Key Electricity Safety Concepts

Here are some basic safety concepts that one should understand about your home electricity system.

How Single Phase Electricity Works

Electricity from the council or Eskom is usually supplied to households in the form of single phase 220V AC (alternating current).  What this means is that the electricity flows backwards and forwards constantly in a wave.

Alternating current is quite a complex subject on its own, so is three phase power – both subjects are beyond this brief discussion.

To simplify the concept, think of single phase electricity as flowing “IN” via the “Live” (brown) wire, and “OUT” via the “Neutral” (blue) wire.  If you can picture that, you can understand why all single phase appliances need those two wires (Live and Neutral) connected to them in order to run.  If you interrupt the circuit, the electricity cannot flow and thus the appliance stops.

The Earth Wire

Anyone who has looked closely at the wiring of many appliances will notice that there is often a third wire (green/yellow).  This wire is called the Earth wire.  It does not play a part in conducting electricity to or from an appliance in normal operation.  So all appliances will run without an Earth wire.  The role of the Earth wire is primarily for safety.

The end of all Earth wires in your house should be connected to the main Earth wire in your DB board.  Which in turn is connected to the main Earth wire in your local substation.  Its called the Earth wire because in the substation, its literally connected to earth via a stake driven into the ground.  It is also connected to the neutral point on the transformer in the substation (this enables electricity that “leaks” out to the Earth wire at a fault, to flow straight back safely to the transformer).  Please note, for this reason, its NOT good enough to simply connect an Earth wire to the nearest water pipe in your home, it must be connected to the main Earth wire in your DB board.

The other end of the Earth wire is usually connected to the shell of the appliance (the part you hold or can touch).  What this does, is enable an alternative path for the electricity in the event of a fault.  For example, if say the Live wire insulation wears through and it happens to touch the shell of your appliance, the electricity will flow back to the transformer through the Earth wire, and not through you, if you are holding the appliance.  This simple thing is a crucial safety mechanism that is often misunderstood, which can tragically lead to serious injury or death when neglected.

The simple cartoon below illustrates how an earth wire can sometimes be the difference between life or death.

It is thus crucial that all house wiring includes properly connected Earth wires that all lead back to the main Earth wire in the DB board.  Earth wires are so important that the wiring code requires that they are joined with a special crimping lug to ensure a sound connection.  Do not neglect Earth wires where required.

Earth Leakage Breaker

The role of the Earth Leakage Breaker is to check that all the electricity that flowed “IN” through the Live wire, flowed “OUT” through the Neutral wire.  If there is a difference, then some of that electricity must have “leaked” out to earth somewhere, which could only be due to a fault.  If that event is detected, then the breaker trips and stops the current flow.

This is another key safety device, because it stops a fault from persisting for a long time.  As soon as a discrepancy is detected between electricity flowing in vs electricity flowing out, then the breaker trips.  So for example, if you happen grab a live wire and the electricity flows through you to Earth, then the breaker should pick up the “leakage”, and then it should trip, so the shock you experience should only be brief.

However, if there is no Earth wire, and the electricity flows through you and then finds a path back to Neutral, then the breaker will not detect any difference  and it will not trip, and you might be shocked until you die.  Which is why its so important to properly Earth everything.

Wire Thickness and Current Limiting Breakers

Its important to use the correct wire for the correct applications.  The wiring code is very strict about which types of wire you need to use in your home.  The reason is that if the wire is too thin for the current it carries, then it heats up.  As it heats up, its resistance increases, so it heats up even more as the current flows through it.  This can result in what’s called “thermal runaway” in which the conductor gets exponentially hotter until it eventually melts, or the insulation or something else starts burning and causes a house fire.

For this reason you must stick to the correct wire types and thicknesses as per the code.

This is also why you see a whole row of switches in your DB board.  Each one of those switches is a current limiting breaker that will trip if the current flowing through the switch exceeds it’s limit.  This is required to protect the wire fed by each of those breakers.  Provided you use the correct wire, the breakers should not let the current flowing through the wire get high enough to cause thermal runaway.

Alternative Power Sources

If you have any alternative power sources permanently installed in your home, you should remember that each one of those power sources needs to have the same protective measures in place as mains power.  This is because each one of those alternative power sources is just as capable of killing someone or causing a house fire.  So all the protective measures discussed above should be applied for any permanently installed alternative power sources.

You should also be very aware that portable alternative power sources often do not have any of the protective measures discussed above.  In particular, be very careful of using long extension leads that are too thin (check to see that the wires are not getting hot).  Also be aware that if a portable power source does not have an Earth Leakage Breaker, then if it starts shocking someone, it may carry on doing that until it runs out of fuel/energy!

Fundamentals of Electricity

A long video that covers most of the points above.  It references the US system but electricity is electricity, the principles are the same regardless of the country you happen to be in.

Disclaimer: Please note the above discussion was a simplified overview of the subject to explain the basic safety principles to laymen.  It should not be relied upon in detail because certain aspects were simplified to enable the discussion for non-technical people.  For further detailed information please consult the House Wiring Code.