follow the conductors. The only time induced voltages get out of whack is when contacts are opened and you stop the flow of electricity producing an inductive kick and that is short time and easily managed.

kuppahalli,

If you have access to the manufacturer's installation instructions, I expect you will find in them the requirement to keep at least 2-inches away from any high-voltage or high-power wires. Also, there usually is a requirement to run the wiring in a separate conduit (cable...). A good approach to solve this would be to replace these wires with a shielded (screened) cable; but make sure its insulation is rated for the maximum voltage on any wires in the cabinet. Then ground the shield (screen) at one end only--I usually would choose the cabinet with the BMS/PLC.

The other posts that suggest adding a resistor across the contacts are valid approaches, but you would have to make sure that this did not cause an error in reading the contact status.

--John M.

where is the switching waveform and pick up waveform. I know that on both rising and falling edge you may find pick up voltage spikes of milli second order.