An understanding of electromagnetism can help you troubleshoot problems in many common situations, and prevent them in others. After all, motors depend upon electromagnetism to operate, and motors are “the lifeblood of industry.” You probably went over the principles of electromagnetism in engineering school or trade school, depending on what educational path you took. But can you recall them now?
One of the key principles is that when you pass current through a wire, an electromagnetic field develops around it. Another is that you can induce current in one wire simply by passing current through another wire that is in parallel with it. This is why, for example, you can measure a voltage in a circuit you thought was de-energized because you locked out and tagged out its supply breaker. Let’s examine how this can happen.
Conductors from your circuit run through a panel with conductors from another circuit. The conductors are touching and laid out in parallel with each other. You can’t insulate against magnetism (not by any practical methods), so the magnetic field of the other circuit induces current to flow in your circuit.
If you have this understanding, what will happen during your de-energization assessment? You won’t be satisfied that simply opening your circuit’s breaker leaves it de-energized. You will test to verify that no voltage is present. If a voltage is present, you’ll figure out where it’s coming from. Induction will probably be a suspect, but that depends upon the magnitude (if it’s the nominal voltage, for example, the cause is almost certainly a direct connection such as a backfeed).
The “ghost reading” problem in control systems is nearly always due to inductive coupling. For example, an extrusion process PLC was seeing high process pressures and temperatures. This situation caused improper control responses. The sensors were calibrated, so they weren’t the cause. This pointed toward inductive coupling. It turned out that the installers had run a significant length of the 0V to 10mV sensor wiring in the same cable tray as the 480V motor wiring, and had even cable-tied that wiring onto one phase conductor.
Another principle of electromagnetism is that field intensity decreases with the inverse square of the distance. This means if you encounter inductive voltages, a little separation will go a long way to eliminate those. This principle is also why the NEC has so many conductor spacing requirements in various Articles. For example, Art. 610 (Cranes and Hoists) is full of these references.
Electromagnetism can also cause conductors in a raceway or cable tray to move or jump, because of field interaction. This has all kinds of implications, and it must be considered during design and construction. In the realm of troubleshooting, it can also be a factor.