Page 20 - Occupational Health & Safety, April 2019

ELECTRICAL SAFETY
Active Indicator for Absence of Voltage
An active indicator is required to visually convey when the absence of voltage has been confirmed. Use of an active indicator is an im- portant fail-safe feature of AVTs. This is because the lack of illu- mination does not guarantee that a de-energized condition exists. While the lack of illumination of an indicator may be the result of the system being de-energized, it could also be due to a faulty device, improper installation, or bad indicator.
This absence-of-voltage indicator is required to be green. No other indicators on the AVT may use green, to ensure standard- ization and eliminate confusion between products from multiple manufacturers. Additionally, the absence-of-voltage indicator will illuminate only if all phase-to-phase and phase-to-ground voltages are below a predetermined threshold that maintains personnel safety while operating reliably. The threshold must be low enough to avoid any injuries to personnel and high enough to avoid nui- sance indications when small amounts of residual voltage are due to radio frequency interference or noise on the ground plane. For AVTs, a de-energized condition is defined as when the voltage is measured below 3.0 V.
Installation of the AVT
If the tester is not in contact with a circuit part when the voltage
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that it is monitoring. To address this, UL 1436 listing requirements include several provisions regarding installation of the AVT. The absence-of-voltage indicator cannot illuminate if the tester is not in direct contact with the circuit part being tested or if phase and ground connections are reversed.
Test Circuit to Verify Functionality
With an installed device, the operator must have a high degree of confidence in the performance. UL 1436 requires a supervisory test circuit to verify that the AVT is functioning properly. The supervi- sory test circuit is activated before and after the absence-of-voltage measurements are taken and requires a known voltage source. Like the process used to validate the functionality of a handheld tester, verifying that the tester is functioning as expected before and after the test ensures that the tester was not damaged during the test, leading to a false indication. Because the AVT will normally be activated on a de-energized system, an independent power source is required. A battery is one example of such a power source, but other technologies also could be utilized.
Safety Integrity Level Rating for Safety Functions
UL 1436 also requires that electronic components related to safety functions meet IEC 61508 SIL 3. This ensures that dangerous fail- ures are detected and controlled in a safe way. Functional safety certification requires quantitative and qualitative analyses by an
measurement is taken, no voltage will be detected. To ensure reli- ability, it is critical that the tester be in contact with the circuit part
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