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Mysterious Voltage on steel frame?
Question

Good Evening,

 

We have a customer we installed a UPS and generator system for around 2 years ago. While completing some maintenance earlier this year, one of the on site maintenance team alerted me to a potential hazard.

 

The steel battery frame is painted/coated, but there was a nick in the coating, someone had leaned on the frame, at the location of the nick, leant on an earthed panel besides them, and had a shock.

 

All that is on the frame is the batteries, but we measured an AC voltage of around 170V between the frame and earth.

 

Above the frame are the main incoming cables from the transformer which is 2MVA - 3No per phase 500mm, I think they are AWA but cannot be certain (the site is 400 miles away so I cannot easily check).

 

There doesn't appear to be any cables touching the frame.

 

Any ideas where the voltage might be coming from, the phases are pretty well balanced - could an induced magnetic flux cause the frame to act as transformer? 

We are due to complete more maintenance next week, we will be running the site on generator, the incoming cables will still have the same voltage (LV - 230V L-N), but there will be no current in them, I have someone on site who will check to see if theres a potential on the stand at this time. Is there anything else you would recommend checking?

 

Thanks, Shaun

 

 

2 Replies
grubbym
13 Posts

Hi Shaun,

you may want to check all earthing to the battery box, and the panel that the person leaned against? with a reduced earth path, you could get that sort of voltage on the earth and nothing would appear to be wrong.

Unless the area is very wet, you are unlikely to get an induced voltage of that magnitude, but if you were, you should see a difference in voltage between to top and bottom of the cabinet.

I hope this helps?

Marcus

 

mapj1
4264 Posts

Is this voltage relative to the ground it stands on ? If the 170v is measured with a high impedance meter, then a random voltage like this is quite possible with stray capacitance to other parts nearby that are live - we think of a capacitor as a cylinder with two wires coming off, but any pair of conductors spaced by any insulation (and that may be a frame and air respectively) will have some capacitance between them.

I'm assuming this was more of a tingle than a belt - that is to say the shock current was limited.

If someone there can see if the voltage varies when the ‘load’ in the form of a person leaning against it changes, that would clinch it as being a pick up effect.

Also is one side of the batteries earthed or do they also float relative to ground.

Is it deliberate that the battery cage is not earthed ? if it was then a shock from it to earth would not be possible, but then a shock between something live and it would be….

also, when the other panel is earthed, is that to a local electrode as well or just to the neutral of the system?

 I'd be inclined to do a few wander lead tests with a volt meter between terra firma (that could be a test electrode, or just a garden fork or an old screwdriver stuck in a few inches into some exposed mud/grass whatever) and things you might expect to be at (or very near) earth potential - your CPC my not actually be that close to ground voltage..

 

Mike

 

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