##### X/R Ratio and DC%

X/R ratio is simply the ratio of the system reactance to the system resistance, looking back to the power source from any point in a power circuit, assuming that a short circuit is applied to the system at that point. It is another way of stating the power factor of the source system.

For a given level of symmetrical fault current and a given circuit breaker contact part time, this X/R ratio establishes the value of asymmetrical fault current the breaker is required to interrupt. A higher X/R ratio, with its slower decay rate, will result in a higher asymmetrical fault current at contact part time. If the X/R ratio is too high, the asymmetrical fault current may exceed the breaker's interrupting capability.

In both the ANSI/IEEE and the IEC standards, the time constant of dc decay for distribution applications is standardized at 45 ms, which corresponds to a X/R ratio of 17 for a 60 Hz system.

Your Hz power grid network with
X/R = has a time constant **τ =
ms.** If you are using high voltage circuit breaker with min. opening time of
ms and rated
short circuit current kA, rated DC%
of this circuit breaker for the normal high voltage network conditions, τ = 45 ms is **
%**