What is the Maximum Allowed (Safe) Voltage for Exposed DC Terminals?

I should also add that where I work, a 50V standard was set because some "visitors" were claiming that kilovolts of electricity were acceptable because it was only DC and that they were only going to use supply "A" for their project. They never thought that somebody might use DC supply "B", "C", "D", "E" or "F" that would turn flesh into jerky or worse before limiting power at those voltages.

I think that was probably a trick question on an exam and they're still baffled by it

The reason why "low output impedance of the battery will quickly liquefy or vaporize parts of the wrench in a spectacular fashion" if "the wrench on the positive terminal touches the chasis"is because of the TOTAL ENERGY in the battery & NOT due to 12 V. If a similar situation happens with a small two wheeler battery, then the result is not too spectacular!!, Jokes apart, normally upto 48 V NOMINAL voltage ( as in telecom requirements ) is generally considered safe, beyond that it is case specific.

I believe redfred and myself are well versed in electricity. And don't kid yourself, a little small two wheeler battery can and will, have just as much of a spectacular result, just, not as long in duration. Don't forget, Big Surprises can come in small packages.

LOL!!

And very true!!

Current record I am aware of is someone electrocuting themselves with a PP3 9V battery.

Potentially possible, and in this case the person (an adult) apparently punched the probes of a continuity test meter into each of his hands and was able to create a circuit with a low enough resistance across his heart to cause the mA necessary to stop it. It is believed he was trying to measure his body's internal resistance after learning about it in a basic electrical class.

So no more putting 9V batteries across your tongues people.

Whle in the army and working in maintenance the example You mentioned about

the wrench hitting ground while disconnecting the battery happened to a friend of mine. The wrench completly melted within 5 seconds.

oilcan13

GA from me redfred, I didn't see anything about electrocution either. And to share a story that I was involved in years ago. The Master Mechanic who taught me auto mechanic's when I was 12 years old also demonstrated why you should never wear metallic objects while working on electrical systems! His wedding got caught between the battery side of a Ford starter solenoid and metal fender well. His wedding ring went from a gold color to white in less than a millisecond. I had a hell of time disconnecting the battery, as he was bent over the battery also. He came close to losing his finger because of it. Even thou, he never got electrocuted, he did get FRIED from the "Low Voltage"!!!

Well, needless to say, I was impressed for life about dealing, with low voltage "DC". And, I still hate the smell of burning flesh to this day!!

As already written earlier, this also happened due to the energy discharged within a millisec. V*I*t, Current I is extremely high due to very low battery resistance. Thus the product of V, I & t is very high, sufficient to do the damage which you saw.

Original question was for the voltage alone.

It was for the SAFE voltage:-

"Can somebody tell me or show me where I can find maximum allowed (safe) voltage for exposed DC terminals....."

So anyone here could correctly assume electrocution might be one of the problems anticipated and to be prevented.......burning is another of course but at least both are meant!!!

Even a fire might be something that needs to be prevented.....I am sure my list is not complete either....

A colleague of mine lost his little finger in the same way when working with a gold ring on it in a computer printer with a heavy duty 5 volt supply.......1978 or thereabouts!!!

True, True, True......how often must we say it on this Forum???? Never ending...

To add to the excellent answer above I suggest you can experience a 9v dc to wet skin by putting a 9v battery on your tongue very very briefly. The harder you press the more current will flow and great the pain. Imagine a 50vdc to wet hands! Just think - AC dangers and your should be safe.

If you meant that AC is safer, you are completely wrong.

Do not forget that a 50 volt DC line is maximum 50 volts.

A 50VAC line is called such because its the RMS value (0.707) of Peak.....so the peak value will be around 30% higher....around 65 volts.

That may not seem much of a difference at 50 volts, but for example a 240VAC line peaks around 312 volts.....

But also, as someone has mentioned already (sorry I forget who) the AC is also more damaging than DC at almost any voltage above around 50 volts.

A 50VAC line is called such because its the RMS value (0.707) of Peak.....so the peak value will be around 30% higher....around 65 volts.

Just a minor clarification. If the voltage is 50 Vrms, then the peak will not be 65 volts, it will be 70.7 volts (50 * √2). Which means it's not 30% higher than 50, it's 41.4% higher (√2 +41.4% of √2 =1.0).

