Why are panel mounted circuit breakers rated 480Y/277 Volts as opposed to just 480V ? 2

[bdn2004]

Attached is a cut sheet of an Allen Bradley DIN rail mounted circuit breaker. This is the latest style that replaced a similar circuit breaker. The old one was only rated 480V, this one has the 480Y/277V rating.
What's the purpose of this rating? Can it not be used on a delta system ? The panel these are going in have no 277V.

 

[dpc]

https://www.ecmweb.com/content/understanding-circuit-breaker-markings

Depends on the system. They cannot be used on a corner-grounded delta. For a high-leg delta, they cannot be used on the high-leg.

 

[STEPE2]

Phase to phase is up to 480Volts RMS AC, Phase to neutral is 277Volts RMS AC

 

[LionelHutz]

"Can it not be used on a delta system?"

The answer is no. It must only be used on a solidly grounded system where the line to ground voltage can't exceed 277V.

 

[jraef]

The 1489-M replaced the 1489-A series, it too was rated 480/277V so I don't know what you are comparing it to. All IEC style DIN rail "Miniature Circuit Breakers" (MCBs) on the market under any brand are what's referred to as "slash rated", meaning 480/277V, not straight 480V. The internal clearances and separation, as well as the asymmetrical fault strength (withstand) are all based on a maximum line-to-ground voltage of 277V. In a 480V delta system, the theoretical L-G voltage could be 480V, so a grounded fault would have more energy in it and the breaker may not hold together as it attempts to interrupt it. If you know for SURE that the system the MCBs will be used in will be 3 phase 4 wire services, you can use MCBs (however they have MUCH lower AIC ratings). If the installation location is unknown, you must err on the side of caution and NOT use an MCB.

To get a true 480V (delta) rating (or when you don't know), you will have to use a Molded Case Circuit Breaker (MCCB), and even then some brands are still slash rated so you have to be careful. But in A-B breakers, you can use the 140-G MCCBs at 480V, no slash, and there is a DIN rail adapter for them


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