Why Use Shielded Cable in Substations?

The right shielding for substation wire and cable continues to be a critical factor in enabling substations to become smarter, more sophisticated and more reliable. New sensors, distributed computing, microprocessor-based relays and other sensitive electronics increasingly appear behind substation switching panels and even in switchyards. But all that new functionality has created big challenges for engineers and technicians - solid-state circuitry, whether home grown by the utility or imported from other industries has a legacy of false operation and even limited survival in the harsh substation environment unless properly "hardened" and shielded.

The need for shielded wire goes back to the late 1800's. Interference between telegraph and telephone wires sharing the same poles led to the practice of greater spacing and using twisted pairs for service drops. That did the job until radio came along and turned the lines into unintended antennas. Reportedly, radio broadcasts sometimes emanated from overhead telephone wires! Engineers and physicists knew that, although twisted pairs could exclude stray magnetic fields, they didn't exclude electric fields. Some sort of metal covering was required, sort of a miniature Faraday cage surrounding the conductors.

Simply wrapping a grounded third conductor around the length of the twisted pair seemed to work well enough. Then braided tinsel and braided wire shielding was...