Electrical Apparatus
The Magazine of Electrical &
Electronic Application & Maintenance

May 2004 featured article

Use (and misuse) of "voltage testers"

From Electrical Apparatus'  May 2004 issue ...
When technicians service electrical equipment or diagnose system problems, they usually seek to deenergize all circuits. Sometimes, however, electricians must work directly on, or near, energized circuits—and this calls for simple and safe means of determining which terminals or conductors are energized (and therefore dangerous). This important article walks readers through the various types of voltage detectors that can be used for this purpose, highlighting the advantages and disadvantages of each.

By Richard L. Nailen, EA Engineering Editor

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Summary: Use (and misuse) of "voltage testers"

With trilingual summary

Servicing electrical equipment or diagnosing system problems usually requires deenergizing the circuit involved. Sometimes, however, the electrician must work directly on or near energized circuits. In any event, a technician needs some simple means of safely determining which terminals or conductors are energized (and therefore dangerous) and which are not.

Several types of <I>voltage detector<P> serve that purpose. The oldest, and one of the simplest, is the solenoid version. When the user places its two leads across a voltage source, the solenoid magnetically moves a pointer on a scale to indicate the presence of voltage and its approximate magnitude. Other visual or audible alarm signals may be provided. Voltmeter accuracy is not required.

That device has some disadvantages. One is that it is inductive. Removing the leads from an energized circuit causes a slight arc, creating a voltage transient that can disturb sensitive electronic apparatus on the circuit. Also, simply making that contact runs the risk of an accidental short-circuit.

Other types of detector require no direct contact with the circuit being tested. Some of them respond electromagnetically (by induction) to current flow through a conductor when the detector is placed nearby. An obvious problem is that conductors can be energized at a dangerous voltage even if no current is flowing. Another detector design involves capacitive coupling to the circuit under test, responding to the electrostatic field surrounding an energized conductor even when no current flows.

Such instruments are useful only for a-c circuits. Others contain Hall Effect sensors, allowing non-contact evaluation of d-c voltages.

Associated with each non-contact detector design is a working range (some instruments provide multiple ranges for user selection), typically with a minimum or threshhold level of response. One difficulty with any such detector is in checking grouped or bundled conductors, some of which may be energized while others are not. This emphasizes the electrician's need to understand the limitations of whatever type of voltage detector is used.

From  "Use (and misuse) of 'voltage testers'" ...by Richard L. Nailen, EA Engineering Editor  - 
published in Electrical Apparatus May 2004  © 2004 Barks Publications, Inc.  All Rights Reserved.