The Mudcat Café TM
Thread #147462 Message #3419026
Posted By: JohnInKansas
13-Oct-12 - 05:12 AM
Thread Name: Tech: electrical shock danger to be aware of
Subject: RE: Tech: electrical shock danger to be aware of
Different numbers required to kill someone are subject to a lot of argument; but conservative estimates speak more of what can kill you and attribute survivors at the levels they claim to "dumb luck."
Some sources say that 10 mA is "potentially dangerous" in the right path through you, but most such reports are talking about DC currents rather than AC. For "power frequencies" in the range of 50 - 60 Hz somewhat higher currents may be (there are arguments) non-lethal, but may cause very serious burns with damage to other innards that it's a good thing to have in working order, even when people survive.
Building standards are necessarily based on a concept of "acceptable risk" but are usually fairly conservative.
Just for talkin' purposes, in the US the RCD limits are seldom incorporated by that name in safety standards, but similar(?) results are imposed by requirements for "Ground Fault Interrupter" (commonly called GFI) outlets in certain kinds of circuits. Usual limits are somewhere in the range of about 1 mA to 5 mA difference between the goes-in and goes-out current for the two primary wires, with some styles additionally breaking contact based on the "leakage to ground" on the usual third wire (possibly(?) closer to the RCD spec, but I haven't pulled the UK std for comparison).
Most US codes require GFI protection on all outdoor outlets (sometimes with exceptions for "low voltages" - from 12VDC to about 70VAC? depending on where you are), and usually on all outlets within some distance (6 foot is about the usual) of any sinks, potties, etc, connected to water lines; but the specifics of the codes in any particular place are subject to "local interpretation" and lots of places allow "grandfathering" of exisitng wiring installed before anyone knew much of anything about electrical hazards. In some old stuff, circuits disconnected can stay "live" via "ground loops" with adjacent circuits - a VERY BIG HAZARD for "modern electricians" since recent codes pretty much prevent them, so even pros aren't familiar with the idea - and hazards.
"Nail protections" are required in most codes, but the specs vary quite a bit on this.
Handbooks on "Repairing Old House Wiring" can give you nightmares with some of the stories about things done - and still in use - before "modern" standards appeared. In some cases you can keep using them until something burns up, but most of the really scary things have to be replaced, often very extensively, before an electrician who wants to keep his/her license can do much of any sort of "repairs." The books give some advice on how to not get killed, but mostly end up with "rip it out and replace it all" for the very old stuff. (How to rip it out without getting killed is a big part of some of the references, sometimes it ain't easy.)
The accident at the beginning here is almost certainly related to the electrolytic capacitors in HV DC stuff. It's poorly covered in many elementary (e.g. High School?) classes, and I've known graduate electrical engineers who "never heard of it;" but it's something that IT IS NECESSARY to know about - at least enough to be a little scared - if you even think about opening the boxes inside the boxes.
No matter how well you know it, it's best to remain a little bit nervous. You may live longer with a little bit of fear, no matter how "good" (professional?) you are.
John