Dangerous electricity

Victor

Which bit is dangerous - total current or total power?

RoundyMooney

In brief, it's the voltage that pushes the current (which is the amount).

It's current flow that does the damage, but a sufficient voltage is required to drive that current. In the right circumstances, 50v up can kill...

10-10-20

Isn't it also true to say that it's also a factor of frequency?

cast_iron

10-10-20 wrote: »

Isn't it also true to say that it's also a factor of frequency?

Not really.

The Luas lines operate on DC (750V). That would kill you just as easily as 750V AC @50Hz.

Stun guns can be up to 50,000V and more. They don't kill as there is very little current involved - the small square PP3 9V battery doesn't have much current capacity.

10-10-20

Hmmm - I just seemed to remember being trained on this, and a Google confirmed it...

"The frequency of the AC has lots to do with the effect on the human body. Unfortunately, 60 cycles is in the most harmful range. At the house voltage frequency, as little as 25 volts can kill. On the other hand, people have withstood 40,000 volts at a frequency of a million cycles or so without fatal effects."
http://pchem.scs.uiuc.edu/pchemlab/electric.htm

OK, if you are comparing 750DC to 750AC, it's no contest... but for smaller voltages, I believe 60 hertz is more disruptive to the heart than 10Hz or 2kHz.

10-10-20

It's horrid to think of where the background study for those charts showing the effects of shock came from... Concentration camps, unfortunately...

mathias

Isn't it also true to say that it's also a factor of frequency?

In a way , but any mains electricity your going to come across will be either 50hz or 60 hz , either is lethal if careless.

1 milliamps , thats 1 /1000 of an amp across the heart can be lethal , Voltage , current and resistance are a relationship that determines what current will flow in any circuit ,

The human body is generally taken as having around 50,000 ohms resistance , and if you take the 1 milliamps current into account , the ohms law ( Voltage = Current X resistance) gives you a Votage of 50v as a potentially lethal level if you make yourself part of the circuit by accident.

So , if your working on anything rated over 50v , be careful , make sure its switched off and if possible locked out. If its at all possible to work on something while its switched off and locked out , then thats the way it should be done. You can never be too careful with electricity.

The above values are taken from CoHe ( control of hazardous energies ) guidelines for work , and would be type 3 EEW , ( energized electrical work) Type 1 being equipment thats not powered up at all . Type 2 is less than 50v etc. Work rules for type 3 and above say you must have someone with you , you cant work on such equipment alone , the person with you is not supposed to do anything except watch you and disconnect you from the circuit if you start to get a shock.

Once you get higher than normal domestic mains , then other hazards present themselves , such as flash burns from high tension lines etc. ( this is common enough in factories , Ion implanters in a Semiconductor plant for instance , have 40 000 V lines and you must wear a flash suit etc. while working on them) Guidelines are listed below , to be safe working with electricity , you should be properly trained and never work on electrical equipment if you dont know what your doing , the only safe piece of electrical equipment is a disconnected piece of electrical equipment. ( and even at that , there are capacitors ,inductors etc which can store charge for hours ).

http://books.google.ie/books?id=u9sN06pNaHoC&dq=energized+electrical+work&pg=PT77&ots=OO97mha_08&sig=C7noHRxbQRMhVP9ieCCXCfbS2fc&prev=http://www.google.ie/search%3Fhl%3Den%26cr%3DcountryIE%26client%3Dfirefox-a%26channel%3Ds%26rls%3Dorg.mozilla:en-GB:official%26hs%3Djan%26sa%3DX%26oi%3Dspell%26resnum%3D1%26ct%3Dresult%26cd%3D1%26q%3Denergized%2Belectrical%2Bwork%26spell%3D1&sa=X&oi=print&ct=result&cd=2&cad=legacy#PPT87,M1

cast_iron

10-10-20 wrote: »

Hmmm - I just seemed to remember being trained on this, and a Google confirmed it...

"The frequency of the AC has lots to do with the effect on the human body. Unfortunately, 60 cycles is in the most harmful range. At the house voltage frequency, as little as 25 volts can kill. On the other hand, people have withstood 40,000 volts at a frequency of a million cycles or so without fatal effects."
http://pchem.scs.uiuc.edu/pchemlab/electric.htm

OK, if you are comparing 750DC to 750AC, it's no contest... but for smaller voltages, I believe 60 hertz is more disruptive to the heart than 10Hz or 2kHz.

