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Participant

Join Date: Oct 2012
Posts: 1

### Switchgear Question

10/20/2012 9:30 AM

why LA of 60kv is used for 66kv line.

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Guru

Join Date: Oct 2007
Location: India
Posts: 1272
#1

### Re: switchgear

10/20/2012 9:45 AM

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

Take a look at this:

QUALIFYING REQUIREMENT DATA - Gujarat Electricity Board

Guru

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 863
#2

### Re: Switchgear Question

10/20/2012 10:52 AM

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.

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Guru

Join Date: Oct 2009
Posts: 1884
#4

### Re: Switchgear Question

10/21/2012 6:57 AM

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.

Guru

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 863
#8

### Re: Switchgear Question

10/22/2012 8:27 PM

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.

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Guru

Join Date: Oct 2009
Posts: 1884
#3

### Re: Switchgear Question

10/21/2012 6:51 AM

LA Voltage is selected based on the formula: HSV * Co-efficient of Earthing

For 66kV, the HSV (Highest System Voltage) is 72.5kV.

66kV System is soildly earthed. SO, the Co-efficient of Earthing is 0.8.

Hence, LA voltage will be: 72.5 x 0.8 = 58kV

The nearest standard LA available is 60kV. So, it is 60kV.

Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 2054
#5

### Re: Switchgear Question

10/22/2012 5:40 AM

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

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Guru

Join Date: Oct 2009
Posts: 1884
#6

### Re: Switchgear Question

10/22/2012 7:15 AM

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.

Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 2054
#7

### Re: Switchgear Question

10/22/2012 9:09 AM

OK thanks

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