Hi I witnessed something at work the other day and it got me to thinking which led me to this site. So one of my colleagues modified a disposable camera to make a tazer removing the bulb and leaving the wires exposed. He was trying to discharge the capacitor with a metal screwdriver at one stage and was certain even if he held the metal handle he would not get a shock. I know electric current will take the path of least resistance but is there any way that you could get a shock in this situation. I think it was approx 350v dc discharge. Anyway second question that I can't find a definite answer which came from this event was this. Say I have a high voltage dc power supply. Say a 400v electric vehicle battery in perfect condition with no connection to earth. Can a dc power source go to earth or does it only go from positive to negative (conventional). Say for example the battery is in a completely insulated box and there is a positive cable that I can touch. Will I get a shock through earth or as long as I don't become a conductor to the negative circuit (short) the battery by touching the negative will I just have the same potential as the battery +ive.
Sorry if this is a silly question but I never really did much science in school and those days were long ago. We have electric vehicles where I work but we have to stand on insulated mats as part of ppe which always led me to believe dc would short to ground but I was thinking about a dc electric fence and if the earth bar is removed there it won't work. Anyway any insight would be greatly appreciated. Thanks in advance

 

Short answer. Most likely.

A solar instillation of 200-250 volts can be extremely dangerous even before it is fully connected and earthed. In this case they function exactly like your battery scenario. High voltage DC with no connection to earth.

Best course of action is to respect ALL sources of voltage above 50 and treat them as deadly. Caution never killed anyone working with electronics.

 

I don't want to minimize the risks of making a mistake with such things, but my short answer is NO.

If there's no return path for current, no current flows and there's no danger. It should be perfectly safe to grab one pole of a hypothetical vehicle battery at 400V, as long as there is no way for your body to connect to the opposite pole.

The danger of course, is that there's a return path you're unaware of. Pants leg against the fender, which is grounded, for instance.

Don't ever rely on the "path of least resistance" idea. It's not really valid. Just like water flowing down a hill, electric current will mostly flow the easiest path, but a little will flow through every path, not just the lowest resistance. Think about a 1K resistor and a 10K resistor in parallel, in series with a battery. Will there be current in the 10K resistor? You bet! Don't let that be you.

 

Theoretically only of course if I was on the floor without insulated ppe holding that cable would the "earth" then be the same potential as battery +ive and say somebody else touched the opposite side of the battery under the same conditions as me then we are conducting yes?

 

Ever seen birds sitting on high voltage power lines?

 

one of my colleagues modified a disposable camera to make a tazer

There is enough energy stored in a disposable flash capacitor to KILL someone. It should NOT be used as a taser.

 

I know right. He said he shocked his friend and he dropped to the floor. He told me it was 20kv but from researching it seems more like 350v. More substantial amount of current I'd imagine. Basically I thought all high voltage would go through you to earth as that was the impression given on our electric vehicle training but then i thought about it in depth and a bit of research led me here. Every day is a school day. This guy however is walking proof of darwinism.

 

This guy however is walking proof of darwinism.

If he's still walking, he's disproof.

 

If he's still walking, he's disproof.

Haha that's true.

 

In hard facts, electricity needs 2 conductors to make a path because a voltage is always a voltage compared to someplace else.
In the school of hard knocks, if electricity can find a path, it will, and it won't tell you except by hurting you.
So we tell the noobies to use top quality safety precautions until they learn the rules. Only when you know the rules can you know when to break them and survive. For instance there is capacitance, which is not a DC path, but when you're working with AC, it's a killer. Consider the neon screwdriver used to test for live wires. It seems to not have a second path, but it does, through the capacitance in the soles of your shoes. Higher frequencies require less capacitance to hurt you. The old TVs had a 15,750 Hz running around at hundreds and thousands of volts. That is a bad place to find out how many picofarads are in your tennies.

 

There's always insulation resistance, di-electric breakdown and capacitance to consider. Each one can be relatively unknown.
Some dust. Some moisture. A point. A crack. Whatever. Can change the perception. All voltages above 25 V, should be really respected. Any potential high power source should be respected. One piece of equipment I worked with had a 6.3 VAC, 3000 A secondary voltage, but at 3000 A, it's not.

A car battery should be respected. 1) because of hydogen gas formation and 2) the huge short-circuit current.

Also, look up GFCI. The amount of current across the heart muscle that can do some serious damage is in the neighborhood of 10 mA.

You should look up "arc flash" too.

 

Thanks for the reply. I have seen some nasty arc flash videos alright. At the moment we are only trained to deenergise the lithium ion battery lockout and test for absence of voltage and work in a zero energy state on electric vehicles and unfortunately the technology is way ahead of the training.
So does a dc supply only ever go to its opposite pole to create a circuit. I know there are so many variables but if all conditions were perfect insulation wise. Take for example the image found on this site.

