The Skinny on Switchgear and the New NEC

摘要

It must be understood that the short time delay from not having an instantaneous trip can allow fault currents to flow for several cycles (typically up to 30 cycles), which can subject the electrical system to high mechanical and thermal stresses. The system equipment must also be coordinated to ensure it can withstand this tripping time delay. Switchboard construction will handle its rated fault current for only three cycles, while switchgear construction can handle its rated fault current for 30 cycles. There is, however, one particular manufacturer that I know of who makes a hybrid type of switchboard that will handle the rated fault current for 30 cycles. But in general, if the engineer chooses to go the route of providing breakers with no instantaneous function, the system must use more expensive switchgear con-struction.