Also, the 240 VAC would not be a peak of 312, but 340.

My comments were made from (a faulty) memory! MINE!!

Thanks for the correction....

Other posts correctly identify that even low voltages can be dangerous, but in the manner that I think your post was intended, I'd suggest, with no formal standards backing is that:

Voltages up to over 24 volts are absolutely safe - short of arc induced burns and the like, with maybe up to about 40 volts as fairly safe. The latter being based on the fact that DC welding voltages are often in excess of 30 volts and generally give no more than an unpleasant tingle with fairly heavy contact. The design of this sort of equipment is also quite open to "accidental touching".

Once DC voltages approach 90-100 v are generally regarded as dangerous and potentially lethal so far as I know - and beyond that level human contact should not be readily possible.

I haven't any suggestions about what to do in the middle ground, except to ere on the side of caution - although that said, I cannot immediately think of an application where the 40 - 90 volt range is used.

Having served on both AC and DC ships of the Royal Navy in the 60's & 70's, AC 440AC 3 Phase (no Neutral), NATO standard since the 50'.

DC 220V, I can assure you that even 220VDC is for most people not dangerous as when shocked, the muscles close generally pulling your had away from the voltage.

We used to search for grounded wires with two fingers across each removed fuse position, when you got a belt, that was the grounded circuit......but it was not painful.

AC puts the muscles in a sort of paralysis that does not allow movement of any type....been there, done that and NOT got the T-Shirt either!!! VERY painful indeed.....

There are several "works" around from Doctors saying that DC is more dangerous, but they are 100% wrong (none of them have tested the effects using their own fingers, I have!)

I also have a table somewhere from a PROPER Scientist showing the "killing" ranges of both types of current, I got it sent to me by a great colleague on CR4 a year or so back, if you want to have a copy I will search it out for you.....

DC 220V, I can assure you that even 220VDC is for most people not dangerous as when shocked, the muscles close generally pulling your had away from the voltage.

Only if you have been properly trained and use the 'back of the palm' method so your fingers don't involuntarily close on the live DC part or object and stay locked preventing escape and death.

That is a comment from someone who has never touched 220vdc........just dreams about it!!

I have had my fingers on 220VDC thousands of times, it was a quick to hand test tool. We all (electrical ratings) did it, it was the first thing you learnt when working on a 220VDC ship......

As an aside, apart from the aircraft carrier ring main systems which is and was a closed face system, all 220VDC RN ships had "open" switchboards. You could touch the live bus bars easily. There was only a safety rail, for when it was rough, to hang onto in front of the live bus bars and equipment.

I never heard of anyone being electrocuted on 220VDC, and we had thousands of such ships over the years from the 19th Century till the 1950's, but many were in servive through the 70's, some till 2001.......

The RN "Daring" destroyers, laid down at the end of WWII were built some with DC power and some with AC power. See here:

http://en.wikipedia.org/wiki/Daring_class_destroyer_%281949%29

There you will find the following sentence:-

The Royal Navy (Daring class) ships were built in two groups, one with the traditional DC electrical system (Daring, Dainty, Defender and Delight) and the remaining ships with a modern AC system. They were known as the 2nd and 5th Destroyer Squadrons, respectively.

The first AC Daring class ship was also built originally with an open faced switchboard. There were so many electrocution deaths onboard during the first commission, 7 or 8 if I remember correctly.

This ship was if I remember correctly:-

http://en.wikipedia.org/wiki/HMS_Diana_%28D126%29

The ship had the open faced switchboards replaced at the end of the first commission and the RN has never had an open faced AC switchboard ever again........far too dangerous!!

Just compare the 60 odd years with DC open faced switchboards and as far as I am aware, no deaths by electrocution (might be some from burning though, I knew someone who lost all the skin on hands as well as being blinded for a few days after dropping a screwdriver across the bus bars of a 22,000 amp 220VDC generator, while searching for an earth on the main switch!! He fell and put on hand on the positive bus and one on the negative!! But he lived to tell the tale !!!)......and the Daring destroyer with the first AC switchboard.....now that should tell you something really important.........

Very few people nowadays work on high current DC voltages of 220VDC or more, where did you get your experiences?

My experience was having 12 x 22,000 amp generators feeding a carrier's ring main.......working temperature in the engine rooms on a cool day was around 92°C......kept us slim and fit!!! I served on that ship twice......64 -66 and 73.

