Short-Circuit / Fault Current in Network Users’ Premises

Prospective short-circuit/fault current and external source impedance at the output point of the EAC's meter.

Translation:

The 16th Edition Regulations of the United Kingdom’s Institution of Engineering and Technology for electrical installations require that, in the event of a short circuit/earth fault, every protective device installed in the electrical installation satisfies the following criteria:

a) Its breaking capacity must not be lower than the prospective fault current.

b) Its operating time and fault isolation time must be within the limits specified by the Regulations.

In order to make it possible to determine both the breaking capacity and the operating time of a protective device, it is necessary for the designer of the electrical installation to know the fault current at the point where the device will be installed.

The Electricity Authority of Cyprus has examined typical supply cases and, for each case, has calculated the short-circuit level and the earth fault current, as well as the external fault source impedance at the meter output point (the origin of the installation).

The results of the above calculations, upon request, should be made available to designers, who bear the ultimate responsibility for calculating the short-circuit/earth fault current at all installation points of the protective devices.

2.1 The calculations for determining the maximum fault current are based on a Transmission Network System with a maximum fault level of 350 MVA at the 11 kV busbars of the transmission substations from which the Distribution System under examination is supplied.

2.2 The calculations took into account the normal operating conditions of the system prior to the fault, with typical loading conditions of the Medium Voltage (MV) and Low Voltage (LV) feeders, as well as of the electrical installation under study.

2.3 The 16th Edition Regulations of the IET for electrical installations permit the construction of electrical installations based not only on the existing TT earthing system, but also on the TN-C-S earthing system. In cases where a designer intends to design an electrical installation based on the TN-C-S earthing system, approval from the Electricity Authority of Cyprus must first be obtained (See Directive TM/59/2007).

2.4 The Electricity Authority of Cyprus network is subject to continuous dynamic development in order to meet both the present and future needs of its customers. This reality entails continuous reinforcement of the network through the establishment of new Transmission and Distribution Substations, as well as through the replacement of existing Power Transformers and network conductors with transformers and conductors of greater capacity and cross-section respectively.

As a result of this dynamic development, there is a continuous increase in short-circuit/earth fault current levels at various points of the network. Therefore, when determining the breaking capacity of protective devices, it is imperative that the worst-case scenario (maximum fault current) that may arise in the future be taken into account, so as to fully accommodate the operational lifespan of the protective devices to be installed. In this way, there will be no need to replace them every time the short-circuit/earth fault current increases as a result of network upgrading and reinforcement.

Furthermore, for electrical installations constructed based on the TN-C-S Earthing System, the calculation of the operating time of protective devices and the conductor cross-section of the installation must take into account the actual external fault impedance. Where this is not known, the worst-case scenario may be assumed, in which, according to the aid ON-SITE GUIDE (BS 7671:2001(2004)), the fault impedance is the maximum permissible value of 0.35 Ω.

2.5 The Miniature Circuit Breakers (MCBs) installed by the Electricity Authority of Cyprus for the protection of its meters also provide supplementary (back-up) protection to the electrical installation, since they are capable of limiting the fault current in accordance with the characteristic curves presented in APPENDIX ‘A’ (ABB manufactured MCBs) and APPENDIX ‘B’ (G.E. manufactured MCBs).

In APPENDIX ‘C’, the operating characteristic curve of the ABB manufactured MCB is presented, while APPENDIX ‘D’ presents the operating characteristic curve of the G.E. manufactured MCB.

Electricity supplies to consumers’ premises are provided either through overhead lines or through underground cables. In both methods, depending on the magnitude of the supplied load and the distance from the Electricity Authority of Cyprus network, conductors of various cross-sections and lengths are used, resulting in variations in the short-circuit/earth fault current.

For the purpose of calculating the maximum short-circuit/earth fault current, the Electricity Authority of Cyprus assumed that the installation would ultimately be supplied through the largest transformer it may install (1000 kVA) and carried out calculations for various distances between the transformer and the installation, as well as for various supply conductor cross-sections, so as to cover all typical supply cases.

3.1 Three-Phase Supply from a 1000 kVA Power Transformer

The prospective short-circuit current Ip at the Substation Distribution Board is 25 kA and the external source impedance is Zp = 10.2 mΩ.

The prospective asymmetrical Phase-Neutral short-circuit current at the Substation Distribution Board Ipn is 24.9 kA and the external source impedance is Zpn = 9.7 mΩ.

The prospective symmetrical short-circuit current Ip and the external source impedance Zp in the event of a Phase-Phase-Phase-Neutral short circuit, as well as the prospective asymmetrical short-circuit current Ipn and the external source impedance Zpn in the event of an asymmetrical Phase-Neutral fault, have been calculated for distances from 5 m up to 75 m from the Substation Distribution Board for the following four different supply cases:

3.1.1 Cable 3 × 300/110 mm² Al.

For the results, see TABLE 1.

3.1.2 Cable 3 × 185/90 mm² Al.

For the results, see TABLE 2.

3.1.3 Cable 3 × 70/35 mm² Al.

For the results, see TABLE 3.

3.1.4 Cable 3 × 35/25 mm² Al.

For the results, see TABLE 4.

Note: In the TN-C-S earthing system of the installation, Zpn = Ze.

3.2 Single-Phase Supply from a 1000 kVA Power Transformer

The prospective short-circuit current (Ipn) and the external source impedance (Zpn) at the Substation Distribution Board are as follows:

Ipn = 16.0 kA

Zpn = 15.2 mΩ

The attached TABLE 5 presents both the prospective short-circuit current (Ipn) and the external source impedance (Zpn) for distances from 0 m up to 20 m for the case of using a 35 mm² Al supply cable.

The maximum source impedance value that may occur at a customer connection point on the Electricity Authority of Cyprus low-voltage network is 0.35 Ω.

Note: In the TN-C-S earthing system, Zpn = Ze.

The prospective short-circuit current Ip at the Substation Distribution Board is 50 kA and the source impedance Zp is 0.005 Ω with a fault angle of 59.53° (R = 0.0012 Ω, X = 0.0048 Ω).

The prospective short-circuit current Ip at the Substation Distribution Board is 36 kA and the source impedance Zp is 0.0069 Ω with a fault angle of 77.3° (R = 0.0015 Ω, X = 0.0068 Ω).

All data and information referred to in this Directive regarding the prospective short-circuit and earth fault current, as well as the external source impedance at the origin of the electrical installation, can also be found on the website of the Electricity Authority of Cyprus.

In the event that a designer does not have the required information in order to classify the electrical installation being designed into the correct supply category, they should contact the relevant Regional Office of the Electricity Authority of Cyprus, which will provide guidance regarding the supply category applicable to the case under study.

Short-Circuit / Fault Current in Network Users’ Premises

Prospective short-circuit/fault current and external source impedance at the output point of the EAC's meter.

1. Introduction

2. Network Conditions During the Fault

3. Supply Methods and Prospective Fault Current