The transient period $\tau$ is the number of hours of the loading cycle, usually 24 hours.
With a period of 1 year, a soil diffusivity equivalent to the general and a load factor approaching unity, the current rating approaches steady-state load as defined by IEC. A longer transient period would reach a current rating below the steady-state value. Because we allow to select the soil diffusivity for each system separately, this may cause problematic effects for long transient periods. Therefore we limit the transient period to 1 year for buried cables.
|seconds||of the transient period|
|31557600||1 year = 365.25 days|