Interesting. I'm not sure what exactly that article is comparing when it says "40,000 volts at a frequency of a million cycles or so without fatal effects."
Possibly a stun gun - it operates at very high frequency, but low current.

A valid comparison would be comparing 40kV, 500mA @DC and 40kV, 500mA @10kHz. I'm guessing prolonged exposure to either will result in a fatality.

mathias

A valid comparison would be comparing 40kV, 500mA @DC and 40kV, 500mA @10kHz. I'm guessing prolonged exposure to either will result in a fatality.

500ma , half an amp , at 40Kv would be fatal in a matter of seconds for either of the above.

Stun guns deliver a high Voltage with a very small current , typically micro or nano amps ( even pico amps ) , its a very short high voltage small energy burst that scrambles the nervous system. In terms of Cycles ( usually meaning cycles per second , the shot is so short that it probably would be in the range of 1 megahertz or a million cycles per second.)

It would be nothing like 500ma , thats half an amp , in both cases above your talking 20,000 watts , a huge amount of power and no one would survive that , no matter what the frequency.

danjo

RoundyMooney wrote: »

In brief, it's the voltage that pushes the current (which is the amount).

It's current flow that does the damage, but a sufficient voltage is required to drive that current. In the right circumstances, 50v up can kill...

Yes, it is the current that does the damage. The voltage is not necessarily a factor e.g when the source is a constant current.:eek:

RoundyMooney

At the risk of sounding dramatic, 40kV at half an amp would kill you before you started to smoke or show any outward sign of injury! (Frequency notwithstanding).

Frequency is a factor of course, but voltage and current, a la Mr. Ohm and his omnipresent law are the main parameters to consider.

As per the post above, the voltage has to be of a sufficient value to drive the current through the resistance of the body in the first place. Another variable too is where the entry and exit points on the body are. Across the chest cavity is far riskier than across two fingers on one hand for example...

cast_iron

RoundyMooney wrote: »

At the risk of sounding dramatic, 40kV at half an amp would kill you before you started to smoke or show any outward sign of injury! (Frequency notwithstanding).

Frequency is a factor of course, but voltage and current, a la Mr. Ohm and his omnipresent law are the main parameters to consider.

Indeed, my 40kV example was probably a touch on the extreme side.:)

I suspect that linked article refers to stun guns, where the minute amount of current is why a 40kV shot can be survived, as opposed to the high frequency of the blast.

At what sort of level does frequency become a practical consideration?

RoundyMooney wrote: »

Another variable too is where the entry and exit points on the body are. Across the chest cavity is far riskier than across two fingers on one hand for example...

Certainly. I was beside someone who got a blast from the mains, as his arms were resting on metal worktop. He was grand, but his chest was sore for a while after.

A question for ye: RCBO was working, why didn't it trip?

RoundyMooney

The current flow may have been across the supply (phase-phase/neutral).

Assuming it was a 30mA unit of course, and not an older 300mA jobby (they're still out there), he may have been wearing rubber soled shoes, effectively isolating him from earth.

Lighting circuit, inadequate earth system...

TBH, there could have been a number of reasons.

Andrea B.

"A question for ye: RCBO was working, why didn't it trip?"

1. Did he get a blast from "mains" which was preceeding RCBO (RCD?).

2. Was the blast from a lighting circuit which is not RCBO (RCD?) protected?

3. Did he release himself in less than 25milliseconds?

On a last note, low voltage (230v) domestic electrocution risk is heightened by the body not being able to fall away from the contact point. Areas such as attics or immersion cupboards, in my opinion, pose the most risk. Muscle fibrillation impedes you from moving or releasing grip. When completely unsure of the status of a wire, or for final clarification, I flick the back of my hand against it.

cast_iron

I should have given more details, sorry.

He was working on a (live) socket after the RCBO. On a wooden ladder with work boots on. He was disconnecting the neutral and touched it by accident.

Stoner

OP Current kills, 50V and 1 mA have been addressed already.

