What do you mean by " LA of 60kv " ?

Take a look at this:

QUALIFYING REQUIREMENT DATA - Gujarat Electricity Board

Because 66kV is the line to line voltage, the line to ground/neutral voltage is 66 kV / √3 = 38.1 kV. Lightning arrestors are applied line to ground/neutral.

Mr. RamConsult,

If a LA used in a 66kV system is to be selected on the phase-to-ground voltage of 38.1kV, then one could go for a 40kV arrestor (which would be much cheaper) rather than a 60kV arrestor. If such an LA is selected, it is sure to blast even under normal system conditions, leave alone during lightning strokes. Your answer is grossly incorrect. I am surprised that you got couple of GAs too for this answer. Pl. see my previous answer (post 3) for calculating the LA Voltage rating.

I wish correct advice is given on such crucial issues of High Voltage Installations.

Thanks for your feedback. Please note that the OP did not ask "how to select a LA" but appeared to me was instead looking for the rationale behind the apparent (to him) illogical use of a LA rated below the system voltage. I'm assuming that once he was made aware of the difference between system voltage and line to neutral voltage he quickly realized the error of his ways and gave GA for clearing that up for him. Simple question easy answer, and I never made any statement about how to choose a lightning arrester.

You chose to read more into the question, which is your prerogative, but please do not read more into the answer than what was stated. Had I done the same I would have asked he where he was located since I believe that it borders on prejudicial to use a person's surname to do so. I would have also sent him this link from GE with the caveat that it's use is highly dependent upon various utilities' practices and their locations. It is an decent primer on LA selection and will further any reader's understanding of this complex subject.

Interestingly the methods and nomenclature that you use do not appear in this document, nor are the results identical, further pointing out the difference between US and international standards, and that location, location, location should always be stated first by posters and respondents alike.

Not disputing what you say, but where does the HSV 72.5kV come from? From a formula or a published table?

It is based on the IEC Standard on Insulation Co-ordination. Generally, the highest system voltage is 10% above the Nominal System Voltage. Thus, for 66kV, it would be 66 + 6.6 = 72.6kV, rounded off to 72.5kV.

OK thanks