Why is 110v safer?

Discussion in 'Electricians' Talk' started by Dan E., Sep 28, 2005.

  1. Dan E.

    Dan E. New Member

    Please enlighten a humble chippy and explain why 110v doesn't fry you? All 110v tools have the same wattage as their 240v equivalents, which must mean that the ampage is doubled. I thought it was the amp level that was dangerous, not the voltage?

    Also, transformers - 'this device has a duty cycle of ten minutes off and 20 minutes on over a 3 hour period'. What the bloody hell does that mean? Everyone uses them and no-one knows...
     
  2. Wilb

    Wilb New Member

    Please enlighten a humble chippy and explain why 110v
    doesn't fry you? All 110v tools have the same wattage
    as their 240v equivalents, which must mean that the
    ampage is doubled. I thought it was the amp level
    that was dangerous, not the voltage?

    It's the amps going through YOU which are dangerous. The voltage is a measure of how hard they get pushed through you. A building site 110V system is actually arranged so that only 55V is available to push amps through you into the earth. This means that it's likely to only push about a 4th of the amps as 230V.

    Also, transformers - 'this device has a duty cycle of
    ten minutes off and 20 minutes on over a 3 hour
    period'. What the bloody hell does that mean?

    It means roughly that after using it flat-out for 20 minutes, you have to give it 10 minutes to cool down. Presumably after three hours of 20-on-10-off you have to give it longer to cool, but it's not clear from the text you quote. 'Duty cycle' is a fancy way of saying how much WORK can be done in a certain amount of TIME.

    Everyone uses them and no-one knows...

    Not quite true. I use them and I know...
     
  3. MechEng

    MechEng New Member

    Yes it is the current that kills you. But, the important thing to remember is that you need the voltage to push that current through you. So a lower voltage can't push as much current through a given resistance (i.e. you!) than a higher voltage.

    Second is that the 110v feeds are through an isolating transformer. This means that you would have to simultaneously touch both the live and neutral to draw a current. If you touched between live and earth, you will not be able to draw a current through the transformer.

    I think that's correct. There is also something about the transformer being earthed at the centre point, so the voltage is never higher than 55volts above earth potential...don't fully understand this bit so maybe someone can explain....would this mean that it's not 110v RMS?

    With regard to the duty cycle, it simply means that if you connected a 3kVA load to a 3kVA transformer and ran it constantly for 1 hour, it would melt. But if you ran it for 20 mins on/10 mins off then it won't melt, as the heat has time to dissipate in the off time.
    In reality, you aren't going to be running your saw or drill constantly for an hour or even 3, so it shouldn't be a problem.
     
  4. MechEng

    MechEng New Member

    beat me to it! can't work and post at the same time!!!!
     
  5. Wilb

    Wilb New Member


    Second is that the 110v feeds are through an
    isolating transformer. This means that you would have
    to simultaneously touch both the live and neutral to
    draw a current. If you touched between live and
    earth, you will not be able to draw a current through
    the transformer.

    That would be true, if the secondary of the transformer was floating (i.e. not connected to ground). A shaver socket is like that.

    I think that's correct.

    In general, yes. For a 110V tool transformer, no.

    There is also something about
    the transformer being earthed at the centre point, so
    the voltage is never higher than 55volts above earth
    potential...don't fully understand this bit so maybe
    someone can explain....

    The secondary (output) winding in the transformer is earthed in the middle. This gives you a maximum of 55V between earth and either line of the circuit.

    would this mean that it's not
    110v RMS?

    It's 110V between the two lines, and 55V between either line and earth. While one line is positive, the other is negative.
     
  6. sinewave

    sinewave Screwfix Select

    I wouldn't worry about burning out ya ****** Dan.


    As you well know Chippies only do about 20 minutes work between half hour tea breaks so your ****** is well up for the job eh!

    :^O :^O :^O :^O
     
  7. Short Fuse

    Short Fuse New Member

    somewise man said it not voltage that kills u it be the current and where it flows.
     
