maninthesea

I was working on a compressor for one of my customers and doing some training with their boat mechanics at the same time earlier this week. While we were working the mechanics talked me into going down to the dock to try and figure out why they were gettting shocked sometimes when touching the shipping container they use for a workshop.
They felt it had something to do with the latest security camera that was installed. Aparently it has burned out more than once and it was right after it was installed that they started getting shocked. I think mostly they wanted me to check it out as I am one of the few guys they know with a fluke that knows fairly well how to use it.
I went down and took a look. The container is grounded to a grounding rod with some grounding strap bolted to it. There is no voltage I could find between ground on the outlets and the container. Then I decieded to check the outlet of the "power conditoner" that feeds the cameras. When I checked from the ground on the outlet of the power contitoner to the body of the container I got 118Volts.
I showed them and told them they should get the company that installed the cameras out to fix that problem.
I am just wondering how that is happening. Should the ground on the output side of the power conditioner be tied to regular ground inside the power conditoner? Or is it a "Floating Ground" Would this indicate that one of the devices that is plugged into the power conditioner is using ground for the power return instead of Neutral?
Trust me I am not planning to fix this problem for them as its outside my scope of work and knowlage and a man should know his limitations. I just want to know for my own edification.

Inner10

I'm going to assume your conditioner is a UPS.

The UPS, assuming it has a floating output acts as an isolaion transformer to the incomming power, these can probably give you some weird reading when comparing to earth ground. These are used alot on distributed control systems. By burned out do you mean the UPS keeps blowing fuses? Could be realated to the mounting of the security cameras, some cameras are not isolated from their metal housings, you put a metal screw throught that to a conductive grounded surface and guess what... a ground potential.

I could be wrong but I would connect my system to earth ground or lift all but one common grounding point (weeks of my life have been devoted to trouble shooting ground potential problems).

maninthesea

The conditoner does not appear to be a UPS as it is only about 6" cube so not much room for a battery bank (unless it had wires to external battery bank I did not see as its kind of cluttered) I assumed it was probably an isolation transformer with some other filters and protection circutry.

Burned up was the term the guys used and from what I understood is the cameras were what were getting replaced not fuses. They also said that you could see arc marks or something along one of the new cameral lines(so I was wondering if there may be some RF voltage that was causing all this until I read 120V to ground from the ground on the output of the conditioner and figured that must be the culprit)

I was not plannign to work in it for them but I am glad to hear that my thoughts of grounding the output of the line conditioner were not out of line. I was wondering if that would defeat the purpose of the isolation transformer. But then again I cant figure out where else the isolation transformer/conditioner would connect the outlet ground to? maybe the output has a hole for the ground prong just so you dont have to cut it off??

My understanding of the purpose of an isolation transformer is theoreticly if you touch any of the wireing and ground you wont get shocked as there is not return path.

I did not follow you about lifting all but one common grounding point.

mrmike

In electronics testing, troubleshooting and servicing, an isolation transformer is a 1:1 power transformer which is used as a safety precaution. Since the neutral wire of an outlet is directly connected to ground, grounded objects near the device under test (desk, lamp, concrete floor, oscilloscope ground lead, etc.) may be at a hazardous potential difference with respect to that device. By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained within the device.

I copied this info from site thinking it may help. I had a similar problem a month ago with people getting a shock from a SS Pasturizing tank. When I checked for voltage at the tank to the Equiptment ground of the tank-nothing. But when I checked from the Tank to another tank next to it - 58volts??? Took me a while to figure this one out-but found a ground wire to the case of a chart recorder was hooked to a hot point on the circuit board by the factory. Not enough to trip anything out, but bleeding voltage to ground! Checking the tank to the equiptment ground was at the same potential-thus no voltage reading, just like checking voltage on the same "hot line".
Yours sounds like a similar problem, Hope this helps.

ABLE1

Not going to attempt to try and figure out where the problem is on this issue. But I am going to ask if you have left the shock hazzard in place as is when you left the area???

Me personally would have pulled the plug and left them know that before plugging it back in they need to get someone there to fix the problem before some one gets dead.

Remember you are the LAST one to have touched the thing.

Just my .02 cents.

Les

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ABLE1

ABLE1

Whoops!!

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ABLE1

maninthesea

MR Mike
From what I understand your saying is if I would have thought it out more I probably would have been able to unplug all 4 devices from the conditioner and then the 118VAC to outputground would go away. Indicating one of the devices had a short to its ground and was feeding back. Probably not the conditoner at all.
Thanks to all and I will check back with the guys and see if the problem got fixed and what the camera installers said was wrong if I can.
Cheers Jim

Inner10

Hmm I'd have to see the power conditioner, most are just a box of snake oil.

The only time I've seen something similar was when my father had a call that a HO was getting shocks from his stove when he touched his fridge, it wasn't heating up right etc etc. Ends up the ground was wired to the hot!