John Larkin wrote:
On Wed, 21 Jan 2009 07:17:57 -0800, "Tube Audio"
wrote:
Hello
I have an Industrial Installation of a string of about 20 SPST Emergency
Stop Switches in series, They are wired Normally Closed. 24VDC is connected
to the string and the switches will interrupt the 24V when pressed in and
Emergency.
The wire is 18 GA stranded and the total length of the string is about 800
feet.
The wire runs in conduit.
I believe I have an intermittent short to the conduit, usually the short
occurs just momentarily so troubleshooting is difficult.
I was thinking of Hi Potting the Switch String. I was going to disconnect
each end of the string and Hi Pot the cable run along with the installed E
Stop Switches. To verify if some part of the conductor is very close to the
conduit.
I believe the wire is rated at around 600V.
The question is what Voltage should I set the Hi Pot tester to and why. The
other end of the Hi Pot tester will connect to the conduit.
Crank it up until something breaks. The objective here is, if
possible, to turn the intermittent into a hard failure, so you can
find it and fix it. I'd use a neon sign transformer, but you may not
be allowed to do that.
Have you inspected all the switches? A wire short inside a conduit is
much less likely than a problen at or in a junction box.
John
Don't do that. If the fault is intermittant in nature but does not occur
during the test, you'll just burn out what was perfectly good cable. Or
other devices in the circuit.
Also, use a tester designed for the application. Rent one if necessary.
If you damage the e-stop system in such a way that it fails during a
subsequent emergency, the sh*t will come back on you.
Crank up the voltage to a level compatible with the wire and device
insulation ratings. If that doesn't reveal the failure, start shaking
components and operating each switch (with insulating gloves!). Heck,
try the shake test with a continuity tester first before fiddling with
the hi-pot.
Here's one link:
http://www.cirris.com/testing/guidel...t_testing.html
--
Paul Hovnanian
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