The world has changed greatly since the 1980s and the requirement for MV distribution in buildings has increased significantly, and Chartered Engineer Brendan Dervan sets out the main issues that an M&E consulting engineer needs to consider when planning for MV distribution for a typical building project.

When I started out as an electrical engineer in the mid-1980s the vast majority of buildings I worked on were supplied at low-voltage (LV) ie directly from the ESB LV network. This LV supply was usually derived from an onsite ESB substation or pole transformer.

However, there were a number of buildings which required supply at MV such as hospitals, shopping centres and industrial facilities. As a young engineer I felt unprepared for this aspect of building services design.

Very theoretical

While some elements of MV was covered on the academic courses I attended, they were very theoretical and did not prepare me for what I needed to know as a specifier of MV equipment such as transformer, switchgear, protection, cables etc.

The world has changed greatly since the 1980s and the requirement for MV distribution in buildings has increased significantly. While the vast majority of MV projects would have still a single circuit breaker and transformer arrangement, larger commercial and industrial complexes would have multiple transformers fed from a single MV distribution board or via a ring main system.

In Ireland data centres, specialist manufacturing plants and other large energy consumers with demands in excess of 10MVA are connected at high-voltage (HV) ie, 38kV or 110kV and in some cases at 220kV.

A demand of 500 kW is usually regarded as the border line for a low voltage supply. In other words above this level a customer is expected to take supply at MV ie, either 10 kV or 20 KV and distribute at MV within their own premises transforming down locally to 400/230V as required.

There is an ever increasing presence of MV distribution in non-industrial buildings and this is set to increase substantially in the coming years. The main driver for this is our climate action plan and in particular the proposed replacement of fossil fuel energy for heating and transport with renewable energy.

This will increase electricity demand in buildings which will now require electrically powered heat pumps and charging points for electric vehicles etc. In many buildings this will increase the demand beyond the threshold of 500 kW for a normal LV supply and customers will have to take supply at MV and instal their own MV switchgear and transformers.

This article sets out the main issues that an M&E consulting engineer needs to consider when planning for MV distribution for a typical building project.

Figure 1 below shows a simplified diagram of a MV ring main distribution system which is typical of what might be specified on a building services project.

The system comprises a main MV Switchboard; 4 X Ring Main Units – (RMU 1- 4); 2 X oil transformers (T1 & T2); 2 X cast resin transformers (T3 & T4) and 4 X MDBs, only two of which are shown for clarity.

MV standards and codes of practice

The international/global standards for 'Power installations exceeding 1 kV AC and 1,5 kV DC' is IEC 61936-1:2021. Part 1 of that standard specifically deals with AC. This standard has been adopted by CENELEC and in turn by Ireland as IS EN 61936-1:2010. In addition to this NSAI has published SR 61936 – 1:2019 – Guidelines on the application of IS EN 61936-1:2010.

In addition to the above standards, designers and installers must comply with the ESB’s code of practice – 'Conditions Governing Connection to the Distribution System: Connections at MV and 38kV and Embedded Generators at LV, MV and 38kV'.

IS EN 50110-1:2013 – 'Operation of electrical installations – General requirements' is another CENELEC standard which deals with the operation of electrical installations. It applies to all voltages from LV to HV. It evolved from safe systems of working used by distribution system operators (DSOs) and transmission system operators (TSOs) worldwide. 

Figure 1 - Typical MV Ring Main Distribution System

In Ireland all customers’ transformers connecting onto 10kV network must be 'dual ratio', ie, they must have dual wound primaries to allow connection at either 10kV or 20kV. Single ratio 20/0.4kV transformers are acceptable where connection is at 20kV. Single ratio 10/0.4kV transformers are acceptable where connection is at 10kV to a private MV network.


The first consideration is the primary MV switchgear to which the ESB supply will connect. Many options need to be considered in terms of the level of redundancy, available space, radial distribution versus ring distribution at surgery.

The choices of MV switchgear have changed considerably in recent years. There are two broad categories of switchgear; air insulated (AIS) and gas insulated (GIS). The latter is considerably more compact and requires less plant room space.

Many switchgear systems use SF6 gas which has very high environmental impact in terms of global warming and ozone depletion. As a result there is a drive to move away from SF6 and use hybrid systems using vacuum circuit breaker technology and air or resin insulated busbars etc.

A typical AIS MV circuit breaker has the following compartments in it as indicated in Figure 2.

  • Cable compartment (CA) to facilitate connection of incoming underground cables; 
  • Circuit breaker compartment (CB);
  • Busbar compartment (BB);
  • Low Voltage (LV) compartment (LV) which contains the protection relay (PR). 

The cable compartment usually houses the current and voltage transformers (CT) and (VT) and earthing switch (ES).

The two most important aspects of specifying MV switchgear are the Loss of Service Continuity (LSC) and the Internal Arc Classification (IAC). The LSC defines the segregation of the switchgear from the busbars and the cables etc and to what extent the assembly can be worked on when the busbars and or cables are live.

LSC1 is the most basic level of service continuity while as LSC2B is the highest allowing access to the switchgear compartment while the busbars and cables are live. LSC is comparable in some ways to the forms of segregation (one to four) that that we specify for LV switchgear.

