Switchgear Explosion Due to Ferroresonance

03 January 2023

Electricity can be difficult to understand, even for experienced electrical engineers, as electricity plays by simple, general rules until it doesn’t. 

Lightning, for example, does things that electricity typically doesn’t do. People are often surprised by the bizarre things that lightning can do, like go through rubber or other insulating materials. Lightning is just one electrical phenomenon that seems to break the general rules by which most electrical engineers live by and there are others that many people may never see.

The Explosion

Metal-enclosed medium-voltage switchgear is a long name for the specific type of heavy-duty electrical equipment that was being installed in a medical facility that was under construction. The equipment was rated for 35,000 volts but was only operating at 25,000 volts. The equipment didn’t have much current flowing through it since the building was under construction and most of the equipment wasn’t installed. Yet somehow, early in the morning, there was an explosion that sent a 75-pound panel flying across the electrical room, through a door, and another 15 feet down a hallway, leaving a cartoon-like imprint of the panel on the doors.

Inside this piece of equipment were a few damaged fuses and a damaged transformer. Oddly, some of the resin that encased the transformer seemed to have melted and spewed out from the inside. Further inspections from Envista’s Electrical team revealed that the interior of the transformer had in fact heated up and shorted out, which resulted in the expulsion of hot ionized gasses and vapors inside the switchgear. These gasses don’t insulate like clean air and resulted in the arc flash that blew the doors off.

A Rare Phenomenon

At the time of the explosion, the transformer had almost no load on it, which was found very odd, as it is very unusual to have a transformer fail when it’s not being stressed by a lot of electrical current. What became clear during Envista’s investigation was that this particular installation had all the components necessary for a rare phenomenon known as ferroresonance

Ferroresonance can only take place in electrical systems where there is very little load, such as a building under construction. In this case, there was a three-phase system with long underground wires to supply the necessary inductive and capacitive components for the phenomenon to take place. The result was resonant voltages far in excess of the designed 25,000 volts and an eventual explosion.

When things work as they’re intended, forensic engineers do not get involved, as forensic engineers typically spend their time in the world of the unlikely and weird. This case is just one of many bizarre and unusual losses that Envista’s forensic engineering team, the experts at identifying the things that many people may never see, encounters in our day-to-day.

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About The Author
Brent Powell
Brent Powell, PE, CFEI
Senior Project Engineer

Mr. Brent Powell is a Senior Project Engineer with over 10 years of experience in electrical engineering including investigations of electrical system breakdowns and malfunctions, electrical power distribution, lighting and photometric, and fault current analysis. He has experience in many different industries including industrial manufacturing, industrial food manufacturing, healthcare, aviation, education, commercial retail and shell, and oil and gas.

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