Induced Voltage

Back in my operating days, we had a tech get shocked while performing maintenance on what should have been a de-energized piece of equipment. After sending them off to get checked, the investigation began into how this happened. “Obviously,” we thought, “there was an error with the tagout.” The tech must have failed to open a breaker. We were wrong – the circuit’s breakers were all open, but voltage remained in the circuit. How could this happen?

Requirements to Create Electricity

Recall your electrical fundamentals. Three items are required to generate current flow:

  • A magnetic field
  • A current-carrying conductor
  • Relative motion between two

This principle was demonstrated by my instructor waving a magnet over a wire to light a bulb. Later, this same principle would be demonstrated in a classic turbine-generator setup. A thorough article explaining these principles is shown here.

Throughout my early career, generating electricity was rooted in a physical machine rotating a magnet near the current-carrying stator. Items 1 and 2 satisfied! The rotation of the rotor creates the motion between the two and, with Item 3 complete, electricity flows. No magic here – I can see mechanical things happening and it results in electricity. Simple. Right?


As my career advanced, I was attempting to qualify as an operator and quizzed about how a transformer worked. I had never considered what was happening in those “magic boxes.

My worldview of electrical generation was being challenged by the same transformers I had walked past for months! I understood that there is a magnetic field in transformers. Current traveling through a wire generates a magnetic field. But where was the motion? There’s no tiny spinning turbine in that transformer box – is there?

In a wire carrying Alternating Current (AC), the magnetic field expands and contracts several times per second depending on the frequency (e.g. 60 Hz in the US). Item 1: satisfied. Therefore, any current-carrying conductor (Item 2) in the vicinity of these lines of magnetic flux will have a voltage induced within them as the relative motion of the magnetic field (Item 3) cuts them. Remember the keyword is RELATIVE motion. Now the transformer made sense.

Induced Voltage

Apply this transformer principle to a standard cable tray of wires. We see these cable trays every day in industrial settings. In some trays, tens of wires lie in direct contact with high amperage flowing through them. The result is a tray full of expanding and collapsing magnetic fields in the vicinity of current-carrying conductors. In this setting, the voltage can be induced in a wire that is otherwise properly locked out. This principle held the key to the shocked tech.

An investigation revealed that the system was properly locked out and tagged. However, the cable to the component being worked on lay in a tray full of other energized cables. These cables had induced sufficient voltage in the cable to injure the tech.

Proper Lock Out / Tag Out (LOTO) principles are the basis for safety but should not be the end of checks. Every electrical safety program should include checks to avoid injuries like the one described. These checks should ensure that any electrical work verifies that no induced voltage is present in the circuit to be worked or that the circuit is grounded before commencing work. Proper procedures, coupled with electrical fundamentals training, could have prevented this accident in multiple ways.

Does your plant need additional checklists for preventing dangerous situations for plant personnel? Fossil Consulting can create proper Lock Out/ Tag Out procedures along with additional paramount safety checklists.

Let us know about your plant’s needs.