A Guide To your Fuse Box
A = Main switch B = Circuit breakers C = RcdA consumer unit or fusebox is a particular type of distribution board comprising a co-ordinated assembly for the control and distribution of electrical energy, principally in domestic premises. A consumer unit incorporates a manual means of isolation on the incoming circuit(s) – a main switch, and an assembly of one or more fuses, circuit-breakers or residual current devices.

Circuit breakers, fuses and RCDs are protective devices that will interrupt the flow of electrical current if the circuit concerned develops a fault. Instead of a modern unit like the one shown above with circuit breakers and RCDs, some older houses have fuseboxes with rewireable fuses.


Fuses
Rewireable fuses have a piece of special fuse wire running between two screws. The fuse wire generates heat when a fault current flows through it and it melts when the heat exceeds the acceptable level. The melted fuse breaks the circuit and stops the electricity supply.


Circuit Breakers
A circuit-breaker is an automatic switch fitted to each circuit in the consumer unit which switches off in the event of a short circuit, an overload or a fault to earth on the circuit. It is not usually required to operate very often.
A circuit-breaker is much the same size as a fuseholder, but give more precise protection than cartridge fuses. When they 'blow' or 'trip', they are simply reset instead of needing replacement or rewiring.


RCDs
A residual current device is a mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specific conditions, such as under earth fault conditions.


Main Switch
The main switch allows you to turn off the electricity supply to the electrical installation. Note that some electrical installations may have more than one main switch. For example, if your house is heated by electric storage heaters, you will probably have a separate main switch and consumer unit arranged to supply them.
It is important to know where the consumer unit is located and that it is accessible. It is also important that you know where the main switch(es) are in order to turn it (them) off in the event of an emergency.
If your electrical installation includes one or more residual current devices (RCDs), it is important that they are checked regularly. You can do this by following the instruction label, which should be located near the RCD. The label will read as follows:

the phenomenon of ageing in short-circuit and overload protective devices doesn't get a lot of discussion. Schneider Electric's Richard Walley believes that it's time for this to change since, as he explains, ageing can severely compromise protection system performance:


Square D brand MCBs in application


A fused system.


Fit it and forget it - that's what every user wants to do with protection devices in electrical installations. And, in fact, that's often what happens. Everyone forgets about the protection until a fault occurs, and then it has to work flawlessly under even the most extreme conditions.
Can we, however, be sure that devices which may have been fitted years, if not decades, earlier will be up to the job? Equally, can we be sure that the devices we fit today will last? The real answer is that it all depends upon the type of protection, and the conditions to which it has been subjected since it was installed.

1) Circuit breakers:

Let's look first at circuit breakers. Although these are fairly complex devices, under normal operating conditions they have no built-in ageing process. Certainly, the mechanism has a finite mechanical and electrical life. For modern breakers, however, this is measured in tens of thousands of operations and is, therefore, unlikely to be a serious consideration in any real application.

In fact, IEC 60947-2 lays down high minimum standards for circuit breaker life and even these are substantially exceeded by the best available products, such as those in Compact NS range from Merlin Gerin, a brand of Schneider Electric.

Under conditions of moderate overload, which is the most common type of fault, the circuit breaker once again experiences no significant ageing effect. In conventional breakers, the bi-metal elements are subjected to thermal cycling, but they are designed with this in mind, and their characteristics are not affected to any extent which could be considered significant. In breakers fitted with electronic trips, even thermal cycling is eliminated.

Now, what about the acid test, a serious short circuit? According to IEC 60947, a circuit breaker must clear three faults equal to Ics, its service breaking capacity, and still remain fit for further service. In other words, neither the contact assembly, nor the characteristics of the device, must be adversely affected by these stringent tests. The breaker must also be capable of clearing one fault equal to Icu, its ultimate breaking capacity, and then be fit for use, albeit with reduced performance.

