Greener Learning
Introduction
The immense energy available in the modern electrical distribution system poses a great risk of damage to property and death or injury to persons, hence the importance of Clause 1.5 Fundamental safety principles of the AS/NZS3000, Wiring Rules. It specifies requirements to ensure an electrical installation is protected against the detrimental effects of current, dangerous fluctuations in voltage and hazards of electric shock. In practice this means using Circuit Breakers or Fuses to protect circuits against overload and short-circuit currents, and to limit touch voltage when an earth fault occurs; using devices for protection against voltage surges and excessive voltage reduction; and using Residual Current Devices (RCDs) for additional protection to limit the rise of touch voltage when an earth fault occurs.
This Learning Outcome explains the principles of how each protective device works. This is a topic of high importance for all Electricians to understand as correct selection and installation of Protection Devices is essential in satisfying the intended requirements of AS/NZS3000...To protect persons, livestock and property from electric shock, fire and physical injury hazards that may arise from any electrical installation.
Overcurrent protection
AS/NZS3000, Appendix B3.1 classifies the term “Overcurrent” as including both overload current and short-circuit current. Both of these are explained later on. Clause 2.5.3 is Protection against overload current (coordination between load requirements, conductors and protective devices). Clause 2.5.4 is Protection against short-circuit current (determination of prospective short-circuit current).
Overload protection
Overload current in circuit conductors must be cut off by a protective device before the overload currents cause a temperature increase that might damage joints, insulation or other material surrounding the conductor. The AS/NZS3000 Wiring Rules stipulate that, in the event of an overload, a protective device (circuit breaker or fuse) must disconnect the supply before the temperature rise caused by the excess current damages the cable insulation, circuit connections or adjacent cables.
The requirements for overload protection in the AS/NZS3000 Wiring Rules are intended to limit overheating of cables to acceptable limits. The note to Clause 2.5.3.1, however, warns that the requirement does not protect against the longer term deteriorating effects on cable insulation of small overloads (less than) of long duration. Circuits should be designed according to all expected operating conditions. The protection should also be immune to causes of ‘nuisance tripping’, such as an overload of short duration, must fully protect the wiring against overloads that could cause damage if allowed to continue.
Short-circuit protection
So far only overload current has been considered, where a measure of the overload is usually known or assumed. Using normal full load or maximum demand current as the base, an overload is expressed as ‘twice rated current’, or ‘150 per cent overload’. However, in the case of a short circuit, the only limit to the value of current present is the impedance of the faulty circuit and the available short-circuit energy with no component of the load having an effect.
Prospective short-circuit current
The interrupting capacity of the protective device (circuit breakers or fuses) must be adequate to enable the interruption of the highest value of current available at the point of installation of the protection (Clause 2.5.2). This value of current is known as the ‘prospective short-circuit current’. The protection will be at the commencement of the circuit, which is usually at the main switchboard or at a distribution board and must operate to interrupt the short-circuit current before the temperature of conductors reaches the admissible limit.
Temperature limits for cables under short-circuit conditions are given in the AS/NZS 3008.1 series Section 5. AS/NZS3000, Clause 2.5.4.1 stipulates that the prospective short-circuit current must be determined at every point of an electrical installation, the relevant points being mainly where a protective device is installed. The prospective short-circuit current decreases as the distance from the point.
Circuit breaker characteristics
The two main protection functions of a circuit breaker are to protect wiring from overcurrent, whether an overload or a short circuit – each requires a different time response. When a short circuit occurs, AS/NZS3000, Clause 1.5.5.3 specifies that the protective device must disconnect the supply within 0.4 s for final subcircuits supplying socket-outlets rated up to 63 A, hand-held Class I equipment and portable equipment intended for manual movement during use. A maximum disconnect time of 5.0 s is specified for circuits such as submains, final subcircuits and those supplying fixed or stationary equipment. As mentioned under overload conditions, AS/NZS3000, Clause 2.5.3.1 of the Wiring Rules requires coordination between conductors and protective devices.
Short-circuit and overload protection functions of circuit breakers are represented in graphs showing their time-current characteristic curves. Fixed-setting circuit breakers (typically MCBs) are intended to protect wiring against both overloads and short circuits in domestic or commercial wiring where operation (switching on or off or resetting) is possible by an uninstructed person. They are designated by their instantaneous time-current characteristic curves, which classify these circuit breakers into three types as shown in its Time - Current Characteristic Curve. It is worth noting that the short-circuit function of the modern circuit breaker is a current-limiting characteristic similar to that of an enclosed fuse link.
For a Diagram on Time - Current characteriestics on Circuit breakers, please click here.
Fuses
The fuse was the first device developed to protect electrical wiring, and dates back to the early telegraph systems of the late 1800s. Since then the modern fuse, with its current-limiting characteristics, has become the first line of defence against high short-circuit currents and provides protection of sensitive components.
More recently developed circuit breakers with similar current-limiting characteristics have caused a major decline in the use of fuses for the protection of most common circuits. Add to this the convenience of resetting a circuit breaker against the need to replace a ‘blown’ fuse element.
Nevertheless, fuses are still considered an important component in circuit operation. They are used extensively in distribution networks and as fault current limiters to back up downstream circuit breakers that do not have the prospective fault-level capacity required in particular situations. They are incorporated in switches that provide an isolation function similar to circuit breakers. In circuits where the likelihood of a fault current is low, but require a high degree of reliability, fuses have an advantage because they have low initial and ongoing costs.
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For a diagram on Fuse Characteristics, please click here.
Protection by residual current devices (RCDs)
RCDs had been used as supplementary protection in some industrial activities, medical treatment areas and the construction industry for many years before they became a common mandatory requirement. These devices were also known as ‘core-balance units’ or ‘current-operated earth-leakage circuit breakers’. Originally RCDs were promoted and aimed particularly at domestic consumer as ‘safety switches – a misnomer as they only provide protection against indirect contact between an active conductor and exposed and extraneous conductive parts.
Their qualified use is mandatory for particular circuits in all installations. Irrespective of the terms used to be described this type of protection, their purpose is the same: to rapidly cut off the power to a circuit when measured conditions in the circuit indicate that a level of current likely to cause physical harm is leaking to earth. RCDs can also provide protection against current leakages likely to cause damage or start a fire.
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For a diagram on Wiring rules for additional protection by RCDs, please click here.
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For a diagram on How RCDs operate, please click here.
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For a diagram on Typical Arrangement of curcuits in an installation, please click here.
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For a diagram on How to test an RCD, please click here.