Andy,

That Graph looks to be very useful, could you please let us know the source.

I got it from another colleague here on CR4 (as I said in the post!!).

It's obviously from a book or white paper......that's all I know......

50 Hz in particular confuses the sinoatrial node

So it may not be the current frying, but the fibrillation that causes death.

However, as said, anything above 32 V (ELV threshold) in normal dry industrial conditions should be regarded as dangerous voltage.

You may like this;

"An external pacemaker was designed and built by the Canadian electrical engineer John Hopps in 1950 based upon observations by cardio-thoracic surgeon Wilfred Gordon Bigelow at Toronto General Hospital . A substantial external device using vacuum tube technology to provide transcutaneous pacing, it was somewhat crude and painful to the patient in use and, being powered from an AC wall socket, carried a potential hazard of electrocution of the patient by inducing ventricular fibrillation."

source Wiki

Hi Andy, what a refreshing post, I have been designing and installing what I call "Battery Centric' renewable energy systems for years now, in Australia, using Solar and now more and more Wind as the power source. As systems get bigger, larger equipment is required, often higher voltages, DC voltages, and everywhere I hear from associates, engineers, Standards committees, electricians, "Oh No, Dangerous dangerous, DC is dreadful, you can't control DC". The Standard in Australia is Max. 120 volts DC, (Open Circuit) and now there are moves to limit it to 48 volts, (60 OC) and whilst I have installed a system with 820vDC (100kW inverter) I have designed now several with 1100vDC and some of those will probably happen, - in the range 1 to 4 meg output. I have been treated as a leper, a murderer, almost a satanist, by some of these folk, despite that I have instituted every requirement to Standard, using all approved, certified, equipment, and never a trace of a problem, just because it was DC. Sure, electricity can be dangerous, so can cars, rifles, axes, all sorts of things, but it is so wonderful to hear of your experience where not only did you use your fingers to detect not 120 but 220 volts DC and no deaths, but the AC was less safe, - so often fear is used to control others, and the basis for that fear is not what is talked of, ie not the real risk but the magnified imagined risk, - yes as child I got a shock from 6 volt, (standing knee deep in salt water intercepting load,) and as an adult my arm hairs stood up when close to 820 volts DC, but with the proper precautions these voltages can still be a friend to humankind. Thanks Andy, Geoff Thomas.

I fully agree with every word you wrote, you were the target for an ignorant bunch with "no hands on", though I must admit, I won't go hands on with over 800 VDC either!!!

Did you like the table, if you would like an original (readable one for you own use, just send me your private email address via the CR4 email....same for anyone else of course.

By the way, we have seen the same type of "hounding" here on CR4 already, sad to say, its nothing new......

I can't believe there is a standard for such an installation. Any contact by both terminals and a metallic conductor will either short out the DC supply or battery. Just put some insulating covers on the terminals ( like they do with automobile batteries) and forget looking for a standard "allowed" voltage.

Just think about touching the spark plug leads on a car engine. About 30000 volts: very unpleasant but not lethal, it all depends upon the total charge available.

There's been more than one man killed automobile ignition system's!

This is a bit of a vague question, but I would start at Exposed, meaning an innocent bystander may somehow grasp these terminals, suffering the consequences thereof? As a youth, powering a tiny dc motor in a plastic hull in salt water, - standing in the water, (age 11) I received a very strong shock from the 6 volt Volkswagen battery I was using to power it, so I would not flagrantly talk of a minimum voltage for DC as when the circumstances are favourable, even very low DC voltages can be fatal. Don't expose your DC terminals at any voltage, may be a good start, but moving on from there, one has to understand, (as maybe you do already,) that with DC there is a problem that with a break in the circuit there is possibly a setting up of an Arc, - and it will, as opposed to AC, not self extinguish. So for higher voltage DC there are stipulated High Rupture Current fuses, designed to break that arc, - cheaper ones use sand which falls into the cavity created by the vaporised fuse wire, others use air blasts, magnetic fields etc. DC, if properly designed for, is great, many advantages in particular situations, - solar output is DC, batteries live in the DC world, - I have put in systems 820 volts DC, undersea power transmission is better with DC, - huge voltages, but just don't expose your DC terminals, one flash and you're ash, the only safe voltage is No Exposed Terminals, (imho), Cheers, Geoff Thomas.

