Using a Tone and Probe Kit

for Finding Cut Alarm Wires

A tone and probe kit is used to trace wires or cables by sending a signal from one end of a wire along its entire length. This is usually done to trace the location of a cut or missing door, window, or other alarm wire. A tone and probe set can also be used to identify unlabelled wires that run to remote locations from a central alarm panel.

A tone and probe kit includes two parts:

• The first is an audio-frequency tone generator, which is connected to one end of a cable to inject a signal.

• The second part is the inductive amplifier probe, which detects the signal anywhere along the length of the wire and emits it as an audible tone.

The tone generator unit has alligator-type clip-on test leads. These are connected to the bare copper ends of a pair of wires at one end of the cable that needs to be located. The probe, sometimes referred to as a “rocket”, picks up the signal wirelessly through induction. The signal is then amplified to create an audible tone that sounds through a speaker built into the probe.

I started out using a tone generator and probe with a classic tone and probe kit from Progressive Electronics, based in Mesa, Arizona. They are still making tone and probe tools under the “Tempo” brand name, and I now see them distributed by Greenlee. I’ve since used various other brands of tone generator/probe tools, including some by Fluke, Klein, and Cen-Tech.

Keep in mind that toners and probes are fairly generic, and can be mixed and matched between brands. All tone generator tools create an audio frequency signal, just like the left or right speaker outputs of a stereo amplifier. Likewise, the amplifier probe picks up these audio frequency signals, and “plays” them through the built-in speaker.

This is good to know if one part of your tone and probe kit fails. You can borrow a toner or probe from another kit, and get back in action until you have time to fix or replace the failed part.

Using a Tone and Probe Kit

All tone generators that I’ve ever seen also include a continuity test function. This is a necessary feature, because sending a tone down a pair of shorted wires will kill the tone, so much so that the probe won’t be able to pick it up.


The continuity test has another function: It’s a quick way to check the tone generator’s battery. Before the first use of the day, flip the toner to “continuity”, and touch the test leads together. If you don’t get a light, or it’s very dim, replace the toner battery. If the light is nice and bright, you’re ready to go.

To begin toning out a wire, connect each tone generator lead to a different conductor of the “target” wire to be toned out. Briefly switch the generator to the “Continuity” function, to make sure there is no short (light stays off), then proceed with toning. If the light is lit, there is a complete or partial short circuit on that pair of wires. (See the section on ”Shorted Wires” below.)

After verifying that there is no short, switch to “Tone” mode, and move to the area you suspect the wire is located. Activate the probe, and move it across the wall or other surface nearest to the wire.

If the target wire is in the area, you should be able to hear the tone. Keep moving the probe around until you find the path or spot with the loudest tone. That is the location of the wire.

In alarm work, you’re most likely using a tone and probe kit to locate a cut or buried wire. In that case, you now know where to cut a small hole in the drywall and fish the end(s) out. If both ends of the target wire run are accessible, you can tone first from one end, then from the other. This will allow you to zero-in on both of the damaged ends and pull them out.

You may have to make two holes in order to fish out both ends of the wire, since there might not be enough slack to pull them out of the wall enough to make connections. If that’s the case, you’ll also need a short piece of the same kind of wire to allow enough length to make up the splices.

Boosting Tone Generator Volume with a Signal Ground

Sometimes when using a tone and probe kit, you’ll find that the tone is faint and difficult to pick up. In that case, adding a signal ground can help.

A signal ground is just a point of reference with respect to the other end of the tone generator. This need not be an actual electrical ground, although it can be. We’re just looking for anything metallic and separate from the wire we're testing. 

This can be an electrical outlet box or screw, metallic conduit, or a metal window frame. You can even use yourself as a signal ground, by holding onto the bare metal of one alligator clip lead. In fact, the capacitance of your body works quite well as a signal ground.

Here's how to do it:

1. Connect one lead of the tone generator to one leg of the wire, as usual.


2. Connect the other lead to a signal ground.

A variation on this is to connect both wires to the toner leads, just as you normally would, but  arrange it so that one of the leads also touches a signal ground. This will sometimes give a slightly less boost to the signal, but with less bleed onto surrounding wiring.