Its your resistance that will limit the current and the time you are exposed to that current that will determine the damage.
WRT power you'd have to consider the about of energy in question, i.e a large amount of power for a very very short time, or less power over a sustained period. Although this has to some degree turned into a Frequency arguement, and frequency deals with the cycle time of AC current, but just basic old time comes into it a lot.

This is why there are all different types of MCBs and RCBO's all with different response times///curves. Knowing the device and conditions in which you are trying to protect against over current is very important, not only to limit the current but to limit the time that someone would be exposed to current, regardless of the frequency of the voltage source.

On a slightly different topic, I have an electrical problem with my heart, ironic etc. anyway I have been cardioverted many times (i.e stop the heart and zap it with an electrical current to start it again). The docs now ask me how much current did we use the last time?
I tell them I dont know and say no more, because when I ask them afterwards they give me the answer in joules.

cast_iron

Would:

P=V*I and W=P*t

solve that for you?

UrbanFox

Non-technically, the way that I have always understood the issue was that current (amperes) is a measure of the quantity of electrical charge that is being delivered to the point at which it is measured.

I always think of current using the hosepipe analogy as being the flow rate of the water i.e. the quantity of water delivered per unit of time as, for example, 1,000 gallons a minute.

Voltage can be likened to the pressure of the water in the hosepipe as measured in pounds per square inch. Naturally enough the pressure (volts) influences the flow rate (amperes).

Incidentally, is there a difference between AC and DC in terms of it's ability to fibrilate the human heart ?

aido79

referring to the frequency dc is more likely to kill you because your muscles will contract and you will stick to it whereas with ac your muscles will contract and retract and you may be thrown back depending on the amount of current.

Jnealon

It's a combination of both but 50V @ 30mA is enough to kill

Victor

I'm disturbed by the lack of coherence of the answers here.

Stoner

Victor wrote: »

I'm disturbed by the lack of coherence of the answers here.

it is current that kills, the amount of current is a huge component in the amount of power, we all have different internal resistance to current.
Like I said WRT total power, this would be the amount of power over an amount of time. The longer the time this total power was used over would dilute the power at a given time.
power = volts by current or
power = current squared by the resistance,
as you can see that without current , you have no power, so they are both dangerous, but it is increasing the current increases the danger in both cases.

We have just gone a bit OTT with some of the answers thats all. If a shock kills you, you wont be wondering if its AC or DC, or what frequency it was.

RoundyMooney

Victor wrote: »

I'm disturbed by the lack of coherence of the answers here.

For brevity check out post #2, or Stoner's reply just above mine. I wouldn't say any of the replies to this thread are incoherent Vic, they are all relatively accurate, although some have more bearing to the OP than others.

I enjoy the discussion tbh.

Victor

RoundyMooney wrote: »

For brevity check out post #2, or Stoner's reply just above mine. I wouldn't say any of the replies to this thread are incoherent Vic, they are all relatively accurate, although some have more bearing to the OP than others.

With respect, while all the answers together may be a complete answer, there is no choerence. Other thsn some vagaries there is no "you cannot be shocked by this", "you may be shocked by this", "you will be shocked by this".

I enjoy the discussion tbh.

Surely there should be a standard straight forward (multi-part if necessary) answer? What with is being a physical law and stuff.

Stoner wrote: »

WRT power you'd have to consider the about of energy in question, i.e a large amount of power for a very very short time, or less power over a sustained period. Although this has to some degree turned into a Frequency arguement, and frequency deals with the cycle time of AC current, but just basic old time comes into it a lot.

You are confusing power and energy. Power = energy divided by time. http://www.sengpielaudio.com/calculator-ohm.htm

Stoner wrote: »

We have just gone a bit OTT with some of the answers thats all. If a shock kills you, you wont be wondering if its AC or DC, or what frequency it was.

Personally I would like to have gained from Darwinism and the knowledge built up over the years as to what is dangerous and what isn't.

Now from what I've learned and assuming a momentary shock.
* You will need a certain minimum voltage, so as to overcome the body's electrical resistence.
* AC is more dangerous than DC. The level of danger will vary depending on the frequency. In a sustained shock, DC may cause a muscle contraction that you can't escape.
* A shock through the heart / chest cavity will be more severe than one through the extremities.
From the above, surely it is possible to plot pK or pI levels of voltage against current, for DC and AC at different frequencies.

