Electrical System Design

On receipt of the client's brief the first consideration for the design is to determine the maximum demand and how many circuits will be required and thus the number of ways in the consumer unit.

From the kitchen, and where appropriate the utility room, appliance layouts the positions of the fixed appliances can be determined. The first question is obviously whether items such as the cooker/ovens, washing machine, tumble drier, dishwasher etc. should be fed from common socket circuits or from their own ways in a consumer unit. If each of the appliances electrical loading is listed it will become apparent that if all the appliances are running at the same time the load is likely to exceed the rating of any socket circuit supply MCB, even when diversity is allowed for, which will cause it to trip. A better solution is to supply each fixed appliance with a load of 13A or less (16A or 10A if the suggested IEC plugs and sockets are used) from an individual way in the consumer unit via a double pole switch local to the appliance then to a socket outlet behind the appliance. For appliances with a load exceeding 13A (16A if the suggested IEC plugs and sockets are used) a double pole local switch is still required with a connection unit behind the appliance. Another benefit of utilising separate ways in the consumer unit is that where the circuits are protected by RCBOs; which is likely in domestic installations due to changes in the IET Regulations; the trip of any one circuit breaker causes only one appliance to be isolated; to enable the loss of power to an appliance to be observed it is essential that the local isolator to an appliance is equipped with a neon indicator.

MCBs and RCBOs are mentioned throughout this article, some will ask about fuses, it would be unusual to find a modern dwelling where fuses would meet the requirements of the IET regulations therefore MCBs and RCBOs are referred to throughout. Due to the changes in the IET Wiring Regulations virtually all circuits will need to have RCBO protection that protection should be for each circuit and not an overall RCD (residual current device), the reason is that all circuits 'leak' due to capacitive and inductive coupling. Leakage occurs within the equipment using electricity and the cabling to the equipment, therefore as the amount of equipment and the length of cabling increases so does the natural leakage using an overall RCD will have a lower trip threshold and will therefore be subject to nuisance tripping. There is another reason to avoid overall RCD protection as a trip will affect multiple circuits which will increase both inconvenience and danger; the inconvenience is due to the amount of appliances that 'go off', a refrigerator or freezer losing power in not only inconvenient there is also a cost and health implication, the danger arises from the entire lighting circuit or an item of medical equipment such as a dialysis machine that may be 'plugged in' failing due to a power loss. A word of warning if the distribution system could be configured differently to protect against such failures then any injury or worse due to that failure may attract criminal proceedings against the system designer and/or the installer.

Having established the principle that fixed appliances in the kitchen/utility room should each be fed from their own dedicated way in a consumer unit it follows that other fixed appliances such as the aerial distribution system, central heating system, computer network system, intruder alarm etc. should each be fed from their own dedicated way in the consumer unit and be protected by an appropriately rated RCBO with a 30mA earth fault trip.

The next decision to be made is regarding the socket circuits, as stated previously the normal approach was to use a ring main for each 100m² of floor area, that approach was initiated in the late 1940s and the early 1950s, since then there has been literally an explosion of appliances and devices that require power and a change of regulations which in most instances requires each of the socket supply circuits, whether ring or radial, to be protected by a 32A or 20A 30mA RCBO, this requires a new approach to be considered. Instead of a ring main serving a number of rooms where an earth fault trip will isolate the entire ring main and therefore all those rooms served by the ring main, each room should have its own socket circuit which, providing the room has an area of 50m² or less, can be fed radially from a 20A 30mA RCBO; this floor area can rise to 75m² if a 32A 30mA RCBO radial feed is used; thus if the occupant of a room causes a trip only that room is affected; there is another benefit if a child is sent to their room as punishment switching off that room's sockets at the consumer unit will prevent the miscreant from using their games console, television or computer whilst under punishment without affecting other occupants of the household. There are also benefits when carrying out decoration as it is easy to isolate a particular room so that socket faceplates can be removed whilst decoration takes place without affecting any other room.

