engineer

tim   .

                                                      


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.   hunkin

                                                         


cartoonist

 

SWITCHES

 

THE COMMENTS

I enjoy reading all the comments but apologize that there are so many I canít answer individually. Iím delighted that my new videos have been so enthusiastically received, which is reflected in the majority of the comments. The videos arenít perfect though so below each video Iíve compiled the comments about things I omitted or people thought were misleading. A few comments added wonderful additional information so Iíve included them as well. 

Iím 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 Ďthese are isolators, not breakersí. 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 havenít 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 Iím 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 canít resist adding this related tale. At the end of the 19th century Edison was championing his DC supply system against Westinghouseís AC system. Both did experiments electrocuting animals to try to prove their system was the safest. In states that used Westinghouseís system for the electric chair, Edison claimed the prisoners had been ĎWestinghousedí. 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.

 

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