Fault ratings - a brief introduction

All electrical systems are potentially liable to short circuits. If a short circuit occurs, abnormal - and possibly dangerous - current levels are created. Such currents are likely to produce considerable thermal and mechanical stresses in electrical distribution equipment, and can be a threat to human life. Therefore, it is essential to protect personnel and equipment by calculating short-circuit currents while designing (or altering) an electrical system. The electrical system can then also be properly protected using suitable fault current protective devices.

Wherever switchboards, panelboards and distribution boards are located close to packaged substations in commercial, industrial or public buildings, attention must be paid to prospective fault-current levels and the need for panels and devices with appropriate short-circuit performance. This is Eaton's MEM Memform Form 4 cubicle switchboard with short-circuit capability of 100kA and busbar ratings up to 10,000A.

Generally speaking, electrical equipment that has been designed to interrupt current at fault levels should have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Electrical equipment intended to interrupt current at other than fault levels should have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.
Applying, for example, such requirements to a circuit breaker, the maximum three-phase fault current the breaker will be required to interrupt (short circuit current at the protective device terminals) must be accurately calculated.

It is not proposed to go into the relatively complicated business of fault calculations in this short article. But at this stage, some definitions may be useful.
Definitions:

The following definitions are provided by the NICEIC's excellent Technical Manual, which is available as an interactive guide on CD:

• Fault current - Fault current is the current resulting from a fault in an electrical circuit, such as a breakdown or a bridging of the electrical insulation. The fault may be between live conductors or between a live conductor and earth.
• Overcurrent - A current in excess of the rated value is known as an overcurrent. In general, fault currents are also overcurrents. There are, however, exceptions (see NICEIC's Technical Manual).
• Earth fault current - A fault current which flows to earth.
• Short-circuit current - An overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions.
• Prospective fault current - This is the value of overcurrent that would flow at a given point in a circuit if a fault (a short-circuit or an earth fault) were to occur at that point. The prospective fault current tends to be higher than the fault current likely to occur in practice. The design of an electrical installation must take account of prospective fault current in order to avoid danger or damage arising under fault conditions.
• Fault current protective devices - A fault current may cause overheating, arcing, fire and even explosion, resulting in injury to persons and damage to property. To reduce the risks of such injury or damage occurring, BS 7671 requires fault protective devices to be provided.
• Protective device - A protective device shall be provided in a circuit to break any fault current flowing in conductors of that same circuit before such current causes danger due to thermal or mechanical effects produced in those conductors or the associated connections. The nominal current of such a protective device may be greater than the current-carrying capacity of the conductor being protected. BS 7671 generally requires a protective device to be provided to break any fault current in a conductor before such current causes danger. However, there are circumstances where, for reasons of practicability or safety, it is permissible to omit a fault current protective device.

Such protective devices are fuses and circuit-breakers.

• Fuse - A fuse is defined in BS 7671 as: A device which by the melting of one or more of its specially designed and proportioned components, opens the circuit in which it is inserted by breaking the current when this exceeds a given value for sufficient time. The fuse comprises all the parts that form the complete device.
• Circuit-breaker - A circuit-breaker is defined in BS 7671 as: A device capable of making, carrying and breaking normal load currents and also making and automatically breaking, under pre-determined conditions, abnormal currents such as short-circuit currents. It is usually required to operate infrequently although some types are suitable for frequent operation.

Returning to the circuit-breaker example given in paragraph 3 of this article, when applying the above requirements, the maximum three-phase fault current that the breaker will be required to interrupt must be calculated. This current can be defined as the short-circuit current available at the terminals of the protective device. It can be assumed that the three-phase short circuits are bolted, (possess no impedance), and that a three-phase short circuit can be considered a balanced load. This means, essentially, that a single-phase circuit can effectively be used to analyse one of the phases and the neutral.

Distribution equipment, such as circuit breakers, fuses and switchgear, as well as control equipment such as motor control centres (MCCs), have interrupting or withstand ratings defined as the maximum RMS values of symmetrical current. It should be remembered that a protective device (such as a circuit-breaker) cannot interrupt a circuit at the very moment a short-circuit occurs (fast as they are), because there is a time delay (eg: breaker contact parting time / relay time delay) that is unavoidable. Because of this delay, the circuit breaker will interrupt the current after a period of (typically) up to eight cycles. By then, the DC component should have reduced almost to zero, and the fault will be effectively symmetrical.

Further information on the nature of and determination of fault currents, prospective fault currents and ratings, by calculation, by measurement and by enquiry is given in the NICEIC's Technical Manual, the NICEIC book entitled Inspection, Testing and Certification and Topic F13-21 of the NICEIC's Technical Manual, respectively. See also elsewhere in this VoltiBULLETIN.