After looking through my Electrical Engineering Handbook, I regret to say that giving you any more information regarding skin effect calcs than what Chris has submitted would be almost impossible.
The topic is spread across 5 sections of my Handbook, each formula requires a variable value which is obtained from either a database, graph or table. Also, writing the formulas out as text only becomes kind of confusing!
Here are a few basic calcs - which are for Skin Effect Ratios on a Linear Cylindrical Conductor to Sinusoidal AC of a given Frequency.
First is for Effective Resistance [R']:
R' = KR (Ohms)
R = True Resistance with continuous current,
K = Value determined from a Table, in terms of x. The value of x is given by:
x = 2*pi*a* sq rt 2*f*u/p
[a = radius of conductor in Cm's, f = Frequency in CPS, u = magnetic permeability of conductor - assumed to be constant, p = resistivity in abohm-centimeters (abohm = 10^-9 ohm)].
For practical calcs, the x value can be expressed as:
x = 0.063598* sq rt f*u/R
where R = DC resistance at operating temp.
If L' is the effective Inductance:
L' = L1 + K'*L2
[L1 = External portion of Inductance, L2 = Internal portion (due to mag. field within the conductor), and K' is found in the table on some page in another section!!]
The total effective Inductance per unit ength of conductor is:
L' = 2*l*n* d/a + K'* u/2
[a = radius of conductor, d is separation of conductors, l and n are derived from some other formula in some other section!].
Best suggestion is to take a trip to your local Library and reference an EE Handbook for this data. If you can afford to buy a Handbook, that would be a nice investment [if you plan to do advanced calcs in the future - otherwise it's just an expensive paper weight!].
The calcs are really simple once you have all the data and formulas, plus the tables and charts / graphs. This stuff is close to impossible to type out and the tables would take me hours to type in a message post!
Per the Hz of faults, the figures of Intensity are plotted at the Fundamental Hz of the system, and with durations from multiple cycles [1 to 100 cycles] for longer Time-Current curves per Intensity, to fractions of a cycle [1/8 to 1/2 cycle] for shorter Time-Current curves per Intensity.
These peak values are figured with the Fundamental Hz as the plotted reference and time reference.