# 480V AC vs. DC

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#### James Ingraham

I have a basic understanding of electricity. However, there seems to be a lot of misunderstanding about what is "dangerous" to humans. One of the things that I have heard is that DC is more dangerous than AC at the same power level. For example, a 480VDC is inherently more likely to seriously injure or kill a person than 480VAC. Is this true? Let's assume that both lines are fused at the same amperage. Where could I find information relating to this hypothesis?

-James Ingraham
Sage Automation, Inc.

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#### T. Connolly

Thats a trick question James.

480VAC peak to peak line voltage is 554 VAC.
480VAC line to ground voltage is 277 VAC.
480VDC to ground is 480VDC (unless the DC source has a floating reference)

In most cases of accidental personnel contact, the path is usually line to ground, so in that sense it could be said to be less dangerous at 277VAC -vs- 480VDC. But can we truly say one is less dangerous though? 480/277AC is more common so it probably kills more people.

I have heard some people claim that DC "locks" your muscles more than AC, and hence is more difficult to get clear of, but I think that is only an old wives tale. I know of no test of the validity of that claim, nor of anyone willing to test it.

Total energy is what does the injury to a person. Your skin is actually a decent insulator. Were it not for our skin a car battery would be able to deliver lethal current.

I can say one thing from personal experience: getting electro-zapped by DC hurts more than AC.

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#### William Hinton Sr. Electrical Engineer @

The problem with voltage is you will be just as dead either way! Voltage causes pain but current is what kills you. Your dry skin has about 1500 ohms but it is only about 500 ohms when wet. Current of about .01 amps is the "let go" threshold, that is why ground fault interruptor breaker trip levels are set at about .005 amps. Breathing stops with current over .1 amps and the heart stops at about .3 amps. The answer is to work safe and not get in touch with any energized equipment.

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#### sekar

Fuses do not offer any protection against electric shock.
It is the current that is dangerous to humans.
And ac 26 to 150 Hz is more dangerous than DC of same peak voltage. During an electric shock the current flows in to the blood stream and the muscles vibrate at that frequency. It is much easier to recover yourself at very low and very high frequency. In DC there is a steady and strong pull. If the DC is taken from a direct AC source then the muscles vibrate at that ac frequncy. It is easy to force yourself out from the pull in dc shock than ac. I was holding a rf circuit terminal once with 8 Kv at about 800Khz and I did not know that I am holding such a HV with respect to ground. And I won't recommend any body to try also.

There is a good reference in a book "the hand book of electronic safety procedures" by Edward A. Lacy by Tab books. ISBN 0830614206.

Best regards.
Sekar

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#### BobB

Both have the potential to kill you.
AC tends to "hang on to you" due to the AC cycle contracting muscles.

DC usually throws you clear but man does it burn. I have seen some very nasty burns from DC on ships in the old days.

A former boss of mine was a test engineer for GEC in the UK. They built some DC switches for a power authority in Canada. He informed me that the arc shutes, which have to be massive for DC as opposed to AC, were incorrectly designed (too small). When they placed thye things on full load and turned them off, a massive arc was formed. It went straight to a walkway and melted it for a considerbla distance. Nasty stuff at the right voltage and current.

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#### wlmostia

In terms of electric shock at 60 hz, DC is generally considered less dangerous for the same level of current. See the Appendix "B" of the Lawrence Livermore National Laboratory EH&S manual referred to below.

For an electrical shock, it is the current level and duration of current that kills or injures but voltage combined with resistance determines current. Generally, 30 volts rms(42v peak) at 60 hz or 60 vdc are considered the maximum safe voltages.

There is some good material on this subject in Appendix "B" of Lawrence Livermore National Laboratory EH&S manual at:

http://www.llnl.gov/es_and_h/hsm/doc_16.01/doc16-01.html

Bill Mostia
=====================================================
William(Bill) L. Mostia, Jr. P.E.
Partner
exida.com
Worldwide Excellence in Dependable Automation
wmostia(AT)exida.com(b) wlmostia(AT)msn.com(h)
www.exida.com 281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.

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#### Curt Wuollet

Either is more than ample to kill anyone at way below the fuse currents. Actually there are two schools of thought on this subject. One is that AC is more dangerous because the frequency is a multiple of the heart's frequencies causing fibrillation. The other is that DC causes the muscles to lock and you can't let go. I think it's often quoted that a current on the order of 10ma. is enough to kill you. With 480 volts that's a body resistance of about 50k. Once skin resistance is overcome, the only saving grace is if the current flows through nonvital parts of your body. That's why most guys from my generation
formed the habit of keeping one hand in their pocket while poking around line operated equipment. In the tube days, almost any piece of equipment could kill you. A lot of people who have never worked on anything but solid state gear form some really bad habits. From hand to hand is about the worst case except for sticking you head in an enclosure and contacting something. I do see people working with both hands and their head in an enclosure. One would hope that said enclosure contains only low voltage stuff. I get strange looks for my hands off habits, but I'm still around. Surviving contact with 480 V will educate even the most recalcitrant. I've worked on stuff with voltges that actually raise the hair on your head and you learn a healthy respect for electrons that want to be free.