Having determined the socket configuration the next step is to look at the lighting circuits by tradition this was carried out with one circuit for the upstairs and one for downstairs or for bungalows and flats one for the living rooms and kitchens and one for the bedrooms and bathroom, but that was when the lighting was generally by a single incandescent lamp in each room apart from the kitchen which, from the late 1950s, would have a fluorescent fitting. The modern trend is to have a variety of fittings including downlights, pendants and chandelier types with a variety of light sources although the use of LED lamps is becoming increasingly prevalent as their electricity usage is approximately one seventh of that of an incandescent lamp for the same light output. There is also an increasing use of electronic control systems that allow the lighting in the controlled zone to have varying intensity and colour. Again as with the sockets and fixed appliances the IET Regulations have changed in regard to the protection of the circuits which in domestic situations will generally require the circuits to be protected by RCBOs with a fixed earth fault setting of 30mA. The use of RCBOs for lighting circuits although commendable for personal protection against electric shocks also has negative effects, for example an elderly person or a pregnant woman or a person carrying an infant going down stairs would be in far greater danger from falling should the lights suddenly extiguish due to a circuit fault than from the possibility of electric shock, it is therefore necessary to reappraise lighting circuits.

If each room has its own lighting circuit then any circuit fault will only isolate the affected room, it also allows any electronic controllers to be mounted near to the consumer unit. To make staircases safer there should be a landing circuit, hall circuit and a staircase circuit thus there would be at least two circuits covering the stairs. As with the socket circuits it also makes isolation easier to allow switch faceplates and light fitting removal for decoration.

The switching of the lighting points should also be reviewed, by tradition each room had a switch adjacent to the door to control the light in that room, the landing would have a two way switch in the hall and another on the landing so that the staircase lighting could be controlled from the hall or landing. With this system when leaving the kitchen switching off the light would leave the area in darkness until the hall switch was operated, how much more convenient to have the hall light controlled from every room leading onto the hall, it is not rocket science just simple two way and intermediate switching. A similar argument can be made for the landing in that each bedroom should have a switch controlling the landing light. For bungalows and flats it should be that every room leading onto the hallway or other common space should have switches to control the lights in that common space. Whilst considering the bedrooms the bedroom lights should be controlled from the door and the bedside. What is being proposed here is that the switching of the lighting should be designed for the maximum convenience of the house holder. That of course supposes that electronic control systems with remote controls have been discounted, if those are to be installed then the outages due to failures must be considered, electronics can and do fail much more frequently than a switch.

The last decision regarding the lighting and power installation is the cable type to be used. An electrical installation has a 'life' of twenty five years, although there are many installations that are much older, a house or dwelling has a life of over one hundred years. It follows that a dwelling will require a number of rewires during its lifetime therefore the wiring method should reflect the need for those rewires. Unfortunately property developers see profit as the key factor in every decision, it is cheaper as an initial install to use twin and earth cable with capping where the cables run on walls, this method of wiring does not allow for rewires. The use of conduit does allow for rewires but is a more difficult to install, chasing walls is time consuming and therefore expensive, thus affecting the profit, it will not be the property developer that pays for rewires. The decision on the wiring method is therefore taken out of the designers hands and given to the client.

Thus the principle of design for the electrical system within a dwelling is established; fixed appliances are individually fed from the consumer unit and the sockets and lights for each room have their own feed from the consumer unit. That leaves the other items mentioned in the appliance and utilisation list but not covered above, some will be fed via sockets the rest by individual circuits from the consumer unit.

For a new dwelling the public electricity supplier (PES) will supply an external meter cupboard which will need to be built in by the builder, due to the constraints regarding the distance from the meter to the consumer unit the consumer unit will need to be sited close enough to the meter cupboard so that the meter tails are less than two metres in length, although it must be noted that different PESs have different rules regarding the maximum length of meter tails. Where the maximum length has to be exceeded then a MCCB needs to be fitted within the proscribed distance and a suitable cable used to connect from the MCCB to the consumer unit.

The maximum load in a modern dwelling is in the kitchen or where separate kitchen/utility room so the locations of the meter cabinet and kitchen will require consideration as to whether there is one consumer unit or two, one adjacent to the meter cupboard and one in the kitchen. The optimal solution is for the meter cabinet and therefore the consumer unit to be located as near as possible to the kitchen or kitchen/utility room.