Leakage current - shocks and problems

[paulears]

Do you work with any of those people who claim to get shocks from pieces of electrical kit, yet nobody else ever does? One of our spot ops lurched from time to time, and his latest excuse was he was getting a shock, and it made him jump. Obviously, nobody took it seriously, he got some abuse for being useless, and we carried on. A week later, somebody else reported their working lights flickering, just a small flick - but I figured that it wouldn't harm to get the spark in to check things. Needless to say, nothing happened. So in a moment of boredom, I stuck a multi meter across the metal connector that feeds the control panel on the follow spot, and the metal clad power outlet it was plugged into. It was a bit of a stretch, but the meter indicated 110 Volts which was a surprise. I've had a few power supply problems in the past on various bits of kit and ended up with half mains voltage on an exposed part - but no current - still a tingle though. As the meter was dangling in mid air - suspended between the two test points, the black lead fell out - I shoved it back in the wrong hole - the current test socket. Re-doing the test was pretty spectacular - as the probe touched the slightly rusty outer box of the socket I was treated to a welding display as the rust sparked it's way off. I'm not sure what the current reading was - the meter ended up on the floor, but I unplugged the spot and removed it.

 

The worst thing was I'd in effect, encouraged somebody to use a piece of kit that was evidently dangerous - certainly dangerous enough that there was no way I was going to touch it when it was on. I feel he was very lucky. Sitting on a wooden stool, on vinyl floor covering was probably a reasonable insulator, but the comms packs hang down from a steel beam just above their heads, the tecpro headset has a metal grill on the mic - I wonder if this is earthed via the cable? Imagine getting the shock via your lip!

 

Damn lucky we didn't have a serious injury - I've no idea what current was present, but enough to turn rust into a shower of sparks, at over 100V.

 

I'm guessing that we've had a water issue over the closed season, the spot is directly under a open trap leading to a clock tower - I bet water has dripped straight into the fan holes on the top.

 

I suspect it was a lucky escape.

[J Pearce]

It strikes me that your venue with its location might also suffer from a high resistance to earth in the cpc. Was the followspot powered from an RCD protected circuit?

[KevinE]

I suppose it begs the question of Why the metal connector on the Followspot wasn't earthed, leakage or no leakage! It's either a double insulated fixture or should be earthed if it's an exposed conductor surely?

[Bobbsy]

I've not worked with people who feel shocks other people don't--but my wife is exactly the opposite. She can take a jolt that would knock other people out and not even notice it.

 

The most dramatic example of this was when my mother in law bought a second hand washing machine. It kept blowing the breaker--but my wife used it several times, fishing around in the drum to get wet laundry out. When the repair man finally arrived, he checked it and immediately condemned it--the drum was live at 240V.

 

My wife said the only thing she noticed was a slight metallic taste in her mouth after a few minutes use.

 

Bob

[pritch]

At a friend's request, I popped round to have a look at his kitchen. His girlfriend was claiming that she was getting shocks off the appliances. Dishwasher, fridge, kettle had all given her shocks, yet my friend hadn't noticed it.

 

I walked into the kitchen, neon screwdriver in hand, and touched it on the first bit of metal I spotted - part of the fridge, which caused it to glow healthily. Same with the kettle, dishwasher, and sticking the screwdriver straight into the earth of a socket. We figured out that the extension that his kitchen and spare bedroom was in had no apparent earthing whatsoever and, realising I was getting out of my depth muttered the phrase "better get an electrician to look at this", before accepting his offer of a cup of tea (but on condition that he was the one to use the kettle).

 

Turned out the reason she was getting shocked but he wasn't was due to him always wearing rubber soled trainers around the house, while she preferred to go bare-foot.

[daifuse]

I've not worked with people who feel shocks other people don't--but my wife is exactly the opposite. She can take a jolt that would knock other people out and not even notice it.

 

 

Bob

 

Many moons ago, as a Youth-in-Training with GPO telephones (1956!) we had many over-head lines on what were refered to as EL & P (electric light and power) poles. One of my senior colleagues could cheerfully connect himself to the live wires up the pole without flinching. On occasion, he would check for live connections for equipment by touching bare connectors, even on 3 phase supplies.

[dbuckley]

If I rub the back of my hand on many unearthed things I feel a distinct "stickyness" which can be confirmed with a meter. Its not a shock or a tingle, but a wierd feeling.

[David Lee]

walked into the kitchen, neon screwdriver in hand, and touched it on the first bit of metal I spotted -

 

Throw those horrid things away. Unreliable. Use a meter.

[matt-the-dj]

walked into the kitchen, neon screwdriver in hand, and touched it on the first bit of metal I spotted -

 

Throw those horrid things away. Unreliable. Use a meter.

 

 

I 2nd that, get a proper test meter.

[WooferHound]

A digital meter will lie to you if one of the leads is floating. You may see a voltage on there but it's reading static charges and radio waves. An analog meter generally not affected by these insignificant voltages.

[paulears]

Interesting end result. The fault was simply the mains earth coming loose. Restoring the connection cured it completely, a re-test passed, and trying the fixture on a known 'delicate' RCD protected outlet caused no spurious tripping. This does generate the question of the sparking - suggesting sufficient current was present to make a few rust clumps pop off. Yet no problem with an RCD?

 

Very strange. Ballast capacitors? Just a mystery.

[archibald]

Interesting

I would sugest that if there is this amount of voltage present with the earth disconected there is a partial breakdown of the insulation. I would get this checked with a proper MEGA as a multimeter will not show this. In addition please note that if ther is a transformer between the RCD and the lamp the RCD may be unable to detect any leakage on the secondary side.

[Brian]

I would sugest that if there is this amount of voltage present with the earth disconected there is a partial breakdown of the insulation.

Not necessarily. If the unit relies on Y capacitors for any mains filtering then it's quite possible for the chassis to rise to half-mains when the earth connection fails. The two Y capacitors form a capacitive divider from L to N with the chassis as the half-way tap. Hence Paul's reading of 110v on the chassis.

[KevinE]

Thank the EMC regulations for the current dumped down the earth wire!

 

I suppose it's not good to transmit hum and hash around theatre wiring although I doubt many nightclubs would notice.

 

When I was at college we were told that the earth wire was a secondary form of protection and an item should be theoretically safe to use without it! How things have changed.

 

The leakage current off my uncles portable TV is enough to cause sparks at the aerial socket.

[archibald]

Yes it is perfectly true that a Y capacitance araingment will do that. I am mearly sugesting it should be investigated.