Minimum Clearances for Switchgear

[crthompson]

I work for a switchgear manufacturer and we have been using the attached chart as a guideline for clearances for around 30-35 years. I have been told that the chart came from an old NEMA standard but several engineers as well as myself have been unable to find the original. We have checked IEEE, ANSI, & NEMA. The problem with the chart is when you go from 15KV @ 110 BIL to 25KV @ 125 BIL, the minimum clearance drops... Is there any way that some of you may be able to shed some light on this situation? On the bright side, this is the first time to my knowledge that we have designed switchgear over the 15KV class. Any help will be greatly appreciated!! Thanks in advance!!

 

[PHovnanian]

Just a wild guess here: The figures for 25 and 34.5 kV may have been developed at a later date as these system voltages came into wider use. Experience probably dictated the suitability of the reduced values. The author of the original table (whoever that might be) just appended the new values to an old table.

Being somewhat conservative, I'd use the 15/110 values for a 25/125 system as well. If someone chose to derate the 25 kV gear for a 15 kV system, found the same table and got nit-picky, they could reject it.

 

[crthompson]

Thanks for the input!

 

[jraef]

Isn't that all covered in IEEE/ANSI C37.20?

The original NEMA standards were superceded when AIEE came into being in the 50's (so I'm told, because I'm not THAT old...), which became IEEE. So if yours were from NEMA, they are likely long outdated.

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[crthompson]

Very helpful Jeff! A fellow engineer was tasked with checking our copy of the C37... I will pass on the information and hopefully get this straightened out... Many thanks!

 

[jraef]

So depending on the class of switchgear you have, C37.20.1 is LV, C37.20.2 is MV Metal Clad, and C37.20.3 is Metal Enclosed (Load Interrupter) Switchgear.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376