Delta Breaker

It just is in reference to how breakers are rated, especially for use in the US and Canada. Not all of our power systems are Wye (Star) configured with a grounded neutral, some are Delta, especially 480V (or 600V Delta in Canada). However a lot of breakers, both MCBs and MCCBs made for the IEC market are rated for 415V Y, which means the maximum voltage from phase to ground (earth) can only be 240V. For a while, IEC breaker mfrs were testing their 415V breakers at 440V or some even at 480V, but they were assuming that all US 480V systems were Wye, so the maximum phase-to-ground votlage was 277V. Unfortunately that is not true because of the legacy systems that are 480V Delta. When these breakers were used on Delta systems, ground faults were at 480V potential and the fault energy was significantly higher than it would have been at 277V, so the breakers were exploding. So now we have to be careful to use "Delta breakers" if we do not know the nature of the final installation system. Conversely, IEC style breakers that are only rated for use on Wye systems are called "Slash-rated breakers", because the nameplates must clearly state "480/277 only" to indicate that they cannot be used on Delta systems.