# Induced Voltage - does it affect your plant?



## Wirenuting (Sep 12, 2010)

Older places that had several full boats and control wires sharing home runs have the greatest induced voltage problem in my experience.
Nothing much you can do unless you separate everything.
I’ve seen over the years as we went from 120volt controls and sensors to PLC’s and building automation systems that the problem reared its ugly head with sharing old runs.
Separating things over the years has mostly solved this problem for us.


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## gpop (May 14, 2018)

We see it all the time in older places especially as they shared the conduit with other motors and controls.
A few times it has caused problems like on a very long run there was enough induced voltage to hold a relay. (480v pump had a 120v on/off switch that activated a relay for the contactor at the disconnect and they only run one conduit. Turn the pump on and it stayed on even when the switch was off)

The last place i worked was all allen bradley mcc's which have a plug in the bucket for 3 phase field wiring. I made up a grounding block that the field wires could plug into then clipped a crocodile clip to the metal door catch to bleed off the induced voltage. 

Biggest down side is false readings on meters. You can ohm a line to ground and it fails yet will pass a meg test. You can get a back feed and mistake it for induced, You can get induced and mistake it for real voltage and of course you can get bit which is not fun. (striking wire to ground before touching it becomes a bad habit that will get you odd looks from electrician's that have not worked in this type of field)


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## mburtis (Sep 1, 2018)

I've ran into ghost voltage readings on meters, never realized you could actually get enough power to shock you or hold a relay in.


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## BlackHowling (Feb 27, 2013)

We built a Chinese designed/import lead/silver mill in BC when I was an apprentice. We were getting enough induce voltage in the control circuits to keep relays pulled. 

200-400ft runs in a 24" cable tray literally heaped with cables

Sent from my SM-G975W using Tapatalk


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## wiz1997 (Mar 30, 2021)

I've worked many places where induced voltages have caused problems.
Mainly with AC inducing voltage on DC control circuits.
The biggest problem is electricians not understanding the purpose of shielded cables or using non-shielded cables when shielded cables should be used.
All our machines primarily use gutters for cables where there may be various voltages, 480, 240, 120, and 24VDC
Shielded cables use the shield to "drain" away any induced or transient voltages.
Generally the shield is attached to ground at one end or the other, but not both.
Some attach the shield at the supply end, some attach at the load end.
Can't say which is "better" but I have seen both methods.

Out in the refineries they landed the shield at the instrument end.
It was explained to me that if a transient voltage was to effect the cable, the transient voltage would take out the instrument and not the controller.

Where I currently work, ALL the motors have shielded wire between the drives and the motor, with the shield attached at the supply end, taped at the load end.
Any 4 to 20mA signal wires are also shielded and attached at the instrument end.
Perhaps someone could explain the reasoning behind the two different methods.
I've seen induced voltages provide attention getting "hits" and enough to trigger inputs on PLC's.


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## SWDweller (Dec 9, 2020)

I got sent out to troubleshoot ghosts at a waste water treatment plant working for Eaton. Standing there you could hear and see some drives and contactors turn on then seconds later other not related equipment would turn on or off. The EC decided to do value engineering and put all of the control cables in one conduit. Voltages were from 24 to 120. Most of the cable were THHN shortest distance to a junction box was ~200 feet. Then I found the controls cabinet made in Germany with a German start up guy. No grounds were connected. 
Lastly I discovered the EC had moved the grounding from a single point to a star. I made my recommendations and wrote the report. Called back a couple of weeks later engineer of record and the owner of the EC and the city manager were all standing there. First time I got to see the drawings, someone had wandered off the reservation, as the as built was nothing was like the drawings. They had corrected most of all my deficiencies when a plane flew over and the whole equipment room was changing state. Now the issues were only when the planes flew over. Being this was not a residential area and very near the end of the runway the planes were low.
The planes were from a Top Gun training base for foreign pilots, All F14's and 15's. I made some calls finally got to the commander of the base, explained what was going on and was soundly told the problem was not the air forces problem. The problem stopped, the foreign pilots were landing with their targeting radar switched on. Or so a little birdy whispered in my ear. 400 watts if I remember correctly, the power of the targeting radar. We had a Faraday cage installed on the roof of the controls building. We never got another call. 
Lesson do not put consumer electronics at the end of a fighter base runway and expect no problems.


