DC Arc Flash Calculation Example

Test Project: DC Demo

The DC Arc Flash in PTW utilizes the Maximum Power Method (Annex D, D.8.1.1) from the NFPA 70E 2012 standard.

The bolted fault current used in the equation is obtained from the DC short-circuit study. Since thefault current in DC varies based on time duration, the maximum fault current is used for the bolted fault current.

From the given demo project, we will examine the DBUS-0002 as an example for DC Arc Flash:

DC Short-Circuit Results:

Maximum Bus Fault Current for DBUS-0002:

Maximum Branch Fault Current for DCBL-0002:

Maximum Branch Fault Current for DCBL-0001:

Maximum Branch Fault Current of DCBL-0003:

Arc Flash Study Setup:

Arc Flash Result:

Based on the TCC of the protective device CB2 shown below, the trip delay time is 0.017sec.

Since the branch bolted fault current thru CB2 is more than the specified Clear Fault Threshold of 80%, and no other device is tripping before CB2, then IE of 0.18 cal/cm2 is reported and PPE Level is Category 0 based on the PPE table shown below.

Similar calculation is done for CB1 and CB3 branch but since CB2 has already cleared the fault, it is not considered as the worst case even though the incident energy is higher than CB2.

Accumulated Energy Example:

DC Arc Flash in PTW implements accumulation of energy where multiple contributions feed a single fault location. Using the same sample above, the clear fault threshold will be increased to 100% and set the reported device to be the “Last Trip Device” to exhibit the accumulation of energy.

The updated Arc Flash results for the setting change is shown below:

The total arcing fault current for DBUS-0002 is 8929A. Based on the maximum branch currents from CB2, CB1 and CB3, the three branches adds up to 8924A. The remaining 5A is reported on the 4th row of the table and since there is no protective device to report, the “MaxTripTime@20s” is placed. Notice that note *N2 is also indicated since the sum of the fault current thru the three branches is less than the clear fault threshold of 100%.

From the AF result above, CB2 will trip first at 0.017 sec. During this period, all branches (CB1 + CB2 + CB3 + Other) are contributing to the fault at the bus. See below for the fault current at the bus at certain arcing time periods.

At arcing time 0.017 sec, the calculated incident energy is 0.18 cal/cm2 (refer to previous sample above). Since we set to clear the fault at 100% and the remaining devices all trip at 20 sec, then subtracting the prior trip time, the remaining 43A will last for 19.983 sec.

Thus, the total IE for the bus;

The report is set to show the last trip device from the study options, therefore the “MaxTripTime@20s” is reported as the protective device in the Summary View and shows the total incident energy of 1.2 cal/cm2 with a Category 1 PPE Level.