  8. BashOn

    BashOn New Member

    There's nothing magical about that, for a given voltage V measured OVER a resistive impedance (your body), there is a given current I THROUGH the impedance, related by the size of the resistance R

    That's Ohm's law, V=IR

    The other important thing to remember is that "voltage" is short hand for "potential difference": it's really potential difference that drives a current. If I hold 1000V in one hand (and am insulated from Earth) and grip 990V in the other hand, the resulting shock would be no worse than a 10V battery earthed as the potential difference is only 10V.

    A good analogy is gravitational potential energy: if I stand on top of a building the 'voltage' or height is large. This is equivalent to saying that the potential difference between the top of the building and the ground is large. There are two ways down: a 'low resistance' (impedance) route, fresh air; a 'high resistance' route, the stairs. If I take the low resistance route, the 'current' (in this case my speed) will be far higher than if I take the high resistance route. In short, if I jump, I will hit the ground with a large speed!

    So all this "it's not voltage that kills you, it's current" talk is a bit misleading: you can't separate one from the other in this sort of system, in exactly the same way as you can't separate the speed that I mash into the ground after jumping from the height of the building.

    If you're really interested, the analogy is even deeper; gravitational and electric potential are the result of gradients in gravitational and electric fields respectively. In all such cases, it's differences that count (that's why you get Earth loops, not all Earths are the same).
     
  9. stuff

    stuff New Member

    The issue here is more to do with humans are not resisters.
    Ohms law only works for simple resisters, hence a larger voltage pushes more current thought a resister.

    You are a non-linear resister with a breakdown threshold.

    If you hold the terminals of a battery you do not get a shock, you are a insulator even if sitting in a swimming pool for up to about 12V. At higher voltages, how wet your hands are and what you are wearing on your feet, how you complete the circuit come into play.

    You touch a live terminal, and are standing on something wearing industrial boots. Well, assuming it is not raining then 110V is below the breakdown threshold of you making a circuit - nothing happens. Maybe at 240V you get a small jolt.

    You hold the earth in one hand and touch the live in the other hand. Pray ! You now form a circuit via you heart - in one arm, and out the other via your chest. Now you have a lower threshold potential, and the impedance (term for non-linear resistors with added inductance and capicitance), of you body comes down to how wet/damp your hands are. At 110V there may not be enough current flowing (high impedance at this voltage), to stop your heart. At 240 - a much higher current will flow and you are likely to be buggered if you have wet hands.

    When I say higher current, above about 0.001 Amps through your heart will likely kill you as it only needs to disrupt the heart beat electrical cycle. But the rest of your body can take a much bigger current - it only hurts, and can make you fly.

    The problem is that it comes down to the break down potential of you in a circuit - this depends on rain, footwear etc. The amount of current flowing through you need only be small to kill you, the impedance of your body rapidly drops with voltage increase, hence the idea of using 110V which can make a 10 fold difference in the current - remember you are a non-linear resistor.

    How you make the circuit is important. Make the current go in one hand and out the other is really bad, most TV engineers keep one hand in a pocket so they cannot stick one hand on an earth. If the current goes in one hand and out a foot then you can take a bigger hit. Water is the killer - it really reduces your impedance as it improves the contact with hands.

    OK- so what about 110V.
    Because a human is a non-linear resistor, above a threshold you rapidly conduct more current so 110V can prevent most deaths in dry conditions. Water is the killer, try putting a 9V pp3 battery on your tongue. Of course most builders work outside - so tough luck, if
    you have wet hands, or make a habit of holding that metal
    hand rail (earth) while drilling a hole into electrics you are stuffed.

    I have had a shock off 55V (arc welding ), but only minor cos it was a bit damp while working, and this was unusual. You will "always" get a shock at 240V. At 110V you chance of serviving is better cos you get hit less often.

    More likely this makes it easier to sell US tools in the UK.

    Duty cycle : At the rated current on the transformer you can use it for 20 minutes and then stop working for 10 minutes to let is cool down, then use it again. At lower currents you do not need to have such a long rest. Jokes aside.
     
  10. stuff

    stuff New Member

    Found a document (scientific journal) that gives an idea of the current within the body and effect. This is not the same as the current that the supply can provide - but the actual current that goes through the body - see the last post.

