Perhaps one for Paul Skymes. My understanding of the 110v system is that the transformer is centre tapped to earth, so that theoretically, any shock should be limited to 55v, or thereabouts. I am sure Paul or someone else with more experience of 'wiggly amps' will be able to explain better.

Phil, Thank you for the vote of confidence! However, it is Skyrme, NOT Skymes! ;) Chris it is "volts that jolts" but "amps that kills". I have had many electric shocks over the years in the 10's of thousands of volts. If anyone "played with a Van De Graff generator at school, college or uni that could generate such voltages, however the current is minuscule. "static" shocks from the ignition systems of cars etc are also often thousands of volts but nano, micro or milli amps. Beware of the old magneto ignitions though they had some current behind the spark! My explanations may not be rigorously technically correct, however, I hope that I have "modified" them to explain the situation in a better manner. Ok as far as construction site supplies, Phil is correct the "normal" 110V a.c. on a "site" as supplied from a "yellow" transformer type supply is what is know in the trade as CTE, that is centre tapped earth. In this scenario you only achieve circa 55V to true earth potential, RMS that is. Below 50V a.c. is considered "harmless", think of it, if you put your hands across your car battery which is capable of delivering several hundred amps in seconds to drive the starter motor, however only at 12V d.c. you probably felt nothing, I know I did every time I did it! The joys of youth eh! OK, Ray is also correct that in the wrong situation a current at 50V a.c. can kill, certainly 0.5A can kill, a.c. or d.c. depending on how it is applied to the body. The reason we look at 30mA as additional protection against electric shock in general electrical installations is that "most" humans can withstand this level of current for half a 50Hz a.c. cycle IIRC. Thus it is a survivable shock, trust me it is! Please don't ask any more! If you are serious about getting "into" this you need to refer to IEC TR 60479 which is an IEC (International ElectroTechnical Committee) report on the physiological affects of electric shocks of varying currents and voltages on humans and animals. BTW 4 legged beasts are more susceptible as they have 4 points of contact and can have a larger voltage differential between their points of contact. Impedance is not the collective name for resistance and electric shock it is the collective name for the "resistance" to an a.c. electric current caused by resistive, capacitive, and inductive affects to the flow of said current. Chris, you DO have my mobile no. if you need it, already, else PM & I'll give it to you again as this may be a little difficult to explain on here, though I am willing to try, thus keep this discussion in the general body of info! Not sure at this stage what else to post? The risk of a shock from correctly maintained power tools even on a construction site is quite small really IMHO. Remember 240V a.c. supplies on construction sites should be protected by a 30mA RCD. These will then be supplying the 100V a.c. CTE step down transformers. Thus the energy at the 110V side will be limited to the 30mA incoming earth leakage current at 55V to earth, "not rigorous I know" also as the equipment is already in good condition the operator should not come into contact with the electrical supply should they?

Hi Paul, Very informative post, cheers. Can you shed any light what the maximum length of extension lead (110v from source to power tool) should be?.....If any. thanks Darach.

Phil, It could be, or it could also be the bit out of the Black, Grey Yellow, Blue or White bit! That depends on which phase and which version of the regs it was wired to. Also IF it is a machine it could be orange, violet,... I can't go on any more! Loosing the will to live! ;)

John, I did say it was not rigorously correct, I also did say that it depends on how the shock is applied to the body. As far as the Xenon headlamp drivers go yep, I'd not want to get in the way of one of those! I must also say that the last time I had an ignition shock was from an old Bosch Motronic engine management system, it still had a single coil and a ht distributor, things have move on since then and the new coil per plug or 2 systems I have not worked on, if I understand things correctly we are still in the 10’s of kV range around the 50 to 60kV mark? Though I’m not sure what the current is. Motronic systems used to be around 30kV IIRC. However, the current does need a return path, so unless you are "touching" the return path also then the current would need to flow through the vehicle tyres, not much chance of that really? Same as birds sitting on a 400kV overhead line they get no shock unless they contact an earthed part or another live line at a different potential, it does happen, especially with 2 birds during the mating season! Mind at 400kV and with the current flowing it can jump considerable gaps! Don't quite get why you should be at more risk from a 230V inverter as you have no earth path from the live supply side of the 230 back to the true general mass of earth, unless you become that also, as the inverter earth would not be connected back to the general mass of earth as a mains installation would be. This type of isolation is the one of the principles behind safety supplies, earth free zones etc. often used for testing and repair environments.

While the original scenario was of a low voltage (centre tapped 110V), it also asked the question what are the different scenarios for fatal shocks. Here are some: If continued for a long time, a current in excess of the 'let-go' current can produce collapse, unconsciousness and death. Currents flowing through the chest, head or the centres controlling respiration can cause it to cease. When the current is interrupted, respiration may not start spontaneously within the timeframe required for life to continue (to artificial respiration here is crucial) Ventricular fibrillation is caused by moderately small cuurents of moderate voltage which over-stimulate the heart. If this is not halted, death can follow in a few minutes. Heart standstill can be caused by relatively high currents at moderate to high voltages. Relatively high currents can cause fatal damage to the central nervous system. Relatively high currents can cause burns to internal organs. If high enough to raise the temperature of the body death is instant. Electricity kills!

The answer to this is yes, and no - it comes out and it goes back. Electrons do wander around the electric circuit (not particularly fast, actually). They don't escape from the wires. Have a look here http://en.wikipedia.org/wiki/Drift_velocity

Whether or not the shock is fatal is not the only consideration. The shock causes involuntary movements (don't forget that our muscles are activated by electrical impulses), and this can cause fatal accidents as well. I know someone who fell off a ladder as a result of an electric shock.

You beat me to it, Jane, I was about to make exactly that point! Something else to consider with 110v equipment is that it's best to have the transformer (yellow box) as close as possible to the source of 240v, then make the "long run" with 110v. Better still, of course, is to avoid too long a run altogether. A friend of mine tells the story of someone who couldn't find a socket to plug in his 110v "Henry" style vacuum cleaner. So he cut the yellow plug off, fitted a blue one and plugged it in to 240. Apparently it worked very well, for a very short time...

Jane your final sentence is the crux of the matter. There is a batch of Principal Contractors that are reluctant to follow the HSE advice, they want to test annually instead, and use cost as the reason. The Client is agreeing with them because he does not want the cost passed on to him. Your wording "would like to see" is the problem. we cannot instruct them as it is only guidance. Almost every time I see an item tested annually and near the end of that year, there is a defect or some damage with the equipment.