House Electrics Pt1

Understand how your electricity is distributed and supplied to your home.

Simplified view of the overall electrical distribution
Electricity is fed to the consumer over a complex network of cables. Very high voltages are needed to supply the energy over long distances and are carried by lines suspended from transmission towers. Transformers set up these high voltages, subsequently, reducing them to more manageable distribution levels. Eventually reducing to a voltage that can be safely connected to the house.

Connections from the local distribution transformer
Throughout the supply network the electricity is distributed in three phases. Effectively, the system combines three separate generators within the one system. A saving in materials is achieved, only three, (or four), supply lines are required, instead of the six required by three separate single-phase systems. At the house, one single-phase supply is provided, the connection details are shown.

The supply input arrangements for the house
The consumer’s mains equipment is fixed close to the point where the supply cables enter the home. The actual supply arrangement will normally be either:
1. The TT System, (mainly used in rural areas)
2. The TN-CS System, (commonly used for urban districts)
3. The TN-S System, (used in some urban districts)

Short circuit faults
Short circuits occur when the live and neutral become directly connected. They are usually caused by accidents or carelessness in checking that a newly installed circuit is safe before switching on. In the home, the short circuit fault current is not likely to exceed 10kA and is usually less than 2kA, (2000A). However, the damage cause by the short circuit can be extensive and they must be isolated immediately.

Earthing faults
Earth faults occur when the live conductor makes accidental contact with the surrounding earth protected metal casing. The fault current is returned via the protective conductor or finds its own path. Earth fault currents are not usually as large as the short circuit values, but they still can cause extensive damage and need to be quickly isolated. For portable equipment the isolation time needs to be less than 0.4s.

 

Safety considerations when using electrical equipment.

Good workmanship and the use of proper materials are essential requirements.

The need for protection
Electrical circuits in the home are all based on very simple principles. In all circuits, electricity runs from the source to the load via the live conductor and returns by the neutral conductor. In addition, a switch is connected in the live conductor, this can break the circuit to interrupt the flow.

READ THIS! Safety First
The driving force for the flow of electricity is the supply voltage, (symbol V) and in the home has a value of about 230 Volts.
Loading is usually expressed in Watts or Kilowatts (symbol W or kW).
Each load needs a specific flow rate of electricity, usually termed the circuit current, (symbol I), measured in Amperes.

Large fault current will flow either, when a live conductor becomes short-circuited with the neutral or, when the live conductor comes into contact with any earthed protective metal casing. Should fault currents persist, there is a serious fire risk to the home. As a safeguard, protective links are included in the circuit. These will safely interrupt any large fault current instantaneously.

Electrical systems in the home
All the circuits are wired from the consumer unit. Although there is no standard arrangement, there will usually be two separate lighting circuits, two 13A socket outlet ring main circuits, plus a separate circuit for the immersion heater and a circuit for the electrical cooker. Extra circuits may also supply instantaneous shower units, smoke alarms and the central heating equipment.

Plans of the electrical installation
When specifying the arrangement of the electrics for a new home or for a large extension, an electrical installation floor plan should be drawn up. Using standard symbols, details of the positions of the various electrical points will need to be shown.

Electric shock
Electric shock will result when a person becomes a part of a live electrical circuit. The severity depends on such factors as age, fithess and how the particular circumstances affect the current through the person. A person’s resistance varies with the dryness of the skin, females are more sensitive and wet situations increase the danger. A shock current of 8mA is painful, the lethal level is about 50mA.

Dealing with electric shock DO NOT PANIC. Never touch the victim until he/she is clear of any live parts. Isolate the electricity, or if not practicable, push the victim away from the source using a long dry wooden brush handle or equivalent. If unconscious but breathing, place in the recovery position before calling the emergency service. When breathing has stopped, vital first aid is essential, familiarize these procedures.

Using electrical tools outdoors or in wet conditions
Take special care when using electrical power tools outdoors. Preferably, use power tools supplied by a 110V transformer, these can be leased from Tool Hire Suppliers. Otherwise, only connect extension leads to sockets protected by a 30mA rcd and only work when the conditions are dry. If working off an aluminium ladder, stand on a substantial dry wooden board. Do not allow the ladder to sink into the ground.

Basic safety rules for ladders
Check the ladder is not damaged. Stand on a firm base at an angle of 4 up to 1 out, or 75° . Do not let the ladder touch any overhead cables or lean against any pipework. Where possible, securely lash the ladder at the top and bottom, otherwise, station someone at the base to hold stable. Avoid slipping on the rungs, clean mud etc., from footwear.

Walking on joists
When working in the roof space, never step on to any of the plaster board or lath and plaster surfaces fixed up to the joists. Walk or crawl on boards spanning between the joists. The boards should be 40mm thickness and 350mm wide. Be careful not to step on any overhang. Walking on the joist is potentially dangerous, heavy persons should not put all their weight on a single joist.

The hazards of naked flames Dry flammable materials can collect in the roof space area and beneath floorboards. Avoid smoking, using candles, or taking any other form of naked flame into these areas. Keep the area around electrical joint boxes and loop-in ceiling roses clean and clear of any flammable material.