 

Also where could I find more material on the subject matter. Could you recommend some literature

 

So does a dc supply only ever go to its opposite pole to create a circuit.

That's a difficult question because it requires examination of every possible scenario. Theoretically, electricity never goes anywhere except back to its source. Lots of noobies come here without their stuff working because they didn't connect the grounds of their 5V Arduino and their 12V switching transistor. On the other extreme, lightning jumps from the sky to the earth, or the other way around, and where's the other conductor? The other conductor is a fluid kind of thing. When the charge is developing, the flow is in one direction, across time and distance. When the big capacitor in the sky discharges, it's quick and direct and doesn't seem to have the presence of the other conductor.

Ok. Enough with the magical mystery tour. You need to consider the practical aspects. Low voltage, low frequency electricity obeys Ohm's Law very nicely. Actually, it all does, but the different ways it works requires a broad experience base. Your job is to survive first and accumulate experience second. The first rule is, "Never bet your life."

I don't care what you believe or what you calculate, Never bet your life that you're right. Never bet your life that somebody else turned the disconnect off. Then, never bet your life that the disconnect isn't defective! Been there, done that, and it was only my semi-paranoid insistence that at least 3 layers of safety were in place that meant only the screwdriver was vaporized. I recommend a slightly paranoid respect for electricity in all of its forms.

Calculate, visualize, know what to expect, but never bet your life that nothing is in play that you don't know about.

 

Having worked on very lethal systems:

1. A 1 KW tube transmitter with 3000 VDC in the box
2. A 15 kVDC @ 1.5 amp regulated power supply. A tube was the series regulator.
3. A 100 kV 0.1 A power supply for x-ray generation.
4. A lamp that operated at 22 V, 40 A, but required a 40 kV ignition pulse.
5. And a whole bunch of insignificant stuff compared to the above.

They are systems to respect.

 

At one time, I was involved in really updating safety about the time MSDS sheets became common place. One other aspect was to complete a job-hazard analysis which turned out to be a little harder and what was decided, was there might be a general one, a specific one and one for say vacuum systems. This was a research lab and not a production environment. So, someone decided the PPE you need.

Ideally, the policy should be safety glasses everywhere. It wasn't. Formally it was. The policy was ALWAYS enforced in the chemistry lab. The machine shop, it basically depended on what you were doing. The table saw was considered the worst piece of equipment, probably followed by the 5 HP engine lathe. The main rule here was "Hand always stays on the chuck key". When you operated the lathe and the mill, you had safety glasses on and no sloppy clothing that could be grabbed by the machine.

I had two minor shop accidents; One with a file. I was making a cutter for the lathe and the file slipped. So, I guess I should have been wearing certain gloves that we didn't have anyway and it was for a personal project. The other was, I did not clamp down a piece of sheet metal on the drill press. This turned into a rotating knife.

Two machine shop accidents I remember were from the machinists: 1) An eye splinter (safety glasses were worn) and another being a result of not keeping the hand on the chuck key rule. It was actually the quill wrench, but it's the same principle.

I also dropped a quartz container that contained zinc and red phosphorous that I was sealing under vacuum and it caught fire. I smothered it and all was mostly well.

In my youth I had a car engine fire. I sacrificed my jacket. The jacket is still around as a reminder.

Close calls can heighten your senses like a chain-saw kickback injury.

It's good to assess the hazards of the job your going to do and be prepared.

My mom is elderly and I have migraine issues, so therefore, a timer is set that's loud and will ring a fairly long time initially and then in spurts until turned off) when a pot or teapot is put on the stove. That timer is for safety. If I do wash, I might set another kind of timer that dings once for how long I expect the wash or dry cycle to take. This one is for convenience.

I use that job hazard analysis technique a lot of times.

 

Hi,

The real problem lies in not being able to identify the return path, if any, because assumptions are always made in every real life case and they may not be accurate.

For example, for a 200v Li-ion battery that is just sitting on a perfectly non conducting table with no connections, you should be able to go over and touch any ONE terminal and not get a shock even if you are standing on the ground. The real question though is not whether you can get away without getting a shock, the real question is should you actually try. Note i use the word "try" which implies an element of possible failure

The problem is we dont know if something else is at work, such as a slightly cracked case and high moisture which causes a more low resistance to actual ground, the ground you are standing on.

An experiment would be to measure the voltage from each battery terminal to a grounded metal rod that was hammered into the ground using a high impedance voltmeter. If you measure a voltage, you know it's there, but even if you measure zero there's no reason to touch it without some protection. There are just too many things that can go wrong.

Also interesting, if you turn the battery on one end so that one terminal is physically closer to the ground then the other, the potential at the other terminal is now higher than the former with respect to ground. If it is a 200v battery than it is 200v higher, but the impedance it so high you may not be able to measure that without a special meter.

For more information, do a search on how workers deal with this when they work on high tension wires that have voltages in the thousands of volts. Some very interesting stories there.

 

Thanks for the answer.