RoundyMooney

Your summation is correct.

It's difficult, if not impossible to predict what will result in a fatality and what won't, because the variety of human physiology makes such a generalisation impossible.

For voltage and current calculations, the resistance of the human body is taken as 50k. Even if we were all exactly alike, no fault current could be subject to exactly that resistance, because entry and exit points will vary with each posited incident. In practise, it has been proven that some are more resistant to electrical shock than others.

Like I say, a generalisation is impossible. All we can really agree is a safe minimum, which is where the electrical safety standards come into play (and even they vary from jurisdiction to jurisdiction).

Victor

RoundyMooney wrote: »

It's difficult, if not impossible to predict what will result in a fatality and what won't, because the variety of human physiology makes such a generalisation impossible.

So that why science usually resorts to percentages - At X volts and Y amps DC Z% will be killed.

Like I say, a generalisation is impossible. All we can really agree is a safe minimum, which is where the electrical safety standards come into play (and even they vary from jurisdiction to jurisdiction).

So, what are those standards?

/rushes off to check for stickies.....

2 stroke

I don't consider electricty dangerous, people are dangerous. Anybody working on electrics should remember that other peoples lives depend on their actions. I've recieved a few shocks due to another persons carelessness.

Copper

, ere are so many variables when it comes to classifying electric shock that its impossible to list them all but from my understanding of it;

1) 50mA is the internationally accepted minimum level of lethal current. So to avoid risk of death, the current flowing through you should be kept under 50mA. This is why the standard rated current of RCD's is 30mA, and the whole point of electrical personal protection is to ensure this level of current is never delivered to a person.

2) The resistance of the human body varies from 1,000 - 10,000 ohms depending on several factors including where the resistance is measured from, e.g. hand to hand, hand to foot, head to foot. The biggest single factor, or one of the biggest factors is the resistivity of the skin. Wet skin = low resistance = greater chance of the current exceeding 50mA. Women generally have a lower resistance than men because their skin is thinner, and men with rough, callused skin particularly on their fingers have the highest resistance. So women, and men with thin, soft skin will be more vulnerable to electric shocks.

3) From this, the lower value of resistance of the human body, i.e worst case scenario, was assumed (1,000 ohms). V = IR, so V = (50mA x 1000ohms) = 50V. So 50V is the accepted minimum level of lethal voltage. This means that 50V could pass a potentially lethal current of 50mA through you if your resistance was 1,000 ohms (although in reality a body resistance of 1,000 ohms is unlikely, you would probably have to be soaking wet). This is why exra low voltage is classified in the E.U as <50V, and in most countries you need an electricians license to work on voltages above 50V - because 50V is the minimum lethal voltage. Phone lines for example are 48V so unqualified people can work on them safely.

The biggest danger in receiving an electric shock is ventricular fibrillation, the heart seizes and stops pumping. A.C is far worse than D.C in this regard because the A.C current flows in and out of the heart 50 times a second and the heart can't contract and release this quick and so seizes (this is why A.C was used in the electric chair, and why A.C was branded in the media as far too dangerous when it was first used). A hand to hand shock directly across the chest cavity is probably the worst kind (both A.C and D.C cause muscle contractions, but A.C causes ventricular fibrillation).

After that comes the risk of electrical burns. Electrical burns can vapourise the skin right down to the bone and lots of heat can be generated when you get a shock. You end up with blisters and burnt skin.

What happens when you get a shock depends on the level of current that is passed through you, what part of the body it is passed through and the length of time it is allowed to pass through you. If you touch a live wire with the back of your hand your muscles will contract and your hand will jump away, so the contact time is minimal. But if you grab it with your hand your muscles will contract and you won't be able to let go, so the contact time will be larger.

50mA, although enough to kill is a miniscule amount of current (hardly 1/5th the amount of current to light a light bulb. My understanding is that it doesn't contain enough energy in itself to harm the body (apart from burns), but its main danger is that it messes with the nervous system and causes vital muscles such as the heart to contract.

