Coordination Time Interval

[timm33333]

When coordinating an upstream fuse with a downstream LV breaker, the IEEE buff book table 15-3 requires a minimum time interval of 0.2 sec between the curves of fuse and breaker. Should this 0.2 sec interval be maintained throughout the lengths of the two curves, or only at the bottom of two curves? Thanks for help.

 

[davidbeach]

Minimum time. As currents get smaller and times get longer there will be a greater interval.

 

[timm33333]

Thanks! Also, the concept of momentary and interrupting is little confusing. Why are they sometimes called “duty” and sometimes “current”?

In short circuit analysis, the momentary occurs at 1/2 cycles and the interrupting at 4 cycles, this makes sense. But I see that when they put the 1/2 cycle rating on the nameplate of the breakers and fuses, they call it interrupting (instead of momentary). Why do they call it interrupting on the nameplates of breakers and fuses? Thanks for help.

 

[LionelHutz]

Momentary (or sometimes called withstand) is the fault current an item can withstand. Interrupting current is the fault current an item can interrupt.

Items that can't or shouldn't be interrupting current (disconnect switch, overloads, wire, etc) need a withstand rating. Items that can interrupt the current (fuses, circuit breakers) would have an interrupt rating.

There are some items that can have both, such as a contactor that may be capable of interrupting a lower level fault current but can also have a higher withstand rating when used with a circuit breaker or fuse so the contactor withstands the current and the fuse or breaker interrupts it. A case I can think of is a vacuum contactor that is rated for 400A and can interrupt around 6kA itself or withstand 50kA when used with fuses.

 

[wroggent]

I may be wrong here (someone please correct me if so) but I believe the 1/2 cycle or 4 cycle current will be used to verify proper interrupting duties of protective devices based on the operating speed of the protective device in question. I believe it is appropriate in some cases to use the 4 cycle current as the interrupting current for medium voltage breakers with relaying because they are relatively slow to operate compared with low voltage breakers and fuses (current limiting type especially) which would use the 1/2 cycle current. Since I am unsure I always use the 1/2 cycle for all interrupting ratings.