OCPD settings for Generator Main Breaker

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Machiavelli999

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IEEE Buff Book (relevant section attached) recommends that the protection curve for the generator main breaker fall to the right and above the generator decrement curve in the instantaneous region. Why is that the case? With the breaker my generator is equipped with, this limitation means I will not have an instantaneous region for my main breaker. This creates a high AF hazard category for my emergency distribution switchgear.

The reasoning for the protective device curve being above the decrement curve is confusing. If the decrement curve represents the maximum fault current from the generator, why would I want my protective device be in front of this curve?
 
My understanding is that the generator decrement curve is an unregulated representation of the generator's output capability. Note the "GEN SC" point, labeled as such in the SKM representation. That point is above and to the right of the decrement curve, and represents the maximum output capability of the genset.
You typically don't want instantaneous on the gen main so that it can have a better chance of running when a fault occurs downstream. And generators are usually microprocessor protected against overload and damage.
There's nothing that says you can't introduce instantaneous, thereby lowering the AF level, but it will likely be at the expense of coordination.
 
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