Four things to consider when replacing a consumer unit

There are a number of reasons why property owners change their consumer unit. Perhaps one of the MCBs has blown and replacements are no longer available. Maybe they have taken possession of an old property and the consumer unit is not compliant with current regulations (soon to be the 18th Edition). Or maybe they are undertaking a rewire, refurbishment or larger renovation project.

Whichever is applicable there are four key things the electrical contractor should consider.

1. Your client is not a qualified electrician

The single most important thing for the electrical contractor to remember is that regardless of how much research they’ve done or how intelligent they are, your client is not a professional electrician, has never before replaced a consumer unit and is unlikely to be aware of all the issues.

The electrical contractor should always encourage the customer to have an Electrical Installation Condition Report (EICR), which should be carried out before replacing the consumer unit. This is to establish that the latest regulations are met and will ensure that the contractor isn’t left with faults to clear that were not included in the original contract.

The EICR test results will then help with the design and selection of protective devices for the new consumer unit.

Educating your client regarding the level of protection that is required under BS7671 will help you design a solution which matches their requirements, budget and expectations and need not be any more onerous than briefly explaining the following:

a. In general the UK Wiring Regulations require that all circuits are protected against both:

  • Overload – the fault which can result in fire in cables and damage to appliances.
  • Residual Current (or Earth Leakage) – the fault which causes electric shock, which can result in injury or death.

b. The Circuit Protection Devices: RCD, MCB and RCBO. What they are and what they do:

  • RCD – Residual Current Device
  • Protects a bank of circuits from residual current or earth leakage.
  • MCB – Mini Circuit Breaker - Protects an individual circuit against overload and fault currents
  • RCBO – Residual Circuit Breaker with Overload - Protects an individual circuit from both residual current and overload. It combines the aspects of both RCD and MCB and is therefore more expensive than an MCB.

2. Future proof …like there’s no tomorrow!!

There’s not a single building project in history during which the client did not, at some point, change their mind or the specification. As the project progresses they may change the gas hob to an electric one, decide that they want to put an electric shower in the guest room or decide they want to install a security system.

The electrical contractor must remember this is entirely reasonable and prepare for rather than react to it. Giving yourself plenty of wiggle room at the outset will avoid hassle and wasted effort later in the project when the inevitable happens. Explain at the start of the project that the consumer unit will serve the property for some considerable time and should be able to comfortably handle any changes and additions to circuit layout in coming years.

If during the design the contractor defines 10 ways is required then at the very least fit a 12 way consumer unit. If a 12 ways is required then go for 15, if 16 go for 20 etc.

3. High priority circuits and circuit separation

In order to properly specify a circuit protection solution which fits with the client’s expectations it is vital to ascertain which circuits require special consideration in respect of RCD’s.

Separation of high priority circuits is important because it removes any chance of that circuit being knocked out by an earth leakage fault on any other. This occurs in a standard RCD/MCB configuration when the RCD cuts the power to all MCBs it is protecting upon detection of a residual current fault on one. ..and of course, this problem is exacerbated when a greater number circuits fall under the protection of one RCD.

Common high priority circuits include smoke alarms and security systems but every client will have a different attitude to what constitutes ‘high priority’: the freezer in one home for example, may just be host to some out of date fish fingers, while another household’s freezer contains expensive cuts of venison! Home offices containing PCs, tropical fish tanks, stairwell lighting, swimming pool pumps and heating systems are all examples of high priority circuits.

4. Which type of consumer unit?

A number of factors determine which consumer unit you will ultimately fit, including the number of circuits, types of circuit and client’s budget. The three main models are:

i. Fully Loaded Consumer Unit


This is a popular solution due to its low cost and comprises a dual RCD board supplied complete with MCBs. They are suitable for smaller properties with less complex circuits and are available in a number of ‘ways’, depending on the manufacturer:

  • Hager: 6 way (VML733H), 10 way (VML755), 12 way (VML766), 16 way (VML716)
  • MK: 10 way (K7666SMET), 15 way (K7668SMET)
  • Wylex: 10 way (NHRSS10SSLHI), 15 way (NHRS15SSLHI)
  • Contactum: 10 way (DDS10166MS-PO1), 12 way (DDS12188MS-P010), 16 way (DDS16166MS-PO1)

The main drawback with fully loaded consumer units is that they usually offer limited configuration flexibility and circuit separation. Indeed, some of the boards above are entirely fixed with no provision for high-priority circuits, such as the Hager VML755 which offers 5 fixed MCB ways on each RCD and does not allow for the fitting of RCBOs.

ii. Main Switch Consumer Unit


This is considered by some as the best circuit protection solution available as it offers total circuit separation but there is a cost implication. It is supplied with just the main switch and allows for every circuit to be RCBO protected. Whilst the consumer unit itself is not particularly expensive the installation of RCBOs makes this a premium solution. Such units are available in all sizes from 5 way up to 40 way. Hager and Wylex have a particularly good offering in the larger units over 20 ways.

iii. High Integrity Consumer Unit


This solution is becoming increasingly popular as it combines the best aspects of a dual RCD unit and main switch consumer unit. Built with three neutral bars, and supplied with 2 RCDs, the HI consumer unit allows for both 2 banks of MCBs and a further bank of RCBOs for high-priority circuits.

This means that standard circuits such as lighting and ring finals are given commensurate residual current protection, whilst high priority circuits are afforded total circuit separation. HI Units were introduced by Wylex about ten years ago and originally were only available in larger duplex arrangements of over 20 ways. Nowadays however, all the major manufacturers offer HI solutions with models as small as 10 ways, offering both the homeowner and professional electrician excellent flexibility over circuit design at affordable prices.

We hope this article helps with your decision making when replacing a consumer unit and we would like to thank Gil-Lec Electrical Wholesalers for supplying the content for this article.

Categories: A Guide To Part P / Domestic Installation

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