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Anonymous Poster #1

MCOV Rating of LA

11/19/2017 10:16 AM

Hi,

For a Rated voltage of 120 kV lightning arrester - what will be the MCOV rating?

I have read in books that this ranges from 0.8 to 0.9 of rated voltage.

But in electrical equipments a single value is given. Can sb please help?

Regards,

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#1

Re: MCOV Rating of LA

11/19/2017 11:18 AM
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Anonymous Poster #2
#2

Re: MCOV Rating of LA

11/19/2017 12:12 PM

Try this.

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#3

Re: MCOV Rating of LA

11/19/2017 12:53 PM

You have to know the class and standard....

..."What's in a class? ANSI/IEEE C62.1 (IEEE Standard for Gapped Silicon-Carbon Surge Arresters for AC Power Circuits) and C62.11 (IEEE Standard for Metal-Oxide Surge Arresters for Alternating Current Power Circuits) separate LAs into four classes: station, intermediate, distribution, and secondary.

Each type provides different levels of protection and energy diversion. The station class offers the best protective level and is capable of diverting the most energy. The intermediate class has the next best level; but a lower energy diversion capability than station-type LAs. The distribution class provides the worst protective levels and lowest energy diversion. Because the secondary-type LA doesn't overlap in voltage range with any of the other classes, it's difficult to make a direct comparison. Keep in mind: As you progress from station to distribution class, the cost significantly declines, but so does the protective level. By comparing cost to benefit, you can get the most efficient arrester for the application.

What's in a rating? A metal oxide varistor (MOV) arrester has two voltage ratings: duty cycle and maximum continuous operating voltage, unlike the silicon carbide that just has the duty cycle rating.

Duty cycle rating. The silicon carbide and MOV arrester have a duty cycle rating (in kV), which duty cycle testing established. This testing subjects an arrester to an AC rms voltage equal to its rating for 24 min, during which the arrester must withstand lightning surges at 1-min intervals. The magnitude of the surges is 10kA (10,000A) for station class arresters and 5kA for intermediate and distribution class arresters. The surge waveshape is an 8/20, which means the current wave reaches a crest in 8 ms (8 microseconds or 0.000008 sec) and diminishes to half the crest value in 20 ms.

Maximum continuous operating voltage rating (MCOV). The MCOV rating is usually 80% to 90% of the duty cycle rating. Table 2 lists the MCOV ratings of various MOV arresters."...

http://www.ecmweb.com/content/arrest-your-lightning-protection-problems

http://www.nemaarresters.org/understanding-arrester-datasheet/

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#4
In reply to #3

Re: MCOV Rating of LA

11/20/2017 2:45 AM

SolarEagle, not a correction just a note. In South Africa we use the lowercase "u" to substitute the "mu sign" that signify Micro to limit the confusion created by using m for millies and micros. 8uS in stead of 8mS to distinguish between micrs and millies. Just a note no correction intended. I understood what you wrote.

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#5
In reply to #4

Re: MCOV Rating of LA

11/20/2017 10:08 AM

He bloody well confused the shank out of me. How can it be 20 milliseconds when the whole entire thing takes 8 microseconds?

I have always hated when people stopped using μ to signify "micro". It is a perfectly good Greek symbol. Why do we keep edging out the Greeks? They deserve royalties on their alphabet. LOL

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#6
In reply to #4

Re: MCOV Rating of LA

11/20/2017 2:18 PM

Yes it certainly doesn't hurt to over-explain terms nowadays...

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