Technical Bulletin
Differences Between Functional Earth and Protective Conductor
1. How Do Functional Earth and Protective Conductor Colours Differ in Electrical Systems?
In electrical systems, earthing is a critical aspect that ensures safety and proper functioning. Two primary types of earthing, functional earth and protective conductor (PE), serve distinct roles. Understanding these roles, and the colour coding associated with them, is essential for maintaining system integrity and safety.
2. Functional Earth
A functional earth is employed for the correct operation of electrical equipment, rather than for safety purposes. This type of earthing is crucial in systems that require earthing for their functionality. For instance, it is used in circuits involving interference suppression or signal reference points in communication systems.
According to BS 7671:2018+A2, functional earths must be clearly identifiable to avoid confusion with other earthing systems. The standard specifies that functional earth conductors should be identified with pink sleeving (clause 514.4 & Table 51) – this extends to fire alarm installations. This colour coding is a recent amendment aimed at ensuring that functional earths are easily distinguishable from other conductors, thereby preventing misconnection (e.g. inadvertent routing of a protective earth via a functional earth cable) and ensuring the correct operation of the equipment they serve.
In practice, the pink coloured sleeving is applied to the insulation of the conductor, making it visibly distinct from other wires within the electrical installation. This is particularly important in complex systems where multiple types of earthing are present, as it helps technicians and engineers quickly identify and correctly handle the functional earth conductors. This is especially relevant in fire detection and fire alarm systems, where extra-low voltage (ELV) circuits also require functional earths to be identified with pink sleeving to ensure clarity and proper function of these critical safety systems.
3. Protective Conductor (PE)
In contrast, a protective conductor is primarily used for safety, specifically to protect against electric shock. The PE ensures that exposed conductive parts of electrical equipment do not reach dangerous voltage levels during fault conditions. It achieves this by creating a path for fault currents to flow directly to earth, which triggers protective devices like circuit breakers to disconnect the faulty circuit.
The PE connects various parts of an electrical installation to the earthing system, including exposed conductive parts, extraneous conductive parts, the main earthing terminal, earth electrodes, and the earthed point of the source or an artificial neutral. This extensive network of connections ensures that all potentially dangerous parts are kept at earth potential, thus preventing electric shock hazards.
The colour coding for protective conductors is also clearly defined in standards to prevent misidentification. BS 7671 mandates that protective conductors must be identified with green and yellow sleeving, with the most common use being Circuit Protective Conductors (CPC). This dual-
colour scheme is internationally recognized and helps ensure that protective conductors are easily distinguishable from other types of wiring.
The green and yellow sleeving must be consistent throughout the entire length of the protective conductor. This uniformity is crucial during installation and maintenance, as it allows for quick identification and verification of the protective conductor’s presence and condition.
4. Importance of Colour Coding
The colour coding of functional earth and protective conductors plays a vital role in maintaining the safety and functionality of electrical systems. The use of pink sleeving for functional earths and green and yellow sleeving for protective conductors ensures that these different types of earthing can be easily and correctly identified. This reduces the risk of errors during installation and maintenance, which could otherwise lead to equipment malfunction or safety hazards.
5. Risks of Misusing Functional Earth as Protective Earth
It is crucial to understand the dangers of using a functional earth as a protective earth. The protective earth's primary role is to provide a path for fault currents to be safely directed to earth, allowing a large fault current to flow through the line conductor. This rapid fault current flow triggers the protective device to operate quickly, typically within 0.4 seconds. If a functional earth is included in this path, it can result in high fault currents passing through fire detection and fire alarm equipment. This can induce EMC spikes, which may damage the sensitive electronics of these systems. Therefore, a well-designed earthing system that keeps functional and protective earths separate is essential to ensure both safety and functionality in electrical installations.
6. Implementation in Standards
BS 5839-1, which provides guidelines for fire detection and fire alarm systems in buildings, in conjunction with BS 7671, helps ensure that electrical systems are both safe and operationally reliable.
The amendment 2 of BS 7671 underscores the importance of correct identification and segregation of these conductors. By mandating the use of specific colours, the standards aim to prevent malfunctions and ensure safety. For example, the pink sleeving for functional earth helps in distinguishing it from the green and yellow sleeving of the protective conductor, aiding in maintaining system integrity and functionality. Additionally, fire detection and fire alarm systems using extra-low voltage (ELV) circuits must also comply with these colour coding rules to ensure their proper operation and safety.
