A working knowledge of electrical theory and practice is useful for wood workers.
Electricity is the flow of electrons in a conductor which provides among numerous applications including communications, heating, cooling, light and motive power to machines.
TERMINOLOGY
CABLES - carry the electrical current and have a voltage and amperage (load) rating.
CONDUCTORS - conduct electricity very well and are typically made of a metal best suited to efficiently, affordably and safely do this job. Copper, aluminium and brass are common. One of the best conductors is gold used on plated printed circuit board connectors - a couple of atoms thick however. The elements with the fewest electrons in their outer shell are the best conductors. Conductor cables have a maximum voltage rating up to which the plastic insulating material will protect users and a rated load capacity.
NON - CONDUCTORS aka insulators - do not conduct electricity very well although technically, with sufficient voltage, any matter can conduct electricity.
SEMI CONDUCTORS - outside the scope of these notes - as the name suggests devices which use materials with electrical characteristics able to be tweaked between being conductors or non-conductors
VOLTAGE - measured in VOLTS - the electrical potential or electro-motive force. EMF measured above Earth which is at zero potential. In Australia single phase mains power is nominally around 230 volts. In the USA the mains voltage is 110 volts somewhat safer in the electric shock stakes but because 110 volts is approximately half 230 volts cables have to be twice the diameter to carry the same load. The rule of thumb is that if you halve the voltage you double the current flow for a given load - higher currents mean greater heat and fire risk. During a recent visit to the USA I was very concerned at the number of two pin power points in motels which would not charge a phone - loose obviously over heated sockets - definitely a fire risk. Cables have a rated maximum voltage which is largely down to the thickness and nature of the insulation around the internal connectors.
AC - Alternating Current is the type of electricity distributed by the grid and to homes and workshops via electrical mains. It starts life at the point of generation and will be stepped up and down in voltage by transformers. To move electricity efficiently the voltage is raised for long distance using thinner cables at much higher voltages then stepped down by transformers at street locations close to end users.
DC - Direct Current is the type of electricity typically derived from batteries and much safer to be around. The highest voltages common in DC power tools is still about 24 volts. Watch this space - the authorities who concern themselves with work place safety look forward to a day when power tools on sites are all low voltage DC.
STATIC ELECTRICITY - electricity which accumulates on a object which discharges when the body is earthed or when the charge overwhelms the ability of the body to accumulate more charge. The obvious example is lightning which is static electricity 'going to earth' from a cloud.
SOLAR CELLS - produce DC voltage which is then processed by an inverter which converts the electricity to AC for use in AC appliances. When feeding the electricity back to the grid through a two way meter the frequency of the back fed AC must be compatible with the frequency on the grid.
VOLT DROP - occurs in all circuits to a greater or lesser extent. The longer the cable run and the smaller the cross sectional diameter of the cable are causal as is the cable environment - clipped in the air or in enclosed ducting etc
LOAD - measured in watts and is the measure of the consumption of an appliance. There is a sub division of inductive load and resistive load for those who wish for further research.
PHASES - A single phase installation requires an active and a neutral wire off the street pole. Earthing is managed on site. The resulting maximum voltage is about 230 volts. The load capacity offered by single phase installations is typically less than in a poly phase installation Three phase means three taps off the rotary generating source at 60 degree separations and requires three actives off the street mains. A three phase installation in a serious workshop means the presence of AC voltages between phases of typically 415 volts which is lethal if not given respect. Three phase installations are becoming the norm because of the number and size of appliances in a home. A three phase motor is more efficient, available in larger sizes and far simpler to make. In some countries the regulations about physical separation of phases is mandated - tales, tall or not, of a typist on an electric typewriter on one phase with a bar heater on a long cord on another phase being electrocted cause this level of regulation. Phase separation may be desirable in any case notwithstanding the need to achieve a balanced load across phases.
AMPERES - the unit of measure of current flow in a conductor.,
WATTS - the measure of the load of an appliance.
HERTZ - Hz - (aka frequency) is the oscillations per second of Alternating Current. Australia uses 50 Hz, the USA uses 60 Hz. Use a 50hz motor on a 60 hz supply and it will run quicker
CIRCUIT - Typically a conductor with overload and residual current protection carrying a voltage higher than earth and appropriate to the load it will serve going through a switch then to a load where the voltage is exhausted doing the work and the current is then carried via a neutral wire usually in the same cable bunch back to the source.
FUSE - a weak link in a circuit designed to melt when the circuit is either overloaded or a short circuit occurs (typically where the fused active touches it's paired neutral or contacts a metal enclosure which is earthed). Did not and does not offer any sophisticated personal protection. Better than nothing.
CIRCUIT BREAKER - an improvement on a fuse (or fusible link). A re-settable device which also reacts to overload or short circuits and isolates the circuit. An MCB is a miniature circuit breaker. Better than a fuse.
ELCB - Earth leakage circuit breaker - a precursor to the MCB/RCD. Better again
SAFETY SWITCH - a vernacular term - an evolution of a circuit breaker where a circuit breaker and a RCD (Residual Current Device) are combined in the one protective device in a switch board. Provides both current overload and ideally protection from electric shock.
