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How To Test Low-Voltage Wiring

by Jeff Fisher


So you've pulled miles and miles of cat 5 and RG6. You've spent hours terminating all the wires to neat rows of jacks in the wiring closet and pretty wall plates all over the house. Will they work? Did you get any wires crossed or jacks mislabeled? Did any of the cables get "screwed" by a sheet-rock screw? Are any of the wires or jacks just inherently defective? Why not spend an hour or so now, before everything gets buttoned up and all the furniture is moved in, to find out and put your mind at rest. It's quick and easy.

There are two steps to testing low-voltage wiring. The first is a strictly visual inspection you must do before the sheet-rock goes up. (Thus, before the cables are terminated.) Don't skip this step as it is really the more important of the two...as it tends to find more problems and potential problems.

The second step is performed after the sheet-rock is up and the cables are terminated. This is where you actually "buzz out" the cabling to verify that it is connected properly.

Pre-Sheetrock Visual Inspection

Just before the sheet-rock goes up, you should make a final visual inspection of all low-voltage wiring. This procedure is actually the most time-consuming of the low-voltage tests. Look for the following:
  • Places where the cat 5 is kinked. Straighten out the cable and "massage" it where it is "flattened out" to work the pairs back into their normal position.
  • Places where the RG6 is kinked. Straighten it out also. Severely kinked RG6 can short the center conductor to the shield at worst, at best it changes the impedance of the cable. If the cross-section of the coax is very deformed, consider re-pulling that run.
  • Make sure all cables are securely tie-wrapped or clamped at their end points (wall-boxes, mud-rings, etc.) so they won't fall back into the wall later.
  • Staples, brackets, and tie-wraps. You should remove mostif not allof the hold-down devices used during installation (except the ones at the end points listed above.) We prefer not to use them in the first place, but sometimes they are necessary to keep everything from "coming unraveled" during construction. Once the sheet-rock is up, the cables won't be going anywhere. And if you ever need to pull in a new cable, using the old one as a pull-wire, you'll be really glad it isn't stapled in several places inside the wall!
  • Check for cuts or gouges all along each cable. Replace any cable with damage that made it through the "cladding" (outer insulator).
  • Make sure that all holes in studs, rafters, and floor-joists have at least 1 1/4" of wood between the edge and the hole. Any less, and a sheet-rock screw could go right through the wire. If there is less than 1 1/4", put a 1/16" steel nail-plate over the area. (This is straight from the National Electrical Code, and is good advice for plumbing and AC wiring as well.)
  • Is each wire marked or labeled in some manner at each end? Doing so now will be much easier than after the sheet-rock is up.
  • Make a final visual comparison between your plans and your wiring. Count wires at each location. Does the count match your plans? Trace out some or all of the cables, making sure they go where they're supposed to.
  • Did you forget to put any wires on your plans? Front and rear doorbell? Security cameras? Thermostats? Runs from the wiring closet to the "point of demarcation" (where the phone and cable companies bring in their lines)?

If you pass the above tests, you're all set. Some feel the need to do a complete "buzz" test of all cables to make sure that they are good. But it has been our experience that this takes a lot of effort (you have to temporarily terminate the cables) and, as long as the cables do not have visible external damage, they're fine. In other words, most damage to cables comes from the outside-in, and will leave visible evidence such as cut cladding and bare copper visible inside. Inherent defects inside the wire, such as shorts and opens, are unheard of when using quality wire such as Belden or IES.

Let the sheet-rocking begin! While the dust settles from all that taping and sanding, gather up your test equipment.

Test Equipment You'll Need

Hopefully, you've budgeted for this not-inconsiderable expense. Over the next few weeks, you can start gathering all the necessary equipment and familiarizing yourself with its operation.

Just kidding! Actually, all you need to do a really bang-up job will cost under $80. Plus, you'll find all kinds of uses for this equipment later. The whole "kit" we suggest is pictured to the right. It is made up of the following items (all available from HomeTech, of course):

  • 400008Cable Tester
  • AFPAFFF-female to F-male push-on right-angle adapter (qty 2)
  • AFM2FMF-Male to F-Male adapter (qty 2)
  • AFF2BMBNC male to F-female adapter (qty 2)
  • 414030RS 8-Pos Modular Plugs for Solid Conductor Wire (qty 4)
  • BAT9V2 9V Battery
  • 1583A1 Foot of left-over cat 5 cable.