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## macmikeman (Jan 23, 2007)

Sometimes what bites you is the capacitors . You turn off a vfd for a 40 horse motor you better wait a 15 minutes or thereabouts to let it bleed off or else manually discharge it. Same thing can happen at LPS and other types of luminaires that have a big cap. I say can, but it never happened to me yet.


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## gpop (May 14, 2018)

wiz1997 said:


> I've worked many places where induced voltages have caused problems.
> Mainly with AC inducing voltage on DC control circuits.
> The biggest problem is electricians not understanding the purpose of shielded cables or using non-shielded cables when shielded cables should be used.
> All our machines primarily use gutters for cables where there may be various voltages, 480, 240, 120, and 24VDC
> ...



We ground our 4-20 at the mcc as every analog by spec has to have a surge suppressor and that comes with a din rail ground for the shield. It also makes life easier when the instrument is mounted to a plastic pipe.


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## SteveBayshore (Apr 7, 2013)

When I need more accuracy than using a Wiggy, I use the Fluke Stray Voltage Eliminator (#SV225)





















on my 787.


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## SWDweller (Dec 9, 2020)

I have the 787 but have never seen the stray voltage eliminator. 

Way cool thanks Steve


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## just the cowboy (Sep 4, 2013)

wiz1997 said:


> Where I currently work, ALL the motors have shielded wire between the drives and the motor, with the shield attached at the supply end, taped at the load end.
> Any 4 to 20mA signal wires are also shielded and attached at the instrument end.
> Perhaps someone could explain the reasoning behind the two different methods.
> I've seen induced voltages provide attention getting "hits" and enough to trigger inputs on PLC's.


This is actually wrong. VFD cables shields should be connected at both ends I believe due to the fact they are trying to squelch the spikes coming out of the cable. Signal cables are connected on one end because they are trying to protect the wires inside and if both ends are connected the shied acts as a core and can induce more spikes. We use all 4-20 control so induced voltage does not effect it as bad. Here is a good write up on shields.








2 Rules for Properly Terminating VFD Cables | Rockwell Automation


VFD cables require special termination so users can optimize VFD cable system performance, reducing EMI, creating a controlled path for common mode current, and helping improve operations efficiencies.




www.rockwellautomation.com





Cowboy


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## gpop (May 14, 2018)

Sometimes induced voltage can be used to your advantage. 

Wire in the field is at 8v. wire in the mcc is at 14v. That a good clue that you are on the wrong wire. 
120v wire to coil reads 5v when its off. Thats a good clue that the coil is open or your on the wrong terminal. 
One of the b-o-y Wires in the mcc reads Zero its time to break out the mega.


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## MoscaFibra (Apr 15, 2021)

My last plant had 2 anneal furnaces, they played absolute havoc on the inductive roll steering. The building was on reclaimed land, so we are pretty sure the current grounding system had failed. Lots of fun. Nothing like massive magnetic fields and very sensitive equipment.


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## MoscaFibra (Apr 15, 2021)

just the cowboy said:


> This is actually wrong. VFD cables shields should be connected at both ends I believe due to the fact they are trying to squelch the spikes coming out of the cable. Signal cables are connected on one end because they are trying to protect the wires inside and if both ends are connected the shied acts as a core and can induce more spikes. We use all 4-20 control so induced voltage does not effect it as bad. Here is a good write up on shields.
> 
> 
> 
> ...


I don't think I have ever seen this article before. Interesting read. I know at my last place we inspected bearing, and had vibration monitoring on our important motors. Anything that came up with pitting we put ground bushings on the shaft. It solved most of those issues, but with added cost and maintenance. One of those things I kind of wish I could back and try this theory and see if it solved it.


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## paulengr (Oct 8, 2017)

Often it’s not induced voltage unless you have long parallel cable runs.

There are many causes for “ghost” voltages. Induction is just one. Here are others.

