What is the bus gap usually use for 138kV and 345kV. Does anyone know and also the fault duration.

The most common approach for defining the arc gap is to calculate it by using the phase-to-ground ground voltage of the circuit and dividing by 10. 10 is the dielectric strength of air: 10 kV per inch from IEEE Std. 4

This would mean for 345 kV:

Voltage line-to-neutral = 345 kV / sqrt (3) = 200 kV

Arc Gap = 200 kV / 10 kV per inch = 20 inches.

Some tables show a range of voltages and the calculations are based on the highest voltage in the range.

Thank you so much!!!

OK for bare minimum theoretical but for those voltages normally you should have a full blown insulation coordination study to reference.

Don't forget to consider transients. I think the general rule is to multiply by 1.3 p.u. when you don't know but it depends on grounding and for transmission voltages they are generally ungrounded so the transient overvoltage factor is much higher.

Does anyone know the working distance for 69kV , 138kV and 345kV for Arc Flash Analysis . I have used 31 inch for 69kV,27inch for 138kV and 62 inch for 345kV. Is it right?

OSHA 1910.269 (2014 final rule) Appendix E states: "employee will be the minimum approach distance minus twice the arc length from the electric arc." which is for developing the calculations for Table 7 - heat energy values . The minimum approach distances can be found by either using tables or by using equations in the standard.

Be careful to use the most current edition (2014) because the MAD values have gone up in general at higher voltages. This has had several utilities that I work with scrambling.