Non-Contact Voltage Detector Limitations

Perhaps better known as “tick tracers” or “proximity sensors,” non-contact voltage detectors have become a popular tool used to identify the presence of AC voltage on live conductors and circuit parts.  These inexpensive devices are easy to use and can provide qualified electrical workers with information necessary to assess circuit conditions for basic on / off status only.  They should NOT be the only instrument used for determining if a circuit is safe to touch during LOTO procedures.

For example, we recently had an incident involving an existing MC cable that was too long and needed to be cut back a few feet to be installed into a 277 V lighting J-box.   The conductors were visible at the blunt cut end of the cable and when the employee placed their tick tracer at the end to check if it was safe to cut, the instrument did not light up or beep.  Assuming the MC was de-energized, they began to cut off the excess cable and now own a pair of pliers with a large hole in the cutting jaws. Fortunately, the CB tripped and no injuries occurred.

Why did that happen?  The answer is found right in the instructions (which everyone reads right?) that come with non-contact voltage detectors.

I recently reviewed the operating instructions from 5 of the leading manufactures of these devices.  Very clearly, they informed the user that:

• The tester will NOT detect voltage if the wire is shielded, the operator is not grounded or is otherwise isolated from an effective earth ground, if sheathed NM cable is saturated, or the voltage is DC.

• The tester MAY NOT detect voltage if the user is not holding the tester, the user is insulated with a glove or other material, the wire is partially buried or in a grounded metal conduit, the tester is at a distance from the voltage source, or if the field created by the voltage source is being blocked or interfered with.

The most interesting information found in the instructions was to never assume a no voltage indication means that the circuit is de-energized.

The convenience of use of these detectors has also caused electrical workers to sometimes be complacent about the consequences of a false negative voltage test. I have heard a number of electricians also refer to these devices as widow makers.

I recommend that you share this information with your crews and remind them that all meters have unique operating characteristics.  They must be inspected before use and always follow the live-dead-live testing procedures, along with effective LOTO measures, before making direct contact with electrical equipment.