Electrical services Fault reduction to help reduce electrical services technical risks It is a legal requirement that an

electrical installation should be designed for safety under normal and abnormal [fault] conditions

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fault currents at relevant distribution points, designers and installers are strongly recommended to apply BS EN 60909-0 to perform detailed calculations of acceptable accuracy to select the settings for correct coordination of protective devices. Although there are many useful guidance materials

on BS7671 from various authors available to assist designers and installers to calculate the prospective fault currents of an electrical installation, they are generally more applicable for smaller installations. l

References 66kV ~ 0.1Ω 350MVA Fa Tx1 Uk-tx=5% 25MVA Fb Tx2 Uk-tx=4.5% 700kVA Fc Figure 1 – Single-line diagram of a Fault MVA calculation case study 66kV ~ MVA 350 Fa

500 MVA

1210 MVA

Fb Figure 2 – MVA equivalent single-line diagram

When the same set of base data was used, often the results of the Fault MVA calculation method were found to be accurate within a few percent of computer calculations using industry standard electrical design software. Since most standard quality assurance procedures in engineering require designers and installers to use a different method to verify the calculated results from the one originally used, the use of Fault MVA calculation method would be a useful aid for verifying e.g. BS7671, BS EN 60909-0 or IEEE calculation results derived from proprietary software. The reader should note that the Fault MVA calculation method shown in

here does not give precise fault kA values when compared with the classical fault calculation method (ie the per unit method1

). It is suggested that the Fault

MVA calculation method should be used by engineers and installers as a quick checking tool only.

References

(1) The Fault MVA calculation method uses a lumped impedance approach, which is only an approximation. The classical per unit method using symmetrical component analysis technique can take the advantage of splitting the impedance into resistive and reactive components, hence BS EN 60909-0:2001 is more accurate.

Tony Sung is chairman of CIBSE Electrical Services Group; Patrick YP Du is associate professor, Department of Building Services Engineering, Hong Kong Polytechnic University; Kevin O’Connell is head of the Department of Electrical Services Engineering, Dublin Institute of Technology

www.cibsejournal.com September 2010 CIBSE Journal 59 11kV

15.556 MVA

17.223 MVA

Fc 0.415kV

4.667 MVA

11kV 0.415kV 0.01Ω G

700kVA Uk-g=15%

(1) In an electrical system such as a power station, with a near-to-source fault, if the system’s zero sequence impedance is small when compared with the positive and negative sequence impedances, the single-phase to neutral/earth fault can produce a fault current higher than the three-phase prospective fault current. (2) The UK’s Electricity at Work Regulations 1989 – Regulation 11, says: ‘Efficient means, suitably located, shall be provided for protecting from excess of current every part of a system as may be necessary to prevent danger.’

CIBSE Electrical Services Group quiz

What is the p.u. value of the equipment shown in Figure 1 (in the main article) if the base MVA is chosen to be 100MVA: 1. Grid supply – (a) 3.5, (b) 0.286 or (c) 1.0? 2. Transformer T1 – (a) 0.01, (b) 0.125, or (c) 0.2? 3. 0.1Ω 11kV cable – (a) 12.1, (b) 0.0827 or (c) 6.05? 4. Transformer T2 – (a) 6.428, (b) 12.85 or (c) 0.1556? 5. 0.01Ω 415V cable – (a) 0.581, (b) 58.1 or (c) 5.81? 6. Standby Generator – (a) 0.047, (b) 21.43 or (c) 7.14?

It is left for the reader to show that the p.u. analysis outcomes are similar to the one derived by the Fault MVA calclation method.

The answer will be posted on the CIBSE Electrical Services Group website: www.cibse-electricalservicesgroup.co.uk

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