Power Cable

Selection of Conductor Size

- current and loading cycle
- Emergency loading and its duration
- Short-circuit rating and fault clearing time
- Allowable voltage drop

Resistance and Skin Effect

Skin effect is an ac phenomena whereby alternating current tends to flow more densely near the outer surface of the conductor than the center

The effective resistance of the conductor is higher for AC than for DC

Reactance of Cables
Electrostatic Capacitance
C = 0.00736 K/log (D/d)
where:
C = capacitance, µF/1000 ft
K = dielectric constant
D = outer diameter
d = core diameter


Cable Shielding

Functions of Insulation Shield
- Confines the dielectric field within the cable
- Equalizes voltage stresses within the insulation by making the electric field - -uniform and radial
- Reduces the hazard of shock
- Helps protect cables installed overhead against induced voltages

Types of Insulation Shield

Shield Jacketed Cable - Consist of semi-conducting tape applied over the insulation followed by a copper tape both of which are spirally wrapped
Concentric Neutral Cable - consist of extruded layer of semi-conducting material over which an annealed cooper coated wires are spirally applied and equally spaced in sufficient numbers.

a. Full conductivity
b. One-third conductivity

Insulation Thickness for Cross-linked, Thermosetting, Polyethylene Insulated Cables
Based on 100% and 133% insulation level