Yes, you read that correctly. Anyone involved with electrical safety knows that testing for absence of voltage is a key requirement for ensuring a circuit is de-energized and safe to work on. But does absence of voltage always mean the circuit is safe? It depends on the type of circuit. When an incident occurs, especially if there is an injury or worse, an investigation inevitably follows. Even minor incidents and near-misses should be investigated. The results can be used to adjust work practices and operating procedures as well as possibly modify designs.

NFPA 70E 110.2(A)(1) requires hazard elimination as the first priority in the implementation of safety-related work practices. Concepts such as “de-energized,” “test before touching” and “electrically safe” have been drilled into us.

Testing for absence of voltage is the cornerstone of establishing and verifying an electrically safe work condition as outlined in NFPA 70E 120.6. Step seven requires using a portable test instrument rated for the application to test for the absence of voltage of each phase conductor. Section 110.2, Electrically Safe Work Condition (B) When Required, states that the electrically safe work condition is required for circuits operating at equal to or greater than 50V when the employee is within the limited approach boundary or interacting with equipment that can increase the likelihood of injury from an arc flash. The 50V threshold also appears in a few other locations. This could imply it is not necessary for voltages below the 50V threshold.

However, some circuits operate well below 50V, yet they can still produce a potentially hazardous current. Current without much voltage? How can that be?

Electrical circuits are considered to operate at fixed voltages such as 480V (or close to it—voltage drop happens). The current varies depending on the connected load. No voltage means no electrical hazard. At least, that’s the assumption—and in most cases, it’s valid.

The overlooked hazard

However, not all circuits behave this way. Some circuits behave as a constant current source. In this case, the current remains essentially constant and the voltage varies—and can be quite low. These circuits may not fit into the traditional “absence of voltage equals safety” mindset.

Early research into the physiological effects of electric current established just how dangerous this misunderstanding can be. Researchers such as Charles Dalziel determined a very small current can be lethal, particularly when it passes through the chest, resulting in respiratory paralysis or ventricular fibrillation. For current-driven circuits, voltage may be minimal, but sufficient current creates a very real hazard.

One example of a current-driven circuit is a current transformer (CT). The voltage across the secondary terminals of the CT is normally quite small. It is based on the CT secondary current and the connected impedance of relays, meters and other devices. Even when the voltage appears very low, the secondary current can still be hazardous. Testing for absence of voltage alone may not adequately identify the hazard associated with CT secondaries.

Airfield lighting circuits

Airfield lighting systems are intentionally designed to regulate current while allowing voltage to vary based on the number of fixtures in service and the condition of the circuit. Under certain conditions, measured voltage may appear very small, yet the circuit can still deliver lethal levels of current.

From a traditional electrical safety perspective, a worker may believe the circuit is safe because voltage is minimal, when in reality, it is not safe.

2027 NFPA 70E to the rescue

At the time of writing, proposed language for the 2027 edition of NFPA 70E will begin to address this gap. While the 2027 edition has a few more steps before publication, the intent is clear: to recognize current-driven circuits and require that hazard elimination and verification methods account for voltage and current, where applicable.

NFPA 70E 120.6, Process for Establishing and Verifying an Electrically Safe Work Condition, contains eight steps. Step seven is to test for absence of voltage, and, in the 2027 edition, it will have a new Exception No. 3: if testing for absence of voltage alone does not indicate that electrical conductors and equipment are de-energized, then more testing is required. This new exception also has a new informational note for further guidance. An example of additional methods include testing for the absence of current in current-driven circuits.

This language acknowledges that there are circuits where voltage alone may not be the best indicator of electrical hazards. Recognizing those situations, and verifying voltage and current where appropriate, is a necessary step to eliminate the risk.

In my May column, I’ll introduce the most significant changes. 

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