Domestic Electricity

The standard domestic electricity supply in Ireland is 230 volt AC (frequency 50hz).

There are 3 holes in an Irish standard socket


The live terminal of the mains supply alternates between a positive and a negative voltage, with respect to the neutral terminal. The 230V is a RMS value (root mean squared (more about that later)) so the value of the charge on this wire / this socket oscillates from +325 to - 325 and back in 1/50 th of a second.

Root Mean Squared value

Root Mean Squared is a method of finding the 'effective' strength of an oscillating wave. If we just took the average we would see that the Highs would 'cancel' out the Lows and the average would come out mathematically at zero. Yet if we were to hold the wire that averaged at zero we may be 'shocked'.

So RMS is found by squaring all the values, this will make all the values positive ( + x + = +, - x - = +), then find the mean of all those values, then get the square root of all those values. So 230 V is the RMS Value of Voltage of a high that can be found by multiplying by root(2)


The neutral terminal stays at a voltage close to zero, with respect to earth.


This is the upper most slot on a socket,


Wiring a plug

Prepare the plug:

  • Remove the cover and loosen the screws of the cord grip
  • Loosen the screws from the brass terminals

Prepare the flex:

  • Measure the flex against the plug and strip the outer covering back as far as the cord grip, taking care not to cut through the coloured insulation on the wire strands
  • If the coloured cables have to be trimmed, ensure that they are long enough to reach the terminals without straining. Allow for a little slack on the green/yellow earth wire
  • Strip about 10 mm (1/2 inch) of the coloured insulation from each core

Fixing the flex to the plug:

  • Remember the colour code:
  • Pass the flex under the loosened cord grip
  • Twist the exposed wire strands of each core between your finger and thumb so that there are no loose strands
  • If the plug has clamp terminals, wrap each core around the appropriate terminal; place the washer on top of the twisted wire and tighten the screw nuts firmly. Make sure that the coloured insulation is not pinched under the terminal clamp.
  • If the plug has pillar terminals, double the twisted bare wire back on itself for about 5 mm, and insert it fully into the hole in the appropriate terminal. Tighten the terminal screw firmly on the wire.
  • Tighten the cord grip screws, making sure that the cord grip is clamped on the full outer covering of the cable and not on the inner cores.
  • Replace the cartridge fuse in 13 Amp plugs with an appropriate sized fuse:
    • Blue 3-Amp fuse for lights and small appliances
    • Brown 13-Amp fuse for larger appliances and heaters
  • Replace the cover

Fuses and Circuit Breakers

The Fuse is very thin piece of wire.

The wire has a quite low melting point.

As current flows through the wire it heats up.

If too large a current flows it melts, thus breaking the circuit

Use appropriate fuse size/rating

Simple fuse calculations.

Appropriate calculations.

Ring and radial circuits, bonding, earthing, and general safety precautions.

No drawing of ring circuits required.

The kilowatt-hour.


The kilowatt-hour is the measure of the amount of electrical energy we use. While in physics we use the Joule, the people who sell electricity think we need a different unit.


we see that to determine the amount of kWh's someone has used

Appropriate calculations.

a 2kW heater on for 5hr uses,

a 14000W motor for 3hours uses,

a 2.2kW kettle on for 6minutes

a 220w playstation on for 24 hours

if electric Ireland rate 21/11/17 is 17.17c per kWh find out the cost of the use

Electricity at home:

• fuse box

• meter, etc.

A minature circuit breaker works in one of two ways when the current is larger than a preset value

  1. a bimetallic strip curls away from its contact
  2. an electromagnet pulls the switch open

MCB has several advantages over fuse:

  1. MCB is more sensitive to current than fuse. It detects any abnormality in the current flow and automatically switches off the electrical circuit.
  2. In case of MCB, the faulty zone of electrical circuit can be easily identified. Faulty circuit trips to the off position. On the other hand in case of fuse, the complete fuse wire needs to be checked by opening fuse grip for confirming the faulty zone.
  3. With MCB it is very simple to resume to the supply. You just need to push the knob of MCB back to on position. But in case of fuse, the entire fuse wire needs to be replaced.
  4. MCB provides a better interface with the help of knob than a fuse. In case of fuse the compete handle needs to be taken care out.
  5. Handling MCB is electrically safer than handling a fuse.
  6. MCB is reusable and hence has less maintenance and replacement cost. Whereas a fuse needs to be replaced whenever it goes faulty.

RCDs (residual current devices).

That is the algebraic sum of all the electric currents meeting at any point in a circuit must be zero, otherwise the circuit will be broken. In a RCD the incoming supply, phase and neutral, passes through an iron core or toroid, which acts as the primary winding of a current transformer.

An RCD, or residual current device, is a life-saving device which is designed to prevent you from getting a fatal electric shock if you touch something live, such as a bare wire. It can also provide some protection against electrical fires. RCDs offer a level of personal protection that ordinary fuses and circuit-breakers cannot provide.

Electrical safety.

Mains electricity can kill if not used safely.

Never overload mains sockets

Never touch electrical connections with wet hands

Replace frayed cables

Don't use dirty plugs – they have a high resistance and may overheat

Switch off TVs at night

Electrical appliances are connected to the mains using a cable containing two or three copper wires that are covered in flexible plastic.

The following links are concentrated on the juggling act that power companies have to employ to balance Voltage, Energy & costs

There is a delicate balancing act performed by Eirgrid (would have been the ESB in the past) between supply and demand. I remember hearing that many years ago when everyone watched the Late Late Show on a Saturday that on one occasion it ended five minutes early. Apparently the whole power system nearly crashed as everyone got up and turned on lights and put on the kettle - the ESB would have relied on the RTE Guide to help them predict such surges in demand and were not ready for it.