Secondary racks usually support three spool insulators mounted
on a common shaft attached to a steel backing that is bolted to the
pole by one or more bolts (though sometimes, depending on the stress
to be accommodated, a lag screw may take the place of a bolt). The
spool insulators are usually spaced 8 in apart, although sometimes
less. If the rack is properly mounted, the insulators may fail before
the rack.
Loadings
The loading on secondary racks has both vertical and horizontal
components; see Figure 5-13. The vertical loading consists of the weight
of the conductors (with the coating of ice) of both the mains and services
attached to them.
With spool-type racks, the load is carried by small arms
acting as cantilever beams, supporting
the insulators. Generally, the insulator
is located between two such arms, but
only the lower one carries the load.
With the knob-type rack, the vertical
load is transmitted to the rear plate by
the U bolts attaching the insulators to
the plate, and to the pole by the bolts
attaching the rack to the pole.
The horizontal load is due to wind acting on ice-laden conductors
and the tension in them, for both the secondary mains and the services.
If the rack is attached to the pole by a bolt under each insulator, the
strength of the back plate plays little part, as the stress is transmitted
almost directly by the bolts.
Where the spool-type rack is mounted with
the bolts at the ends only (or by a bolt and a lag screw), the back of the
rack acts as a cantilever beam supported at both ends, with the load applied
at each of the points where the arms are attached to the back. The
greatest stress is at the point in the back where either of the two arms
supporting the middle insulator is attached, the bending moment being
greatest at that point. In the knob-type rack, both the vertical and horizontal
loadings are transmitted to the back of the rack and through the
mounting and bolts to the pole.
Where the conductors dead-end on a rack mounted on the side of
a pole (or where there is an unbalanced pull from broken conductors),
the side pull on the rack is limited by the strength of the arms or the U
bolts supporting the insulators. It is preferable to mount the rack on the
face of the pole, with the arms and U bolts supporting the insulators in
tension, and the stresses transmitted to the pole almost directly by the
mounting bolts.
In general, like pins, conductor fastenings should be able to withstand
a 700-lb stress.
Messenger Clamp
As mentioned earlier, when the secondary conductors are cabled
about the neutral conductor acting as the supporting messenger, a clamp
supporting the neutral is bolted to the pole. Here, both the vertical and
horizontal loadings are transmitted to the pole by the through bolt by
which the clamp is attached to the pole.
on a common shaft attached to a steel backing that is bolted to the
pole by one or more bolts (though sometimes, depending on the stress
to be accommodated, a lag screw may take the place of a bolt). The
spool insulators are usually spaced 8 in apart, although sometimes
less. If the rack is properly mounted, the insulators may fail before
the rack.
Loadings
The loading on secondary racks has both vertical and horizontal
components; see Figure 5-13. The vertical loading consists of the weight
of the conductors (with the coating of ice) of both the mains and services
attached to them.
With spool-type racks, the load is carried by small arms
acting as cantilever beams, supporting
the insulators. Generally, the insulator
is located between two such arms, but
only the lower one carries the load.
With the knob-type rack, the vertical
load is transmitted to the rear plate by
the U bolts attaching the insulators to
the plate, and to the pole by the bolts
attaching the rack to the pole.
The horizontal load is due to wind acting on ice-laden conductors
and the tension in them, for both the secondary mains and the services.
If the rack is attached to the pole by a bolt under each insulator, the
strength of the back plate plays little part, as the stress is transmitted
almost directly by the bolts.
Where the spool-type rack is mounted with
the bolts at the ends only (or by a bolt and a lag screw), the back of the
rack acts as a cantilever beam supported at both ends, with the load applied
at each of the points where the arms are attached to the back. The
greatest stress is at the point in the back where either of the two arms
supporting the middle insulator is attached, the bending moment being
greatest at that point. In the knob-type rack, both the vertical and horizontal
loadings are transmitted to the back of the rack and through the
mounting and bolts to the pole.
Where the conductors dead-end on a rack mounted on the side of
a pole (or where there is an unbalanced pull from broken conductors),
the side pull on the rack is limited by the strength of the arms or the U
bolts supporting the insulators. It is preferable to mount the rack on the
face of the pole, with the arms and U bolts supporting the insulators in
tension, and the stresses transmitted to the pole almost directly by the
mounting bolts.
In general, like pins, conductor fastenings should be able to withstand
a 700-lb stress.
Messenger Clamp
As mentioned earlier, when the secondary conductors are cabled
about the neutral conductor acting as the supporting messenger, a clamp
supporting the neutral is bolted to the pole. Here, both the vertical and
horizontal loadings are transmitted to the pole by the through bolt by
which the clamp is attached to the pole.
