Safely using Multi-Service Distribution Boards (MSDBs) | NAPIT

Safely using Multi-Service Distribution Boards (MSDBs) | NAPIT

NAPIT’s Chief Technical Officer, Frank Bertie, and the BEAMA Cut-out & Feeder Pillar Group, explore the types of equipment and the correct safety procedures that the electrical contractor must consider when carrying out work.

Electrical contractors aren’t considered to be responsible for any installations which involve the supply authority’s equipment, such as service cables, cut-out fuses, or the connection to metering equipment.

There are situations where they become involved and are being contracted to work on the Building Network Operators’ equipment within apartment buildings and complexes.

What is a Building Network Operator (BNO)?
A Building Network Operator is de?ned in Energy Networks Association (ENA) Engineering Recommendation G87 (ENA EREC G87) as: ‘The organisation that owns or operates the electricity distribution network within a multiple occupancy building, between the intake position and customer’s installations. The BNO may be the DNO, another licensed distributor or a third party exempt from an electricity distribution license (e.g. a facilities management company)’ (see Fig 1).


A BNO is responsible for the design, installation and maintenance of the building network. The building shall satisfy the requirements of Building Regulations, BS 7671 (IET Wiring Regulations), BS 8313 (code of practice for the accommodation of building services in ducts) and BS 9999 (code of practice for ?re safety in the design, management and use of buildings). The developer should liaise with all utilities, including the Distribution Network Operator (DNO), at the earliest opportunity to determine what the appropriate arrangement will be.

Building Network Operators’ Multi-Service Distribution Boards (MSDBs) are an essential part of the electrical distribution in multi-occupancy buildings (apartment blocks) (see Fig 2). Essentially, they are a fuseboard receiving a supply from the electricity utility and distributing it via 63 to 100 A fuses to individual dwellings.


Historically, MSDBs were installed and operated by the Distribution Network Operator (DNO). Increasingly, they’re installed and/or operated by the Building Network Operator (BNO) and their electrical contractor.

With the door open, older MSDBs may have large areas of unshrouded hazardous live conductors and provide little or no protection against contact with hazardous live parts. More recent designs usually provide protection against electric shock by barriers, etc. when all fuse carriers are in place, in accordance with IPXXB of BS EN 60529.

When the 63 or 100 A fuse carriers are removed the fuse base provides protection close to IPXXB. However, this is not the case when higher rating circuits are incorporated in larger MSDBs. The larger circuits are usually based on the use of ‘J’ type fuse carriers with ratings up to 630 A, or even 800 A. When a fuse carrier is removed to isolate a circuit, a large contact that can be live, is exposed. In addition, a ‘J’ type fuse handle has no proven load breaking or fault making capability when it is inserted and removed.

MSDBs are very simple, reliable, low-voltage assemblies that benefit from incorporating components manufactured specifically for the application. The 63 or 100 A fuse units are essentially house service cut-outs. Currently these include double screw cable terminals and are stringently tested to more onerous standards than most equivalent industrial products. The ‘J’ type fuse links include a fast-acting characteristic, thereby minimising the energy let-through in the event of a fault and much reducing the risk of a fire.

DNOs are very familiar with operating MSDBs and similar equipment; they have an enviable record to demonstrate that MSDBs can be operated safely. Operations involving switching of circuits and/or exposure to live parts are all carried out following rigorous training, risk assessment and then working in accordance with a strict procedure.

Suitable PPE must be worn while carrying out the tasks (visor, gloves, etc). When appropriate a Fusemate, or equal, is used to check a circuit isn’t faulted before a fuse carrier is inserted. Inevitably, the training required is of a specialist nature and is best provided by an organisation with experience in operating such equipment. Most DNOs and a number of private organisations offer this type of training. PPE is available from specialist organisations. The level of protection provided by the PPE is dictated by the risk associated with the task to be undertaken.

To demonstrate it is safe to insert a ‘J’ type fuse link, typically a device such as Fusemate is inserted in place of a ‘J’ type fuse (see Fig 3). These are designed to carry maximum load current for a short period of time in order to allow a circuit to be tested. When closed under ‘normal load’ current conditions the device will complete the circuit and generally display the current level. If closed under fault current conditions, the fuse in the Fusemate blows, clearly indicating it isn’t safe to insert the fuse link in a standard ‘J’ type fuse handle.


Safety precautions for working on MSDBs
Anyone involved in working on or near live equipment needs to discharge their obligations under the Electricity at Work Act. BNOs, and when relevant their contractors, need to operate MSDBs safely. This needs to include the earlier and less well shrouded MSDBs. Working practices at least equal to those used by the DNOs are essential.

When it is necessary to work on energised MSDBs, more in-depth guidance on the practises necessary is available from a number of sources. These include the Health & Safety Executive’s Guide HSG85 and BS EN 50110. When followed, these documents provide guidance on good practise when working on energised equipment and a means of quantifying risks and the level of PPE required to mitigate those risks.

MSDBs are a very reliable, time-proven form of low voltage distribution assembly that requires speci?c safety considerations to ensure the safety of operators and anyone adjacent to them. This includes specialist training, use of the appropriate procedures and PPE, and suitable test equipment, when necessary (see Fig 4). The necessary measures to ensure the safe use of MSDBs are available and should always be used to ensure there are no issues.

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