Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1942-08-15
SAYS
AppLHinq Arresters

Just because your equipment has
not already been nit doesn't make it
immune from lightning. And If it has
been hit, don't think it won't get hit
again. So, apply arresters and give
your equipment fighting chance
against the tremendous force Of light
ming
The logical place to start is on the
line and transformers serving your
plant. The recommendations of au-
thorities vary somewhat and also the
increased knowledge about lightning in
recent years has changed ideas on
protection, so don't be surprised If
some of the ideas proposed herein dis
agree with your previous ideas.
Three sets Of arresters are generally
recommended on the incoming line
and transformer primaries. The first
set should be located approximately
$500 feet from the transformer, the
second set 1900 feet away, and the
third set at the transformer
The purpose Of the arresters remote
from the transformers is to discharge
to the ground the heavy strokes with
steep wave fronts. It might be hkened
to the situation existing between the
protection afforded you by power poles
from a runaway automobile. If the
auto is coming at you at 5 miles per
hour, you could safely get behind a
power pole and depend on the pole
to stop it. But If the car is coming
at 10 miles per hour, it would very
probably shear off the first pole 1t hit
and keep coming but would be stopped
by the second pole.
A set of arresters consists of one
arrester for each "hot" or ungrounded |
on the incoming line. For
example a $500 volt 3-phase, 8 wire
Delta connected circuit requires three |
9 900 volt arresters per set. A 4,16O
volt, 3-phase, A wire star connected /
circuit with grounded neutral requires
three 5000 volt because the line to
ground voltage is only 2.400 volts) ar
rosters per set. ArreSters are rated In
muItiples of 5000 volts, and it Is nee
sssary to select the next higher volt
age rating above the line voltage or
line to ground voltage for grounded
star circuits
All sets of arresters should be con-
nected to a good low resistance
In a rocky location 1t may be
almost impossible to obtain low re-
;istance ground even If several ground
-ods are driven and connected to-
yether. In a swampy or marshy loca-
ion, one ground rod may be very ade-
Iuate.
mum allowable ground resistance as
here are so many variable conditions,
tut here is what it means. II the
round resistance we're very low or !,
me ohm and small lightning stroke
# 10000 amperes discharged into 1t,
voltage OF 1x10,000 or 10000 volts
you'd build up between the line and I
he ground. If the ground were rela- :
iyely poor, or 100 ohms, a voltage OF !
90. 200 volts would build up which
yould probably damage your equip
nent. Your local electric utility Is
erhaps your best source of informa- I
ION on grounding requirements as
hey have had experience in your To-
atty. The manufacturers representa- I
ives can also help you.
Particular care should be gwen 10
occurring a good ground connecJlon (
1r the arresters at the transformers
S this is the vital location. The ta9ks I
f the transformers also should De
onnected to this ground for maxi r
lum protection against lightning. The
nemen and electricians are now di-
ided about $3.50 on the safety angle
{ grounding transformer tanks, but
ne practice of grounding tanks has
rown steadily indicating that 1t Is
afe and sound. II
SsS---------- II