I enjoy reading all the comments but apologize that
there are so many I can’t answer individually. I’m delighted that my
new videos have been so enthusiastically received, which is reflected in
the majority of the comments. The videos aren’t perfect though so below
each video I’ve compiled the comments about things I omitted or people
thought were misleading. A few comments added wonderful additional
information so I’ve included them as well.
I’m delighted that many people commented on the giant
microswitch because Rex made it 30 years ago for our Secret Life of the
Lift film. Far too good to throw out its been waiting in my stores for
this moment of glory. Its made of layers of MDF, cut to shape on a bandsaw.
There were several explanations why electricity
sub-station switches move so slowly. First that ‘these are
isolators, not breakers’. The power is normally
switched by the breakers, which are totally enclosed and do move fast
using compressed sulphur Hexafluoride gas to quench the arc. Isolators are
normally used when the supply is turned off, though obviously as in the
video clip there are exceptions. Though Nick Abbot wrote: The high voltage
switches move slowly to prevent thunder. If they moved at high speed,
there would still be arcing but it would be a loud cracking sound, like
thunder. Plus, the slow moving switches increase the power to the line
slower, using the air gap as a resistor. Finally, these switches often
have 'horns' to arc across. Only when touching and the arc is extinguished
do the load carrying contacts meet.
Scamin Sam rightly told I got the suppressor circuit
wrong. I haven’t used one for a while and forgot that the resistor goes
in parallel with the capacitor, not in series. The idea is that resistor
drains the charge out of the capacitor so its fully discharged when the
contact is made or broken. I have used them correctly, but the
contacts in switches and relays still arced when switching DC motors,
though possibly not quite so much.
A lot of people wrote to explain my mystery mercury wall
switches. They are explosion proof because the arc is totally enclosed in
the glass enclosure. One person had a mercury delay switch, just a tiny
tube to restrict the flow form one contact to the other. A retired surgeon
told me that operating theatres always have explosion proof switches
because for long operations an inflammable anaesthetic is used and that
there have been rare cases of patients exploding.
Linux Lovah asked why are DC ratings for switches so much lower than
their AC rating (on the toggle switch I showed in the video the AC rating
was 15 amps and the DC only 1 amp). Such a good question. I have a
possible explanation but do write if I’m wrong. As AC has zero voltage
100 times a second, or 120 for countries with 60hz supplies like the US,
this probably encourages the arc to extinguish. I can’t resist adding
this related tale. At the end of the 19th century Edison was
championing his DC supply system against Westinghouse’s AC system. Both
did experiments electrocuting animals to try to prove their system was the
safest. In states that used Westinghouse’s system for the electric
chair, Edison claimed the prisoners had been ‘Westinghoused’. But with
the benefit of hindsight AC was safer because the reversal of the current
has the involuntary effect on muscles of repelling and releasing grip,
whereas DC just tightens the grip.
