Here's my usual speech in regards to working on live circuits:
1: If you are sweating really good, a system pushing current at 40 Volts will cause a harsh shock - harsh enough to make a person lose their footing and fall off a ladder.
If the Voltage comes closer to 50 Volts, this can indeed be lethal to a person when high salt / mineral contents are perspired [sweated out]. Body Resistance may drop as low as 1K ohm when sweating badly.
2: a shock from 120 VAC is just as hazardous as if it was from 208, 230 277 or 480 VAC. The Voltage is high enough at 120 VAC to push currents of 5ma and higher - without a conductive perspiration situation [sweating hard]. As mentioned by others, the lower voltage of 120 VAC can at times "Sneak Up On You" and place a person into a progressively increasing shock situation. The higher levels of current cause the muscles in the hands to grab even tighter - hence the drop in body Resistance will follow.
3: Not only is there the hazard of shocks, but also the blinding flashes that accompany faults.
4: And most important - Some faults [more like a lot of faults] can easily exceed the max. rating of branch breakers - which will then cause the Main Circuit Breaker for that panel to trip [unless the fault has exceeded it's rating too!]. This REALLY gets clients mad! If the fault exceeds the panel's main breaker's rating, it too will not be able to trip.
This leaves the option for calling the Fire Department
In the last 4 weeks, I have been asked by five separate Electricians, why did a certain ground fault [or in 2 cases a L-L fault] trip the main breaker instead of the branch breaker.
Mostly the breakers in question were "No-Trip Zinsco" F frames [the typical ones], so there's one reason
A few thought that the locking clip [prevents someone from turning off dedicated circuits - such as file servers...] was keeping the handle from moving and therefore could not trip during the fault.
It's difficult to explain to some people that the handle does not actuate the on/off or trip actions, but that it resets or opens the contacts through cams and not a direct link.
It's also difficult to explain why the really sparky ground faults will trip branch breakers, but the ones with minimal sparks [but lots of noise] will not trip a branch breaker.
This one takes about 35 days to cover
Well, I feel better now that all this ranting is done!
Moral to story: It takes just one mistake to ruin your day / week / life [or someone else's day / week / life].
Scott SET