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  #1  
Old 04-23-2011, 11:50 AM
Dog80 Dog80 is offline
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How to test for short-circuit in a long run of cable?

I have 16 CCTV cameras. They get their power from a single 12V power supply unit that looks like this: http://www.pkcctv.com/UploadFiles/CC...3A_4_group.JPG

This power supply has four different 12V zones and the cameras grouped by 4 are connected to each zone. While the current draw in the first three zones is about 0.5 Amps, the draw on the fourth zone is 3.5 Amps and the fuse gets burned after a while.

Obviously either one of the cameras in the group is defective and draws a lot of current, or there's a short circuit in the wiring. I am going to disconnect the cameras one by one to see if one of them causes the problem. But what happens if the problem remains after that? The total cabling of this camera group is several hundreds of meters and most of it not easily accesible.

Is there a method for detecting shorted wires in situations like this?
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  #2  
Old 04-23-2011, 11:52 AM
beowulff beowulff is online now
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Yes, but it's expensive: TDR.

Last edited by beowulff; 04-23-2011 at 11:53 AM..
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  #3  
Old 04-23-2011, 11:53 AM
Joey P Joey P is offline
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Detach the wires at both ends and test for continuity from each wire to all the others and from each wire to ground, there should be none.

IOW, un-hook all the wires you think may be shorted, at both ends. Then with a continuity tester, touch one wire, then with the other end, touch all the other wires and a ground, one at a time. If it lights up, you've completed a circuit, but since you deliberately broke all the circuits before hand, there's a short somewhere.
Does that make sense?
Then move on to the next wire until you've checked each possible combination.

Last edited by Joey P; 04-23-2011 at 11:57 AM..
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Old 04-23-2011, 12:57 PM
jtgain jtgain is offline
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Quote:
Originally Posted by Joey P View Post
Detach the wires at both ends and test for continuity from each wire to all the others and from each wire to ground, there should be none.

IOW, un-hook all the wires you think may be shorted, at both ends. Then with a continuity tester, touch one wire, then with the other end, touch all the other wires and a ground, one at a time. If it lights up, you've completed a circuit, but since you deliberately broke all the circuits before hand, there's a short somewhere.
Does that make sense?
Then move on to the next wire until you've checked each possible combination.
This. If you don't have a continuity tester, use a multimeter and check for resistance. With both sides disconnected, there should be infinite resistance (signified by a 1 on my meter). If it shows zero, then there is short somewhere between the two wires.
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Old 04-23-2011, 01:21 PM
panamajack panamajack is offline
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Far cheaper than TDR, but less capable, is a continuity tester with a remote probe. Like this one (~$33).

With any continuity tester, you're only getting a result of whether the wire is below a certain resistance, which may not necessarily be enough to let you know what the problem is. (If there is a direct short between two wires, it should detect that, but that may not be what happened.)
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Old 04-23-2011, 01:24 PM
beowulff beowulff is online now
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Determining if a long cable is shorted is laughably trivial. It's finding out where the short is that's hard...
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Old 04-23-2011, 01:34 PM
jtgain jtgain is offline
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Forgive my ignorance, but the situation that the OP is describing sounds like one that is NOT a short, no? If there was a short, then there would be zero amps at the receiving end, not too many, right?
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  #8  
Old 04-23-2011, 01:46 PM
Joey P Joey P is offline
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Quote:
Originally Posted by panamajack View Post
Far cheaper than TDR, but less capable, is a continuity tester with a remote probe. Like this one (~$33).
You wouldn't actually need one like that. Normally, with a long run, you can just twist the wires together at the other end, which is what the remote part does on that tester. However, since we are testing for a short, we are using the continuity tester in sort of the opposite way. We're testing the circuit to make sure that there is no continuity. Basically, a short in the wiring will take the place of the remote probe.

Last edited by Joey P; 04-23-2011 at 01:47 PM..
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  #9  
Old 04-23-2011, 01:48 PM
Joey P Joey P is offline
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Quote:
Originally Posted by beowulff View Post
Determining if a long cable is shorted is laughably trivial. It's finding out where the short is that's hard...
In a case like this, once you figure out which wire is shorted it's often easier to abandon it and run a new wire.
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  #10  
Old 04-23-2011, 04:08 PM
Crafter_Man Crafter_Man is offline
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Zone 4 supplies power to four cameras, correct? And each camera pulls about 125 mA?

