What Do You Know About Capacitive Voltage Sensors?

Tick tracer. Glow meter. Sniffer. They go by many names, but capacitive voltage sensors are all designed to do one thing: detect the presence of voltage in a wire or piece of equipment without actually making direct contact with the conductor or energized part. These test tools are popular because they're inexpensive, easy to use, and small enough to fit in a shirt pocket. Recently, however, more

Tick tracer. Glow meter. Sniffer. They go by many names, but capacitive voltage sensors are all designed to do one thing: detect the presence of voltage in a wire or piece of equipment without actually making direct contact with the conductor or energized part. These test tools are popular because they're inexpensive, easy to use, and small enough to fit in a shirt pocket.

Recently, however, more and more end-users have raised questions about the principles behind the devices' design, their correct application, and appropriate safety precautions associated with their use. How does this thing work anyway? How can it detect voltage without making metallic contact? Can it detect live conductors inside a grounded metal conduit?

As with all pieces of test equipment, it's a good idea to first understand the basic principles of operation of a device before you go out and try to use it. This way you'll keep yourself out of harm's way and know exactly when and where it will and won't work. And the following FAQ should give you a good head start toward that goal.

Q. What is the principle of operation behind this test instrument?

A. The test instrument works on the principle of capacitive coupling. To understand this, let's return momentarily to electrical circuit theory and recall how a capacitor works. A capacitor has two conductors, or “plates,” that are separated by a non-conductor called a dielectric. If we connect an AC voltage across the two conductors, an AC current will flow as the electrons are alternately attracted or repelled by the voltage on the opposite plate. In other words, there's a complete AC circuit even though there's no “hard-wired” circuit connection. The electrical field inside the capacitor, between the two plates, is what completes the AC circuit.

Capacitors are often thought of as individual circuit components, but in reality the world is full of small stray capacitors you might not normally think about. For instance, suppose you're standing on a carpeted concrete floor directly under a 120V light fixture and the light is on. Your body is conducting a very small AC current because it's part of a circuit consisting of two capacitors in series. The two conductors or plates of one capacitor are the live element in the light bulb and your body. The dielectric is the air between them. The two conductors for the second capacitor are your body and the concrete floor. The dielectric for the second capacitor is the carpet, plus your shoes and socks. This second capacitor is much larger than the first. Understanding how the voltage divides between these two capacitors in series is critical to understanding how the capacitive voltage sensor works.

Let's return to the electrical circuit theory again. In a series circuit, the largest voltage will develop across the largest impedance (Ohm's Law). With capacitors, the smaller the capacitor, the larger the impedance, which is known as capacitive reactance. Thus, when two capacitors are in series, the largest voltage will develop across the smallest capacitor.

In the above example, only a few volts will develop between your feet and the floor (the large capacitor) while the remainder of the 120V will be between your head and the light bulb (the small capacitor). Now if you hold the barrel of a capacitive voltage sensor in your hand and place it near the conductors of the light fixture, you're basically inserting a high impedance-sensing element into a capacitively coupled series circuit. Your hand, arm, body, and feet form a relatively large capacitor coupled to the floor. The sensor tip is a small capacitor coupled to the live conductor. As a result, the sensing circuit in the tester detects voltage and signals to you via a lamp, tone, ticking noise, or buzzer that voltage is present. Thus the nicknames: glow meter, tick tracer, and sniffer.

Q. Will the sensor still work as designed if I'm not holding it? What if I'm standing on a wooden ladder?

A. The simple answer is no. The reason for this is that you've opened the capacitively coupled circuit by removing your hand from the sensor or sufficiently isolated yourself from ground. And as noted above, capacitively coupled circuits have to be closed (complete) for current to flow. Whenever the dielectric between two conductors is increased so much so that the field between the two conductors is diminished to the point of ineffectiveness, the circuit opens.

Another way to conceive of this is to think of the effective capacitance at that point in the circuit as becoming smaller and smaller because of the increase in dielectric. Capacitive reactance (Xc) increases as capacitance decreases and continues to grow to the point where virtually all the voltage drop occurs across this now open element of the circuit. Any remaining voltage drop in the capacitive sensor portion of the circuit is now too small to activate the test instrument.

In practice, this means that the capacitive sensor must have a sufficient ground connection — typically via the operator — for the tester to operate as intended.

Q. Will the tester work on a shielded cable or through a metal enclosure?

A. No. The tester won't work if the capacitive sensing path is interrupted by a metal shield, such as a metallic conduit or a metal enclosure (assuming that both of these are effectively grounded). It can't detect voltage inside a metal enclosure, grounded metal conduit, or shielded cable. On the other hand, if the sensor does activate on a metal conduit or a metal enclosure, this indicates that those surfaces are not effectively grounded and pose a shock hazard.

