Ask the Engineer - At what amperage does a 20-amp fuse blow?

Question from Antonio Esparza: At what amperage does a 20-amp fuse blow? Is it exactly at 20 amps or is there a variance?

Engineer Reply: STI Cabinet PDU's utilize Bussmann SC-20G fuses to protect the outlets at 20 amps. Fuses don't have a fixed point for tripping, though they are consistent from one fuse to the next. So, don't think of it as a variance. The 20A rating does not actually mean that it will trip at 20A like flipping a switch. Fuses are resistive devices that get hot with current flowing through them. They can only withstand so much heat, so the higher the current flowing, the faster they blow. The chart on page three of the spec shows the tripping time for any particular current. For instance, 30A will take roughly 100 seconds, 73A will take roughly 1 second, and 120A will take roughly 0.1 second. Keep in mind that a short circuit fault is going to attempt to draw thousands of amps, so tripping will be much faster. Indeed, one of the benefits of using fuses over circuit breakers is the reaction being as much as 5x faster for a short circuit fault.

Comments

[Mark]

Thank you for clarifying, as I never realized that. From time to time, I would hear from the data center folks our breaker is drawing 21 amps on a 20 amp circuit, and didn't realize why it wasn't tripped. Thanks.

[Robert]

Yes, a somewhat similar trip curve can be seen with hydraulic-magnetic circuit breakers wherein a slight over-current will take a very long time to trip. In both cases of the circuit breaker and fuse, the idea is to delay a slight over-current trip(which won't typically cause actual damage), thus avoiding the "nuisance trip".

[Jospeh Jones]

Can you explain selective coordination and how it effects when a circuit breaer trips and why?

[Robert]

Selective coordination is the act of isolating a faulted circuit from the remainder of the electrical system, while maintaining uninterrupted power to the unaffected circuits. Proper selective coordination eliminates unnecessary power outages and reduces costly downtime. The faulted circuit is isolated by the selective operation of only that over-current protection device closest to the over current condition. Fuses, by acting faster than the upstream breaker, ensure selective coordination. Additionally, lower amperage rated fuses require less energy to open the circuit than higher amperage rated devices. This allows fuses to be very easy to selectively coordinate. Circuit breakers require a coordination study to ensure selective coordination. Overlap of circuit breaker trip curves between the upstream and downstream devices often results in simultaneous operation of both devices. A circuit breaker system will clear the fault condition and open the circuit, but it will also remove power to all of the remaining loads being served by the PDU.