The standard's use of two breaking capacities, Ics and Icu, does give rise to a potential problem. How is a user to know whether a particular fault produced a current less than Ics, or in the band between Ics and Icu? The straightforward answer is to eliminate the problem by designing breakers with Ics equal to Icu, as has been done with the Merlin Gerin NS range.

The user knows for certain that a breaker with Ics equal to Icu can remain in service until it has interrupted a maximum of three short-circuit faults. This is a major benefit as, even after the most serious faults, the supply can be quickly restored as soon as the problem has been rectified. No replacement parts or spares are needed.

One question remains, however: how is it possible to determine whether this is the first, second or third short circuit which the breaker has experienced? There is, of course, no readily apparent indication, but it should be borne in mind that we are considering SERIOUS short circuits. These are exceptionally rare, and when they do occur, they are sufficiently important to merit proper documentation in the maintenance records, if nowhere else.

It should not be difficult, therefore, to check on any individual breaker's short-circuit history. It is also worth mentioning, perhaps, that few people easily forget either the occurrence or the results of a 50kA fault!

Practical experience of circui

Increasing electrical safety in the home has been a prominent theme of late, due in part to the Building Regulations, and the revisions to the IEE Wiring Regulations. Alan Roadway of ABB discusses how electrical installers can help ensure safe electrical practice.


The latest generation Housemaster consumer unit

Most drivers accept the idea of vehicle MOTs as an important safety measure. Yet how many home owners apply the same principle to having their domestic electrical installations professionally checked on a regular basis? According to the Electrical Safety Council, over 40% of the British population do not know or have never had their electrics checked and almost a quarter (24%) of people do not know how old their wiring is. And when it comes to paying for checks, a significant minority of people (28%) say they would not be prepared to pay for an inspection by a registered electrician.

The National Inspection Council for Electrical Installation Contracting (NICEIC) recommends that a domestic installation should be inspected a minimum of every 10 years, but the message is obviously not reaching everyone.

On average, there are 10 deaths and over 750 injuries* as a result of faulty electrical installations in the home each year. Added to this, electrical faults cause 12,500 fires, resulting in 25 deaths and 590 injuries every year.

Regulating safety
In theory at least, the situation should have improved with the revisions of the Building Regulations in January 2008. The Government’s regulatory impact assessment estimated that 30% of electrical accidents were caused by poor workmanship. The Building Regulations stipulate that any major electrical work carried out since the regulation came into force should have been done by a “competent” person. Very simple jobs, such as replacing a plug or changing a light fitting in the living room, can still be tackled by DIY enthusiasts. But jobs such as adding a new circuit or any electrical work carried out in a “special” location such as a bathroom must be approved and so should not pose a safety problem.

However, changes in the regulations cannot compel householders to get their electrical systems checked or bring older installations up to date. In addition, there are more demands being placed on domestic electrical systems than ever before as people rely increasingly on new electrical goods, ranging from garden gadgets to IT systems. Any extra burden only increases the risk of problems.

Encouraging consumers
Responsible installers therefore have a key role to play in promoting the role of inspection to potential clients. A handy checklist can be a useful first tool in making people aware of when it’s time to call in the professionals. For example, any or all of the following points may indicate the need for an inspection:

• Is the wiring more than 15 years old? Standards and cables have changed and the safest option is to renew. If the age of the system is unknown, there are telltale signs to look out for, such as any round-pin plugs and sockets, or lights that use the old, twisted type of flex.

• Has it been longer than ten years since the wiring was checked? All electrical systems deteriorate gradually over time and the latest guidance recommends ten years as the maximum time between safety inspections.

• Has electrical work been carried out by amateurs and never checked? Although this may have been perfectly legal at the time, it wouldn’t comply with today’s regulations, and there may well be safety implications.

• Do plugs or sockets get hot? Overheating is usually a sign that too much current is being drawn.

• Do fuses blow regularly and for no obvious reason? This indicates potentially dangerous current surges in the network.

• Does the householder use adaptors routinely to run multiple appliances from a single outlet? Older systems often provide insufficient sockets to cope with modern lifest