Electromagnetic induction and eddy current effect: When alternating current passes through a single-core cable, an alternating magnetic field is generated around the cable. If this cable passes through a steel pipe, since the steel pipe is a conductor, the alternating magnetic field will generate electromagnetic induction inside the steel pipe, and then eddy currents will be produced in the steel pipe. Eddy currents are generated by the circular motion of free electrons in a steel pipe under the influence of a magnetic field. This process consumes electrical energy and causes the steel pipe to heat up. If there is excessive heat, it may cause safety issues such as fires.
Magnetic fields with opposite current directions cancel each other out: If multiple cables in the same circuit (such as three phase lines and one neutral line in a three-phase four-wire system) are run through the pipe together, due to their opposite current directions, the generated magnetic fields will cancel each other out, and thus no significant eddy currents will be produced inside the steel pipe. However, since single-core cables have only one conductor, the magnetic field they generate cannot be cancelled out by other conductors, so the eddy current effect will be more obvious.
