Having addressed the importance of the distribution board’s location and the labelling of the various circuits, in this edition we will focus our attention on the control of the various circuits within the distribution board.
Each circuit requires protection against overload, as well as fault conditions. The rating of each circuit is labelled in amps.
Typically, the lighting circuit is rated at 10 amps and the plug circuit at 20 amps. The rating of the circuit breaker protects the size of cable that is wired to the respective outlets.
In addition to the protection of overload and fault current, we have a separate circuit breaker called the Earth Leakage Unit – the function of which is to protect against the “leakage” of current.
In the case where the insulation of the cable supplying a particular appliance becomes impaired, the risk arises where metal or conductive material of the appliance becomes “alive” and the user of the appliance gets a shock. Accordingly, all metallic appliances are supplied with an earth terminal which must be correctly connected and the earth terminal securely bonded.
Item A
To ensure optimum circuit protection and avoid “nuisance tripping” there are certain circuits that should not be connected to the Earth Leakage Unit. These include all access controlled circuits, dedicated computer circuits, stove and some light circuits. Please note that this statement is based on the assumption that the installation is correctly wired in accordance with SANS 10142.
In many instances, the main switch on the distribution board is an Earth Leakage Unit.
This truly is “over-protecting” the installation and exposes the installation to unnecessary circuit tripping.
In the event of an earth-related fault, the entire installation becomes inoperative. This is most serious in the event of a fire when the premises are engulfed in smoke and panic is at the maximum.
Item B
The circuits that must be protected by the Earth Leakage Unit in addition to the defined circuit breaker are the plug circuits, water-related circuits, including the geyser, spa and jacuzzi and swimming pool, as well as lights that are directly over the bath or shower.
It is prudent to ensure that the circuits in the distribution board are segregated as mentioned in “A” and “B” above. That way, the main switch controls the entire distribution board and each respective circuit breaker protects its designated circuit with the Earth Leakage Unit providing additional protection to the plugs and water-related circuits only.
All too often when a new or altered installation is wired, very little thought is given to future electrical requirements.
Given the limited capacity within the distribution board, any additional circuits end up by being “tacked on” to existing circuits.
This leads to the overloading of the circuit and in some cases gives rise to “mixed circuits” where typically lights and plugs are protected by the same circuit breaker.
Mixed circuits must be avoided at all costs as these soon prove to be electrically unmanageable.
When drawing up the plans for a new or additional electrical installation, careful thought should be given to the electrical requirements.
A good rule of thumb is to limit the number of plugs on a given circuit to no more than five and, in the case of lights, limit the number of rooms to a particular circuit.
Upon completing the circuit requirements, add up the number of circuit breakers required and thereafter allow provision for an additional 25 to 30 per cent extra circuit breakers.
Also insist on additional spare conduit wireways into the distribution board, which will allow for the wiring of additional circuits at a later stage.
Another important element in the optimising of the electrical installation circuit management is to make use of sub distribution boards.
The sub distribution board is a sibling of the main distribution board and it is preferable to have a separate Earth Leakage Unit in this board.
By doing this, any earth-related fault on one of the circuits in the sub distribution board will trip at the sub distribution board and will enable prompt location of the fault.
Well that’s all for this edition.
The next “blog” topic of concern will be focused on the correct manner of the wiring in the ceilings of your homes.
This is a serious safety issue and, as an end user, you are ultimately responsible for work carried out in your home.
The points that I will raise will enable you to define the acceptable standard of installation work.
In conclusion, please feel free to contact me for any advice or queries relating to the electrical installation. Also, having spent 10 years lecturing in electrical engineering, I am available to give talks to interested groups. My email address is anthony.murdoch7@gmail.com.
Here’s to your electrical safety.
Tony Murdoch
