This "Cut the Copper" blog examines how the medium-voltage (MV) vacuum circuit breaker came into surface and its benefits over air-magnetic breakers.

Joe Guentert

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In the 1970s, GE dusted off a fault interruption technology initially developed in its labs 45 years earlier, introduced this new product, then placed it into commercial service—the medium-voltage (MV) vacuum circuit breaker. This breaker came onto the market with significant advantages over the medium voltage air-magnetic breakers it replaced (as well as MV oil circuit breakers prior to that).

The new technology reduced the size, weight, cost, complexity and maintenance requirements of previous technologies, while increasing the durability and longevity of the breaker, and the new breaker quickly gained wide acceptance in all markets. Other manufacturers quickly followed suit with new vacuum breaker designs of their own.

Fault interruption was crisp and fast and precise, and after interrupting even a large fault within just microseconds after contact opening and the first current-zero on each pole, the contacts that had just interrupted the fault arc inside the vacuum bottles instantly healed themselves, by condensing and re-depositing the plasma of the arc back onto the contacts as fresh new, smooth metal.

This was fantastic for interrupting a large fault, but turned out to be not so good for switching on and off inductive loads (like MV transformers and motors) – as users soon learned. The main problem was that upon opening a breaker, the current that had been flowing through the inductive load stopped flowing instantly. The interruption of current flow went from SOMETHING to NOTHING in virtually zero time, and the energy trapped inside the inductor instantly displayed itself as a huge transient voltage across its outer winding terminals, and could also stressfully distribute the excessive voltages deep within the interior windings and winding layers of the transformer.

This was a far different current interruption than had been provided by air magnetic breakers, which had drawn the fault arc comparatively slowly through a long and large “arc quencher” path, causing a relatively lengthy interruption and arc cooling process. The abruptness of operation of the new vacuum breakers caught the industry off guard and unprepared for a new phenomenon - frequent catastrophic failures of downstream transformers, in ways that had almost never been experienced before.