Voltage detection and discharge: Similar equipment, different function

Two indispensable pieces of equipment when working around medium- and high-voltage lines are voltage detectors and discharge sticks. They look similar and are sometimes confused but have distinct functions that complement (not replace) each other.

What does a voltage detector do?

A voltage detector is used before the testing or maintenance of voltage lines is performed. It provides the operator with a visual and audible warning if voltage is present. These detectors are typically long insulated poles, made from dielectric materials that retard current flow and protect the operator from shock.

Mounted on one end of the pole is the detection head, which operates on a minute capacitive current to ground, incepting both visual and audible indicators. Pole length is critical. It is regulated by IEC standard 61243 in order to keep the operator safe and out of the arcing distance.

Detector heads are also sold separately and can be mounted on industry-standard “hot line” poles. There are two types of mounting, Type C and Type K. Type C is a male-female connection. Type K is a ribbed semi-circle that somewhat resembles a small hand. These connectors are standardized so that any head will fit any pole, as long as the types match. When providing a separate pole, the operator must know the IEC safe working distance against the rated voltage of the equipment in order to provide an appropriate pole length (this can vary by a few meters).

Non-contact detectors are also available, but don’t give the most accurate results. These small, handheld devices are often called “tick tracers.” They will warn of voltage in the vicinity, but run the risk of being less than accurate. Without contact verification, the operator may think that the voltage is emanating from an incorrect source. This type of detector is most often used in building wiring situations (120, 240 480V) rather than in medium voltage or high voltage applications. Tick tracers can warn that a circuit in a piece of equipment is live, but run some risk in that different current sources in reasonable proximity can be confused.

What does a discharge stick do?

Discharge sticks, or discharge rods, are applied at the conclusion of a test. Various categories of test equipment, especially in insulation testing, will store a static charge in the insulation of the item under test (IUT). This voltage can be lethal, and must be discharged to a safe level (<50 V) before the IUT can be approached.

Do not apply a direct short to the IUT, as in the form of a wire or other metallic link. Applying a direct short is dangerous, causing cascading discharge current, heat, sparking, equipment damage and risk of fire.

Proper and safe discharge is implemented by a discharge stick. Similar in appearance to a voltage detector, the discharge stick consists largely of a dielectric pole of standard-regulated length. But instead of an indicator head on the pole, the discharge stick has a regulated circuit to ground.

Inside the pole is a resistor network that safely diverts current from the test item to ground in a controlled fashion, without sparking, heat or any other safety risk. The pole is applied to the test item by a metal hook on one end. Discharge current is diverted to ground by a heavy grounding conductor and robust clamp on the other end. The grounding clamp is first connected to system ground and then the hook end applied to the test item. When discharge is complete, then, and only then, is it safe to apply a “dead short” via a second hook midway down the pole. This will bleed off residual charge and can be left in place to prevent spontaneous recharging.

Look before you leap

Both voltage detectors and discharge sticks are necessary items for electrical repair and maintenance. Be familiar with what each does, and don’t confuse the two or the results could be disastrous.