Exposed Copper in a Public Place


Old 05-31-14, 06:14 PM
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Exposed Copper in a Public Place

Hi guys,

I know very little about electricity, so please excuse my poor vocabulary.

The museum I work at has an "Edison Section", where kids/adults can operate a flood light, create a pretty decent spark with a "Jacob's Ladder" (it is a spark gap), and play with one of those Plasma Ball things.

The entire wall where all of these things are plugged in is operated by a huge copper knife switch mounted on the wall within arm's reach, much like this one. After an incident involving the Jacob's Ladder, it occurred to me that a lot of electricity is probably flowing through this switch.

Are museum visitors going to be electrocuted if they touch the copper on this switch? Could it create a dangerous spark or arc flash?

If it will just unpleasantly shock people then I can put up a sign, but in general I don't strive to kill our guests.

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Old 05-31-14, 06:57 PM
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If it were my place I would have everything energized behind glass. Not worth the risk.
Old 05-31-14, 09:41 PM
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When wired properly the terminals on the right of that switch are always live. I would not want that kind of liability by leaving that out in the open.
Old 05-31-14, 10:00 PM
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Some of the exposed knife switch parts, if energized with 120 volts, are hazardous to touch whether the switch is open or closed.

Are you sure that the knife switch controls more than a low voltage circuit?

A low voltage circuit could in turn control a relay (contactor) that switches "house current" on and off for demonstration lights, fans, etc.

If you pull the switch open slowly and it was carrying, say, 200 watts at 120 volts, there will be a very noticeable spark (arc). If the switch were carrying about 25 watts at 12 volts (about 2 amperes) the spark could still be seen as the switch contacts open but would be quite small.

I find it hard to believe that a knife switch with exposed live contacts would be displayed within reach of people. Another possibiility is that the knife contacts are completely dead and there is a hidden plunger or lever activated physically by the knife switch and that is connected to a real switch behind the wall or panel.

Last edited by AllanJ; 05-31-14 at 10:33 PM.
Old 05-31-14, 11:06 PM
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Allen beat me to it. I was about to post what he posted.

Last edited by ray2047; 06-01-14 at 09:58 AM.
Old 06-01-14, 07:50 AM
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Building on Allen's post, using a multi-meter and reading between the terminals of the knife switch will tell you the voltage present at the switch. Start with setting the meter 240 volts AC and work your way down. Anything more the 24 volts, or so, should not be exposed.

Who designed/built this display?
Old 06-02-14, 01:18 PM
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Ah thanks, Allan. There is no spark when it is thrown, though I usually don't throw it slowly. It didn't occur to me that it might simply be attached to a lever.

Unfortunately the man that built the display passed away before I even started working there, so I will have to use a voltage meter as you guys suggested. He worked at an Edison power plant, so I assume he knew what he was doing, but now I know what to do to double check that.

Thanks again; I will update you guys.

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