Question Damaged FTP Cat5 cable - a cheap way to detect fault point

Oct 9, 2019
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Newbie sysadmin here. I've got a 40 meter run of FTP (the entire cable is shielded, not individual wires) Cat5 cable in which two of the pairs are damaged somewhere. The cable is an old one, and was already run by the previous tenants, with me redirecting it some 20 meters in. Unfortunately, it didn't occur to me to test it before using, and now I'm stuck with it.

I know that it's generally recommended to simply replace the entire cable, but it's already attached in some hard to reach places (stairwell), so I would like to find the fault point and insert a passthrough adapter there or something (I don't need it to be super-reliable, the cable is going to host one 100 Mbit Ethernet run and one POTS line). The Internet says there's devices for this sort of thing (specifically for network cables), but they're quite expensive for my purposes. I've found a device like this one at a local store, but I wonder if it will help me since it seems to be made for locating power lines. Suppose I remove jacks from the ends and put a crocodile on each faulty pair from one end. Will it work? Will it work if the cable is "shielded" (quotes because there's no grounding in the entire building)? I'm not even sure how these things work when the wire is cut off and there's no electrical curcuit to speak of.

Thanks in advance!
 
You can buy the simple led testers that pretty much put a battery one end and 8 leds on the other. They are like $10. They will show if the wires are connected end to end on the proper pins and will show if you have broken pairs. The device you list is more to find a cable you do not know where goes. On ethernet cable the signal is so strong it will bleed into the other pairs in the same cable.

What a home user generally can not afford are meters that actually runs tests on the cables ability to pass signals. The also make devices that can find cuts or breaks in cable. These are mostly used on fiber but the same units generally work on copper also. Generally they are called TDR meters but again unless you want to spend near $1000 it is not a option.

Good news is cables almost never break in the middle. It would have to be something like a nail or maybe a rat that damages the cable. It is almost always the ends that cause the issues.

Crimp on ends you just blindly cut them off and replace them until you get lucky and fix it. Since your are a more permanent install I would use keystone jacks. You can get tool less ones designed for home users. You can redo 1 wire at a time if you need to.

If this was a new purchase you would never buy shielded cable. It is massively hard to properly install. It must be grounded on both. In addition the ground can not be the electrical ground it must a completely separate system. Not grounding it causes it to act as a antenna increasing interference...the opposite of why you buy it in the first place. Lucky in home installs there is no interference anyway so there is nothing to increases or prevent it is just a waste of money.
 
Reactions: Atmaks and Mandark
Oct 9, 2019
2
0
10
0
You can buy the simple led testers that pretty much put a battery one end and 8 leds on the other. They are like $10. They will show if the wires are connected end to end on the proper pins and will show if you have broken pairs. The device you list is more to find a cable you do not know where goes. On ethernet cable the signal is so strong it will bleed into the other pairs in the same cable.

What a home user generally can not afford are meters that actually runs tests on the cables ability to pass signals. The also make devices that can find cuts or breaks in cable. These are mostly used on fiber but the same units generally work on copper also. Generally they are called TDR meters but again unless you want to spend near $1000 it is not a option.

Good news is cables almost never break in the middle. It would have to be something like a nail or maybe a rat that damages the cable. It is almost always the ends that cause the issues.

Crimp on ends you just blindly cut them off and replace them until you get lucky and fix it. Since your are a more permanent install I would use keystone jacks. You can get tool less ones designed for home users. You can redo 1 wire at a time if you need to.

If this was a new purchase you would never buy shielded cable. It is massively hard to properly install. It must be grounded on both. In addition the ground can not be the electrical ground it must a completely separate system. Not grounding it causes it to act as a antenna increasing interference...the opposite of why you buy it in the first place. Lucky in home installs there is no interference anyway so there is nothing to increases or prevent it is just a waste of money.
Thank you for a detailed reply. Yes, I do have one of those 10$ testers which is how I found out that 2 pairs are not connected (only started testing after running it). It also allows only 100 Mbit/s, but I think this could also be because the run exceeds 50 meters.

I hoped that I could just attach the device to one of the disconnected wires and move the receiver part of the device along the cable until it stops bleeping or something like that. Guess that won't work for me.

I'll try recrimping the ends a few times to see if it helps. Don't see anything wrong with the current ones at a glance, but I can see how they would cause trouble, since the shielding strongly resists getting pushed into the connector. Really hope it actually turns out to be the issue.

On a side note, do you think this device would work? It even has a 8P8C port and everything, though it costs around as much as a LED tester. I'm considering buying one for the future.
 
I bought this about 6-7 years ago Fluke network tester kit when i first started doing IT work for a small company, has saved my butt more then once.

The building had older 25 pair trunk line ran everywhere for ethernet with punch down blocks at one end. Being able to tone to trace the line and the scanner to check cables breaks, length, and correct wire orientation became invaluable in trouble shooting.
 
You likely don't need to fix anything for 10/100 and one POTS line, because that only requires 6 good wires which you probably have already. You just need to know which wires are bad--which you can find by just cutting off one end, twisting all the wires together and checking continuity at the other end with a multimeter. If they are wired into jacks, then you could make a stub with all the wires twisted and plug that in.

10/100 only requires four wires, and while it's best to use twisted pairs, ethernet really isn't that picky for only 40 meters:

Obviously you don't need to use those colors, but they have to be located in those places. Use whichever two leftover good wires you have for the telephone. Sounds like you were extremely lucky and the broken wires just happened to be the ones only needed for gigabit.

The expensive cable testers will actually tell you how many meters away the break is, so can actually tell you if the problem is right at the jack or plug you are testing.

Unless you work in an aluminum smelter, don't worry about the shielding. It's supposed to be grounded on only one end because its purpose is to ground stray EMI signals, not conduct current between slightly different grounds in a building.
 

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