Seok Jin Wong

How to detect stray voltages with a handheld multimeter

Blog Post created by Seok Jin Wong Employee on Apr 25, 2018

Have you ever encountered a scenario in which an AC voltage signal is measured on an electrical circuit that has been completely disconnected? Isn’t it confusing when voltage is measured in the dummy circuits?

 

Stray voltage, sometimes referred to as ghost voltage, is a voltage that appears in an electrical conductor such as a wire, even though the wire is disconnected from an electrical circuit. You may spend hours troubleshooting this circuit and end up realizing that it’s a stray voltage, even though all wires are disconnected!

 

Where do stray voltages come from?

It is very common for electricians and technicians to pull extra wire when facilities or buildings are built and wired. This is just like renovating your house - you will pull extra wire from the conduit for future usage. Normally, these wires are left unconnected. These are the areas where phantom voltage will appear in the circuits.

Wires left unconnected are most likely to be the areas where stray voltage will appear in your circuits. 

 

Why do stray voltages appear?

Stray voltage readings can be caused by capacitive coupling of energized conductors with nearby unused wire. This capacitance increases as the length of the conductor increases. The longer the wire, the more prevalent a stray voltage.

 

Current in an active circuit can also trigger a stray voltage reading; the higher the current in the active circuit, the higher the stray voltage. Stray voltage readings caused by active circuits can range from a few volts up to the voltage of the adjacent conductors. It should be noted that according to Underwriters Laboratories Inc. (UL), stray voltage is not real voltage and it cannot cause physical harm to a person. This is because, even though the voltages may be high, the amount of energy stored in the capacitive coupling is very low.

 

UL also states that care must be taken to ensure that the voltage reading is a stray voltage and not a result of a cable defect or improper installation; as such a situation may result in a shock hazard.

 

Here is an example that illustrates the overall situation. Imagine that you are installing low voltage lighting in a warehouse office, as shown in Figure 1. The warehouse is equipped with two wires running in parallel to the conduit. One is for light A, which is ON, and the other pair of wires will be used to install a new light using a new expansion cable that runs parallel with light A.

 

installation in building or facilityFigure 1: Installation of low voltage lighting in a warehouse office.

 

Before beginning the installation, you check the voltage on the wire using a normal handheld multimeter with high input impedance, and the measurement result shows as 40 volts even though the line is disconnected from the main switch. Now, you suspect that touching conductors has formed a short circuit, causing voltage to leak through the conductor’s insulation. You spend a lot of time troubleshooting and investigating. However, after a thorough investigation, you find that there is no short circuit to ground! The 40 volts displayed on the measurement reading is a phantom voltage reading formed by the unused wire. After all the hard work troubleshooting, you realize you have lost a lot of time troubleshooting a stray voltage.


From this example, we can conclude using a normal handheld digital multimeter to measure such circuit can make it difficult for you to differentiate ghost voltage reading from legitimate readings. Most handheld digital multimeters have high input impedance compared to the impedance of the circuit being measured. The handheld multimeters with high input impedance that is greater than 1 MΩ are designed to place very little load on the circuit under test. In this capacitive coupling situation, a phantom voltage reading is measured by this high input impedance multimeter.

 

In our example, if a low input impedance multimeter had been used to perform the AC voltage measurement, the electrician would have found virtually zero stray voltage. This is because stray voltage is a physical phenomenon involving very small values of capacitance; it cannot energize a load. Using a multimeter with low input impedance will short out the capacitive coupling effect, while using a high input impedance multimeter will not.


The solution

Certain models of Keysight’s handheld multimeters, for example, the U1242C, have a unique feature: a ZLow function (Figure 2) that allows you to switch from high input impedance mode to low input impedance mode to check for the presence of stray voltages. This solution eliminates the need to carry both a low impedance meter and a high impedance meter.

ZLOW function on U1242C handheld DMMFigure 2. U1242C handheld DMM with ZLow function

 

The ZLow function acts like a backup voltage indicator and eliminates the need to carry additional tools for troubleshooting. If a real voltage is measured using the ZLow function, the positive temperature coefficient (PTC) thermistor that is designed as an over current protection will ensure the multimeter always operates in high input impedance.


Use a multimeter with flexible input impedance

Now you know how to detect stray voltages efficiently and effectively using a handheld multimeter with low impedance mode. Keysight offers different handheld multimeters that come with ZLow function that can remove stray voltages from your measurements by dissipating the coupling voltage. Use ZLow to reduce the possibility of false readings in areas where the presence of stray voltages is suspected.


To learn more, download the Stray Voltage Testing Made Easy with U1272A application note.
Check out Keysight.com for more info about Keysight’s handheld digital multimeters.

 

Keysight handheld multimeters

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