BOOK_CONTENT
From Protection of Electrical Networks

10.1 Network protection

Notations

? t

selectivity interval (generally set at 0.3 seconds: see section 9.2)

I setA

current threshold of the protection located at A ("A" for arrival)

I setDi

current threshold of the protection located on the outgoing feeder Di ("D" for departure)

I sc2,min, A

minimum phase-to-phase short-circuit current with protection located at A

I sc2,min, Di

minimum phase-to-phase short-circuit current with protection located on the outgoing feeder Di

I ?

resistance limited current

I C

total capacitive current of the network when a phase-to-earth fault occurs,

I Ci

capacitive current generated by the outgoing feeder Di when a phase-to-earth current occurs, I Ci=3 jC i ? V n

I p

permanent current that the earthing resistor can withstand

I nCT

current transformer rating

t H

time delay of the protection located at H, H1 or H2

t A

time delay of the protection located at A, A1 or A2

t D

time delay of the protection located on the outgoing feeders Di

t N

time delay of the protection located on the neutral earthing connection

V set, A

threshold of the residual overvoltage protection that trips the circuit-breaker at A, A1 or A2

t R

directional protection time delay

I set, R

directional protection current threshold

We will first of all determine the requirements of the network and equipment for earth faults relevant...

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