Use of solid state switches especially PLC TRIAC outputs. It’s a semiconductor, not an insulator. The current is low but even if it’s 1-2 Megaohms this is no match for a 10 Megaohms input resistance meter which often shows nearly full voltage. Same with VFDs and soft starts unless there’s a true disconnect.

Cable depolarization. Charges exist on shielded cables in the insulation. Even if you short it out especially after a PI test (apply DC for 10 minutes) even if it is shorted to “zero” it can recharge itself from polarization and give you a nasty shock. Always discharge for 3x longer than you charge or leave a ground cluster on it or treat it as energized.

Capacitive dividers. Very narrow gaps which is especially true in vacuum bottles have a high insulative value to be sure but also two conductors separated by an insulator is the definition of a capacitor. Very common to measure even hundreds of volts on otherwise “open” terminals. That’s what isolation/disconnect switches are for.

Welded contacts. A three phase breaker/disconnect or especially a contactor will operator “properly” with one phase welded since the circuit is not completed. This can go unnoticed by operations. ALWAYS test for voltage with a meter and check visible gap disconnects before touching anything, locked out or not. The lock is just there to prevent someone from accidentally re-energizing. Use work site grounding if you need to.

Capacitors. Do NOT believe those stupid indicator LEDs on VFDs. They are supposed to discharge in 15 minutes if the internal resistors are working properly That’s a big if. Not all of them have one and replacements might not be the same as factory originals. A 100 HP or larger drive can cause heart fibrillation.

Ground potential rise. Ground is not ground everywhere except at long distances of miles (remote Earth). This is usually more of a transient issue but having tens of Volts difference is common. Again…grounding and bonding is your friend.

Ground loops. Big problem in electronics (EMF). Grounding at two points created a circuit, a loop antenna that sucks up every bit of electrical noise. In power systems it also creates a one turn “CT” that carries significant current. If you have effective grounding, ground one end only. If you don’t ground both ends. See GEMI for guidance on maximum conduit lengths before bonding is no longer effective. Be heavy handed but consistent about how you ground.

Bad grounding. It’s surprisingly common. Using a clamp on tester usually I find about 1% of structural grounds are defective. About the same amount are ripped off or broken. Cable trays are often not bonded but then used as grounds. Those are probably the two most common issues I find in grounding systems. In plants where PVC ducts or conduit is prevalent, grounding is nearly always an afterthought. So it’s no surprise you get all kinds of things going on.


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## Jlarson (Jun 28, 2009)

If you're in one plant you get used to it over time and kinda know what's gonna cause what, what long ass conduit that's stuffed with control runs is gonna cause problems, what line uses a brand of SSR's that has high leakage.... stuff like that. 

It's more interesting when you run service and see diffrent crap all the time, a lot of it hacked up lol


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## just the cowboy (Sep 4, 2013)

MoscaFibra said:


> My last plant had 2 anneal furnaces, they played absolute havoc on the inductive roll steering. The building was on reclaimed land, so we are pretty sure the current grounding system had failed. Lots of fun. Nothing like massive magnetic fields and very sensitive equipment.


We had a non destructive welding tester once. It would jolt thousands of amps thru the test weld before an x-ray. It had maybe 20 flex 500MCM on each side and you could watch the cables dance. What fields were flying was a lot I guess.


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## Don_Farr (Sep 28, 2018)

Greetings - I was working at a Church, all I was tasked to do was to add a 120V -30A branch circuit for a workroom. When I opened the junction box I discovered a serious problem. All of the Neutrals were twist connected (bare wire twisted together with pliers) and covered by tape. Seriously that's what I found. When I asked the maintenance man, he told me that they just grab any neutral they can find and splice to it., because they all meet at the same place. And by the way, he told that just disconnecting the circuit you were working on still might get you shocked

It took me over 30 minutes to explain that this practice was a code violation and why they were getting backchannel shocks. I was told to leave the neutral connection as is. So I just went about my task then redid the neutrals connections at this junction box. Next time I was there he told me that the random shocks in this area of the property had stopped. He asked what I did, so I told him and he still did not believe me.

Moral of this story is that Old properties may have mis-wired neutrals which can put unwanted voltage anywhere it wants to.


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