Regards
cww

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#### Anonymous

One thing is not debatable: There are not varying degrees of dead, hence there are not varying degress of danger. We've all heard the stories and wondered if they were urban legends or not. Best practice is to isolate the circuit. I learned that the hard, and lucky, way. I (stupidly) reached into a 480 live panel to retrieve a tool, and brushed the edge of a fuse clip with my forearm just as my hand closed on the tool. Fortunately for me I was on a non-conductive platform, but my hand was grounded on the tool. I had entry/exit burns on my forearm and hand, and my arm hurt for nearly half a year, no exageration. There was a burn path running internally down my arm from the entry burn to the exit burn. By the way, that was actually from 277V, (480 line to ground) as an earlier post points out.

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#### Jim Moser

This is very interesting. T. Edison and G. Westinghouse had diametrically opposite views of the relative safety of DC vs AC. To prove his point, Mr. Edison invented the electric chair.

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#### Dobrowolski, Jacek

Hi,

DC is more dangerous if you survive the electrical shock, because DC makes electrolysis of body fluids and produces many chemical agents which can kill a human even few hours after the shock.

Regards,

Jacek Dobrowolski, M. Sc. E. Eng.
Software Eng.

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#### James Ingraham

Thanks, everyone! Particularly helpful was the link to Appendix "B" of the Lawrence Livermore National Laboratory EH&S manual. Highly recommended reading.

-James Ingraham
Sage Automation, Inc.

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#### wlmostia

> DC is more dangerous if you survive the electrical shock, because DC
> makes electrolysis of body fluids and produces many chemical agents
> which can kill a human even few hours after the shock.

Could you provide references that support this statement?

Bill Mostia
=====================================================
William(Bill) L. Mostia, Jr. P.E.
Partner
exida.com, LLC
wmostia(AT)exida.com(b) wlmostia(AT)msn.com(h)
www.exida.com
281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.

M

#### Mark Blunier

Mr. Edison had diametrically opposite opportunities for financial gain based on the acceptance and purchase of AC or DC electrical equipment and power. The 'safety' issue wasn't about safety, it was about marketing, and who held the patents.

Mark Blunier
Any opinions expressed in this message are not necessarily those of the company.

D

#### Daniel Scott

If you don't survive the shock, who cares?

--------------------------------------
Danny Scott
Fredericton, N.B.
[email protected]

D

#### Dobrowolski, Jacek

I have paper in Polish ( probably no much of help for you ), but authors refer to the IEC Report Publ. 479-1 and 479-2.

Jacek Dobrowolski, M. Sc. E. Eng.
Software Eng.

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#### Mark Lochhaas

I served as a nuclear electrician on submarines in the Navy some years ago. At that time we were taught that 0.1 amps for 1 second will kill. The regulation was that you could not work on a voltage greater than 30 volts without protection. Using Ohm's law you can figure that the Navy felt the resistance of the human body could be as low as 300 ohms. They also taught us the exhaustion, lack of nourishment, and exposure to salt air, among other things, could significantly reduce the resistance of the human body. They further taught us that the danger also came from the path of the current through the body. For instance, left hand to left foot through the heart, or a path through the head increased the risk. I suppose the message is that a surprisingly small electrical potential could kill anyone under the right circumstances. The moral, don't do that; be stubbornly and methodically safe.

mcl

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#### Curt Wuollet

Yes, if you're hot and sweaty enough, 12 V can be very uncomfortable, though probably not dangerous. Salt would definitely lower your skin resistance drastically and you would get shocks from potentials that you would not notice under normal conditions. I agree that one should studiously avoid becoming a conductor.

Regards

cww

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#### Anonymous

Having been an electrician, been in electronics, and having been shocked by multiple types and voltages of both AC and DC, I can tell you that I'd rather be hit by AC any day. The reason I would give for this would be involuntary muscular contractions. In getting shocked by DC (especially if its around the hand), there's a strong potential the current will cause you to tense up on a muscle and physically grab the conductor, sort of adding to the problem. With an AC shock you have the voltage swinging from a positive level to a negative level, reversing the polarity (and achieving a point where theres a zero or closer to zero current). I don't know if this alters the way your muscles tense up, but it definatelly feels that way. Its easier to let go of AC as opposed to DC.

Now as far as the overall danger of electricity, I think its probably safer than many people believe, given the fact that I'm still alive. I do have to say I think it can seriously mess a person up. You may think I'm making it up, but there was an old electrician at a supply house one day that came in. We got on the subject of being shocked and he told us he'd gotten shocked so many times that he could not wear an electric watch. I watched with my own eyes as he put on two different people's watches and they all malfunctioned and ceased to function properly. I'm not sure exactly what causes such a thing, but it can occur.

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#### Chris Jackson

"Mr. Edison had diametrically opposite opportunities for financial gain based on the acceptance and purchase of AC or DC electrical equipment and power. The 'safety' issue wasn't about safety, it was about marketing, and who held the patents."

I couldn't have said it any better myself. On a farther note... strapping someone down to a chair and applying lethal current through them doesn't really show how safe or unsafe AC or DC is, but rather current, if given the opportunity, will kill you. Especially when you're strapped down with no way to keep yourself out of its path.