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.

 

How Power Cable Interfere with Instrumentation and Communication Cable

Power Cable Interference with Instrumentation, Communication  or Control Cable and  Reductance This Effect

Let’s know the basic difference between Power Cable and Communication Cable.

Power Cable: The cable which uses to transfer electrical energy from one point to others is known as a power cable. Comparatively higher sizes of Aluminum or Copper conductor is used as a power cable in electrical power distribution and transmission system.

Power cable construction may different type’s like- single-core, dual-core or multi-core.

Core materials mainly use as Copper or Aluminium.

Metallic sheath or armour uses to reduce induced voltage and mechanical protection from any external damages. Steel, Copper, Aluminium or Led uses for cable outer cable sheath.

Metallic screen normally earthed in one or both ends to reduces the external electric field to zero.

Communication Cable: The cable which is used to communicate low-level signal in-between two or more instrument is known as communication or instrumentation or control cable. Special type, smaller sizes of wire used as communication cable.

Communication cable construction mainly coaxial or metallic screen type.

Both power and communication cables core positioned look like in a cylinder.

Feature of Power Cable and Instrumentation Cable:

                               I.            Power cable carry high voltage and current;

                             II.            Instrument cable carry low-level signal;

                          III.            Power Cable generally bulk size where instrumentation cables are smaller sizes.

Why Instrumentation Cable interfered by Power Cable?

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The instrumentation cable can pick-up some noise signals which is hindered from nearby sources- like power cable. Communication or Instrumentation cable generally lays very near and in parallel with power cables which generate noise signal to interfere with electronic devices and signals.

If we look at the above figure were combined all information from the power cable and communication cable construction, laying position, equivalent electrical circuit and different mutual coupling that takes part to interfere.

Mutual coupling between power cable core, communication cable core and screen and grounding or earthing circuit of a three-phase cable is shown in the figure in which unbalanced currents flow. The unbalanced current can be replaced by one equivalent current. Let consider the three-phase cable is armoured and it is earthed at one or both ends.

How Power Cable Interfere Instrumentation Cable?

Power cable generate a stray signal for control cable by mutual induced current because-

a.     Single-core cables run in groups;

b.     High magnitude fault current flows through cables;

c.      Unbalanced currents flow through cables;

d.     Surge current flows through cables;

e.      Harmonic currents flow through cables.

Stray or noise signal generated by power cable may interfere with control or communication cable in different forms of coupling way. Here is some of them are as below:

                                i.            Common circuit conduction;

                              ii.            Mutual Capacitive coupling;

                            iii.            Mutual Inductive coupling.

Electrostatic and Electromagnetic effect is not fixed for all, it varies from cable circuit and laying condition. Some of the common circuit condition is as below:

Condition-1: Single conductor above a flat plane-

-Leakage capacitance.

-Self-inductance.

Condition-2: Two conductors in parallel-

-Coupling capacitance between the conductors.

-Self-inductance of each conductor.

-Mutual inductance between the conductors.

Condition-3: One screened conductor in a cylinder:

-Coupling capacitance.

-Self-inductance of the conductor.

-Mutual inductance between the conductor and the cylinder.

How Reduce Interference Between Power Cable and Control Cable?

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There is no single solution to remove or reduce Power Cable Interference with Instrumentation, Communication or Control Cable, but taking some measures interferences may reduce.  Separate the control cable from power cable as following tender way, it is possible to reduce the power cable interference for the control cable.

Type of Power & Control Cables	Minimum Distance Between Cables (mm)
110 Volt or 110 Amp 240 Volt or 50 Amp 420 Volt or 500 Amp 3300 Volt to 33,000 Volt Current above 200 Amp	300 450 600 1000 1000

Screen Earthing: Cable screen earthing is another way to reduce induced noises, because in this case, the signal current flows along in the inner core and returns in the screen at opposite direction.