Dangerous Capacitor

Pommie said:

If my memory serves me correctly, anything over 48V is considered lethal in the UK. On farms I believe it is 25V!!

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Neither of the voltages you've listed are considered to be lethal but they might be deemed to be dangerous depending on the current (AC is more dangerous than DC) and the environment.

As mentioned above, AC is much more dangerous, firstly it has a higher peak voltage, secondly it interferes with the heart's pacemaker more effectively.

Extra low voltage is defined as <120VDC or 50VAC and is considered to be a low risk.

The IEE regulations specify that in dry conditions voltages under 60VDC don't need insulating and in damp conditions it drops to 30V.

Furthermore even 120VDC is not considered to be lethal. Enclosures containing voltages between 60VDC and 120VDC only need basic insulation between the live parts and the user.

Capacitors charged to <120V don't need to have bleader resistors connected to them because it's considered too low to give anyone more than a mild shock.

For AC the voltages are much lower. Voltages above 25VAC need to be insulated, dropping to 12VAC in damp areas.

Enclosures containing voltages between 25VAC and 50VAC only need basic insulation between the live parts and the user.

Basic insulation is defined as an insulator that's not 100% fail safe, i.e. the sheath on most wires used in electrical equipment.

Fatality of electrical shock is a big variable. If you have a bad heart a few MA can kill you. IF your in good health and fisicaly fit you may take a lot more than 20ma to actualy kill you.
I have had countless shocks in my life and after a while you adjust to the feeling. I still avoid it at all costs but I do not panic when they happen.

Those little joy buzzer shockers are typicaly under 1 ma output at most. I have a freind of mine that has some shocking flashlights. I am used to getting shocks so I can hold the button down and act like I am not getting a shock. Then I hand it back to him and say the battery must be dead.
Then he tries it! Man! Thats still funny every time!

Would I be being pedantic to point out that 150VDC is greater than 120VCD.

Mike.

Only by 30V though.

Yes, it might kill you but I wouldn't say it will kill you, I'd still say it's unlikely to kill, even the mains is unlikely to kill you, 150VDC is much safer than 230VAC.

It may be only 30V but isn't that like saying only just dead. I've had many a shock from 230VAC and survived. I regularly test our electric fence by touching it. However, I do not go near that fence when the ground is wet - I once got a shock of it when I was barefoot in mud and that was much worse than any 230V shock.

End of the day, to advise anyone that 150VDC is not a hazard to life is a bad idea and one I felt I had to challenge.

Mike.

Pommie said:

Played that game and it is fun. However, I wouldn't try it with a 4700uF cap.

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Except 4700uF is likely to be only 63V working - so not any concern (and no fun! )

hi,
I feel that we giving out some doubtful advice on what constitutes a dangerous voltage level.

The degree of injury due to an electrical shock will depend greatly upon the person receiving the shock and the local conditions.

My advice is to treat any voltage greater than 50V as potentially lethal.

There are lots of 'official' documents for guidance regarding the avoidance of electrical shock injury and I would recommend that they be read.

They could save your life or prevent you from killing someone else.

Electric Shock Hazards

Pommie] End of the day said:

My advice is to treat any voltage greater than 50V as potentially lethal.

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Sorry but I think that's dangerous advice.

50VAC is pretty nasty but 50VDC is pretty safe.

I think it's pretty important to know that AC is three to five times more dangerous than DC of the same voltage and current.

Four car batteries connected in series is unlikely to give you more than a tingle even if you have moist skin. The 50V secondary of a transformer is capable of giving you a nasty shock, possibly even deadly if your skin is moist.

Don't forget that a 50VAC shock is really a 70.7V shock, before you even consider the other physiological effects.

ESD Journal - Electrical Hazard

So, in the end, if I have a 50v 4700uF cap, it's not enough to do any damage?
I don't think I have the means to charge it that far anyway...

No, at 50V you probably won't even feel it, even if you doubled the voltage the shock will be unimpressive.

Use a 5kV 10nF capacitor for a safe but painful shock.

5000 volts?
How would I charge it to 5000 volts?

Sorry but I think that's dangerous advice.

50VAC is pretty nasty but 50VDC is pretty safe.

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Hero,
How can safety advice be dangerous advice.

With respect hero, the words pretty nasty and pretty safe are meaningless.!

Any voltage over 50V in certain circumstances can kill, are you saying this statement is incorrect.?

EDIT:
**broken link removed**

With all due respect to Hero, I once landed myself on a 48VDC 10A bus bar and found myself in the ER. Low DC is dangerous if at the right amps. Docs said I fibrillated my heart.