If your feet are in contact with water, from 6 to 12 volts, according to your age and your physical state.

If you are in a dry place, without conducting dust (like in a foundry, or a metalwork), your heart will define the upper value: I used to know some men able to support up to 110 volts DC (from those ones, one percent able to support 220 volts DC). Generally, if you are young and not fat, with short hairs and a clean working dress, standing on a dry place, you should be able to support up to 50/60 volts DC, the lowest welding voltage for steel, in a welding post.

These values, with a current not exceeding 1, 2 or 3 amps DC, according to the fibrillating value of your heart.

Do a check-up of your heart before playing with DC voltage. If your heart beating is under 70/80 and your blood pressure definitely 12/8, then you should be able to touch critical voltages, up to 50 VDC.

Standards define 3 levels: 12 VDC (generally SAFE value EXCEPT in marine or wet places), 24 VDC(common value for portable tools) and 48 VDC (common value for telecomm bateries). For dental operations, a DC voltage of 6 volts is recommended.

See your local standards before playing with some DC voltages.BE CAUTOIUS.

BRZK

I believe that the Canadian and American Electrical Codes specify that 30 Vdc are considered 'safe'. I imagine that there are conditions that go with this: for example, that the voltage source be a high (enough) impedance one (so excluding things like current transformers).

Based on this link, I'd have to say less than 50 volts.

Agreed.

Welcome to CR4 Vladdd. You should use licensed electricians to avoid a lawsuit.

All to often it seems the OP's think that we are Oracle's or have a Crystal Ball sitting in front of us and be able to fill in the missing key information regarding their questions.

I agree that it better to be safe than sorry, it is more important to be locally compliant. If IEC standards are what your location requires for compliance then you are properly exceeding your local standards. You must identify which standard is correct for this installation and meet or exceed that standard not some standard that you are just familiar with or that meet your budget.

If the sheet metal covers are held in place with screws, then the panel would not be considered "readily accessible" because a tool is required for access. If the sheet metal cover is hinged and has a handle, then I would prefer to have a quarter-turn flathead screw or a lock and key if that's the only mechanism that meets your requirements.

Being one who trouble-shoots a lot of electrical equipment, I would suggest that if you decide to go with shields, then be sure to put small holes in them for access with a multimeter. Having to removing small guards inside an electrical enclosure while trouble-shooting creates a lot of opportunities to drop little screws into live components.

NEC Definitions of "Accessible":

Accessible (as applied to equipment). Admitting close approach; not guarded by locked doors, elevation, or other effective means.

Accessible (as applied to wiring methods). Capable of being removed or exposed without damaging the building structure or finish or not permanently closed in by the structure or finish of the building.

Accessible, Readily (Readily Accessible). Capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, and so forth.

Generally speaking 32 Volts is the industrial norm.

This is also "ELV" (extra low voltage) threshold.

I'm a Little surprised that you cite the Standards, yet ask the question.

There has been much truth about electrical safety shared in this discussion - sobering and otherwise, but I believe the relevant standards may fall within:

EN 60950

EN 60335.1

EN 60335.2.( .....)

depending on the type of equipment.

I suggest that the inquirer speak to his local electrical safety/regulatory authority to determine the relevant standard(s) for his specific equipment in his location.

In addition to the standards there is that commodity known as common sense and personal responsibility which these days are not so common and are becoming less so as more and more 'nanny' legislation comes into effect and more frivolous cases elicit exorbitant damages!

Or AS/NZS 3100:2009

But as you say, it depends on where you are.

No mention so far of battery banks in submarines - but given time .....

You mentioned subs......

I was told of an American conventional sub - one of the last that limped into Williamstown dockyard (Melbourne Aus) on the last of its battery power - DGs having failed - and asked if they could connect to the 110 DC power on the pier to trickle charge their batteries while waiting for spares for the DGs, having been given approval they series/paralled their batteries to 110V and connected ..... the the dockyard fuses vaporised, arced over and kept drawing current, the rotating machinery in the powerhouse tried to tear itself out of the concrete foundations and the only way they could shut it down was to shut down the rotating machinery ...... then someone did some calculations on that battery configuration in the sub - trickle charge was in excess of 2000 Amps ......

or so the story goes ......