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1. Junior Member Pro Subscriber
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Dec 2020
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A system with an operating voltage of 13.8 kV line-to-line utilizes a wye-connected source with a 400 ampere grounding resister. Which of the following is the most suitable rating for lighting arresters?

a. 15kV
b. 5kV
c. 8kV
d. 10kV
e. 12kV

The answer is 12kV, but I am unsure as to why. Can someone help me out on this?

2. Junior Member Pro Subscriber
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A system with an operating voltage of 13.8 kV line-to-line utilizes a wye-connected source with a 400 ampere grounding resister. Which of the following is the most suitable rating for lighting arresters?

a. 15kV
b. 5kV
c. 8kV
d. 10kV
e. 12kV

The answer is 12kV, but I am unsure as to why. Can someone help me out on this?
I briefly looked this up and found the following in Testguy in "Surge Arresters: Selection, Application and Testing Overview"

I found this in the article.

MCOV Example 1: 13.8kV Solidy Grounded System
The continuous operating voltage is 13,800 divided by the square root of 3, or 7970 V. This is above the MCOV of 7,650 V for an arrester rated 9 kV.

Depending on the magnitude and duration of system overvoltages, it may be necessary to use a 10 kV arrester with an MCOV of 8.4 kV or a 12 kV arrester with an MCOV of 10.2 kV.

MCOV Example 2: Resistance-grounded 13.8 kV system
Depending on the time needed for protective relays to clear ground faults off the system, the choice will be between arresters rated 12 kV, 15 kV and 18 kV.

MCOV Example 3: 13.8kV Ungrounded System
The 12.7 kV MCOV of a 15 kV arrester is not adequate for a nominal voltage of 13.8kV. Use an 18 kV arrester with an MCOV of 15.3 kV.

There are multiple choices in each, but your question said 400A ground resistor, so I am assuming Example 2.

This still does not answer your question of "why" but may head you in the correct direction.

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