# Induced voltages from ground or?



## Norwest (Apr 15, 2015)

Hi, so I am completely stumped.
While commissioning we found that a 30 conductor from the plc to jb had 56v on one of the wires ( core number 9 ) which was also a neutral. We then checked the rest of the conductors in that cable and found that all of the wires running to the inputs had a voltage ranging between 11-47v including the spares. I then isolated the 30c in the JB and measured them again with there being no change. I then isolated them in the PLC as well one at a time and found that there was no change with the exception when I pulled two of the fuses. Each fuse when pulled would reduce the voltage by about half in all the spares as well as the now isolated wires.
At this point I also started checking other spares in the plc that were from different cable runs and they also had voltage in them. Some of them would go to zero when those same two fuses were pulled. 

My first thought was an induced voltage from a high voltage cable running too close or parrelled. I then followed all the cables and determined that yes they had all been separated in the tray properly and had tray divider installed.

I was also thinking it could have to do with the input cards. But then this morning a local control station was not working properly and the Inputs on this go directly back to a soft start in the MCC. I was also finding 56v for the stop input which would not allow the drive to drop out. When I turn off the drive this voltage on the inputs goes away but it does not affect the induced voltages in the plc.

When I went back to the plc this morning I found that the first wire I had found with 56v now has 87v to ground and pulling the first two fuses drops the voltage to ground by 30v each respectively and I then went through all the other fuses and found a third which had no affect yesterday when pulled would drop the voltage to zero.
All the spares also have about double the voltage now too.

I have tried pulling off the neutrals as well as the grounds and there seems to be no difference.

Also by the end of today it had become worse so that motors which were running fine yesterday and even this morning will no longer stop when the stop button is pushed at the hand station.

Any ideas would be appreciated!


----------



## macmikeman (Jan 23, 2007)

Norwest said:


> Hi, so I am completely stumped.
> While commissioning we found that a 30 conductor from the plc to jb had 56v on one of the wires ( core number 9 ) which was also a neutral. We then checked the rest of the conductors in that cable and found that all of the wires running to the inputs had a voltage ranging between 11-47v including the spares. I then isolated the 30c in the JB and measured them again with there being no change. I then isolated them in the PLC as well one at a time and found that there was no change with the exception when I pulled two of the fuses. Each fuse when pulled would reduce the voltage by about half in all the spares as well as the now isolated wires.
> At this point I also started checking other spares in the plc that were from different cable runs and they also had voltage in them. Some of them would go to zero when those same two fuses were pulled.
> 
> ...


How many mounting screws stand between that PlC and the rubbish bin?


----------



## varmit (Apr 19, 2009)

This is common on long multi conductor cables. If there is any current on any of the conductors, there will be an induced voltage on the other wires. If these wires go direct to a PLC, you will need to add pull down resistors on the I/O to help eliminate the induced voltage.


----------



## Norwest (Apr 15, 2015)

Thanks. I will look into the resistors.


----------



## NacBooster29 (Oct 25, 2010)

Are you mixing inputs and out puts from your plc? In the same cable? 
I.e. start stop, and status ?


----------



## Norwest (Apr 15, 2015)

Ya, both input and output are in the same cable. I can see this as being a possible issue as well...


----------



## KennyW (Aug 31, 2013)

Not likely. 
Is this 120v ac io? How long is the run?


----------



## Norwest (Apr 15, 2015)

The runs are a couple hundred feet. 

I think i have decided it has to do with the i/o cards and there being no resistors as a previous poster mentioned. The engineers who are responsible for making any decision though say they have never heard of putting a resistor in and think it's more likely that the ground grid has been damaged in the last couple weeks...although i can't see it happening with it being completely buried and there having been no demolition or excavation in the area since the construction and testing of the ground grid by the contractor who installed it with the engineer present to verify tests as well as a separate test by an independent power systems tech.

thanks for all the help.


----------



## NC Plc (Mar 24, 2014)

I see resistors on plcs all the time. I'm not at a point that I completely understand the purpose yet though.


----------



## drsparky (Nov 13, 2008)

Is it shielded cable?


----------



## varmit (Apr 19, 2009)

The induced voltages, on the multi conductor cables, exist because there is no load on the wires to shunt out the voltage. Adding most any sort of resistive load will eliminate the "ghost voltage". 

The scenario is similar to the false readings that a digital meter will indicate when testing a non energized circuit. Most 120 VAC I/O will gate at about 60 volts. These electronic inputs do not have enough of a load to shunt the induced voltage, so the input turns "on".

Another answer is to have the field wiring operate interposing relays and wire the PLC inputs though the relay contacts. The PLC outputs can operate relay coils and the field device is turned off-on by these relays.


----------



## pjholguin (May 16, 2014)

How are the resistors connected to PLC inputs?


----------



## Jhellwig (Jun 18, 2014)

A wiggy or othe low impedance voltage tester would confirm that it is just ghost voltage. If you take the wiggy and go to measure voltage on the input it will most likely shut the input off and when you take the wiggy off it might come back on. It is a common issue with newer Plc hardware. I have seen resistors used but I have liked using snuffers more. 

It can happen with 24 volt dc also if wiring is not properly isolated from ac wiring. 

Allen Bradley also had that issue on power flex 700s early on. You didn't even have to have wires hooked to the inputs. If you left the unused it puts at their factory defaults it would lock the drive into one speed till you disabled the input.


----------



## pjholguin (May 16, 2014)

Most facilities do not allow Wiggies(solenoid type testers) to be used on their sites due to issues with GFPE systems. You just need to use a low impedance meter or tester.


----------



## don_resqcapt19 (Jul 18, 2010)

pjholguin said:


> Most facilities do not allow Wiggies(solenoid type testers) to be used on their sites due to issues with GFPE systems. You just need to use a low impedance meter or tester.


Wiggies do not normally draw enough current to trip a GFPE. The one I use draws less than 20 mA. Most GFPEs operate at 30 mA and above.


----------



## varmit (Apr 19, 2009)

pjholguin said:


> How are the resistors connected to PLC inputs?


The resistor is connected between the input wire and the neutral. This is usually done on a terminal strip located in the same panel as the PLC. There is not much room to connect anything on the I/O cards. It seems that the resistor value is 1/4 watt, 250 ohm, but my memory could be wrong on the value.


----------



## pjholguin (May 16, 2014)

varmit said:


> The resistor is connected between the input wire and the neutral. This is usually done on a terminal strip located in the same panel as the PLC. There is not much room to connect anything on the I/O cards. It seems that the resistor value is 1/4 watt, 250 ohm, but my memory could be wrong on the value.


Is this a parallel connection?


----------



## pjholguin (May 16, 2014)

don_resqcapt19 said:


> Wiggies do not normally draw enough current to trip a GFPE. The one I use draws less than 20 mA. Most GFPEs operate at 30 mA and above.


I understand what you are saying, but I have seen it happen first hand. Granted this was during start up, once it was found to be a possible problem...Wiggies were not allowed on the sites.


----------



## varmit (Apr 19, 2009)

pjholguin said:


> Is this a parallel connection?


Yes, on each input.


----------



## NC Plc (Mar 24, 2014)

Meds make me loopy, ignore this.


----------

