The slash rating, under the United States National Electrical Code, is given to circuit interrupt hardware and specifies a maximum line-to-ground voltage rating in combination with a maximum line-to-line voltage rating. One common application would be for a three-phase

Three-phase

In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...

 electrical loads. For example a 120/240V rating can disqualify a circuit breaker for use with a delta system load that would otherwise work with a wye system load.