3.6.1 - Introduction

'Overcurrent' means what it says - a greater level of current than the materials in use will tolerate for a long period of time. The term can be divided into two types of excess current.

1 Overload currents
These are currents higher than those intended to be present in the system. If such currents persist they will result in an increase in conductor temperature, and hence a rise in insulation temperature. High conductor temperatures are of little consequence except that the resistance of the conductor will be increased leading to greater levels of voltage drop.

Insulation cannot tolerate high temperatures since they will lead to deterioration and eventually failure. The most common insulation material is p.v.c. If it becomes too hot it softens, allowing conductors which press against it (and this will happen in all cases where a conductor is bent) to migrate through it so that they come close to, or even move beyond, the insulation surface. For this reason, p.v.c. insulation should not normally run at temperatures higher than 70°C, whereas under overload conditions it may have allowable temperatures up to 115°C for a short period during transient conditions.

2 Short circuit currents
These currents will only occur under fault conditions, and may be very high indeed. As we shall shortly show (see {3.6.3 and 3.6.4}) such currents will open the protective devices very quickly. These currents will not flow for long periods, so that under such short-term circumstances the temperature of p.v.c. insulation may be allowed to rise to 160°C.

The clearance time of the protective device is governed by the adiabatic equation which is considered more fully in {3.7.3}.