Checking that the circuit is dead
Before starting to work on a circuit, ensure that it has been isolated and is dead. To deter anyone from re-connecting the circuit, remove the circuit fuse and/or tape across any isolating switch, and post a danger warning. Do not rely on the appliance switch for isolation, parts within the equipment may still be live when this switch is in the off position. Preferably, use a circuit tester to prove the circuit is dead.

 

Safely terminate flexible cords and cables:

Tools Pliers, side cutters, blade knife, wire strippers, screwdrivers, multi-meter

Materials 13A plug, suitable cartridge fuse, 2-core and 3-core flexible, flexible connector

Flexible electrical cords
Flexible cords provide the means for connecting all portable appliances, etc., to the permanent wiring in the house. Usually the connection is made via a plug and socket. The flex conductors are formed from twisted strands of fine copper wire. Each conductor is insulated with a colour coded pvc covering and an additional outer pvc sheath provides double insulation and extra mechanical protection.

Stripping and connecting flexible cords
Slit the sheath with a sharp knife, taking care not to damage the insulation of the individual conductors. Hold the conductors and peel back the sleeving, fold over and cut off neatly. Separate and cut the conductors to length. Use wire strippers to remove about 10mm of insulation from the end of each conductor.

Connecting a fused 13A plug
Always make sure the cable clamping arrangement is secure. Connect the cables to the correct terminals, live is the brown conductor, neutral is the blue conductor and the earth is the green and yellow. Before connecting to a terminal, twist the exposed filament strands together. Ensure that all the conductor filaments are clamped under the terminal, leaving no loose exposed strands.

Extending a flexible cord
WARNING when fitting a detachable two part flexible connector, never connect the part with the exposed pins to the plug side of the flex or in any situation where the exposed pins can remain live. Ideally, replace the existing flex with one of more suitable length. Only where connections are of a moulded sealed-in form of construction should two-terminal or three-terminal connectors be required.

Connecting a lighting pendant drop
Make sure the conductors pass round the support guides, (hooks), otherwise an excessive pull could be applied to the terminations. At the ceiling rose the brown flexible conductor connects to the outside switch terminal, the blue conductor connects to the corresponding outside neutral terminal.

Cables for permanent wiring
The cables for the fixed wiring in the house will normally be of the pvc, insulated and sheathed, twin and earth, type of construction. The size of the cable and its nominal current capacity is determined by the conductor cross sectional area ( mm² ). Each conductor is colour coded, the live conductor is red, the neutral is black and any exposed earth conductor is insulated with a yellow and green sleeving.

Terminating cables
Do not cut the wires off too short. Always leave a small loop of cable at each accessory. Remove the outer sheath to leave just 15mm beyond the entry into the mounting box. If more than one conductor is to be inserted in the same terminal, do not twist the conductors together.

 

Earth Bonding Arrangements

You may need to improve the earth bonding arrangements in your home to conform to current I.E.E. wiring regulations. Or, if you are installing a new shower unit or similar, you will need to install a new earth bonding arrangement to suit.

Tools Pliers, side cutters, cable stripper, blade knife, steel tape, electric drill, screwdrivers

Materials Earth bonding clamps, 4mm 2 earth cable, electrical fittings suitable for bathrooms

The main earth terminal
The main earth terminal needs to be mounted close to the supply company’s incoming service fuse. This substantial, earth terminal bar should be labelled with all connections visible and clearly identified. Arrangements for supplying and connecting the short length of supply earthing lead would normally need to be approved by the supply company.

Main equipotential bonding
Services that pass into the ground may pick up different ground potentials, depending on the various earth leakage currents passing in the vicinity of the buried pipes. These voltage differences must not be carried by these services into the home. Consequently, the water service pipe, gas installation pipe and central heating system are all bonded to the common potential of the main earthing point.

Bonding Clamp
An earth clamp is used to make the connection to the metal pipework. For the gas and water services connection is made close to the point of entry, see Step 2 for location of the bonding connections. Scrape off any paintwork from pipe and polish with wire wool before securing the clamp, attach the bonding conductor and make sure the safety label is fixed. Allow reasonable access to these connections.

Devices for isolating and switching circuits in a bathroom
Except were the controls for electrical shower units or pumps comply with the appropriate British Standard, all other switches must be positioned so as to be inaccessible to a person using a bath or shower. Isolating switches should be of a pull-cord-operated type.

Connecting portable equipment in a bathroom
Except for the provision of a special electric shaver supply unit, there must be no socket outlet or any other arrangement for connecting portable equipment. In a room, other than one used specifically as a bathroom or shower room, any socket outlet must be installed at least 2.5m from the shower cubicle.

Luminaries for Bathrooms
Light fittings must be at least 2.5m from the bath or shower. Totally enclosed luminaries, fitted up to the ceiling are recommended.

Equipotential Bonding in Bathrooms
Unfortunately, during the short period needed to clear a circuit fault, the exposed metalwork in the bathroom can become charged to different voltage levels. Simultaneously, touching two separate metal surfaces, each charged to a different voltage, can cause a shock. Equipotential bonding of exposed metal surface limits the likely voltage differences to a safe level.