Too Modest

Rob Eagle:

Only 2 shocks in you life Mike?  I've lost count, it would top one hundred I guess.

 

Well, no, like you,  rather more than 2 shocks, just those 2 that I consider near fatal as opposed to just a warning nip and a few choice phrases uttered .  Not sure how many probably not 100, but  a few per decade.


Oddly not during intentional live working (of which I have done rather more than would probably be approved, such as odd light switch swaps with the lights on and so forth.)


Quite a few back EMF tingles from 28V relays and things, car ignition, oh  and the odd electric fence while camping, and back indoors dropping one TV tube having scuffed the EHT sucker point. That was very memorable from an instant of panic, but luckily it landed intact ,and worked ok after a re-focus.


Not so many mains shocks, though I have been bitten by a hot water tank external thermostat where the bimetallic element is exposed when it is not on the tank and it is the part of the moving live contact... nasty design that one, and wet  cables outdoors.

I'm sure many of us are the same.

Mike.

Helios:

The RCD is undoubtably one of the real biggest saftey improvements the electrical buisness has come up with , but sensative to every tingles , I dont think will work , in part because at the moment the national grid , is living with the reality of 40% (at times) of electricty generation comming from arrays of turbines that may vary output , and this causes /can cause variations in quality of supply , and the thought of main RCDs tripping out on voltage variations strikes me as being a lot more common if you have the wrong sort of sensativty in RCDs , certainly into the future of electricty generation at the moment.

Why would an RCD trip on a voltage variation?  There's nothing in them that measures the voltage.

Coby:
412-06-01 A residual current device shall not be used as the sole means of protection against direct contact.


Explain please.

An RCD used in conjunction with an 'earthed wrist strap' could be effectively used as "protection against direct contact".

mapj1:

Not as well documented that it isn't, unless you are considering shocks of 70-80 year duration, when I agree most will have died, but not specifically  from electrocution.

If you restrain yourself to considering external shocks, those applied through the skin, and ignore shocks during surgery and from deeply penetrating electrodes, as not being typical of domestic shocks, the following curves apply to humans and low frequency AC. The kink between half  a second and perhaps 0.2 of a second relates to the period of a human heart beat. Shocks that are of short duration c.f. a heartbeat are  can be higher magnitude without effect than those that span a heartbeat or more. although this refers to IEC 60479, that in turn is based on the works of Dalziel and others in the 20th century.

The 'Supply Voltage' is missing from the graph above!

Simon Barker:

Coby:

Andy Millar:

Hey, I've just had a brilliant idea, how about if we put some non-conductive material around live conductors, we could call it something like "insulation", as a primary protection, and then in case that fails put some sort of secondary protection in such as an RCD?


I'll wait for the Nobel prize to arrive in the post...

In which case; when your 'primary protection' fails, you are therefore reliant on 'a residual current device being used as the sole means of protection against direct contact'...that doesn't work!


I think you'll find that primary and secondary 'protection' is provided by 'two layers' of insulation.

Only if you're using double insulation.  Class I appliances may contain single insulated wires.  And there's always "reinforced" insulation, to confuse matters.


And the pedants will argue that most electrical cable doesn't have two laters of insulation - it's "insulated and sheathed" - even though the sheath is made of the same stuff af the insulation.

There are always at least two 'layers' of material between live conductors and the outside world, twin & earth, 'single insulated' singles within a conduit/trunking system, mineral insulated, armoured... the list goes on, then terminated inside a box of some description.

This is the most basic of our basic principles!

Helios:

mmm yes they are fault devices , must be my reading of a fault , it just says current , which iI guess is why they are called residual current device , and are for fault protection , so yes voltage variation in itself does not operate them , however in fault voltage may no doubt be going somewhere else , which i believe (as its no longer in the line conductor) and this lack of voltage in the line conductore is what causes the trip . A voltage drop (I think therefore) would also cause a trip ??

I think you maybe missing the point:

If the supply voltage associated with that graph is in truth for example 110v (55v to earth), then the graph may well be accurate.

But to claim (or try to give the impression in this example) that 300mA @ 230v, for a split second is "perceptible but causes no muscle reaction" is absolute nonsense and is therefore a very, very dangerous thing to upload to this thread.

The author of that graph has deliberately tried to mislead our readers and should, if anyone is hurt or even killed as a result, be prosecuted at some point in the future.