24 Nov
2014
24 Nov
'14
12:52 a.m.
On 2014-Nov-23, at 5:46 PM, Pete Turnbull wrote:
On 23/11/2014 22:12, Brent Hilpert wrote:
Well, I'm willing to entertain counter-intuitive notions but I'm really not
convinced.
Some reading suggests going beyond the current level at which one loses intentional muscle
control, while it will or can result in involuntary muscle contractions, those
contractions may be more likely to result in hanging on rather than getting kicked off,
and you're more likely to exceed that current level with the higher voltage.
Getting 'thrown off' also has it's own set of associated risks.
But isn't there an internal logic problem in there to begin with? I thought the point
of the step-down/isolation transformer for portable tools was to reduce the 240V to a
safer level (or is just to make it easier for manufacturers, to only have to manufacture
110/120V tools worldwide?, in which case one would think the onus would be on NA to
change). Those transformers aren't required in NA because we're only operating at
120V.
Once you require the transformers to get down from 240V, you might as well go to the
center-tapped 55-0-55 technique at a small increase in complexity over 0-110 for a further
safety improvement.
The benefit of the North American split-phase
system vs EU/Britain is
you have the energy & copper-efficiency of 240V available for heavy
appliances, but you have the safety factor throughout the house of
never having more than 120V between you and earth.
That's not as much of a safety factor as many people think, indeed possibly not at
all. It's true that 230V will give you more of a jolt than 120V, and more so because
human body resistance drops slightly at higher voltages. Nevertheless, there's a
strong argument that a 230V jolt is more likely to throw you off the conductor whereas
120V may be more likely to make you latch onto it. It depends how you touch it,
obviously, but evidence suggests that (EHT excepted) the region around 100V-130V is about
the most dangerous. I recall reading a discussion of that as part of the European
harmonisation of voltages, in connection with 110V for site equipment. Evidently that was
in part why the idea of one live and one neutral with a single simple MCB or RCD was
rejected in favour of the split 55-0-55 system (proposed by the UK).