How Calculate the Circuit Protective Conductor Size?

What is Circuit Protective Conductor?

The conductor used in an electrical circuit to join together all exposed conductive parts and the main earthing terminal is known as Circuit Protective Conductor in short CPC. It actually includes the earthing conductor as well as the equipotential bonding conductors.

Electrical circuits are used protective devices like fuses, circuit breakers, or RCD to protect the circuit during a fault. While circuit fault event occurs sufficient current will flow around the earth fault loop to operate the of the fault protective devices and disconnect the fault. 

The types of protective conductors are as below:

1. circuit protective conductors (CPC);
2. protective bonding conductors;
3. earthing conductors.

Why do use CPC - Circuit Protective Conductor?

CPC or Circuit Protective Conductors are meant for protection against high contact voltages by leaking fault currents! Protective Conductors actually connect conductive parts of metal casings in electrical appliances, conductive parts of buildings and constructions etc. to good earthing electrodes. 

The function of Protective Conductors to reduce the magnitude of voltages during fault conditions.

How to calculate CPC size or  Circuit Protective Conductor Size?

In order to prevent overheating of the protective conductor during a fault, the cross-sectional area of a protective conductor(s) shall be not less than that determined by the adiabatic formula as follows:

where:

S is the nominal cross-sectional area of a conductor in mm2.

I is the value of fault current in amperes (rms for a.c.) for a fault of negligible impedance.

t is the operating time of the disconnecting device in seconds, corresponding to the fault current. It is found from the protective device characteristic curve. k is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures, see Tables 54.2 to 54.6 of BS 7671.

It should be noted that where the initial cable size has been adjusted following a thermal withstand check, further iterations may be necessary as the new size itself affects the prospective fault current.

Alternatively, the minimum cross-sectional area of a protective conductor can be determined by selection from Table 54.7 of BS 7671 (as below), but this virtually always yields a larger cable.

Protective Conductor is defined in BS7671 under Part 2 Definitions as:-

A conductor used for some measure of protection against electric shock and intended for connecting together any of the following parts:

1. Exposed conductive parts.

2. Extraneous conductive parts.

3. The main earthing terminal.

4. Earth electrodes.

5. The earthed point of the source, or an artificial neural. Protective Conductors are divided into 4 main categories in BS7671

1. Earthing conductor.

2. Main protective bonding conductor.

3. Supplementary bonding conductor.

4. Circuit Protective Conductor.