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Absence of voltage testing: An update on accepted approaches


January 20, 2021
By Andrew Cochrane

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January 21, 2021 – Absence of voltage testing is a vital step in the process of verifying and establishing a deenergized state for any electrical system. As detailed in CSA Z462 “Workplace electrical safety”, Clause 4.2.5., there is a specific and approved approach to establishing an electrically safe work condition:

• Determine all possible source of electrical supply.
• Interrupt the load current, open the disconnecting device for each possible source.
• Where possible, verify that all blades of the disconnecting devices are open.
• Release or block any stored energy.
• Apply lockout device in accordance with documented and established work procedures
• Using an adequately rated portable test instrument, test each phase conductor or circuit part to verify it is de-energized. Test each phase conductor or circuit path, both phase-to-phase and phase-to-ground. Before and after each test, verify the test instrument is operating satisfactorily.

The key is verification… you need to verify the electrical system is deenergized and verify the test instrument is functioning correctly.

Conducting an absence of voltage test inside an electrical cabinet is not only time-consuming but contains risk of shock and electrocution from inadvertent contact with electrical circuit paths inside the panel, incorrect application of test instruments or human error.

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To address this risk, CSA Z462 contains an Exception Note whereby the step requiring direct contact with previously energized conductors with an adequately-rated test device can be replaced by using an adequately-rated, permanently mounted test device, provided the product meets certain requirements, such as:

• It is listed and labelled for the purpose of verifying the absence of voltage.
• It tests each phase conductor or circuit path, both phase-to-phase and phase-to-ground.
• The test device is verified as operating satisfactorily on any known voltage source before and after verifying the absence of voltage.

What is an absence of voltage tester?

Listed and labelled for the purpose of verifying the absence of voltage refers to UL 1436 “Standard for safety: outlet circuit testers and similar indicating devices”. In the Glossary (Section 5), an absence of voltage tester (AVT) is defined as:

A permanently mounted test device that is used to verify that a circuit is deenergized prior to opening an electrical enclosure that contains energized electrical conductors or circuit paths.

An AVT is provided with a test circuit with active indications to verify the absence of phase-to-phase and phase-to-ground voltage.

UL 1436 Section 12 contains specific construction and operation clauses related to the AVT product, including (but not limited to):

• An AVT shall provide the user with a visual indicator to confirm the absence of voltage after the absence of voltage test has been performed. The visual indication shall be Green.
• The AVT shall incorporate a supervisory test circuit to verify that the tester is functioning properly before and after the AVT performs voltage measurements.
• The AVT visual indicator shall only illuminate Green when all phase-to-phase and phase-to-ground voltages measure less than 3VAC or 3VDC.
• The AVT visual indicators shall not illuminate Green unless the phase and ground leads are in direct contact with the circuit conductors being tested.
• The AVT shall comply with the Standard for Functional Safety IEC 61508 and achieve a SIL 3 rating.

Achieving a SIL 3 safety rating requires meeting requirements with respect to failure mode effects analysis, meeting requirements with respect to component selection and probability of failure rates, and meeting guidelines with respect to safety concepts and architecture. This is to assure the AVT product has been designed in such a manner that it will not fail in any critical manner.

Operation and design of an AVT

A listed and approved absence of voltage tester provides a simple means of initiating a test (press Test Button). This reduces the time required to conduct the test (from 20 minutes to around 10 seconds) and allows the test to be conducted prior to opening an electrical enclosure, thereby reducing the risk of exposure to shock or electrocution hazards.

The AVT provides visual indication of the presence of voltage via 3-phase voltage Red lights connected directly to the circuit conductors, thereby indicating that hazardous voltage is present. This functionality is separate from the absence of voltage test and not required by UL 1436, but it is available and provides verification of an electrically unsafe condition.

This direct connection is part of the integral safety architecture and operation of an absence of voltage tester.

To initiate the absence of voltage test, the user presses the Test button on the display unit. Once initiated, the test process will not commence until the supervisory circuit has verified the phase voltage lights are Off and the sensor lead connections have been verified.

The continuity check is a critical aspect of verification and is provided through a continuity monitoring loop where two sensors are provided per phase and per ground. When a short or break is detected in any of the leads, the next test in the sequence will not initiate. To complete the continuity verification, a signal is injected on one of the sensor leads and is read by the other sensor lead.

Once the above conditions are verified, the absence of voltage tester runs through the following verifications, in sequence:

• energy level on the bus is low enough to perform the test
• voltage source is sufficient to perform the test
• phase A, B then C voltage is <3V RMS
• phase A, B then C connection loop has continuity
• phase A, B then C DC voltage is <3V RMS
• phase A, B then C connection loop has continuity
• phase A, B then C RMS voltage is <3V RMS
• phase A, B then C connection loop has continuity
• phase A, B then C DC voltage is <3V RMS

Again, the verifications are performed in sequence; the next test can only initiate once the previous one has passed. Only when all verification tests are passed, in sequence, will all 3-phase Green lights illuminated, can we confirm the absence of voltage.

Test ports for absence of voltage testing

Voltage test ports allow for the user to test for voltage without opening an electrical enclosure, and are excellent tools for safely troubleshooting and determining the value of any voltage present.

However, these test portals cannot be used to verify the absence of voltage as per the Note 1 Exception because:

• They are not labelled for the purpose of verifying the absence of voltage.
• There is no supervisory circuit to ensure the test portal is functioning properly.
• There is no active indication of the absence of voltage.
• There is no continuity check to verify the leads being tested at the port are in direct contact with the circuit conductors being tested.

Reading no voltage at the test port only indicates an absence of voltage at the test port.

Pairing a voltage indicator with a test portal

Even if we were to combine a voltage indicator with a test portal in a single enclosure—both with leads to the energized conductors—it would not suffice to meet the Note 1 Exception for verifying an electrically safe work condition, regardless of whether the work was being performed by a qualified worker or not.

Having two independent connections to the circuit conductors—one set of leads from the voltage indicator and another set of leads from the voltage test portal—provides redundancy, but consider…

Let’s say we observe (confirm) voltage through the indicating lights; we then deenergize and observe the lights are no longer illuminated. We conduct a voltage test at the test portal using an adequately rated portable test instrument and measure no voltage. Even then, we cannot verify an absence of voltage because:

• no supervisory circuit ensuring the test portal is functioning properly
• no active indication of absence of voltage
• no continuity check verifying the leads are connected, all of which are required

As you can see, we cannot and should not rely solely on those indicating lights.

Verified… or not verified

CSA Z462 4.1.4. notes “hazard elimination shall be the first priority in the implementation of safety related work practices”. Using a combination voltage indicator and voltage test port reduces the probability of electrical shock by accidental contact with live conductors (as the test is undertaken outside the cabinet), but does not eliminate the hazard as absence of voltage cannot be verified.

When conducting an absence of voltage test, there are only two options: use an adequately rated portable test device and make direct contact with the circuit conductors being tested, or use a listed and labelled absence of voltage tester.

When it comes to electrical safety and establishing a safe working environment, a high degree of confidence simply isn’t enough; the absence of voltage is either verified or it is not.


Andrew Cochran is president of I-Gard Corp., a provider of neutral grounding resistors, ground fault protection systems and power resistor-based solutions for power protection.

NOTE: CSA Z462-21, 4.2.5. “Exception Notes 1” will be updated to replace the term permanently mounted test device with permanently mounted absence of voltage tester to remove any confusion.

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