How to Use Absence of Voltage Testers During Maintenance to Protect Operators from High Voltages

By Bill Giovino

Contributed By Digi-Key's North American Editors

Electrical enclosures commonly found in industrial facilities can contain high voltages ranging from 110 or 220 line voltage to many hundreds of volts. Regulations covering these enclosures, including Underwriter’s Laboratories (UL) and National Fire Protection Association (NFPA), require that high voltages inside these enclosures be verified to be absent before any updates, repair, or maintenance work can begin.

This requires operators to put on bulky personal protection equipment before turning off power to the enclosure and opening the door. The operator must then go through a manual process of testing the internal voltages. The test equipment must undergo a self-test before and after verifying the absence of voltages. Only then can the operator begin maintenance on the enclosure. This is a time-consuming and error-prone procedure that must be repeated for each enclosure.

This article introduces absence of voltage testers (AVTs) and explains how they can be used to automate the process of ensuring that no voltage is present inside the enclosure. It examines the advantages of AVTs with high voltage industrial electronics enclosures and shows how they keep operators safe while saving time when performing maintenance. It then looks at three AVTs from Panduit Corporation that automate the process of testing.

Protecting human operators from electrical hazards

Industrial automation facilities are required to comply with all NFPA standards applicable for the workplace environment. In particular, the NFPA-70E “Standard for Electrical Safety in the Workplace” enforces safe work practices to protect human operators from major electrical hazards. It includes workplace procedures to protect from injuries caused by live power lines including electric shocks, electrocution, arc flash, and arc blast.

When performing maintenance on electrical enclosures, a technician must first safely verify that there is an absence of voltage inside the enclosure in accordance with NFPA-70E. Typically, this testing for absence of voltage is a manual procedure (Figure 1). Before opening the enclosure, the engineer or maintenance technician must don personal protection equipment, including full-body insulation with a face covering, helmet, and gloves.

Image of maintenance inside an electrical enclosureFigure 1: Before performing maintenance inside an electrical enclosure—shown here with the door open—the electronics must be safely tested to verify that there is an absence of voltage. (Image source: Panduit Corporation)

Some enclosures can contain hundreds of volts and the technician must be properly protected in case the voltages are still present. For example, a power switch may have malfunctioned and is stuck in the “ON” position, or an incorrect configuration has brought in external power that has not been properly switched off.

Next, the engineer or technician must turn off all power to the enclosure electronics using one or more external switches. To test the internal voltages, a hand-held voltage instrument such as a probe or voltmeter is used, which must be tested before use to verify correct operation. This includes checking that the display of the instrument is on and verifying that there are no low battery indicators.

With the technician properly protected and the voltage probe ready for use, it is then safe for the technician to open the enclosure, as shown in Figure 1. Engineers or technicians verify—using the voltage instrument—that no voltages are present at power test points and connectors inside the enclosure, and that any capacitors are safely discharged. This requires them to carefully place the two voltage probes, one at the voltage location to be tested and the other at the associated ground terminal. This must be done without disturbing the existing electrical components inside the enclosure or dislocating any wires. Placing the probes incorrectly can result in a false absence of voltage reading, or in some cases damage internal components. Any electric fans inside the enclosure are visually verified to be stopped, indicating that no residual charges are left.

Lastly, they must once again test that the voltage instrument is operating correctly to ensure that the absence of voltage readings were correct and not an equipment malfunction. Only then can they perform the required maintenance inside the enclosure.

This is a complex and time-consuming process that is susceptible to operator error due to many factors, including distraction, fatigue, or outside interference. It can delay maintenance, and in the case of a malfunction, can expose the technician to hazardous voltages, or even expose an unprotected technician passing by the enclosure to hazardous voltages.

Automated absence of voltage testers

The solution to this manual process is an automated system that can test for an absence of voltage inside the enclosure before the door is opened. Panduit Corporation has a solution with its VeriSafe AVTs. The Panduit AVTs automate the entire process of testing for an absence of voltage inside an enclosure, including a self-test of the AVT before and after voltage testing.

The AVT uses an isolation module mounted inside the enclosure which connects redundant sensor leads to high-voltage areas, as well as neutral and ground lines. The isolation module is securely connected to a battery-powered indicator module mounted in a panel knockout in the enclosure door. When the test button is depressed, the AVT first performs a self-test of its system. If the AVT self-test fails, the failure is indicated by red LEDs on the button and testing stops. If the self-test passes, the isolation module then performs testing for any voltages present, as well as for ground faults.

Afterward, the AVT performs a last self-test. Success or errors are indicated by LEDs on the test button. Only after a successful test is completed, and absence of voltage is verified, is it safe for the technician to open the enclosure door. This saves time and keeps engineers, technicians, and operators safe while increasing uptime and improving productivity.

An AVT designed for industrial enclosures is the Panduit VS-AVT-C08-L10 VeriSafe AVT (Figure 2). The isolation module can be mounted on a DIN rail or directly to the enclosure using screws. This allows flexibility for installation in existing systems. It has three pairs of 10 foot (ft.) sensor leads for connecting to voltage terminals and is safe for systems for up to 600 volts, AC or DC.

Image of Panduit VS-AVT-C08-L10 automated AVTFigure 2: The Panduit VS-AVT-C08-L10 is an automated AVT with 10 ft. sensor leads and an 8 ft. system cable. It can be used on AC or DC voltage systems. (Image source: Panduit Corporation)

It has an 8 ft. system cable that securely connects the isolation box to the indicator module. The bright yellow indicator module (highlighted in Figure 3) has a small momentary push button above the Panduit logo. This is pressed to begin the test. If both self-tests and voltage tests complete successfully, the indicator module will display all green for the L1, L2, and L3 voltage presence LEDs. If any hazardous voltages are detected, one or more of the voltage presence detectors will illuminate red. This provides a safe and reliable method of testing that ensures there are no hazardous voltages inside the cabinet before the enclosure door is opened.

