Some circuit breakers have dual voltage ratings stamped on them, such as 240/120V or 480/277V. These breakers are known in the industry as slash-voltage rated breakers. Slash-voltage rated breakers are tested to interrupt fault current with a higher voltage between phases and a lower voltage between phase-ground. Lower voltage rating comes into play when the breaker is interrupting phase-to-ground fault current at phase-ground voltage (single pole clearing). Higher voltage comes in to play when phase-phase or three-phase faults are interrupted (two or three pole clearing). Slash-voltage rated breakers are not rated to open faults at phase-phase voltage across only one pole. Slash-voltage rated circuit breakers take advantage of the fact that the power system neutral is fixed at ground potential. However, neutral potential not always be at ground potential and could vary depending on the source grounding scheme used, thus putting restrictions on using slash-rated breakers. Only for a solidly grounded source will the neutral remain at ground potential for all conditions. When there is a possibility of full voltage across a single pole, a straight rated breaker should be used.

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In contrast with the slash-rated breaker is the straight-rated breaker, e.g., 240V or 480V. These breakers can be applied in circuits with a nominal voltage between any two conductors that does not exceed the circuit breaker voltage rating, except in corner grounded applications where the breaker has to be marked 1𝞍-3𝞍 to ensure suitability.

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National Electric Code 240.85 restricts the use of slash-rated breakers to systems that are solidly grounded. This is to ensure the phase-ground voltage does not exceed the lower end of the breaker voltage. When there is a possibility of phase-phase voltage across a single breaker pole, fully rated circuit (straight-rated) breakers must be used. A fully rated breaker has only one voltage rating, such as 240V or 480V, etc. An example of when phase-phase voltage can appear across a single pole is when A phase short circuits to ground in a corner B-phase grounded system, as shown in figure 3.  

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NEC 240.85 states, “A circuit breaker with slash rating, such as 120/240V or 480Y/277V, shall be permitted to be applied in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker’s voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker’s voltage rating.”

Voltage is a critical factor in the safe interruption of fault current. A typical circuit breaker in Figure 2 indicates that the manufacturer provides different fault interrupting capabilities (18kA@240V and 14kA@480V) at different voltages.

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Per ANSI C37.16, when circuit breakers are applied on systems where the rated maximum voltage may appear across a single pole, short circuit ratings are 87% of the three-pole rating. This means that for marginally applied breakers, there is a risk of overduty if the ground fault current is high. Corner grounded delta connection requires special attention, and the manufacturer should be consulted for breaker application.

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If a control panel has one or more breakers that are slash rated, UL508A requires panel voltage marking to be slash-rated.  

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For current limiting fuses, the marked interrupting rating is the tested single-pole interrupting rating. There are no slash ratings for fuses. Ensure the fuse is rated for an appropriate circuit voltage, depending on the source grounding. If the source is ungrounded or resistively grounded, select fuses rated for phase-phase voltage, and if the source is solidly grounded, fuses can be rated for phase-ground voltage or higher.

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Ref 1: IEEE Green Book: IEEE Standard 142-1991, Ref 2: IEEE Blue Book: IEEE Standard 1015-2006