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AM18.1 Medium voltage distribution: Methods (2022)
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AM18.1 Medium voltage distribution: Methods (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 first part, Methods, introduces the basic principles; it also provides the most common medium voltage distribution techniques relevant to building services engineers. Upcoming parts will include:

  • AM18.2: Equipment
  • AM18.3: Protection
  • AM18.4: Fault calculations
  • AM18.5: Grading

These will explore protection devices and equipment in greater detail. They will provide typical application calculations and solutions for the protection of transformers, distribution cables, busbars, generators and motors, and will provide examples so that these calculations are clear.

This Applications Manual will not cover high voltage systems (that is, those using power supplies greater than 33 kV). This means that we will not go into any detail regarding transmission and distribution, or specialist 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 to these applications, it is best to refer to manufacturers’ literature.

Table of Contents

1 Introduction

1.1 Distribution voltage

2 Voltage levels: the ambiguous nature of the term ‘high voltage’

2.1 High-voltage direct current

3 The UK electricity grid

3.1 Office of Gas and Electricity Markets (OFGEM)

3.1.1 Energy supply

3.1.2 New connections

3.1.3 The connection process

4 MV distribution design

4.1 Distribution networks

4.1.1 Radial distribution

4.1.2 Dual/duplicate feeder distribution systems

4.2 Ring mains

4.2.1 Open ring main

4.2.2 Closed ring main

4.3 Comparison of power distribution architecture

4.4 Operational aspects

4.4.1 Rural overhead networks

4.4.2 Urban underground networks

4.4.3 Load-transfer schemes

4.5 On-site connections, wayleaves and easements

4.5.1 Substation components and dimensions

4.6 System earthing

5 On-site power generation

5.1 Embedded/distributed generation

5.1.1 Control of embedded/distributed generation

5.2 Private generation: MV or LV?

6 Electrical safety in MV systems

6.1 Designated MV safety personnel

6.1.1 Management

6.1.2 Designated person

6.1.3 Authorising engineer

6.1.4 Authorised person

6.1.5 Senior authorised person

6.1.6 Competent person

6.1.7 Responsible person

6.2 MV safety documentation and procedures

6.2.1 Safety programme (switching schedule)

6.2.2 Permit to work

6.2.3 Sanction for test

6.2.4 Limitation of access

6.3 Security and admittance to substations

6.4 MV safety

6.4.1 MV personal protective equipment

6.4.2 MV temporary and permanent earthing and bonding equipment

Annex A: Load characteristics

A1 Motor size and starting load

A2 Harmonics

Annex B: The Electricity at Work Regulations 1989

Regulation 4: Systems, work activities and protective equipment

Regulation 5: Strength and capability of electrical equipment

Regulation 6: Adverse or hazardous environments

Regulation 8: Earthing or other suitable precautions

Regulation 11: Means for protecting from excess of current

Regulation 12: Means for cutting off the supply and for isolation

Regulation 15: Working space, access and lighting

Regulation 16: Persons to be competent to prevent danger and injury

Annex C: Examples of MV safety documentation and safety procedures

C1 Suggested solution

C2 Safety programme

C.2.1 Cable spiking gun

C3 Isolation and earthing diagram

C4 Permit to work

C4.1 Pre-printed pads

C5 Other safety documentation

C5.1 Sanction for test

C5.2 Limitation of access

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