Defibrillators in a hospital use DC.

Might I add, I have no heart condition. At least not yet...

Mikebits said:

With all due respect to Hero, I once landed myself on a 48VDC 10A bus bar and found myself in the ER. Low DC is dangerous if at the right amps. Docs said I fibrillated my heart.

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That must have been under some seriously strange circumstances? - AMPS are what's dangerous, but won't flow unless there's enough voltage there to make it flow.

It actually happened during a training situation. I was teaching some new techs the layout of a cell site, I was holding the equipment rack with one hand and was pointing out various parts of the system, when I pointed to the 48v bus, my hand felt like it grabbed the bus bar hard, the rest was blank. All I remember is a Jolt, punch feeling.

I was rather thin back then, body mass does play a role.

Mikebits said:

It actually happened during a training situation. I was teaching some new techs the layout of a cell site, I was holding the equipment rack with one hand and was pointing out various parts of the system, when I pointed to the 48v bus, my hand felt like it grabbed the bus bar hard, the rest was blank. All I remember is a Jolt, punch feeling.

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One reason why most people consider DC more dangerous than AC, with AC at even 230V you would have most probably just been thrown clear.

I'm presuming that living where you do it was hot?, and you were suitably sweaty? - for most of the year over here you wouldn't even know you'd touched it

I was rather thin back then, body mass does play a role.

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So there is some advantage been a fat pig then?

No wonder I've never found 230V shocks any concern.

I have been hit with big volts during my career. 600 VDC tube Anode supply, 115VAC, 220VAC, I even got zapped from a radar repeater that was turned off but the broken HV cap supplying the CRT anode was still charged with a few KV of volts. I kid you not, the shock I got from the 10A 48V bus knocked me so hard my grand kids will feel it.

Mikebits,
Do you have a dodgy heart?

Was it possible that the 48VDC supply was actually a much higher voltage?

Was your skin very damp?

Did you have a cut on your finger?

I've experimented with 50VDC before and I didn't even get a shock when I held +V in one hand and and 0V in the other.

I've had a shock from the 30VAC secondary of a mains transformer before but that's AC not DC.

ericgibbs said:

Any voltage over 50V in certain circumstances can kill, are you saying this statement is incorrect.?

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Agreed any voltage over 50V can kill given the correct circumstances.

I think your blanket statement is very dangerous because it implies that any voltage below 50V is safe. This is far from true: for example 45VAC 50Hz is much more dangerous than 55VDC.

The whole idea that AC throws you free and DC makes you stick is also a myth. With AC current the zero crossing time is far too short to allow your mussels to relax and set you free, both AC and DC can cause you to stick. My mum's neighbour nearly got electrocuted by an electric drill because he froze and couldn't let go of it, luckilly she turned off the power and he was all right.

The article I linked to before actually specifies a higher "Let-go" current for DC than AC.
ESD Journal - Electrical Hazard

"Let-go" current

The maximum current that can cause contraction of the flexor musculature of the arm but still permit the subject to release his hand from the current source.

DC - this value is about 75 mA

AC - this value is about 15 mA

and varies with muscle mass.

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Knowing this I'm far more cautious around 30VAC than I am around 50VDC.

Hero999 said:

The whole idea that AC throws you free and DC makes you stick is also a myth.

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Sorry, but your myth is a myth

From practical experience over many years, and experiences of many others, AC is considered far safer than DC in the trade - and does indeed, more often than not, throw you clear.

But in either case, NEVER touch anything metal with the palm side of your hand, ALWAYS brush it first with the back of your hand, if it's live there's no way you can grab it and hold on then.

Agreed any voltage over 50V can kill given the correct circumstances.

I think your blanket statement is very dangerous because it implies that any voltage below 50V is safe. This is far from true: for example 45VAC 50Hz is much more dangerous than 55VDC.

The whole idea that AC throws you free and DC makes you stick is also a myth. With AC current the zero crossing time is far too short to allow your mussels to relax and set you free, both AC and DC can cause you to stick. My mum's neighbour nearly got electrocuted by an electric drill because he froze and couldn't let go of it, luckilly she turned off the power and he was all right.

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hero,
NOWHERE in my post do I imply that less than 50V is safe,

I said:
Any voltage over 50V in certain circumstances can kill, are you saying this statement is incorrect.?

Please dont misquote me.!

Your statement about DC not causing a person to stick [muscle contraction] is incorrect.

I would suggest that you do some more research into electrocution causes.