Q. Can I use the sensor to detect faulty grounds?

A. Yes, an open ground will cause the sensor to indicate live voltage. An ungrounded enclosure or conduit will “pick up” the voltage field of any live conductors. The sensor is an inherently high-input impedance device that allows it to detect voltages that a low-input impedance tester, such as a solenoid tester, won't. The solenoid tester would just “draw down” the coupled voltage of the enclosure.

It's possible to simulate this with a simple lab exercise. Get a desk lamp that has a two-prong plug. Plug it into a live socket; it isn't necessary to turn the lamp on. The sensor will indicate live voltage when brought into contact with metal portions of the lamp, which are the equivalent of the floated metal enclosure.

The tester can also be used to check ground connections at the receptacle. Normally, when testing a wall outlet, the sensor detects voltage in the hot (black wire) socket and doesn't detect voltage in the neutral (white wire) or ground (green wire) socket. However, if the ground wire in a receptacle is floating (i.e., not continuous to the grounding point), the sensor will indicate live voltage in the ground socket. Also, if the neutral is open (not connected to the transformer neutral) or floating (ungrounded due to open neutral-ground bond), the sensor will also signal the presence of voltage.

For situations where the ground has high impedance, but isn't actually open, the capacitive sensor may or may not indicate the presence of voltage. In these cases a loop impedance tester is required to measure the actual ground loop impedance.

Q. Can I use this type of tester to detect an open neutral in a branch circuit?

A. Yes. Let's assume the circuit you're testing is a 120V wall outlet. When you plug in a load, nothing happens. A quick check of the panel shows the circuit breaker is on and your multimeter measures 120V between the hot and ground at the outlet. Next, you take out your sensor, insert the tip into the hot side of the outlet, and it indicates live voltage. Then, you insert the tip into the neutral side of the outlet with the same results: a live voltage indication. How can this be? If the neutral were in contact with the hot conductor, wouldn't you have a short circuit? Wouldn't the breaker trip?

If you think carefully about capacitive coupling, the answer is obvious. The hot and neutral conductors are lying side by side for the complete distance from the outlet back to the panel. In other words, they're capacitively coupled. Each wire is one plate of the capacitor, and the conductor insulation is the dielectric. If the neutral is open at the panel, and therefore not grounded, the neutral conductor will float up to nearly the same voltage as the hot conductor. This is why the voltage sensor indicates live voltage on the neutral.

Q. Can I use the tester in hazardous areas?

A. Because these devices are essentially non-contact and low-energy (high impedance), they should, in theory, be suitable for use in this type of environment. However, don't use one in a hazardous area unless it's formally endorsed by the manufacturer for such a purpose.

Smith is a product specialist with Fluke Corp. in Everett, Wash.

Sidebar: Safety First

The main advantages of the capacitive voltage tester are its speed, convenience, and ease of use. In these categories, it's hard to beat. But that doesn't mean standard safety precautions don't apply. It's crucial that you follow these recommendations when using the device.

  • Always test the sensor on a known live voltage before proceeding with your test. This will confirm the battery is good and the sensor is operating properly. There should be no exceptions to this rule. In fact, good practice recommends you first verify the unit is working on a known live source, then make your test, and then recheck the tester against a known live source.

  • Use the sensor to check AC circuits only.

  • Be aware that live voltage won't be sensed inside any effectively grounded enclosure or conduit. The converse of this statement is that a metal enclosure that should be grounded but isn't will activate the sensor.

  • Use a minimum IEC 1010-1, CAT III-600V rated device when taking measurements around 3-phase circuits. Some newer models have CAT IV-1,000V ratings.

  • Be aware of the minimum sensing threshold of the device. Some sensors state clearly on the packaging and the device itself that its range is 90V to 600V. Since OSHA defines hazardous voltage as greater than 30V, it's possible those hazardous voltages in the 30V to 90V range won't be detected. Fortunately, voltages in this range are unlikely, as no nominal voltages exist in this range. One reason for the 90V threshold is that de-sensitizing the device tends to prolong battery life. Also, a lower voltage threshold would cause too many “nuisance” indications.

  • Make sure you're reasonably well grounded and isolated from the cable or piece of equipment you're testing. With no difference in potential between you and the object you're testing, there's no chance the tester will work as designed.

  • In some cases, the tester may falsely indicate live voltage on a low-voltage conductor due to the proximity of a nearby high-voltage conductor such as an overhead line. When in doubt, use a different test method on the low-voltage circuit.

Sidebar: Thoughts From the Field

Results from a recent online survey on the Mike Holt Enterprises Web site revealed some interesting thoughts on the use and application of “tick tracers.” These candid comments show what a few working electricians think about these simple, inexpensive devices.

“To the trained professional, it is a useful tool.”

“Tick tracers are very useful and help to speed up the troubleshooting process.”

“At my company the rule is that we only use the non-contact voltage tester. Of course you must verify the tick tracer first on a known live circuit, then check the circuit in question, then recheck the tick tracer against the known live circuit again.”

“A volt-tic is a good screening tool, but always use a contact (conductive) tester in addition to verify that a conductor or other object is dead before touching it.”

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