Electricity (Leaving Cert Construction Studies): Revision Notes
Electricity in the home
Electrical connections and the ESB
The Electricity Supply Board (ESB), now called ESB Networks, handles all electrical connections to homes. They install an external metre cabinet on the outside wall of your house. This cabinet contains several important components:
- ESB meter - tracks how much electricity you use so the company can charge you
- Official metre seal - prevents tampering with the metre
- ESB network fuse - protects the main supply
- Main overcurrent device - additional protection if needed
The ESB connects a mains power cable from outside to this cabinet to supply electricity to your home. Only ESB personnel can access the sealed metre.
If the cabinet is more than 3 metres from your home's distribution board (fuse board), you need a separate miniature circuit breaker (MCB) rated at least 63 amps in a non-conductive enclosure.
From the external cabinet, electricity flows to the distribution board inside your house. This board contains MCBs, residual current devices (RCDs), and fuses that control how electricity is distributed throughout your home.
Electrical safety devices
Every electrical connection in your home needs safety devices to protect you from electric shock and prevent damage to appliances. There are three main types of safety devices.
Fuses
A fuse acts as a deliberate weak point in an electrical circuit. When too much current flows through it, the fuse burns out and breaks the circuit. This protects both the circuit and any appliances connected to it from damage.
The main problem with fuses is that once they create the safety break, you have to replace them. This makes them less efficient than other safety devices. Modern systems now place fuses in appliance plugs rather than the main distribution board, protecting both the appliance and the user.
Miniature circuit breakers (MCBs)
MCBs monitor the amount of current flowing through circuits. You can set them to a safe maximum current level. If the current exceeds this safe limit, the MCB will "trip" - a switch physically flips from on to off in one-tenth of a second, stopping the current flow.
MCBs trip when circuits become overloaded with too many appliances or when there's a fault in the circuit. Once you've fixed the problem, you can simply flick the switch back to the "on" position to restart the current.
Different circuits need different maximum safe limits. For example:
- Lighting circuits need much less current (5/6 amps)
- Socket circuits need more current (30 amps)
You can only connect a maximum of ten outlets to a single circuit. This reduces the risk of overloading and prevents complete power cuts when MCBs trip.
Residual current devices (RCDs)
While MCBs respond to current overloads, RCDs detect differences in current flow within circuits. When a circuit becomes damaged, current can leak and flow through nearby objects, creating a danger of electrocution.
RCDs detect any current leakage. If there's a difference of 30 milliamps or more in the circuit, the RCD will trip within milliseconds and stop current flow. Most RCDs have a test button that allows you to check they're working properly as a safety device.
RCDs must be fitted as standard on all socket, water heater, and electric shower circuits.
Grounding the circuit
Electricity always travels along the path of least resistance to reach electrical ground. Electrical ground directs current safely into the earth and prevents high-voltage contact (electric shock) if electrical insulation fails.
The electrical system connects to ground by driving a conductive rod into the earth outside the home. A wire connects the distribution board to this grounding rod. Every circuit in the house connects through the distribution board to this ground wire and rod.
All electrical circuits must have proper grounding for safety. This feature provides a safe path for leaked current to travel to earth and works alongside other safety devices in the system.
Appliance isolation
Appliances that need higher than normal current require additional safety measures. This applies to electric cookers, water heaters, and instantaneous electric showers. These appliances need:
- Higher-gauge wiring - thicker cables from the distribution board to handle the extra current
- Double pole isolation switch - disconnects both the live wire and neutral wire simultaneously
The isolation switch helps with maintenance and installation because you can disconnect these appliances from electrical current without going to the distribution board. It also acts as an additional safety device that you can operate independently.
Any appliance rated 3.6kW or higher must have its own dedicated circuit connected permanently to the distribution board, rather than using a plug and socket.
Circuits in the home
Individual appliances need different amounts of current to operate, but most can run from one circuit on the distribution board. To handle varying current needs, electrical cables come in different thicknesses. Thicker cables can carry greater electrical loads than thinner cables without overheating.
Circuits direct electricity from the distribution board through various routes around the home, ending in sockets or lighting fixtures.
Lighting circuits
Most homes have one lighting circuit, though larger houses may have more depending on size. The lighting circuit starts at the distribution board and passes through all light fittings before returning to the distribution board.
Compared to other circuits, lighting uses relatively little electricity - this is called "low drain".
Lighting circuits can use either junction box systems or ceiling rose (loop-in) systems.
Ring main circuits
The ring main circuit creates a loop of electrical outlets (sockets) used to power appliances around the home. Large houses may need more than one ring main circuit. Kitchens often have two dedicated ring main circuits because many heavy-draw appliances operate there, and several may run simultaneously.
Ring main circuits supply sockets with 13 amps of power. While outlets can handle 13 amps, appliance plugs have cartridge fuses rated specifically for each appliance. The outlet allows 13 amps through, and the plug controls whether this reaches the appliance.
Fuse ratings typically range from 3 amps for low-usage items like radios to higher ratings for electrical devices like microwaves, washing machines, and irons.
Recommended socket distribution
Different areas of the home need different numbers of electrical sockets:
- Kitchen: 10 sockets
- Dining room: 6 sockets
- Sitting room: 5 sockets
- Double bedroom: 4 sockets
- Single bedroom: 3 sockets
- Hall: 2 sockets
- Landing: 1 socket
Bathroom electricity regulations
Water conducts electricity, so strict regulations govern electrical use in bathrooms.
Electric heaters in bathrooms must be permanently fixed and wired, positioned more than 0.8 metres from the bath. Bathroom switches should be ceiling-mounted with pull cords for operation.
The only socket generally allowed in bathrooms is a low-voltage shaving socket, installed away from direct spray from showers, taps, and baths.
Electrical appliances and power consumption
Appliances use different amounts of power when operating. Some appliances show their energy ratings, others don't. Ratings appear in watts or kilowatts and indicate how much power the appliance uses during operation.
A-rated appliances typically use 55% less energy than D-rated appliances.
Common appliance ratings include:
- Light bulbs: 40W, 60W, 100W, 150W
- Electric fire: 1kW, 2kW, 3kW
- Television: 200W
- Electric kettle: 1kW, 2kW, 3kW
- Hairdryer: 1500W or 1.5kW
- Electric cooker: 8kW
- Fridge: 600W
- Washing machine: 920W
- PC: 300W
Calculating electricity costs
You can calculate how much electrical devices cost to run using the kilowatt hour (kWh) formula. Kilowatt hour represents 1000 watts used for one hour and is the standard unit for measuring energy sold to homes.
Worked Example: Calculating Electricity Costs
Formula:
Step 1: Calculate units used if a 2kW electric fire runs for 4 hours:
Step 2: Find the cost by multiplying kWh by electricity tariff rate: At 18.81 pence per kWh:
Home certification
Electricity is dangerous, so qualified electricians must carry out all electrical work in homes. When electrical contractors install new electrical systems or modify existing ones, they must test and certify that the work meets current standards set by the Electro-Technical Council of Ireland (ETCI).
Clients receive a copy of this certification when work is completed. The client then passes this certification to the ESB, who will connect or reconnect the dwelling to the electricity network.
Key Points to Remember:
- ESB Networks handle all electrical connections to homes through external metre cabinets
- Three main safety devices protect electrical circuits: fuses, MCBs, and RCDs
- All circuits must be grounded to provide safe paths for leaked current to reach earth
- High-power appliances need dedicated circuits and special isolation switches
- Ring main circuits supply 13-amp power to household sockets throughout the home
- Bathroom electrical use has strict safety regulations due to water's ability to conduct electricity