What is bleed, you ask?

Read on...

Problems While Using a Tone Generator and Probe

Bleed Onto Nearby Conductors

The tone you inject onto a particular wire can sometimes bleed over onto neighboring conductors and wires, causing confusion as to the actual path. This happens most often when the wire under test runs close to other wires along  a common pathway through a soffit or other wiring chaseway.

You may pick up this signal bleed after the wires have diverged from the common path, giving you a false indication of where your wire runs.

Bleed is almost never as strong as the signal emitted by the wire under test, but if the bled-on wire is closer than the test wire, its signal can overpower it.

 
With experience using a tone and probe kit, you’ll learn when to recognize when bleed is occurring, and under which conditions you might expect it to happen. It isn’t usually a big problem, but some things you can do to reduce the effects of bleed are:

• Don’t connect to only one conductor of a wire you’re trying to trace (unless you have a shorted 2-conductor, as described below). This increases bleed immensely.


• Unless you’re deliberately trying to use a signal ground to boost the tone, make sure neither lead of the tone generator touches any surrounding metal objects. This includes mud rings, window or door frames, electrical outlets, ground wires, alarm panel boxes, etc. Likewise, don’t touch either alligator clip of the tone generator with your hands when trying to locate a wire.

If you think you’ve found the target wire, but suspect bleed is occurring, make a small (dime-sized) hole in the drywall and fish out the wire. If you turn out to have the wrong wire, a small hole is easier to patch and won’t be as noticeable.

Shorted Conductors

If you run into a situation where you need to tone out a shorted wire, try to switch to a pair of wires in the same cable that isn’t shorted.

If you’re trying to tone on a 2-conductor wire, there are no other wires to switch to. In that case, it’s time to violate the warnings I just gave above:

1. Connect one lead of your tone generator to one (or both) of the conductors to be tested.


2. Connect the other tone generator lead to a signal ground

This will greatly increase the tone, and give you a better chance of finding the wire under test. Of course, the drawback is that it will probably also cause lots of bleed onto other nearby metal or wiring.

Using a tone and probe kit is not an exact science. You’ll have to strike a balance between creating enough signal to find your target wire, while at the same time figuring out which signals are just useless bleed.

Twisted Pairs and Noise Cancellation

Also important to know is the fact that twisted pairs found in many kinds of wire tend to reduce or cancel out the tone. Cat-5, Cat-6, and many multi-pair cables contain several sets of twisted pairs.

There is a good reason for this: the very purpose for twisting the pairs in the first place is to cancel noise. The signal from our tone generator is technically noise, so for our purposes, cancelling it out isn’t desirable.

So, what can we do?

The solution: When toning out a cable with multiple twisted pairs, connect the tone generator leads to off-colored wires. So, for example, instead of connecting to the toner leads to the blue and blue/white pair of wires, connect to the blue and orange wires.

Even though the pairs are also twisted around each other, the rate of that twist is very gradual, and won’t kill the tone like the much tighter twist rate of the like-colored wires.

A premium tone and probe kit like the Fluke Pro3000 Tone and Probe set have some additional features that can make your work easier.

Among these are:

• Multiple tones to better identify individual wire pairs
• RJ-11 telephone and/or RJ-45 data connectors
• Bed-of-nails clips to make connecting to insulated wires easier

So, who should own a tone and probe kit?

If you’re in the alarm business, a a good quality tone generator with probe is a must-have. Trying to trace a cut wire without a tone and probe kit is a frustrating waste of time. It's also totally needless, considering most of these kits cost around $100.

You’ll Probably save the cost of the tool in labor alone on your first cut-wire repair job.

If you’re a DIYer or home hobbyist, you may never have a need for one of these tone and probe sets.

If you ever do, but don't want to spend too much money, I recommend the budget-priced Cable Tracker tone and probe kit from Harbor Freight Tools.

It’s not a premium tone and probe set, so it’s not built as ruggedly as the Fluke and similar kits. But, it performs the basics well enough, and costs less than $30.

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