msshel0
Member
- Location
- Evansville, IN, USA
We are a mining and heavy civil construction company. When we put things in underground, we are required to have a resistance grounded system. We have ground fault monitors to indicate when a ground fault occurs. The monitor will then trip the appropriate circuit breaker when a ground fault occurs in less than a second. My question is about slash rated breakers.
NEC 240.85 says: "A circuit breaker with a slash rating, such as 120/240 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 an the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker's voltage rating."
I understand that some people use resistance grounded systems in such a way that they keep their equipment running due to the fact that it's dangerous to shut down their equipment. This leaves their systems open to a second ground fault and a dangerous situation. In our case:
1) we will typically have either a 480V phase to phase and 277v Phase to ground system or 600V phase to phase with 347v phase to ground system.
2) we measure the zero sequence ground current to trip the circuit breaker immediately if the ground fault monitor detects current on the ground
Since we immediately trip our circuit breaker, I don't understand why this is anymore dangerous than the solidly grounded system required in 240.85. Can somebody explain to me why I cannot use slash rated breakers on my systems?
NEC 240.85 says: "A circuit breaker with a slash rating, such as 120/240 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 an the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker's voltage rating."
I understand that some people use resistance grounded systems in such a way that they keep their equipment running due to the fact that it's dangerous to shut down their equipment. This leaves their systems open to a second ground fault and a dangerous situation. In our case:
1) we will typically have either a 480V phase to phase and 277v Phase to ground system or 600V phase to phase with 347v phase to ground system.
2) we measure the zero sequence ground current to trip the circuit breaker immediately if the ground fault monitor detects current on the ground
Since we immediately trip our circuit breaker, I don't understand why this is anymore dangerous than the solidly grounded system required in 240.85. Can somebody explain to me why I cannot use slash rated breakers on my systems?