In Zone 4, connect power to each of the four cameras, one at a time. I suspect three will pull around 125 mA, and one will pull around 3.125 A.

Once you find the camera that pulls 3.125 A, you will then have to determine if it's the camera or the wiring that is causing it. You can use the substitution method to do this.

If it's the wiring, you'll probably need TDR to locate the short. If you don't have TDR, here are a few ideas:

1. Using a good digital ohmmeter that has been carefully zeroed (or better yet, a 4-wire resistance measurement), measure the conductor-to-conductor resistance at each end. If one is much larger than the other, you know the short is closer to one end vs. the other. (The short will be closer to the end that reads the lowest resistance.)

2. Cut a few inches off each end and re-do the ends to see if that fixes the problem.

3. Cut the cable in the middle and check each segment. Replace the bad segment.

4. Replace the whole damn cable.
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  #11  
Old 04-23-2011, 04:21 PM
Joey P Joey P is offline
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Quote:
Originally Posted by Crafter_Man View Post
Zone 4 supplies power to four cameras, correct? And each camera pulls about 125 mA?

In Zone 4, connect power to each of the four cameras, one at a time. I suspect three will pull around 125 mA, and one will pull around 3.125 A.

Once you find the camera that pulls 3.125 A, you will then have to determine if it's the camera or the wiring that is causing it. You can use the substitution method to do this.

If it's the wiring, you'll probably need TDR to locate the short. If you don't have TDR, here are a few ideas:

1. Using a good digital ohmmeter that has been carefully zeroed (or better yet, a 4-wire resistance measurement), measure the conductor-to-conductor resistance at each end. If one is much larger than the other, you know the short is closer to one end vs. the other. (The short will be closer to the end that reads the lowest resistance.)

2. Cut a few inches off each end and re-do the ends to see if that fixes the problem.

3. Cut the cable in the middle and check each segment. Replace the bad segment.

4. Replace the whole damn cable.
Of course, before doing all this, you'd probably first want to connect the camera to the power source with a known good cable. Just move the camera to one of the other locations and plug it in there. If you don't have any problems, it's the wiring (double check this by plugging a different camera into the old location and see if it blows the fuse), if the camera blows a fuse in the new location, it's the camera.

Last edited by Joey P; 04-23-2011 at 04:22 PM..
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  #12  
Old 04-24-2011, 06:08 AM
casdave casdave is offline
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Disconnect everything, check for continuity.

I have to say this, if it was a full on short circuit, none of the camaras would work as the power would be shunted away from them.

The trick on very long runs is measure the continuity at differant points or outlets, you can work out which section has the short because the readings will vary from point to point.If it is just one very long continuous run and you have a dead short then you can do a little bit of cable calc and get an idea of where the cable fault lies.
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  #13  
Old 04-24-2011, 07:01 AM
thelabdude thelabdude is offline
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One other thing is to inspect the area and look for changes. Was the wall damaged here, did somebody pound a nail in there, etc.? Left alone, cables in a wall are less likely to fail unless overloaded.
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  #14  
Old 08-17-2011, 10:29 PM
etbrown4 etbrown4 is offline
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Lots of folks don't have a TDR, and they are expensive.

The problem with an ohm meter is it will tell you which cable is bad, just not where.

If you can get to the cables, why not use a short tester like the one sold at Harbor Freight for about $20?

It's battery operated, and you waive the wand over the cable, and it will pinpoint the short location.

Might this be the simplest solution if there is access to the cable?
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  #15  
Old 08-17-2011, 11:32 PM
Rick Rick is offline
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Disconnect all 4 cameras.
Turn on power and measure amp draw, it should be zero.
If > zero, problem is in cable. See previous posts about locating shorts in cables. Wiggle and flex the wire near the connectors to see if the problem is there ( it might be, particularly if the cameras move)
Assuming cable reading is zero, reconnect cameras one at a time, and observe amp readings. One of them will be excessive.
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