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.

Thanks

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

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
------------------------------------------------------------------
Opinions stated herein are the sole property of the author. Standard
disclaimers apply. All rights reserved. For external use only. If
irritation, rash or swelling occurs, discontinue use immediately
and consult a physician. Void where prohibited.

In article , Tube Audio
wrote:


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.

Thanks


Typical control cable is 600V. When we used to test the cabling for new
control circuits (I was with a large electrical utility), we always
tested them at 1000VDC. Usually resistance testers ('Meggers') are DC
and 600VAC is pretty close to 1000V peak, so that was our logic.
Certainly a 600V control cable should accept 500VDC and still show
several Megohms, even if it is very old and in in a poor environment.
New cable would be 200+.

--
Jim

"Tube Audio" wrote in message
...
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.

Thanks


What makes you think it is a short ?

800M is a long run at 24V. If the circuit is holding a contactor in, small
voltage fluctuations will drop it out.

George.

"George Vest" wrote:

The wire is 18 GA stranded and the total length of the string is about 800
feet.

What makes you think it is a short ?

800M is a long run at 24V. If the circuit is holding a contactor in, small
voltage fluctuations will drop it out.

800m of 18 awg is 17 or 18 ohms. Marginal for a moderately large contactor
maybe but if it was so marginal that voltage fluctuations drop it out he
would never pull it in in the first place.

--

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.

Thanks

After reading the posts I would like to know the current and voltage
ratings of the contactor coil.

18 guage is small for 800 feet with the usual current requirements for
a contactor. With #18 wire and 24VDC you can sink only 1.33 Amps. And
for practical purposes your contactor current is limited to .13 amps.
More than this the voltage across ther contactor coil will be too low
for reliable operation. Not enough current for the contactor.

If you can't replace the wire with 16 or 14 use a transistor as a
driver/switch at the contactor to make the 18 wire a signal conductor
instead of a switch leg. Use the 24VDC for Vcc on the transistor.
There are also Solid State Switch packages for this.

Most technicians should be able to wire this up with no problems with
the switches, connectors and wire. Your switches do need to be
selected for this. Are your switches rated for DC?

Your momentary "short" in this case may be a result of the component
activity consisting of the L of the coil and the RC of the wire and
the conduit. Possibly caused by spiking from an external source. A
diode across the contactor coil will minimize this. 800 feet is a long
way for #18 wire.

My best philosophy at 12:23 AM!

Bob AZ

On Thu, 22 Jan 2009 03:01:31 +0000, nospam
wrote:

"George Vest" wrote:

The wire is 18 GA stranded and the total length of the string is about
800 feet.
^^^ ^^^^


What makes you think it is a short ?

800M is a long run at 24V. If the circuit is holding a contactor in, small
voltage fluctuations will drop it out.

800m of 18 awg is 17 or 18 ohms. Marginal for a moderately large contactor
maybe but if it was so marginal that voltage fluctuations drop it out he
would never pull it in in the first place.

800 FEET, not meters...

5.21 ohms total loop resistance. A drop of about 1/2 volts if the coil
drew 100 MA, which it may. Not significant.

On Jan 21, 9:17*am, "Tube Audio" wrote:
I believe I have an intermittent short to the conduit, usually the short
occurs just momentarily so troubleshooting is difficult.

Get one of those frickin-hot pulse generators from John Larkin and
scope the result. Where it arcs over (from the high voltage impulse)
will reflect voltage back. ;-)

Tim

On Jan 21, 9:17 am, "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.

Thanks

Before i would start chasing down a possible short, I would verify
that my coil is getting enough current to stay in, and that my 24VDC
supply is not overloaded. Do you have other loads on the DC supply?
If you have other loads on this supply, especially intermittent ones,
that could be enough to cause the coil to drop out.
1. I would force all other loads on the DC supply to be on, and then
see how the coil in question behaves.
2. If the supply is ruled out, then I would pull the cable off of the
coil and measure the resistance of the cable, including all switches
(bypass the supply). If the supply voltage divided by the system
resistance is marginal, that would be a good place for a problem.
3. If this isn't the problem... 800 feet of conduit will expand in the
sun. And if you have a problem which shows up or goes away with
temperature, the expanding conduit might be pulling on a loose
connection.
4. Then I would check for the short.

Good luck,
Andy

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.

Thanks




Having read all the replies and good suggestions, what ever
the fault is, intermittent ones are the hardest to find.

Assuming all the cable is the same age, the cable is in
conduit, why not simply buy a reel of cable and replace all
of it, not more than a couple of hours work to pull in a new
cable with the old one as a draw wire at the lengths you
describe and then you'd have solved the problem, made your
insurance company happy that you have serviced and renewed
the safety system and saved a lot of time and effort. Not to
mention that in this condition you really shouldn't be using
the equipment the safety system is there to isolate, H&S
will be all over you if they knew - they wont care that the
fault actually is rendering the system safe, they will only
care that you have a fault with it.

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.

Thanks




I am a firm believer in cause and effect. When is the fault happening?
day night? machines on? machines off? cold? hot?

If the switches are closed, who's to say you don't have a intermittent
switch contact from temp extremes or vibration? Or a loose wire to
switch contact? Corrosion from water damage or humidity?

I would ohm all the switches for closed contract resistance.
and check and re torque all switch to wire connections.


bob

bob