**4.3 Digital Fault Recorder (DFR)**

The Digital Fault Recorder (DFR) acts as the black box of a substation. It records highly accurate waveforms related to faults. The recorded data are huge amount of analog and status data for pre-fault, fault and post-fault conditions (Kezunovic, 2008a). These data may include maximum current, sequence of events, type of fault and the sequence of operation of circuit breakers (Thomas et al., 2006).

The sample rate of the DFR is normally very high and assumed to be 64 to 356 samples per cycle. The DFR data is normally formed in COMTRADE format (IEEE Inc., 2006). Additionally, the data captured by DFRs are offline and they are not used in real time. These data are stored as samples for further offline processing. Generally, the DFRs are installed in the most important substations (Kezunovic, 2008a).
