Observe voltage between multicore cable and ground

Thread Starter

Pravinj

Joined May 26, 2016
2
Hi

I have a connection from PLC cabinet to MOV for remote operation
Details as below :
Driving Control Voltage : 120VAC
Cable from PLC to MOV for command (DO) and feedback (DI) : 19C

Observation :
MOV operation and feedback from remote mode in PLC is fine, no issues.

One observation is, when put multimeter probe between one core of the cable and to ground ,it measures around 45VAC.This voltage is enough to lit feedback LED. Need to understand

1. What is this voltage /Why do see measure this voltage
2. Is this harmful for PLC
3. Can we eliminate this

upload_2019-6-29_20-49-18.png
 

dendad

Joined Feb 20, 2016
4,493
If you have a mains power supply on the device, and the power supply does not have a earth, often around half the mains supply voltage can be measures in the output due to capacitive coupling of the mains filter.
We have run into problems like this in many installations that do not run a common ground. So ensure your network cable has a shield that is connected at both ends to bleed the voltage away. It is usually at reasonable high impedance, so not a lot of current is involved, but it can damage your devises, or at least drive out of the common mode area so no readings can be taken.
As mentioned above, your circuit is too small to see hat you are using.
 

Thread Starter

Pravinj

Joined May 26, 2016
2
Hi All,

Thanks for the response. My apologies for smaller pic :)

First, based on the above response let me clarify, the cable used is 19C without shield/drain wire. So to eliminate is there any other possibility .
Second, for examining the source of this voltage I tried one test, removed all voltage from circuit and then measured the voltage (core to ground ) observed negligible voltage, then closed one fuse(which is going to send 120VAC in one core of that 19C) and observed around 10VAC (in the next core to ground),then next fuse closing shoots the voltage to 15VAC and with all fuse close it reaches 45VAC.Is this observation giving any meaning? What is phenomenon?
Also as dendad described, "it is usually at high impedance with less current" ,and my application these are digital signals(status and command) and terminated on relay side ,in what way it affect the devices.

Attached schematic of circuit
 

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