MODULE 4 — Underground Cables
4.1 Cable vs Overhead Line
| Feature | Overhead (OHL) | Cable (UGC) |
|---|---|---|
| Cost | Low (Base) | High (3x to 10x) |
| Capacitance | Low | Very High (20-40x) |
| Inductance | High | Low |
| Fault Location | Easy (Visual) | Difficult |
| Safety | Moderate | High |
| Charging Current | Low | High (Limits distance) |
4.2 Cable Construction & Dielectric Stress
Dielectric Stress ($E$)
$$ E_{max} = \frac{V}{r_1 \ln(r_2/r_1)} \quad \text{(at
core)} $$
Optimum Radius
$$ r_{1opt} = \frac{r_2}{e} \approx 0.368 r_2 $$
Emax occurs at the surface of the conductor. Emin occurs at the inner surface of the
lead sheath.
4.4 Cable Current Rating & Derating
Current Rating
$$ I = \sqrt{\frac{\Delta\theta}{n R_T^{(ac)} +
\text{losses}}} $$
Overall Derated Current
$$ I_{rated} = I_{base} \times k_1 \times k_2 \times k_3
\times k_4 $$
Derating Factors (Toggle to apply
adverse conditions):
1000A
Base Rating: 1000A
Adjusted Capacity