Post by Johny B GoodOn Thu, 18 Sep 2014 23:23:46 +0100, The Natural Philosopher
Post by The Natural PhilosopherPost by WindmillPost by Johny B GoodPost by DerbyBornPost by Graham.Post by Adam FunkPost by Mike BarnesAAMOI would it be practical/permissible to connect a neon between
switched live and earth?
That would be a very naughty thing to do, & would trip RCDs.
It would indeed be naughty, but it won't trip an RCD (DAMHIKT)
At my last house I once wired a neon between a room stat and earth to show
me when it was calling for heat. The house didn't have RCD. I often wonder
if the new owner ever had a problem and couldn't track it down.
With just one such neon indicator lamp, I doubt the 1 or 2 mA 'earth
leakage' would be a problem (OTOH, 20 or so, _might_ start to cause
trouble).
AIR the typical mains neon is rated at about 1/10 of a watt, so it
would be more like 0.4 mA.
ISTR 1-2Mohm series on those, so 100uA??
AFAIR, the neon screwdrivers use a 220K safety / current limiting
resistor in the handle[1]. I'm not entirely sure but I suspect the
same 220k value is used by neon indicator lamps as well which, with a
60v running voltage on the neon lamp itself, suggests a current draw
approximating 0.8mA rms. I can imagine the use of a 100K resistor to
achieve a higher brightness in such mains indicator lamps, suggesting
a current draw around the 1.8mA mark.
If the neon lamp rating of 100mW is for the whole luminary (neon plus
ballast resistor) then even a 220K resistor is going to exceed this
(177mW) so I expect that this is for the bare neon itself (60v by
1.67mA). I think my 'guestimate' of 1 or 2 mA earth leakage is close
enough to determine the level of risk of unwanted RCD trips.
Almost nil, I would think, in the UK environment. Overhead powerlines
in N. America are said to show occasional 10 KV spikes during
severe thunderstorms, and experience long ago showed fairly common
failure of electronic equipment in such circumstances, so that might
cause RCD aka GFI trips.
Someone said that RCDs incorporated some electronics, presumably to
amplify any current differences between L and N sufficiently to trip a
latching relay, so I suppose that might cause sensitivity to spikes if
response was too fast.
Post by Johny B Good[1] Neon lamps typically strike at about 90v and 'burn' at 60v. The
finger contact on the end of a mains testing neon screwdriver provides
ample leakage/capacitive coupling to earth to achieve both
requirements with ease.
Which has always surprised me, not that I've ever tried to measure my
capacitance to earth.
Post by Johny B GoodThe neon lamp in the screwdriver handle is small enough to remain
struck over a long enough portion of each half cycle of mains to still
give sufficient indication with currents as low as a few tens of
micro-amps. The 220K ballast resistor is large enough in value to let
it do double duty as a safety resistor as well.
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