For a fault on an AC network, both the AC component and DC component must be considered. The DC component is a result of the relationship between the amount of resistance (R) and reactance (X) present on the network. This relationship is known as the X/R ratio.
When a DNO provides a fault level for a given point on their network, an initial short-circuit current (Ik" or Ikss), and X/R ratio (or the R and X values) are normally provided.
By producing a waveform with these values, it is easy to visualise how the X/R ratio affects the amount of DC current present at any point.
Switchgear is rated based on the maximum make and break current it can withstand. The make rating is the peak asymmetrical current (ip) which will be seen at the moment of fault. This can be calculated a number of ways, but the prefered method is based on the formula proivded in IEC 60909:
The break rating is based on the the RMS AC current (Ik") present at the time that the circuit breaker opens (break time).
An often overlooked aspect of switchgear ratings is the amount of DC current present at the time that the circuit breaker opens. Switchgear manufacturers provide this limit as a percentage. This percentage relates to the proportion of DC current within the asymmetrical fault current at this time.
The quicker a circuit breaker opens, the higher the percentage of DC current. The addition of a high fault X/R ratio can on occasion raise the DC current percentage to a value greater than the switchgear rating.
Fault currents:Make current (ip) - kA Break AC current (Ib) - kA Break DC current (idc) - kA Break % DC component (%DC) - %