Using your300018modular crimper (get one if you don't have one, you'll use it a lot) make up two 5" cat 5 jumper cables. Wire them "straight through" so that pin 1 on one end is connected to pin 1 on the other end. They should each look like this:
Plug the "terminator" that comes with the cable tester into one of the BNC adapters. Connect a Male to Male F adapter. Then screw the adapter into the right-angle adapter. The result should look like this:
Screw the BNC adapter onto a Male to Male F adapter. Then screw the adapter into the right-angle adapter. The result should look like this:
The whole shebang will fit into the little 4 x 6 leatherette case that came with the cable tester!

Depending on the size of your home (and the loudness of your voice) you might want to borrow or buy a pair of 2-way radios. Just about anything will do since you'll only need them to reach from the wiring closet to the furthest low-voltage receptacle.

Note that this also implies that you'll need a friend to help with the actual testing. It won't take long, and no special skills are required. A pizza should be adequate compensation. Throw in a beer and I'll come over and help! You could do this alone, but it will take ten times as long and you'll wear yourself out.

Electrical Testing Procedure

Doing the actual testing is simplicity in itself. Station yourself in the wiring closet. You will need the cable tester with a cat 5 jumper plugged into it, and the BNC to F push-on adapter plugged into it. You should also have a pencil and paper. (Note that I'm assuming that you have terminated all the cat 5 cables onto modular jacks. If you've terminated them onto a 110 punch down block, you'll need a modular plug to 110 IDC adapter cable.)

Give your helper the terminator part of the tester, with a cat5 jumper plugged into it, and the coax terminator part of the tester, plugged into the other BNC to F push-on adapter.

Turn the tester on. The red POWER led will flash. Make sure the GND button is OUT/OFF. If you have any problems, check the GND button. If you turn it on accidentally, it'll mess you all up. (I've done this before.) You won't be doing anything that needs that button pressed!

You will call out a room and the cable type/number, and plug your tester into the appropriate jack. Your helper will plug in the appropriate terminator at the other end.

For coax cable: When your helper has plugged in the coax terminator at the other end, press the momentary button on the side of your tester. The BNC led should turn green. Release the button. The BNC led should go out. Just note any discrepancies on the paper at this point. Don't get sidetracked trying to fix things, you'll run out of "helper" time. Plus, having a complete "map" of errors may help you diagnose what you've done wrong (i.e. mis-labeling cables.)

For cat 5 cable: When your helper plugs in the cat 5 terminator, he/she should see all four main LEDs flash green in a repeating sequence moving down the terminator. (The little one at the bottom will stay off.) Again note any discrepancies.

With good communications, and few faults, it should only take a few seconds to test each cable!

Correcting Faults

What if the LEDs don't do what they're supposed to?

For coax cable: If the BNC led stays off when you press the button on the side, you have the wrong cable or the cable is "open." (Remember that the tester must be turned on or you'll see this symptom on every cable!) With the terminator plugged into the remote end of the coax, plug the tester into other cables in the wiring closed that showed "open" on your fault-map and press the side button. If any of these cables light up the led in green, then that is the other end of the cable with the terminator; you have a cable mis-labeled. If you're sure you have the right cable, but it's still "open," cut off the F connectors on both ends and put new ones on it. If you still have an open, there must be something wrong with the cable.

If the BNC led turns RED when you release the button on the side of the tester, you have a short in the coax. This is not uncommon and it usually occurs at the connectors where a tiny piece of braid can get against the center conductor as you're putting on the connectors. Cut off both ends and apply new connectors. If you still have a short, the cable probably got "nailed" somewhere.

For cat 5 cable: If no LEDs flash on the terminator, you probably have the wrong cable! Plug the tester (turn it on!) into the jack at the remote location. Then go through your fault-map and plug the terminator into each "dead" jack. If any LEDs light up, that's your cable.

If a led on the terminator flashes red, a pair is wired backwards. If the LEDs flash out of sequence, pairs are swapped. If a led doesn't flash at all, there is an open connection. If the terminator flashes anything, but not the right thing, then you've got the right cable, but there's probably something wrong in the connectors. Examine your punch-down job at both ends. It's easy to swap a pair when installing a bunch of these. Also look inside the connector, sometimes the "leaf spring connectors" can get in the wrong groove, or be bent up so that they don't press against the contacts on the plug. If everything looks OK, pull the wires out, cut the ends off, and punch it down again. If you still have a problem, the wire itself is probably bad.

That's it! Good hunting!

Jeff(external link)

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