How to repair severed ethernet cable?

June 10, 2008 6:41 AM

How can I repair a severed ethernet cable?

While doing some remodeling, my contractor accidentally severed the CAT 6 ethernet cable I had run inside the walls. Replacing the entire run is not an option, but I can easily get to section that was cut.

The way I know to fix it is to cleanly cut each severed end, attach plugs and connect them via a coupler, but perhaps I am overlooking a better/easier way.

posted by tomwheeler to computers & internet (14 comments total)

I've repaired cables by simply twisting about a half inch of bare wire together and taping them. I'm not sure if there is a performance hit, but I never noticed one.
posted by Fuzzy Skinner at 6:58 AM on June 10, 2008

The way you know to fix it is the correct way. It's twisted pair, not twisted mess of bare wire ends and electrician's tape. With all due respect to Fuzzy. You could even take the opportunity to put a biscuit block there!
posted by mzurer at 7:00 AM on June 10, 2008

Just to repeat, the best way is as you said it: cut the ends, re-crimp and then couple them together.

If you don't, you increase the amount of crosstalk over the untwisted parts, which for long runs will cause dropouts or worse. OTOH, the specifications for ethernet say that you can't have more than half an inch of untwisted cable at either end.

I tried to a bodge a cat5 run of about 20m like this, and the PC at the other end simply couldn't see the network until it was properly done.
posted by gkhewitt at 7:09 AM on June 10, 2008

Ideally you would use a 110 punch block to terminate and extend the run.
posted by doomtop at 7:54 AM on June 10, 2008

Thirded: The way you know is the right way. It's a bit of work, but it's best to do it right the first time.

Off the record: I've seen folks strip ends and tape it all together. Usually it will work, but it's ugly, unstable, and the furthest thing from optimal.

Even so, doing it right is what, a fifteen minute job? Even if you terminate wrong twice and have to cut off ends and start over? That's not so bad.
posted by SlyBevel at 8:05 AM on June 10, 2008

Just to clarify, the only thing that makes a punch block ideal is that it's easier to deal with - particularly with a short amount of slack - than putting heads on the cables. Beyond that there's really nothing superior about the nature of the connection.

If you don't already own the gear to put heads on cable then you might be just as well off going with the punch, however, since you can buy cheap disposable punchdowns.
posted by phearlez at 8:17 AM on June 10, 2008

You could try to solder or properly splice the individual wires together to do something better than twisting and tape, but that's probably about as much work as doing it right.
posted by TheOnlyCoolTim at 8:20 AM on June 10, 2008

repeating: Joins in CAT5 are liable to cause trouble unless you do as gkhewitt says and crimp plus onto the ends then plug the plugs into a connector.

Ethernet isn't like current-carrying wires (which can be spliced easily) -- if you splice CAT5 it's unlikely to work well or even at all. I'm a network engineer and know this from personal experience.

If you don't have access to or want to buy a crimper and plugs then getting a replacement cable will be the least-painful solution.
posted by anadem at 8:51 AM on June 10, 2008

It seems to me that you're overlooking the most obvious and easy solution; require the contractor to fix it. He broke it, he bought it.
posted by Justinian at 9:01 AM on June 10, 2008 [1 favorite has favorites]

I've tried to repair by crimping and putting new ends and connecting via a coupler and could not get it to work. You might want to make sure that the coupler has an impedance of about 100 ohms so that you can prevent impedance mismatching, which if your cable is longer than say 50 feet, will cause it to fail. Also, I found once that I had to put a switch in between so that it would terminate into the correct impedance. If the cable is still in the wall and loose you should be able to tie another cable to it and just pull the new cable through while pulling the old cable out... This will ensure that you never experience problems with it.
posted by ets960 at 9:17 AM on June 10, 2008

In all likelihood, yes, you can do a hack job and it'll work. Generally, it will work at lower speeds better than higher.

It's what is called 'controlled impedance wiring'. Each twisted pair of wiring carries a differential signal, which is (if properly terminated) does not see common mode signals and will not reflect the incident signal back to the source.

When you put in a splice, you insert a discontinuity into the pair, and if you were using an instrument called a Time Domain Reflectometer (TDR, e.g. Tektronix 1503C), you could see the splice as a small bump in the reflection trace.

I repaired the wiring in the Marriott Marquis hotel on times square using a TDR and found a rats nest splice job in an elevator shaft, once, so it happens. This was a 200 meter run of cable to the 14th floor. Once the splice was fixed, I discovered a design error in the comm boards... wrong terminating resistors. The two items fixed the problem, but in my 30+ years in electronics, it's the only time I have had catastrophic impedace mismatch/reflection problems. (Except for RF and video, of course, where they abound!)

Fix it the best way for you, but if you are planning gigabyte ethernet, do a good job and/or replace the cable.
posted by FauxScot at 9:19 AM on June 10, 2008 [1 favorite has favorites]

If you want to do it quick/easy/moisture free, go out and buy yourself some Butt splices down at the hardware store. After all, ten billion telcom installer must like 'em for a reason.(I've used them before in the same situation, and they seem to hold up to 10/100 fine. All bets off for Gigabit.)
posted by Orb2069 at 9:40 AM on June 10, 2008

You could do a splice like this if you're good at soldering. It's a bit of work, but it's cheap.
posted by zsazsa at 10:13 AM on June 10, 2008

I am a software guy, but I've played sysadmin and deputy network engineer before so I know how to crimp the connectors and have the necessary equipment. I was just fishing for some sort of trick I didn't know about, but as others have said, the right way is the best way and so I'll do that.
posted by tomwheeler at 8:50 PM on June 10, 2008