The IAC defines the level of accessibility to the switchgear in terms of access by authorised or unauthorised personnel and also whether it can be accessed from the front only or sides and rear also. It also defines the short circuit withstand capacity of the switchgear assembly in kA seconds.

For example, an MV Switchboard specification of 'LSC2B – AF 30/3' means a Loss of Service Continuity – LSC class 2B,accessible only by authorised persons (A) and accessible only from the front (F) and it is 30kA rated for 3 seconds (30/3).

Figure 2: Cross sectional view of a typical air insulated MV circuit breaker arrangement

Protection relays

The vast majority of MV installation on building services project involve protection of feeders to transformers as shown in Figure 1. Typical protection scheme involves overcurrent and earth fault protection of the feeder cable and transformer primary winding.

An important factor to take into consideration is the upstream earthing arrangement on the ESB network. The ESB operate both 'earthed' and 'unearthed' or isolated systems on their MV network. Early consultation with the local ESB office is strongly recommended as the protection requirements differ considerably.

As a minimum every circuit breaker will have a current transformer (CT) installed usually in the cable compartment. The CT provides the overcurrent and earth fault signals to the protection relay. They can also provide signals for ammeters, energy meters etc.

Voltage transformers (VT) are also required for energy measurement and or grid protection schemes where generators are installed for peak shaving and the like.

Unlike LV circuit breaker, the tripping mechanism in an MV CB requires an external power supply unit, usually a 24V or 48 V d.c. and often referred to as the battery tripping unit. It is very important that this unit is monitored for faults through the building management system, as failure of the supply would result in the breaker being unable to trip in a fault condition.

This would then result in the next breaker upstream tripping which in many cases will be on the ESB network. Besides the obvious widespread disruption to other customers on the network, the excessive let-through energy could result in damage to the faulted customer's installation. 

Ring or radial distribution

The next consideration is whether radial or ring distribution is used. Ring distribution offers a lot of flexibility and allows the network to be easily expanded in the future.

It is a good solution for a campus arrangement where it is understood from the beginning that the site will continue to develop over time. A radial system might be more suitable for non-campus or smaller sites where there is limited scope for expansion.

MV cables

Another important consideration is the type of cabling used. Generally MV can be distributed using single core or three core cable. There are advantages and disadvantages of each type which need to be considered.

For example, on large campuses with heavy loads and long cable runs, the use of three core cable can result in a requirement for massive cable drums which may be difficult to transport. Utility companies such as the ESB uses single core cables extensively.


There are two basic types of transformers used for MV distribution, oil filled and cast resin resin.

Oil filled transformers are generally located in dedicated transformer rooms or external compounds around the perimeter of the building. Because they are filled with oil they are not generally located inside a building.

Cast resin transformers (CRTs) are designed for locating indoors and can be located closely to the actual loads. In many cases they are close coupled to the associated LV distribution board.

This arrangement is often referred to as a unit substation. While CRTs are more expensive their use in unit substation arrangements can result in considerable savings in cables on the LV side.

Transformers are manufactured to IEC standard (kVA) ratings, ie, 400, 630, 800, 1,000, 1,600, 2,000, 2,500, 3,150. It is often considered good practice to use two smaller transformers to feed an LV distribution board rather than one large transformer.

The former arrangement requires two LV distribution boards with a common coupler interlocked with the two main breakers to ensure the transformers cannot be paralleled. There is an obvious increase in cost here for the extra transformer and the extra circuit breaker and associated cabling. However, the advantages in terms of resilience are worth considering.

MV training courses 

Clearly there is an ever increasing presence of MV distribution on small-scale building services projects which previously would have been supplied at LV. There is a need to include MV design on the curriculum of building services engineering courses at third level. Engineers Ireland offers a number of CPD courses relating to MV distribution.

Best Training offers a one day/seven-hour CPD course through Engineers Ireland, which covers all the essentials a consulting engineer needs to know in planning and specifying a MV installation for a typical building service project. This course is run twice or three times a year based on demand. The following topics are covered:

  • Part 1 - MV Distribution – Introduction, the grid, standards, typical networks earthing etc;
  • Part 2 - MV Switchgear- AIS and GIS systems, protection relays, battery tripping units;
  • Part 3 –Transformers – oil and cast resin types, room ventilation, vector groups;
  • Part 4 - MV Cables, structure 1 core versus 3 core, terminations;
  • Part 5 - MV Testing – Switchgear and protection systems tests. 

Author: Brendan Dervan is a Chartered Engineer with more than 40 years’ experience in M&E building services. He established Dervan Engineering Consultants (DEC) in 1999 which merged with Cundall, a global multidisciplinary engineering consultancy firm, in 2016. He has a wide range of experience in the design, installation and maintenance of M&E services in various building sectors. He retired from mainstream consultancy in 2019 and set up Best Training which provides specialist CPD services to the M&E sector. He is the author of a number of publications on the National Rules for Electrical Installations / IS10101. He represents Engineers Ireland on the NSAI’s TC2 Committee and is secretary of Engineers Ireland Electrical Division.