    1 microAmp = 0.000001 amps
    1 milliAmp = 0.001 amps

    100 microAmps : UL limit for hospital equipment not directly connect to a patient
    500 microAmps : UL limit for consumer products (Stray)
    700 microAmps : not perceptible.
    2 milliAmps : mild sensation
    5 milliAMps : Maximum harmless current in body
    6 milliAmps : Sensation of shock
    8 milliAmps : muscular control is not lost
    9 milliAmps : painful : individual can let go (DC)
    10 milliAmps : fatal body current is maintained
    20 milliAmps : Cannot let go : impedance breakdown - current can increase till death occurs (DC current)
    40 milliAmps : Breathing stops : often fatal
    90 milliAmps : Heart fibillation in 1 to 4 seconds
    800 MilliAmps : Heart stop immediately, can be restarted
    7 Amps : Severe burns, still not necessarily fatal unless vital organ burned

    Above apply for DC current where a person cannot let go above 20 milliamps due to grip. AC current is always for domestic & industrial because person is usually thrown clear due to muscle spasm.
     
  11. Dan E.

    Dan E. New Member

    Ouch! Grip - is that because current makes the nerves which give the muscles the command to contract fire, whereas AC creates alternate contract/release nerves to fire, resulting in spasm?
     
  12. BashOn

    BashOn New Member

    Good post...

    Ohm's law always holds, it just needs to recast as R=dV/dI for nonlinear resistors.

    There are two small points I now make

    1. You can't separate voltage and current for a given load, so the phrases "current kills" or "voltage kills" are exactly equivalent.

    2. The "current kills" convention has arisen because of the way these current stats are published. They basically say that if you experience THIS current then you will experience THIS effect. But that doesn't provide any practical guidance on safety because you can experience the same current from different voltages given different conditions (as you pointed out).

    Anwyay, in answer to the chippie, you are safer because:

    1. You are exposed to a lower potential difference
    2. At this lower pd your impedance might be higher so you are safer again.
     
  13. BashOn

    BashOn New Member

    Regarding nerve spasms, that's spot on, ac makes them spasm about more than dc, as they have a chance to relax each cycle.
    Unfortunately, nerves in the human body respond best at about 60Hz, which is a bit close to the mains for liking!
     
  14. stuff

    stuff New Member

    Your nerves work using electricity - (or rather a depolarisation potential that moves along a nerve at about 100meters/second - slow). Hence - it is a conductor - or rather a electric shock results in complete depolarisation of the nerve. Nerves tell muscles to contract using a "pulse modulation" the faster the pulses the harder the muscle contracts. Above a certain rate of pulses (or complete depolarisation (electric shock - and some toxins)), the muscles go into "tetnus" run flat out. Hence the name tetnus for a bacterium that causes lock-jaw or complete tetnus of you body and therefore death.

    Therefore DC electric shock tells all muscles to work flat out. But you have pairs of muscles (open fingers - close fingers) - and one is always stronger (close finger - strong grip). Ie you jump in the direction of the strongest muscle, but your cannot let go of what you have touched.

    AC shock - you do not get tetnus as you are applying a pulse modulation artificially. Muscles go into spasam which is not the same as tetnus, they just vibrate. You still cannot do anything, but you just go rigid. Ie you tend to fly away for AC but in a sort of rigid solid lump that then goes floppy, you neither grip or let go.
     
  15. forky

    forky New Member

    can you use 110v in the rain?
     
  16. dingbat

    dingbat New Member

    Well, this isn't correct at all.

    What you are <u>supposed</u> to do here is to deeply patronise the original poster, then proceed to post entirely incorrect and specious argument and conjecture.

    If these sorts of helpful shenanigans are going to continue I may have to take my trolling elsewhere! Harrumph!

    :)
     
  17. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    55VAC Centre tapped should have bene the answer for the original poster. .

    Eh up then Ding!
     
  18. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    . . .and at what level of voltage starts to creep in and do damage, hence the value...?

    ;)
     
  19. unphased

    unphased Screwfix Select

    Because the cable is yellow. Isn't this an old post, 2005?
     
  20. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    Your right Roy!

    Say, was this not a topic cast to us by an individual in the tea room..?
     

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