Lightning Arrestor Voltage Question

where is the arrestor being applied?

Depends upon the maximum line to ground operating voltage, type of construction, grounding, and what you are trying to protect from impulse overvoltage. Temporary 50 Hz overvoltage under single line fault conditions at the arrester location is also a consideration.

Normally 33 KV rated equipments are subject to 225 KV impulse testing. Impulse testing is to test equipment against lightining voltage withstandability in laboratory condition with wave shape os 1.2/50 microseconds. So, your arrestor should be rated as follow.

(1) Rated voltage - 33 KV

(2) Maximum rated voltage - 1.1 * 33 KV

(3) Rated Impulse level - 225 KV

(4) Rated Maximum Current withstandability - According to potential lightning hazards available to your region

I think, this is enough for your question

Doesn't it still depend on the arrestor's application? Also, KV is different than kVa...

I don't believe that 33,000v rated equipment is hit with a 225,000v impulse...something, and quite possibly everything, is going to be fried.

I'm not sure what the decay(wave shape) would be with a 50hz impulse either...Although, I guess you can hit a 50hz with a 60hz...just not the other way around.

Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage.

a 225 KV impulse is going to be done on 225KV rated equipment(225kv BIL)

the 33,000volt equipment might have a 225KV BIL...in this case it can with hit with an impulse(impulse testing is also referred to as 60Hz withstand testing).

BIL (Basic Impulse (Insulation) Levels)

the type/size(etc..) of arrestor will depend on the application, need more info. from original posting party...

Just a couple of comments. There is no 50 Hz (nor any other frequency) associated with an impulse voltage wave. By definition, impulse is a wave of unidirectional polarity; either positive or negative. The standard impulse voltage wave is 1.2/50 (1.2 microsecond front defined as the slope of a line drawn between 30 and 90% of crest) x 50 microseconds (time to half crest on the tail). Thus, impulse tests are not the application of a high frequency steep wave front voltage and impulse testing is never referred to as 60 Hz withstand testing. They are different tests conducted with different test equipment (transformer, impulse generator).

First off, thank you for the correction on the 60Hz withstand. I believe that is actually HI/Lo-POT 1minutes 60hz testing of transformers' bushings insulation levels at operating freq. and voltage.

high freq. steep wave front voltage is a type of impulse.

but it is not a lightning impulse, you are right.

lighting impulse withstands ARE impulse tests using the standard 1.2/50us lightning impulse with a crest value equal to the BIL or LIWL

BIL is actually going the way of LIWL (Lightning Impulse Withstand Level)

basically the one who started the thread needs to let the application of the arrestor be known. Is it on a 33kVa transformer? Is it on a building? Is it internal? Will only one be require? Is it in on the transmission line?

Insulation and Arrestors are designed to limit lightning surges to below the BIL/LIWL of equipment.

Check out IEC 71-1

Rated voltage - 33/Sq. root 3 KV

Maximum rated voltage - (1.1 * 33)/sq.root 3 KV

Rated Impulse level - 225 KV

And according to potential lightning hazards available to your region

Divide line to line voltage by the square root of 3. Thats your line to neutral voltage. Choose arrester MCOV that is closest next size up.

For 230V line it is 300V, if it is 400V (ie three phase) it is 600v

check out

http://www.cooperpower.com

http://www.hubbellpowersystems.com/powertest/ohio_brass/ohiobrass.html

http://www.joslynhivoltage.com/

these guys can answer any questions you may have...