Consumer Unit Types
Split Load, Dual RCD & RCBO Boards

The consumer unit (CU) is the heart of every domestic electrical installation. It houses the main switch, the protective devices for each circuit, and increasingly, surge protection devices. The type of consumer unit you specify determines how circuits are grouped, how RCD protection is provided, and how the installation behaves when a fault occurs.

Under current BS 7671 regulations, all consumer units in domestic premises must have metal enclosures (Regulation 421.1.201), and almost every circuit requires 30 mA RCD protection (Regulations 411.3.3 and 411.3.4). The question is not whether to provide RCD protection, but how to arrange it — and that decision depends on the type of consumer unit you choose.

There are five main types of consumer unit available in the UK market. Each has distinct advantages and limitations, and understanding the differences is essential for specifying the right board for each job. The choice affects cost, discrimination, reliability, and the client experience when a fault occurs.

A split load consumer unit uses a single RCD to protect a group of circuits, with the remaining circuits connected directly to the main switch without RCD protection. This was the standard arrangement for many years but has become less common as BS 7671 has expanded the requirement for RCD protection to cover almost every circuit.

In a typical split load board, the main switch feeds one side directly (the non-RCD side) and the other side through a single 63A or 80A, 30 mA RCD. Circuits that do not require RCD protection — such as fire alarm supplies and emergency lighting — are connected to the non-RCD side. All other circuits are connected to the RCD-protected side.

Split load boards are rarely specified for new installations today. The poor discrimination and the near-universal requirement for RCD protection make them impractical for most modern domestic work. However, you may encounter them during periodic inspections on existing installations.

The dual RCD board has been the most common type of domestic consumer unit for the past decade. It uses two RCDs (typically 63A or 80A, 30 mA) to protect two groups of circuits, with a main switch providing isolation for the entire installation. Non-RCD circuits (fire alarm, emergency lighting) are connected to the main switch side.

Circuits are distributed between the two RCDs to balance the load and provide some degree of discrimination. The general practice is to ensure that critical services are split between the two RCDs — for example, upstairs lighting on one RCD and downstairs lighting on the other — so that a single RCD trip does not cause a complete loss of lighting.

Dual RCD boards remain popular for budget-conscious domestic installations, but the trend is firmly towards all-RCBO boards for new work. The cost difference has narrowed significantly, and the practical benefits of individual circuit protection increasingly outweigh the material savings.

A high integrity consumer unit builds on the dual RCD design by adding a third RCD to protect critical circuits that should not be affected by faults on other circuits. The typical arrangement uses three RCDs: two protecting the main circuit groups, and a third protecting circuits such as the freezer, burglar alarm, smoke detection, and external lighting.

The concept behind high integrity boards is that certain circuits are more important to keep live during a fault condition. A freezer full of food can suffer hundreds of pounds of damage if disconnected for several hours. A burglar alarm that loses power is no longer protecting the property. By placing these circuits on a dedicated RCD with no other circuits to cause a trip, the risk of nuisance disconnection is virtually eliminated for those critical services.

High integrity boards cost more than standard dual RCD boards (typically 20 to 40 pounds more for the board itself, plus the cost of the additional RCD) but less than a full RCBO board. They offer a good compromise between cost and discrimination for installations where budget is a factor but the client has critical circuits that must not be interrupted.

However, the circuits within each RCD group still suffer from the same poor discrimination as a standard dual RCD board — a fault on the kitchen ring will still trip all of RCD Group A. For this reason, many electricians now prefer to go straight to an all-RCBO board rather than use a high integrity arrangement.

An all-RCBO consumer unit provides individual RCD and overcurrent protection for every circuit using individual RCBO devices. There are no group RCDs in the board — the main switch provides isolation only, and each circuit has its own dedicated RCBO combining 30 mA RCD protection with MCB overcurrent protection.

This design provides the best possible discrimination. When a fault occurs on any circuit, only that single circuit is disconnected. Every other circuit in the installation remains fully operational. The householder experiences minimal disruption, and the electrician can immediately identify which circuit has the fault simply by looking at which RCBO has tripped.

The main disadvantage of all-RCBO boards is cost. Individual RCBOs are more expensive than MCBs — typically 25 to 50 pounds per RCBO compared to 5 to 10 pounds per MCB. For a 10-way board, this adds approximately 150 to 400 pounds to the material cost compared to a dual RCD board. However, this cost difference has been steadily decreasing, and the practical benefits make all-RCBO boards the preferred choice for most professional electricians.

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A main switch only consumer unit contains just the main isolating switch and the bus-bars for connecting individual protective devices. There are no RCDs built into the board — all RCD protection is provided by individual RCBOs fitted into the available ways.

This type of board is essentially the chassis for an all-RCBO installation. It provides maximum flexibility because every way can accept any RCBO from the manufacturer range, and there are no RCD groups to manage. The electrician simply selects the appropriate RCBO (type, rating, and sensitivity) for each circuit and fits it to the board.

Main switch only boards are also used where external RCDs are required — for example, where time-delayed RCDs are used for discrimination in sub-distribution boards, or where specific RCD types (Type B for EV chargers, Type F for inverter-driven equipment) are needed on individual circuits.

Many manufacturers now supply their standard domestic consumer unit range in both dual RCD and main switch only configurations, giving electricians the choice of RCD architecture for the same physical enclosure. When ordering, ensure you specify the correct variant — a dual RCD board cannot easily be converted to a main switch only board, and vice versa.

The choice of consumer unit type depends on several factors: budget, the number of circuits, the client's expectations, the type of premises, and the specific requirements of the installation.

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