Medium Voltage Application Guide

SWITCHGEAR
Exercise
A 17.5 kV secondary distribution, indoor switchgear system requires MO arrestors to be fitted on the incomer side.
The system supply is 15 kV/50 Hz and is isolated from earth. Use calculated ratings and the manufacturers’ data
sheet for selection.
The highlighted numbers 1~4 refer to the solution page.
Step 1: Calculate the MO arrestor ratings
Continuous operating voltage
For an isolated earthed neutral system, continuous operating voltage (U c ) ≥ system voltage (U s )
U c ≥ 15 kV
Rated voltage
Rated voltage (U r ) ≥ 1.25 times continuous operating voltage (U c )
U r ≥ 1.25 x 15
U r ≥ 18.75 kV
Nominal discharge current
For a secondary distribution system, use a lightning-impulse nominal discharge current rating (I n ) of 10 kA.
Residual protection voltage
The residual protection voltage (U res ) developed across the MO arrester, when discharging 10 kA, must be
less than 70% of the equipment lightning-impulse rating (U p )
U res U p x 0.7
95 x 0.7
66 kV
Step 2: Using the calculated ratings, select an MO arrestor from the manufacturer's data sheet
Page 134 Medium Voltage Application Guide 710-12280-00A