Good day all,

The issue around generators has always been an interesting one.I'm just interested to hear how you deal with generator fault contribution , with respect to arc flash studies.

As you all know , when a generator fault happens, there are three phases : sub-transient, transient and steady states.

Which of the fault contributions do you take into account? During the sub-transient state , the fault current AC component is at its highest magnitude BUT it decays at a very fast rate.But during the transient state , the fault current contribution is lower than the previous state however the fault duration is LONGER.This where my question is originating from.Because higher fault currents mean shorter clearing times and therefore lower incident energy; lower fault currents mean longer clearing time and therefore higher incident energy.Is it safe to use the transient fault currents only ?

IEEE 1584 doesn't specify how you calculate the "bolted fault current" which is used as input for calculating arcing fault current and hence clearing time. Commonly the bolted fault current is taken as the RMS symmetrical current, but not accounting for generator decrement. For generators this would be the transient condition.

SKM uses bolted fault current calculated using its "comprehensive" methodology. Also SKM has user inputs for one step of fault current decrement which is entered as a % of FLC after a specified number of cycles. The user can select whether or not the software recalculates the trip time based on the reduced fault current. This is commonly used to account for field forcing by the AVR.

SKM allows you to change the fault current after a period of time. Here is what their help menu states:

Reduce Generator / Synchronous Motor Fault Contribution To – Generators and synchronous motors do not supply the same amount of fault current after a certain number of cycles following the fault. For example, the fault current may be reduced from the initial 1000% of the Rated Current (10 per unit) to 300% after 10 cycles. Enter the percentage of the Rated Current and the number of cycles after which to reduce the fault current to. PTW assumes a step change from the initial fault current to the reduced value and incident energy will be calculated using the initial fault current and the number of cycles specified, then accumulated with rest of the incident energy calculated using the reduced fault current and the duration at which the protective device trips. The Apply To Generator check box controls whether the reduction of contribution should be applied to generators. If unchecked, generator contribution will be the same as the initial fault for the entire arcing duration. Similarly, the Apply To Synchronous Motors check box controls whether synchronous motor contribution should be reduced after the number of cycles.