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what kills? The voltage the current, does the frequency has anything to do with it?
what kills?
- Thread starter xesemesa666
- Start date
lots of things
route through the body is very important (if you ever do have to work live keeping one hand in your pocket is a bloody good idea)
actual current through the body is crucial
voltage is important only because the higher the voltage the more current it can push through your body
duration is very important ie if something has a kilovolt or so of static and it discharges through your body the duration of the current will be too miniscule to do you any harm. rcds used for supplementry protection against direct contact also rely on this fact.
i belive frequecny has some effect too though i'm not exactly sure what. I do know that high power RF equipment will give you nasty burns without ever giving you a shock to warn you!
route through the body is very important (if you ever do have to work live keeping one hand in your pocket is a bloody good idea)
actual current through the body is crucial
voltage is important only because the higher the voltage the more current it can push through your body
duration is very important ie if something has a kilovolt or so of static and it discharges through your body the duration of the current will be too miniscule to do you any harm. rcds used for supplementry protection against direct contact also rely on this fact.
i belive frequecny has some effect too though i'm not exactly sure what. I do know that high power RF equipment will give you nasty burns without ever giving you a shock to warn you!
A lecturer at college used to tell us 'its the amps that clamps and the volts that jolts'
Another useful tip when working with live, is never touch a cable with your open hand first, always with the back of your hand, at least then the muscle contraction will push the arm away from the source rather than clamping you to it.
Another useful tip when working with live, is never touch a cable with your open hand first, always with the back of your hand, at least then the muscle contraction will push the arm away from the source rather than clamping you to it.
Plugwash and nstreet are both correct. It is indeed current that kills but only if it goes through either the heart or the brain. A little thought will tell you that it's jolly difficult to accidently shove a lethal electric current through your brain. The first attemps at a functioning electric chair proved just how difficult it could be. On the other hand, the heart lies nicely on the current path from hand to hand or hand to foot. Consequently, most deaths from (accidental) electric shock happen this way.
A ball park figure for a lethal current is 50 mA but don't assume from this that you will survive 40 mA! The acceptable limit for leakage current from patient connected medical equipment is 50 MICROAMPS.
Frequency does come into it but only when you get up to RF. It's all down to the skin effect (literally in this case) where RF currents flow over the surface of a conductor. Result; your skin burns off but your heart carries on regardless.
Frequency matters in another sense too. A small DC voltage - even if it's too small to notice - will cause damage if applied for a prolonged period. That's because your body doesn't conduct current in the same way as wires do. All current flow is by movement of ions and this means chemical change. (Think electrolysis. Water splits into hydrogen and oxygen.) With AC the chemical changes are (mostly) reversed on alternate half cycles.
PS: Nstreet's back of the hand trick is correct if you know the wire is live but for testing suspicious metal I prefer a neon driver!
A ball park figure for a lethal current is 50 mA but don't assume from this that you will survive 40 mA! The acceptable limit for leakage current from patient connected medical equipment is 50 MICROAMPS.
Frequency does come into it but only when you get up to RF. It's all down to the skin effect (literally in this case) where RF currents flow over the surface of a conductor. Result; your skin burns off but your heart carries on regardless.
Frequency matters in another sense too. A small DC voltage - even if it's too small to notice - will cause damage if applied for a prolonged period. That's because your body doesn't conduct current in the same way as wires do. All current flow is by movement of ions and this means chemical change. (Think electrolysis. Water splits into hydrogen and oxygen.) With AC the chemical changes are (mostly) reversed on alternate half cycles.
PS: Nstreet's back of the hand trick is correct if you know the wire is live but for testing suspicious metal I prefer a neon driver!
B
breezer
i disagree on quantity of current, i believe its something like 10mA or less, but to get that amount of current to flow through your body the voltage has to be very high (to ovecome the resistance) or you have to be wet (which also lowers the resistance)
the bck of hand "trick" is what firemen do when going through a premises they can not see in. also you should not touch anythin you even think could be live.
as for neon screwdrivers they belong in the bin.
the bck of hand "trick" is what firemen do when going through a premises they can not see in. also you should not touch anythin you even think could be live.
as for neon screwdrivers they belong in the bin.
oh they will tell you if mains voltage is there or not
what they won't tell you is if that voltage is from an extremely high impedance coupling (generally capactitive) or something low enough impedance to actually give you a shock
if you do decide to use one however you absoloutely must treat it as a safety critical item. just like with an anti static wrist strap you should either test it before every use or keep it under your control at all times (remember the story of an apprentice that electricuted his master by dropping one in a tank of water and wiping it off?)
i seem to remember a lot of sparks have been put off the things by thier masters giving them modified ones purely to try and convince them the things suck.
