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AM18.4 Medium voltage distribution: Fault calculations (2022)
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AM18.4 Medium voltage distribution: Fault calculations (2022)

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This Applications Manual on medium voltage distribution has been produced to help fill the gap in electrical knowledge in respect of how to safely employ medium voltage for the distribution of electrical power.

Our aim is to provide an understanding of a number of key aspects at different design and construction stages of a medium voltage (11 kV) power distribution system to buildings. Consideration is given to system design and to the selection and erection of equipment, including the associated practical aspects.

It is intended that this Applications Manual will be used by practitioners in conjunction with established international wiring standards and relevant codes of practice. It will also be of interest to designers and authorities who, while not directly concerned with the design or installation of electrical systems, must understand the advice offered to them by specialists.

Further, the manual should be of value to those who wish to enhance their knowledge of electrical power and building services engineering.

Medium voltage distribution is a specialised area of power engineering. This manual is in five parts:

AM18.4: Fault calculations will cover fault calculations and their application on protection devices for industry standard distribution configurations.

The parts of this Applications Manual will not cover high voltage systems (that is, those using power supply voltages greater than 33 kV). This also means that we will not go into any detail regarding transmission and distribution at these voltages nor specialised power applications within the industry (such as the protection of power factor correction capacitors, harmonic snubber circuits, high voltage direct current, high power static conversion, battery storage or smart grids). For information regarding possible solutions for these applications, it is best to refer to manufacturers’ literature.

Note: A corrigendum was issued on 21st June 2023, correcting a small error on Page 3: The heading of Table 1 Column 6 should read 'Impedance' (%). This has been corrected in the PDF available for download. Purchasers of the hard copy are asked to make a note of this change.

1 Fault calculations

2 Three-phase faults and the need for calculations

2.1 Per-unit percentage calculations

2.2 Calculation methodology

2.3 Per-unit calculation example

3 Single-phase line-to-line and earth faults

4 Fault current contribution from motors and generators

4.1 Introduction (asynchronous) motors

4.2 Synchronous generators

5 Symmetric and asymmetric fault currents

6 Discrimination and island mode of operation

Annex A: An additional short-circuit calculation example

A1 Busbar A (II kV)

A2 Busbar B (nominal value 400 V)

A2.1 Revised chosen base values (at 400 V)

A2.2 Circuit breaker

A2.3 Busbars (at B)

A2.4 Short-circuit current(s) at busbar B

A2.5 Motor contribution

A2.6 Instantaneous peak current (400 V) from supply, generator and grouped motors

A1.2 Incoming supply

A1.3 ACSr overheard line (aluminium conductors, steel reinforced OHL)

A1.4 Short-circuit current(s) at busbar A

A1.5 Motor contribution

A1.5.1 Contributed current

Annex B: Symmetrical components

B1 A brief introduction to symmetrical components and their application to fault current calculations

B2 Fault conditions

B3 Example line-to-earth calculations

B3.1 Three-phase fault

B3.2 Single line-to-earth fault

B4 The effect of fault and or neutral earthing resistance

Annex C: Sample cable impedances

Authors: Les Norman (Brunel University London), Adam Rawlinson (PCS Consulting Services Ltd.), Phil Reed (RPS Group PLC)

Peer reviewers: Derek Elliott (Insight PFM Ltd/CIBSE Electrical Services Group), Neil Hitchman (Vinci Construction UK/CIBSE Electrical Services Group), Tony Sung (Energy Reduction Management Ltd/CIBSE Electrical Services Group) (chair)

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