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 GROUND
METHODS AND MATERIALS
Tape Shields
Based
upon field observation, and experience of our Proving Ground, we have concluded that permanent grounds are
best assured by tack soldering to the
cable metal tape. Whether a clamp, strap
or flexible braid is used, it is best to take solder to the
metal tape in order to assure good conditions
since temperature cycling may loosen
the mechanical grip and corrosion may reduce the contact surfaces of the metals.
If
clamps are used, care is required that stiff materials or large
forces do not cause pinching or other deformation of
the cable insulation or disruption of
the metal tape during normal expansion due
to current loading. Clamps or heavy straps should be preformed
around a scrap piece of cable before final application.
When
using braid or light strap, tack solder the end to the metal
tape, wrap 11/4 turns around the cable, tack solder again to
the previous turn and the cable metal tape,
then train the lead away from the cable.
When attaching the ground in this manner, an accidental pull on the ground lead will not likely rip
the cable metal tape.
We
recommend the use of rosin core solder or stearine flux.
DO NOT USE prepared paste or acid core solder.
With zinc taped cables it is necessary
to limit soldering from temperature to approximately
700°F and to use a tin-lead type solder to prevent injury to the zinc tape. Check the solderability on
a length of scrap cable. Use a soldering
iron. DO NOT USE A TORCH on any metal taped cable. A torch may seriously damage the cable.
The
ground leads should have short circuit ampacity equal to
or greater than the cable metal tape. Heavy
tinned copper wire braid or 1/2
" x .030" tinned copper strap are most satisfactory. Tinsel
or light copper wire braids do not have adequate ampacity
and are subject to flexing fatigue and
corrosion.
Where
vibration is expected, special sleeves and heavy leads may be required; refer such problems to a Kerite representative
for special recommendations.
Concentric
Neutrals
Concentric
neutral cables are easy to work with. The individual wires should simply be pulled back and twisted together.
The resulting wire can then be lugged to the
ground conductor or point.
Sealing
For
jacketed cables with shields grounded in outdoor or submersible locations, adequate seals should be made
to prevent water entering under the jacket.
Sealing putty formed around the ground leads with an overwrap of several layers of insulating
tape makes an effective seal. When insulating
tape is used over metal braid, the braid must be solder filled to effect a good
seal. Outdoors, ground leads should be
brought from the bottom side of horizontal cables and downward to form a drip loop on
vertical cables. Friction tape binders
to secure the lead to the cable help prevent
disturbing the seal if the lead is pulled.
Grounding
Practices
Ground
should be applied to cable sheaths at terminals, in accordance with the appropriate terminal instructions.
Possible exceptions where only one ground is
used.
•
Very short lengths.
• Locations (such as pole top) where exposed grounds are
not allowed.
• Situations where a sheath interruption is recommended to
avoid sheath circulating current.
Where
cables fan out from conduit to terminals, and grounds are
to be made to the conduit, the connections to cable sheaths
may be made close to the terminals, and insulated
or bare grounding leads brought down
along the cable to the conduit in an open helix around the cable, or secured with rings of
tape.
In
long runs, cable may be grounded additionally between terminals according to the user’s normal practice. Where
more than one ground is applied to a
cable sheath, there may be sheath currents and consequent losses. Sometimes these losses
are great enough to affect the rating
of the cable, in which case sheath interrupting joints may be considered.
For
more information on shield currents and cross bonding, refer
to IEEE Standard 575.
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