Download Arresters verses Shield Wire

Arresters verses Shield Wire
Do we care that lighting strikes
our lines?
The insulation is self restoring and if the
structure is metal the insulation will flash
and the lightning will be conducted to
ground.
 So if that is the case why do we care?
 It cause an outage to the line. How?

Voltage Sag Caused by Lightning
Typical Network
You want lightning to
strike the shield wire
instead of the phase
conductors. Once the
lightning strikes the
shield wire then you
need ground resistance
low enough to keep the
lightning from
backflashing to the phase
wires. On transmission
structures this is done by
a long ground wire to get
ground resistance below
25 ohms. The
sometimes call this
ground wire
counterpoise.
Lightning can
strike the ground
nearby and cause
the tower to
rapidly discharge
it stored charge
therefore making a
high potential
difference
between the tower
and the phase
wires causing
them to flash to
ground. In this
case the backflash
is only prevented
by the strength of
the insulation.
When using arresters they
will conduct in either
direction so they will
prevent flashing of the
insulators whether the
lightning strikes the phase
wire or strikes the ground.
And you don’t need as low
of ground resistance as
with a shield wire.
From engineering
manual Section 14
SI 1.1 you can
calculate the
separation distance
from one arrester to
another. It is based
on traveling wave
theory.
D is the distance
and Et is the BIL
of the insulator Ea
is the clamping
level of the arrester
and the
denominator is
how fast the
lightning stroke
rises.
Arresters vs. Shield Wire
For Good Shielding:
 Pole has to be raised 8 feet.
 Center phase wire has to be put on crossarm
and may cause conductor slap problem.
 Ground resistance must be reduced to under
50 Ω - 25 Ω if possible.
 Cost $15,000/mile.
Arresters vs. Shield Wire (continued)
For Arresters:
 Can install arrester on phase wires.
 Need 8 sets/mile at $1000/set.
 Ground resistance should be about 50 Ω,
but by installing one on each phase, it
creates equal potential, so ground resistance
can be greater.
 Cost $8,000/mile.