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Safe voltage

03/03/2007 6:10 AM

Can someone clarify the Safe value of voltage (from electric shock point of view to personnel) from both AC and DC systems?

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#1

Re: Safe voltage

03/03/2007 11:31 PM

It depends where the voltage is applied. Even one tenth of a volt (ac or dc) applied directly to the heart at the right point in the cardiac cycle can cause fibrillation and death.

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#2
In reply to #1

Re: Safe voltage

03/04/2007 12:33 AM

It is true it depends 'where' the voltage is applied.

However,direct application of current to heart,is extremely rare and abnormal case of operation.

It is intended to fix safe voltage value for design basis under normal conditions of operation.

Please advise the safe voltage value as accepted in international standards.

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#4
In reply to #2

Re: Safe voltage

03/04/2007 7:38 AM

In industry a safe voltage level is defined as in the EU low voltage directive as being less than 50 volts dc and 30 volts ac (rms).

These are the figures that you are allowed to have exposed to human contact, but only touch contact, not hold or grab contact!!

Once you get into the medical instrument area then vastly smaller voltages and currents are allowed for different classes of instrument protection.

John.

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#3

Re: Safe voltage

03/04/2007 4:21 AM

Hogwash!

There is no such specification for a safe working voltage so long as legal authorities and technical experts coexist.

The military OJT command may demand gloveless 12VDC and gloved 47VDC - consider the military has an anticipated casulty rate. Not even military expectations include the known human skin and heart tolerance for 10,000-40,000 spark plug surge for a high percentage of personnel. The power is a foreknown, controlled and limited range.

Ford may expect 12vdc skin contact for the batty assy line but the skin is controlled ambient RH less than 100%

Normal skin resistance is well documented but highly variable and unpredictable with injuries, insect bites, sores, and stings.

If you desire to formulate a specific controlled voltage, amperage, and energy surge you must also clearly define the acceptable associated parameters and tolerances for specific classes of personnel (sex, age, skin condition, appertances), appropriate dress or attire, and duration of enjoyment. Please obtain localized legal and technical expert advise.

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#5

Re: Safe voltage

03/04/2007 11:29 AM

nvmani:

I refer you to the responses generated by the question from Yogindranath titled "Dangerous Voltage" posted on this forum on 2/24/07.

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#6
In reply to #5

Re: Safe voltage

03/05/2007 4:39 AM

Thank you for information that this forum discussed "Dangerous Voltage" (2/24/07). I shall refer.

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#7

Re: Safe voltage

03/05/2007 8:30 AM

Zero

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Anonymous Poster
#8

Re: Safe voltage

03/05/2007 10:00 AM

Its not so much the potential level at these nominal values, its the current level. Is the device current limited? mA can kill!! A 12V battery is low in potential, but short one out to see what unlimited amps looks like!!

Regards: cgl.

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Anonymous Poster
#9

Re: Safe voltage

03/05/2007 2:56 PM

sorry not the best advise but i got a (good) kick for a milk float and than was and earth return system but shorted through me to the body and this would have only be about half the protental at 40 volts. ............... no. dont glow in the dark but do make clocks work!!

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Anonymous Poster
#10

Re: Safe voltage

03/06/2007 6:11 AM

Speaking of amps, have any of you been involved with Arc Flash studies?

Regards: cgl.

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#11
In reply to #10

Re: Safe voltage

03/06/2007 10:47 AM
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#12
In reply to #10

Re: Safe voltage

03/06/2007 10:53 AM

Plaese refer website www.arcadvisor.com/arcflash/ieee1584.html - 22k It may help

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#13

Re: Safe voltage

12/12/2009 1:41 AM

I feel this discussion is not asking the right question.

I agree its Currant that kills - not voltage! But all other things being equal it's a bigger Voltage that causes the bigger currents.

I agree mA can kill in the wrong place (directly across the heart). But that is less likely than a casual encounter with a bare conductor. So what range of Voltage is likely to cause a fatal current to flow.

If we assume normal dry-ish skin, and casual or accidental contact with an exposed contact, bus-bar or terminal... At what voltage does that sort of dangerous current become more possible?

In other words... For normal everyday applications, at what level of voltage does it start to become necessary to insulate, or cover, exposed contacts etc.

A 9v battery is considered safe for children to put in radios (and even on their tongues - for fun I suspect) And 12V terminals are commonly exposed in vehicles, Yet 120V can be lethal!

What I'm interested in is the dividing line, (or gray area perhaps?), between the two. Is there an accepted level, or even a Legal requirement, to insulate above a certain Voltage & is it different AC - DC?

In the UK before the "Health & Safety Gestapo" took over, the legal limit for uninsulated electrical stuff was 60V AC or DC -- In practice to ensure compliance with the law, stuff at a nominal max of 50V was insulated - to leave a margin of error.

I remember working as an engineer @ the BBC in London with exposed Bus-bars with 50V by my head (50V was used to operate relays, uni-selectors, indicator lamps, and other programme switching equipment for the BBC of the 1960s).

Many times, when as a loose end, in some slack period, The joke would be some guy (me or a mate) picking up forks & screwdrivers walking over to the bus-bar distribution board which was carrying the 50V DC Supply... Then placing hands casually on the metal parts for a brief second or two, which did no harm to anyone. A little tingle perhaps, a small twitch in the finger in use, but otherwise "No Sweat!"

But I know all other higher voltages used in Broadcasting House at that time were always made very safe & kept well covered.

Nowadays the Guidelines, Rules, European Directives, Laws, Underwriter's Labs, Building Codes etc may have developed a legally enforceable book written in stone full of dos & dont's? ....

Or maybe there is no fixed point at which everything must be insulated? Perhaps its still a gray area like it was @ the BBC in the 60s...

But if anyone has a figure (law, guideline, recommendation) for what is considered to be the max safe voltage to be left exposed (AC DC) please let us know.

Nigel London UK

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#14

Re: Safe voltage

03/31/2010 10:03 AM

Hi,

I can't believe it's taken so much side chat without answering a straight forward question.

In Australia/New Zealand EXTRA LOW VOLTAGE is considered safe enough for un-insulated handling by the unlicensed general public.

Published in the Australia/ New Zealand standard: AS/AZS 3000:2007 definitions 1.4.98 Voltage (a) Extra-low voltage:

Not exceeding 50 V a.c. or 120 V ripple-free d.c.

PLEASE NOTE "RIPPLE-FREE"; otherwise it is no longer d.c. ! These standards published for the guide of licenced general electricians are based on accepted international standards with variations for local conditions/authority. As Australia and New Zealand cover a vast territory, with environmental conditions found any where in other parts of the world, the standard should be taken as safe for any part of the world.

Stephen.

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Actranuk (1); Anonymous Poster (4); Bill (1); Bluestone (1); Cornstoves (1); ElectricianStephen (1); Electroman (1); nvmani (4)

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