Stray Voltage - Stray/contact Voltage Detection

Stray/contact Voltage Detection

Stray voltage is generally discovered during routine electrical work, or as a result of a customer complaint or shock incident. A growing number of utilities in urban areas now conduct routine periodic and systematic active tests for stray voltage (or more specifically, contact voltage) for public safety reasons. Some incipient electrical faults may also be discovered during routine work or inspection programs which are not specifically focused on stray voltage.

Equipment used to detect stray voltage varies, but common devices are electrical tester pens or electric field detectors, with follow-up testing using a low-impedance voltmeter. Electrical tester pens are hand-held devices which detect a potential difference between the user's hand and the object being tested. They generally indicate on contact with an energized object, if the potential difference is above the sensitivity threshold of the device. Accuracy can be affected if the user is at an elevated potential him/herself or if the user is not making firm contact with the reference terminal of the device using a bare hand.

Capacitive coupling is the mechanism used by electrical tester pen devices. Because the capacitance between an object and a current source is typically small, only very small currents can flow from the energized source to the coupled object. High impedance digital voltmeters may measure elevated voltages from non-energized objects due to this coupling, in effect providing a misleading reading. For this reason, voltage measurements of normally non-energized objects must be verified.

Verification of a voltage reading is performed using a low impedance voltmeter, which usually has a shunt resistor load bridging the voltmeter terminals. Since very little current can flow from a coupled surface through the small shunt or meter resistance, capacitively coupled voltages will collapse to zero, indicating a harmless "false alarm". If an object in question is in contact with a current source, or coupled by a very large capacitance (possible but unlikely in this context), the voltage will drop only slightly as dictated by Ohm's Law. In this latter case, real power is being delivered, indicating a potentially hazardous situation.

Electric field detectors detect the electric field strength relative to the user's body or mounting platform. By sensing electric field gradients at a distance, they can detect energized objects without making direct contact, making these instruments useful for scanning or screening large areas for potential electrical hazards. A low electric field reading also provides a positive indication that no objects are energized in a tested area. Electric field detectors respond to all field sources, and positive indications must be verified with a voltmeter to eliminate false positives. Electric field proximity sensing also has other industrial applications from manufacturing to building security.

Since stray voltage can not be seen, smelled or heard, there is no easy way for the public to know when a dangerous condition exists. Periodic testing is a first step, but it is possible that a dangerous condition can occur without warning.