A guy I work with has very thick skin and callused finger tips from working with his hands. Wearing rubber boots, he can touch the tops of live breakers and not feel a thing, where I would be dead trying. Similarly, I've worked on and held live cables unknowingly and not felt a thing because I was either on a ladder with rubber ends, or was three or four stories up in a building and the resistance to earth was too great. There are many factors and in identical scenarios, I could die where you would not feel a thing or only suffer minor burns or vice versa.

cast_iron

Victor wrote: »

Now from what I've learned and assuming a momentary shock.
* You will need a certain minimum voltage, so as to overcome the body's electrical resistence.

That doesn't make sense, but a minimum voltage figure considered to be dangerous to the human body is around 50V.

Victor wrote: »

* AC is more dangerous than DC.

Well that depends. Do you mean just in theory or in practical reality?

The reality is that 95% of people will only be have to worry about 220V @50Hz. We developed quite a tangent of the whole frequency issue, but it's mainly theory. For example, I'm not aware of any 220V 1kHz supplies or anything similar.

Victor wrote: »

In a sustained shock, DC may cause a muscle contraction that you can't escape.

Incorrect. It occurs for both and is largely confined to low current levels. The actual situation circumstances most important in this case, but there are some accepted figures.

AC max current that one can release: 10mA
DC max current that one can release: undefined up to 300mA; above 300mA - only after several minutes.

@ Copper: Fibrillation can occur from both AC and DC - the threshold of fibrillation for d.c. is several times higher than a.c. for shock duration longer that the cardiac cycle. If the shock duration is shorter than 200 ms, the threshold of fibrillation for d.c. is approximately the same as for a.c., measured in rms value.

Victor wrote: »

* A shock through the heart / chest cavity will be more severe than one through the extremities.

It's more lethal, but less likely to occur.

Victor wrote: »

From the above, surely it is possible to plot pK or pI levels of voltage against current, for DC and AC at different frequencies.
.
.
The level of danger will vary depending on the frequency.

At higher frequencies, the bodies resistance decreases and there is increases danger. But at 220V, the bodies impedance is the same at DC and AC (impedance used as opposed to resistance as capacitive affects come into play at AC)

Victor wrote: »

So that why science usually resorts to percentages - At X volts and Y amps DC Z% will be killed.

That's over simplistic as as Roundy said, there are a large amount of variables to consider, so such a table wouldn't be of much use.

Stoner

you put a lot of effort into that answer Copper , fair play.

I've been looking at how short and non descriptive the OP was TBH.

Increasing the current increases the electrical power, be that an AC or DC source, therefore in a general answer to a general question, frequency is not really a factor.
Without current , there is no electrical power, the magnitude of the current determines the magnitude of the power, therefore as we have all answered, current kills.
The time that someone is exposed to that current also plays a large part in determining the damage level of the resulting shock, i.e the danger that the OP referred to, so time is a factor in the answer.

Please bear in mind that the OP was a seemingly low level question, and it seems to me that Victor is capable of answering the question himslef .

Is this some sort of initiation for the forum

IMO Victor if I knew you wanted your answer in terms of
"pK or pI levels of voltage against current, for DC and AC at different frequencies."
it would have impacted greatly on the answer . But the OP asks what we all took as simple question.

If as much effort was put into formulating the original question as into exposing peoples efforts of help, this thread would be a lot shorter.

IMO The aim of this forum is to help people out with DIY electrical issues that they do not understand and want advice on, it is also here to watch for poor advice from poorly informed posters, I'm sure it will develop further.
I for one mistakenly assumed that posters will only ask questions seeking knowledge, lesson learned.
The forum is 4 days old, the stickies will come.

A positive from this thread is that we do need some more stickies as per Victors comments.
I'll put up a sticky about ETCI regulations and British standards tomorrow, Roundymooney must have loads internal ESB links, he is among the best trained electricians in the World after all (or so my brother keeps telling me:D)

2 stroke

Wearing rubber boots, he can touch the tops of live breakers and not feel a thing, where I would be dead trying

Are your boots antistatic or insulating? antistatic boots conduct electricty.

Best piece of electrical safety advice was taught to me by a now retired electrician. He called it the electricians rule of thumb.

If you're not using your left hand put it in your pocket

This will reduce the risk of a shock across the heart as it stops you resting your other hand on an earthed surface.