MCB/RCD (RESIDUAL CURRENT DEVICE) - the most recent level of protection available in the one device. The MCB looks at overload of the circuit and includes short circuit activation. The RCD function monitors the SUPPLY current in the ACTIVE supply cable and the RETURN current in the paired NEUTRAL wire and if an imbalance is detected the device will trip within 25 to 40 milliseconds - hopefully avoiding any harm to persons or equipment. The state of the art in personal and built items protection. Not without downsides however. Upgrade an old switchboard to current standards and don't be surprised if the new RCD trips because of undetected faults that may have been in place for years. RCD's are also unforgiving with dodgy appliances. To diagnose a dud appliance which persistently trips a supply RCD, physically unplug all appliances on the circuit (just switching off won't detect the problem because most simple electrical appliances are connected via a standard domestic general purpose outlet - GPO - which has switches which switch only the active wire - single pole switching - and the neutral wire remains connected to the appliance. Double pole switching should be the standard for machines so that they are fully isolated when the ON/OFF switch is thrown.).
Then reset the RCD, plug in and turn on appliances in turn until one trips the RCD and you likely have the culprit. Test fridges first - often problematic because of the water around their insides.
EARTHING - The oldest protection concept. An appliance plug has a longer earth pin intended to make contact before the active and neutral pins and on withdrawal being the last pin to break contact. This will ground everything plugged in first before the shorter load pins make contact so if you have a loose wire in an appliance which rubs on the metal box it should short and trip the circuit. Earthing alone is a double edged sword however - if you come into contact with a hot wire or similar the last thing you want is to be earthed because the current will flow through your body to the earth and this causes electrocution. The golden rule is have effective earthing of your wiring and plumbing but in tandem with RCD/MCB protection. Earth wires are never switched or interrupted other than when in a cable plug. Appliances must have a dedicated earth connection terminal. As discussed the earth terminal in a plug must make first and break last.
EARTH STAKE - check inside your fuse box for a heavier than usual green wire which is connects the earth busbar in your switchboard to a copper rod which is hammered into the ground nearby - this is your earthing point for your house. For those who have some gear and knowledge test the earth cable at the earth stake with an amp meter - ideally zero current. If you get a significant reading call your electrician.
PLUMBING STRAP - It is common place to earth the copper plumbing in a house which means all taps and metal fittings in a bathroom are earth points.
POLARITY - refers to the active, neutral and earth wires in an electrical installation or power lead and their correct connections. Single phase active cables are typically brown (occasionally red in older installations) and carry the supply voltage. Neutral wires are blue (or black in older installations) and in contemporary wiring carry the current back to the same source RCD/MCB to provide residual current protection as discussed above. As the term implies neutral wires should test at zero volts. Earth wires are green or green and yellow striped.
LIGHT & POWER AND MIXED CIRCUITS - Wiring in buildings is defined by cross sectional size of the conductor in the cable. A lighting circuit will typically be run in 1.5 squ mm cable and power circuits in at least 2.5mm squ mm cable. Mixed circuits should be run in 2,5mm.
DOUBLE INSULATION - found on power tools and is indicated by a square inside a square symbol. These are designed to a standard which does not require and prohibits earthing. This means a double insulated appliance has a two wire supply - active and neutral. It may have a three pin plug for only for mechanical connection - the earth is absent.
SWITCHING - A switch opens and closes an electrical circuit.and livens up the circuit at the speed of light. A simple switch such as found in a general purpose outlet (GPO) or an architrave switch switches only the active cable which is why when fault finding an RCD issue you physically have to un-plug appliances - just switching off will leave the earth and neutral connected to the appliance which will prevent a diagnosis.
NO-VOLT SWITCHING - should be the minimum standard for all machines in a workshop. An NVR (no volt relay - also known as a contactor) typically has a GREEN start button next to a RED stop button. The START button closes the coil-activated spring-loaded contactor which picks up an active from a third pair of normally open contacts. In this state the normally open push to make green start button springs open and the contactor is held closed by the third pair of normally closed contacts. It 'latches the contactor' on and the machine runs. Hit the normally closed RED stop button and the contactor un-latches, the switch opens and the machine stops.
Why is this preferable? Because in a workshop environment machines which are simply switched will stop if the mains supply to the workshop is interrupted. They remain however switched on but with no supply. You may be unaware of the power outage and be near or even working on a machine trying to find an apparent fault when the power to the workshop is remotely restored re-starting the machines and serious harm or death can eventuate. An NVR avoids this possibility because when the power is interrupted the coil in the switch de-energises and the switch because it is spring loaded opens. Each machine must then be switched on again to resume work.
This is the same issue for home appliances during a power outage - they are usually simply switched and will come back on when the power is restored. This can be hazardous because operating appliances will stop when the power is down but will start again upon restoration. How ofter do you get up next morning after an outage to find lights on? The other benefits of a NVR contactor are the contactor is a double pole switch which completely isolates the machine (other than the un-switched earth) and the red off switch is typically large and offers an emergency and routine switch function.
BATTERY CHARGERS - have historically been the cause of many fires. Best to design for them to do so and be placed in steel enclosures away from flammable structure
LITHIUM ION BATTERIES - store these where they won't cause harm if they catch fire. The fires are chemically driven and very dangerous. At time of writing we are being flooded with appliances which use these batteries with it seems little regard to quality of manufacture.