Figure 82 example of stabilized backfill pe4 group

Figure 8.2 Example of stabilized backfill
PE4| Group 5_5EEB

UNIVERSITY OF SANTO TOMAS
FACULTY OF ENGINEERING
España, Manila
B. Sheath bonding type
When Installing Distribution voltage cables, it usually is installed with solidly bonded
sheaths to minimize sheath circulating currents produced by magnetic flux on single
core cables that links the conductors and sheaths. To remedy this, A trefoil formation is
usually made, however this proves futile since it has poor heat dissipation and there is
considerable heating effect upon each cable. Although this is not a problem for cable
ratings up to 33 kV, Ratings above 33 kV may have a very tough time. In exchange for
the trefoil formation, specially bonded cable systems are used. The way these systems
work involves earthing the single core sheaths at a single point and insulating all other
points. By doing this, sheath losses are eliminated, and space can be available between
the phase cables and reduces the mutual heating effect. To protect the sheaths from
transient voltages arising from lightning and switching transients, sheath voltage
limiters are fit at all joint and sealing end points where the sheath is insulated. The
graph below shows the induced sheath voltages in proportional with the size and
spacings of cables. There are three variations of specially bonded systems that are
used which are end-point bonding, mid-point bonding and cross-bonding.
Figure 8.3 Sheath Voltage and Spacing /Diameter relationship
a. End-point bonding
- This system earths one termination and insulates the other with inclusion of Sheath
voltage limiters. A separate earth continuity conductor is needed to account for the
fault currents that usually passes through the cable sheaths.
PE4| Group 5_5EEB

UNIVERSITY OF SANTO TOMAS
FACULTY OF ENGINEERING
España, Manila
Figure 8.4 End-point bonding schematic
b. Mid-point bonding
When the length of the cable is too long, A mid-point bonding system is used wherein
the middle of the cable is earthed and all other terminations is insulated and fitted with
sheath voltage limiters.
Figure 8.5 Mid-point bonding Schematic
c. Cross-bonding
- In this system, the route is split into major section sections each of which contains
three drum lengths fitted with insulated flanges. At every third joint the sheaths are
connected, then at all other positions they are joined which ends in all sheaths being
connected in series. The combination of sheaths at the third joint is made inside a link
disconnecting box.
PE4| Group 5_5EEB

UNIVERSITY OF SANTO TOMAS
FACULTY OF ENGINEERING
España, Manila
Figure 8.6 Cross-bonding process
Figure 8.7 Cross-bonding Schematic
C. Low Dielectric Loss Insulating Materials
- Simply changing the type of insulating material your cable uses may have profound
effects on the rating your cable can handle. Having Low dielectric Loss Cables offers
advantages over the conventional paper insulated, fluid-filled cables. Cables such as
PPL or Polypropylene Laminated cable and XLPE or Cross-Linked Polyethylene. The