Safety First

Happy Fourth of July! I thought I'd share some safety related technical info with you today so that when you get around to setting off your fireworks, staying safe will seem easy!

This is a substation:



A substation is where power is transformed from high voltage (161kV) into low voltage (23kV) for meter use.

I've already touched on the hazards for birds, but substations are also one of the most dangerous places that a human can be. MLGW utilizes a variety measures to keep people and equipment safe.

Here's a peek of just one small piece of the pie, breaker repair, compliments of one of our substation engineers.

Substation 67, Power Circuit Breaker 67651

Electricity is like invisible fire. It is extremely useful but must be monitored continuously and shut down immediately if it strays outside manageable parameters. That is the job of power circuit breakers, the silent sentinels of our substation system.


It is estimated that the blink of an eye takes 0.1 to 0.15 seconds or 100 to 150 milliseconds. Newer era power circuit breakers can open a circuit in 50 milliseconds. This means that the breaker can interrupt a problem or fault in the system in half an eye blink.

Breakers are extremely reliable and once in service they usually last decades. Some are still in service after seventy years and are functioning fine; these are the original breakers from the beginning of time, MLGW time. Equipment such as this have been improved over the decades and are no longer manufactured. (Often the only reason it is retired is the unavailability of replacement parts--quite a testament to the original equipment and system designers of the early 1900s.)

Recently, a critical breaker at Substation 67 that serves a number of large industrial customers started showing signs that it was approaching the end of its practical life. All replacement tasks—demolition, foundation, ground grid, electrical equipment, control conduits and protection installation—must be coordinated.

The demolition requires no wrecking ball. It involves the removal of the old breaker and its associated conduits and cables. More often than not the old foundation cannot be used. Field crews use a Ditch Witch with a hydraulic ram to bust the old foundation out. (Old breakers are stored as spare parts for breakers that are still in the grid.)

The next order of business is the new foundation. Memphis is in a seismic zone 3 and all foundations have to meet the requirements dictated by this reality. This particular installation is on very sandy soil at the end of President’s Island which adds another complication. The breaker in this case weighs about 11,000 pounds and is approximately 11 x 14 x 19 feet high and must be anchored solidly to a stout foundation.

After the breaker is set it has to be properly tied into the station grounds. All substation equipment and structures are bolted and bonded onto a huge underground grounding net called the ground grid. This grid is made up of buried conductor that is about ¾ inches in diameter. This provides a low resistance path to ground for any sparks that get unruly and again protects equipment and personnel from electrical shock.

It is the “invisible” safeguards that we all depend on. Engineers have termed this protection “the dark arts”. If they don’t get it right, we’re all in the dark – literally.

The next time you blink your eyes, think of the thousands of breakers working thanklessly for you.