Shocking induced voltage surprise(the importance of grounding) - Page 1
Forum section: electronics / general

author: Kalin
author_rank: Regular Contributor
timestamp: March 30, 2017, 11:08:17 pm
content: Working today I managed to get a quite a good shock off of a cable connector and I find the reason behind it very interesting.I am an electrician working in a plant with 600V 3-phase powering most everything in the mill. In some locations we use PVC junction boxes. I was very surprised when I opened up this PVC junction box to find everything neat and tidy with all of the grounding in place. I opened up the next box downstream of this cable and it was the same thing everything where it should be. But when the motor was running I was still reading 160V to ground from the connector. Long story short, with teck cable there was enough induced voltage in the steel armour on the cable to shock and because it terminated in a PVC box there was no path to ground until I came along. I had no idea that induction was that powerful and it definitely shows why electricity is nothing to mess with.Hopefully someone finds this interesting/funny/informative.

author: Dubbie
author_rank: Unknown
timestamp: March 30, 2017, 11:12:51 pm
content: Quote from: Kalin on March 30, 2017, 11:08:17 pmYou should try grabbing the output terminals of a 600V transformer. There is no connection at all to the input terminals. That will be a nice extension of your education about the power of induction.It may however be the last lesson you ever have about the subject.

author: Kalin
author_rank: Regular Contributor
timestamp: March 30, 2017, 11:18:30 pm
content: Quote from: Dubbie on March 30, 2017, 11:12:51 pmYou should try grabbing the output terminals of a 600V transformer.I should have said I had no idea that incidental induction was that powerful. A transformer is designed to transfer electricity through induction. Cable is not.Sent from my SM-G930W8 using Tapatalk

author: Dubbie
author_rank: Unknown
timestamp: March 30, 2017, 11:20:26 pm
content: I know thats what you meant. Was just poking a little fun

author: calexanian
author_rank: Super Contributor
timestamp: March 30, 2017, 11:22:28 pm
content: There are two modes of coupling. Induction and capacitive. Both are just as deadly. I have had to deal with both over the years and utility companies have had to institute specific procedures to avoid it.

author: Kalin
author_rank: Regular Contributor
timestamp: March 30, 2017, 11:22:39 pm
content: Quote from: Dubbie on March 30, 2017, 11:20:26 pmAll good. I felt like quite the dumbass today so maybe I'm a little over sensitive.In my brilliance after the first shock I reached out and touched it again just to be sure I hadn't imagined it.

author: Cubdriver
author_rank: Unknown
timestamp: March 31, 2017, 12:41:33 am
content: Quote from: Kalin on March 30, 2017, 11:22:39 pmThat sounds like something I'd do, too.Wait, did I really just get.. *bzzzzzt*My vote would be for capacitive coupling, FWIW.-Pat

author: Kalin
author_rank: Regular Contributor
timestamp: March 31, 2017, 12:45:27 am
content: Quote from: Cubdriver on March 31, 2017, 12:41:33 amDo you mean it wasn't inducing voltage it was a result of the capacitive coupling? I will have to do some research to learn about this. I'm still an apprentice so I don't have all the school yet.Wouldn't that much capacitance affect the operation of the circuit?

author: Brumby
author_rank: Unknown
timestamp: March 31, 2017, 01:21:15 am
content: In an environment where you are running 600V 3 phase - you would look like a mosquito in a bug-zapper.

author: Kalin
author_rank: Regular Contributor
timestamp: March 31, 2017, 01:34:23 am
content: Quote from: Brumby on March 31, 2017, 01:21:15 amJust in general? Or if you actually touch phase to phase?I think workmanship make the difference in safety. You can have a 120V Deathtrap or an uber-safe 600V distribution center, the difference being the workmanship and attitude towards safety

author: Brumby
author_rank: Unknown
timestamp: March 31, 2017, 02:38:09 am
content: In my book, the attitude towards safety comes first. No matter how good the workmanship is, with a poor attitude, you can kill yourself - or worse, someone else.Get the attitude right before all else. When there are safety protocols in place - FOLLOW THEM. Don't argue or feel they are slowing you down or wasting time. They are there for a good reason. You don't have to understand the why or how they came to be, although such knowledge might be helpful - just as long as you don't try and rationalise a way around them.Good workmanship is part of this, so you should always endeavour to be spot on with this - but a word to the wise.... If you are approaching an environment for the first time, I would suggest it to be a sensible move to not assume anything, especially if I was going to be working on it. Poor workmanship might be obvious, but it also may not. Good workmanship is still subject to issues if, for example, something has been damaged - especially when that damage is out of sight.As for the the 600V 3 phase environment... Contact from phase to phase may not quite vaporise you - but you aren't likely to get a second chance. At that voltage and with 3 phase, the equipment is going to be drawing a significant amount of power. Get yourself in the wrong place and the power supply and equipment aren't going to suffer.Pretty much the same for any capacitive or inductive coupling... and when you have long runs, the capacitance just increases, allowing for more current to pass. Again, these currents (if they do flow) are going to be small compared to the demands of the equipment, so their impact is not likely to be much on the installation - but they could still be large enough to give you a toe tag.Be safe.

author: Gregg
author_rank: Super Contributor
timestamp: March 31, 2017, 03:33:23 am
content: You may have more of a problem than capacitive or inductive coupling. With all 3 phases carrying relatively balanced currents inside a metallic conduit, the AC coupling should be minimal capacitive or inductive. That is the exact reason why it is mandatory to run all phases through the same conduit to cancel that type of coupling. You might get a high reading on a high impedance digital meter but with a low impedance measuring device like a solenoid actuated meter it should go away. I always carry a Knopp K-60 when working on mains power.

author: Kalin
author_rank: Regular Contributor
timestamp: March 31, 2017, 11:47:06 am
content: Quote from: Gregg on March 31, 2017, 03:33:23 amI put grounding bushings on the teck connectors and it went away. I don't have a solenoid meter but my fluke does have a low impedance setting would that work well enough?

author: Damianos
author_rank: Frequent Contributor
timestamp: March 31, 2017, 07:20:15 pm
content: I think that is capacitive and not inductive coupling.I am thinking that the circuit has somewhere to close...When you touched the connector your body closed the circuit to ground putting yourself in series with the capacitor:ground - power circuit - wires to motor -| |- cable shield - body - groundDue to relatively low capacitance the voltage dropped with even a little load. This can be checked by connecting a resistor of few kOhms in the place of "body" and measuring the voltage again.If it was inductive coupling then the secondary coil (the shield of the cable) has to be connected to ground on the other end. In this case the load (body) is connected in parallel with the that coil, that has very low impedance (it is one turn at low frequency). You can imagine what would happen by touching it or by grounding it.