Medium voltage switchgear application guide

Medium voltage switchgear application guide
5 - CABLES OR
CABLES/LINES(cont’d)
■
The
BREAK
breaking
THE
capacity
INSTALLATION’S
must be higher
FAULT
than
CURRENTS
circuit current possible at the installation point. or equal to the maximum short-
If the device is used as an incomer
The circuit breaking current capacity possible must at the be installation higher than point. or equal to the maximum short-
The on the short-circuit type and current section limited of the conductors. by the overhead or underground link depends
Ik = 3 U•Zk ■for X 0.4 an ohm/km overhead = U
3 • R2 +X2
link
R = ρL
Three-phase ■for an S with ρ= 3.310-6 ohm cm2
underground cables: link
for Almelec
Single-phase R = ρL cables: X = 0.1 0.1 to to 0.15 0.20 ohm/km ohm/km
S with ρ=2.810-6 ρ=1.810-6 ohm ohm cm2
cm2
for copper
for aluminium
If the device is used as an outlet
The protection switch device does not located have upstream a short-circuit which current provides breaking protection capacity. against It is shortcircuits.
the
the It must time however needed be to eliminate able to withstand it. the short-circuit current induced for
If or protection the fuse-switch against combination short-circuits must is required be used. the fused switch-disconnector
the The short-circuit breaker current must at have the connection a breaking capacity point. higher than or equal to
■
Applying
OPERATE
voltage
WEAK
to a
CURRENTS
no-load line
WITHOUT
can be
EXCESSIVE
compared
OVERVOLTAGE
to capacitor switching.
date
11/94
- B•3•2 -
revised
05/2004
page 17