Image of indicator module for the Panduit AVT systemsFigure 3: The indicator module for the Panduit AVT systems has a momentary push button that begins the test. Results of the self-test and voltage tests are indicated by various green, yellow, and red LEDs in the indicator module. (Image source: Panduit Corporation)

The external indicator module is 48.6 millimeters (mm) in diameter, and its bright yellow color stands out clearly on an enclosure door. The mounting diameter is 30 mm top notched, so it can easily mount in a standard 30 mm notched panel knockout found on industrial enclosure doors.

The AVT is powered by an industrial 3.6-volt lithium battery mounted in the indicator module. The battery can be replaced by unscrewing the indicator module. This provides additional safety as the enclosure door can stay securely closed while the battery is replaced.

For added reliability, there are two sensor leads for each of the three voltage contacts, providing redundancy in case a lead is accidentally disconnected. The VS-AVT-C08-L10’s three pairs of 10 ft. sensor leads are for connecting to the hot, neutral, and ground wires in an AC voltage system. In a DC voltage system, these dual leads would be connected to the voltage plus, voltage minus, and earth ground leads. In addition, there is a fourth pair of sensor leads for connecting to physical earth ground. When the AVT is not operational, each of the sensor leads are open in a high-impedance state so as to not interfere with the operation of the enclosure electronics.

For each pair of redundant leads, it is important that each pair not be mechanically connected to the same contact point, or the safety and redundancy of dual leads is defeated. Each pair of leads must be connected to the same cable some distance from each other, providing redundancy and preventing single point of contact failure.

Panduit’s AVT increases operator safety and can reduce the need for an operator to wear personal protection equipment when opening the enclosure. However, it’s important to note that the need to wear such protective gear may still apply for reasons beyond the scope of the AVT’s specified capabilities, and can include factors such as nearby equipment, other activities in the area, and operator training. For that reason, it is up to the safety managers of a facility to decide whether the use of Panduit’s AVT reduces the need for personal protection equipment during enclosure maintenance.

Indicating absence of voltage in the presence of flammable gases

If the enclosure is located in an industrial environment where flammable gases may be present, Panduit offers the VS-AVT2-C02L03 AVT for hazardous locations (Figure 4). It has the same operation and appearance as the VS-AVT-C08-L10 except that it is designated for operation in OSHA Class I Division 2 hazardous locations where flammable liquids are used and contained, but flammable gases may still be present in the atmosphere.

Image of Panduit VS-AVT2-CO2L03 AVTFigure 4: The VS-AVT2-CO2L03 is visually and operationally similar to the VS-AVT-CO8-L10 but is designed for use in OSHA Class 1 Division 2 hazardous locations. (Image source: Panduit Corp.)

The VS-AVT2-C02L03 has 3 ft. sensor leads and a 2 ft. system cable and provides safe indication of absence of voltage so that high voltages are not exposed to flammable gases when the enclosure door is opened.

Retrofitting existing systems

For existing electrical enclosures that do not have built-in sensor points, or for enclosures that cannot be taken off-line for an extended period of time to install sensor contact points, Panduit offers the VS-AVT-RKP1 retrofit AVT. It has all the same components as the Panduit VS-AVT-C08-L10 AVT, including 10 ft. sensor leads and an 8 ft. system cable, but also includes a VeriSafe piercing connector kit (Figure 5).

Image of Panduit VS-AVT-RKP1 retrofit AVTFigure 5: The VS-AVT-RKP1 retrofit AVT comes with a piercing connector kit for enclosures that cannot be taken off-line for sensor contact installations. (Image source: Panduit Corp.)

It provides a space-efficient way of retrofitting an enclosure for an AVT without compromising the integrity of existing conductors. It is appropriate for quickly tapping 14 to 6 gauge (AWG) conductors without cutting or stripping. This saves time and allows for fast and safe installation of the Panduit AVT—without extensive downtime. It also includes twelve cable ties and six cable tie mounts to keep the sensor leads safely out of the way of existing electronics.


Keeping engineers, technicians, and operators safe from hazardous voltages is a top priority in industrial facilities. Electrical enclosures can contain hundreds of volts and so must be verified to have no voltages inside before opening the enclosure for maintenance. AVTs provide a safe, easy, and effective means of automating testing for an absence of voltage inside the enclosure before the door is opened. This ensures safety while saving time and improving productivity.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Bill Giovino

Bill Giovino is an Electronics Engineer with a BSEE from Syracuse University, and is one of the few people to successfully jump from design engineer, to field applications engineer, to technology marketing.

For over 25 years Bill has enjoyed promoting new technologies in front of technical and non-technical audiences alike for many companies including STMicroelectronics, Intel, and Maxim Integrated. While at STMicroelectronics, Bill helped spearhead the company’s early successes in the microcontroller industry. At Infineon Bill orchestrated the company’s first microcontroller design wins in U.S. automotive. As a marketing consultant for his company CPU Technologies, Bill has helped many companies turn underperforming products into success stories.

Bill was an early adopter of the Internet of Things, including putting the first full TCP/IP stack on a microcontroller. Bill is devoted to the message of “Sales Through Education” and the increasing importance of clear, well written communications in promoting products online. He is moderator of the popular LinkedIn Semiconductor Sales & Marketing Group and speaks B2E fluently.

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Digi-Key's North American Editors