High impedance digital multimeters (DMMs) are capable of measuring stray or ghost voltages. Stray or ghost voltages occur from capacitive coupling, which takes place between energized circuits and non-energized non-connected adjacent wiring. Because of this coupling effect and the DMM's high impedance, it's not always possible to determine if the circuit under test is energized or de-energized, and this creates confusion for the person performing the test.
Where are stray voltages encountered?
The most common place to encounter this situation is unused cable runs or electrical wiring in existing conduit. When facilities or buildings are built and wired, it's very common for the electricians to pull extra wire thru the conduit for future use. These wires are typically left unconnected until needed. Another example would be an open ground or neutral on a 120 V branch circuit or an open phase in a three phase power system.
What does a high impedance meter indicate when measuring a stray voltage?
A high impedance meter presents virtually no load to the circuit under test. By design, this is the ideal situation, since you don't want to have the meter loading the circuit and affecting the circuit measurements. However, in capacitive coupling situations, a high impedance meter measuring between ground or neutral to the unconnected cable will indicate some amount of voltage present. Typically this voltage reading may be as high as 50% of the energized voltage in the same proximity. Is this voltage real? Yes, it is, but it's static voltage, containing no real energy or current flow. For users who need to determine whether a circuit or connection is energized or not this stray voltage reading presents a real source of confusion. Is the connection hot or not?
The solution, the SV225 stray voltage adapter:
The stray voltage adapter is a digital multimeter accessory that allows the DMM to measure circuits subject to stray or ghost voltages from adjacent energized wiring. The adapter provides a low impedance load to the measured circuit that desensitizes the meter to low energy spurious sources of interference. It allows a high impedance DMM to make an accurate determination as to whether the circuit under test is energized or not. Using this adapter allows the user to determine whether a circuit, connection, cable or connector is energized or not. If the measurement points are energized with a "hard" voltage, the meter will simply display the voltage reading. If the measurement points contain a stray or ghost voltage the meter will read very close to zero volts indicating the circuit or connection is de-energized. See examples on the next page.
The stray voltage adapter is designed to be used in conjunction with high impedance digital multimeters, to help determine whether a power circuit is energized. The adapter presents a 3 kΩ load to the circuit under test, dissipating any stray voltage present if the circuit is not energized. This adapter should not be used on control circuits or anywhere where the circuit under test could be adversely affected by this low impedance load. The adapter can withstand continuously applied power system voltages without damage, however it's intended for intermittent use.
Figure A is a normal reading for an energized 120 volt branch circuit between hot and neutral. This reading is displayed on the meter with or without using the stray voltage adapter.
Figure B is the measurement displayed with the high impedance DMM between neutral and an unconnected wire in the same conduit as a 120 volt branch circuit feed. Note the high impedance meter is displaying 33 volts. This is a capacitively coupled stray voltage reading.
Figure C is displaying the result of the measurement from Figure B when the stray voltage adapter is placed in the circuit. Note that the reading is now 13 millivolts or very close to zero volts, a de-energized connection.
The low impedance presented by the stray voltage adapter dissipates the stray voltage. If the reading in Figure B was a "hard" voltage, the reading in Figure C would have been the same as Figure B.