Re: Control panels safety questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

One convenient way to do this is to use a relay/contactor for main power.
You set it up as a latching relay with a "momentary push on" circuit, and a
"momentary push off" circuit.  The off circuit is a normally closed button
in series with the coil and the latching contacts.  The push on circuit is
across the latching contacts (Normally open).  Then, your continuous run
switch is set up as a double pole switch. One pole (call it normally open)
is set up in series with the coil power (after the big contactor).  The
other pole, opposite sense (normally closed) is in series with the "turn on"
switch.

With this setup, you can't turn on the main contactor unless the continuous
run switch is in the OFF position.

For what it's worth, this kind of circuit is also nice for interlocks.  The
normally closed interlock circuit is also in series with the contactor coil.
A break in the interlock (i.e. any switch goes open) causes the main
contactor to drop out, requiring another press of the "turn on" push button.
Your key switch could be in the interlock loop.

When working out all those relays, one should give some consideration to
failure modes.  Think about what happens if a switch fails, a wire falls
off, etc., and make sure it "fails safe", i.e. power off.

Suitable contactors turn up surplus all the time at reasonable (<$10)
prices.  New, they're going to be pretty pricey.  A secondary advantage is
you get that nice satisfying "thunk" when you turn on main power.



----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, October 11, 2001 5:14 PM
Subject: Control panels safety questions


> Original poster: "Jonathan Peakall by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jpeakall-at-mcn-dot-org>
>
> Hi All,
>
> I am constructing the control panel for my new TC, and of course, I have
> some questions:
>
> I want to have two run modes on the controller. One will be a momentary
> "dead man" style switch, used for demo purposes. The other mode will be a
> continous run mode, so I can do/adjust/whatever while the coil is running,
> or if the coil is to be run for periods longer than one wants to hold the
> momentary switch. I plan to have a key switch for the main power, and a
> second key switch for the continous mode switch. Want I want to do is make
> it so that if the continous mode switch is accidentally left on that the
> main power switch will not operate, to ensure against operator error. What
> would be a good way to achieve this?
>
> Thanks,
>
> Jonathan Peakall
>
>
>