what they won't tell you is if that voltage is from an extremely high impedance coupling (generally capactitive) or something low enough impedance to actually give you a shock
if you do decide to use one however you absoloutely must treat it as a safety critical item. just like with an anti static wrist strap you should either test it before every use or keep it under your control at all times (remember the story of an apprentice that electricuted his master by dropping one in a tank of water and wiping it off?)
i seem to remember a lot of sparks have been put off the things by thier masters giving them modified ones purely to try and convince them the things suck.
kinda wandering offtopic but
water isn't all that low resistance and you have a resistance quite similar to water especially when you are wet (skin resistance drops dramatically when wet).
as for if it would kill you that depends mainly on how far away you are from it and if it was dissconnected rapidly (ie by a rcd)
you can calculate the current in the water if you know where the current carrying conductors are and if you assume your body makes negligable difference to the current patterns you could therefore calculate how far away you have to be to avoid a dangerous shock.
a metal bath (especailly an earthed one) could make this issue much worse as it would act as an electrode chaning the current patters significantly
water isn't all that low resistance and you have a resistance quite similar to water especially when you are wet (skin resistance drops dramatically when wet).
as for if it would kill you that depends mainly on how far away you are from it and if it was dissconnected rapidly (ie by a rcd)
you can calculate the current in the water if you know where the current carrying conductors are and if you assume your body makes negligable difference to the current patterns you could therefore calculate how far away you have to be to avoid a dangerous shock.
a metal bath (especailly an earthed one) could make this issue much worse as it would act as an electrode chaning the current patters significantly
Actually falling into live salty water can be less dangerous than falling into pure water, although the current is higher, the water is the preferred path. However, don't believe everything you see in films, particularly re electrocution. Its a lot less glamorous than it looks. Usually, if something live drops into a pool of water, at least if the object is earthed, very little current flows in the water outside the appliance, and usually something disconnects the supply fairly rapidly, particularly if RCDs are involved.
More dangerous are wet bodies handling otherwise dry metalwork - there are more electrocutions is summer, and hot countries (scantier clothes, more sweat) than in winter, as a direct consequence of this.
PS old Radio jingoes - its the volts that jolts, the mils that kills.
As already mentioned the exact killing current varies with person, current path etc. One often overlooked in europe is foot to foot - more of a problem in australia apparently.
http://www.ballengearry.com.au/papers/Step_and_Touch_Voltage_update_for_2004_090804.pdf.
http://www.pulp.tc/BiksonMSafeVoltageReview.pdf has a pretty comprehensive summary of the mechanisms involved. Not light reading though.
More dangerous are wet bodies handling otherwise dry metalwork - there are more electrocutions is summer, and hot countries (scantier clothes, more sweat) than in winter, as a direct consequence of this.
PS old Radio jingoes - its the volts that jolts, the mils that kills.
As already mentioned the exact killing current varies with person, current path etc. One often overlooked in europe is foot to foot - more of a problem in australia apparently.
http://www.ballengearry.com.au/papers/Step_and_Touch_Voltage_update_for_2004_090804.pdf.
http://www.pulp.tc/BiksonMSafeVoltageReview.pdf has a pretty comprehensive summary of the mechanisms involved. Not light reading though.
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foot to foot....yeah.
A local woman killed herself a few years ago by mowing the lawn in bare feet...the current travelled up one leg, through the heart and back down the other leg.
A local woman killed herself a few years ago by mowing the lawn in bare feet...the current travelled up one leg, through the heart and back down the other leg.
B
Big_Spark
xesemesa666 said:
A Really Hungry Bear?
What strange anatomy she had...securespark said:
Is that why they say that the route to a woman's ***** is through her heart?
along with what has been said about voltage and current having a bearing frequency does make a difference, the body muscles especially the heart muscles will react to lower frequencies and the 50 hz mark is within this lethal range of stopping the heart muscle working, one example of this is the regulations that govern high voltage neon especially when used in a studio environment the old method using transformers had inherent safety factors and lots of regs covered their use amongst them the use of barriers around the area where the neon was used. nowadays it is driven by high frequency high voltage invertors typically 3 kv at 3-4Khz this high frequency has made the use of neons safer indoors (the transformer type is still used outdoors, height restrictions and isolation rules come into effect then) so now barriers are not required if the HT leads are inadvertantly touched you may receive a very nasty tingle and possibly a burn ( I remember the neon salesman at the time using a very crude method of holding the HT lead in his bare fingers albeit very tightly to show no muscular reaction 
) but the muscles do not get contorted or affected so therefore not lethal. Unfortunately the high frequencies can cause problem in themselves if not installed properly but this tends to be operation issues rather than safety issues.
) but the muscles do not get contorted or affected so therefore not lethal. Unfortunately the high frequencies can cause problem in themselves if not installed properly but this tends to be operation issues rather than safety issues.
