by Hannes Baard
OVER the past month or two, we have looked at conductors, cables and whatever else can conduct electricity in an electrical installation. We also looked at things or conductors that were part of the design to carry electric current from the start and others... well...
If you recall, these conductors come in a variety of guises and disguises...
Some were meant to be part of the electrical installation, as I said, and then others were not intended to be part of the installation or any electrical circuit at all, but come into contact with the electrical installation purely by ‘accident' - such as the metal work of a geyser and the metal water pipes associated with the hot and cold water installation, for example.
Now, under normal working conditions, the insulation of the cables or other methods such as the insulating spacers used for bus bars, does a pretty good job of preventing someone from getting an electric shock, but what if things go wrong? What then?
Let's face it... nothing lasts forever, so insulating material can also deteriorate, perish, melt or otherwise get damaged. It could also happen quite easily that a fixing screw to mount a socket outlet is too long and the installer is too lazy to trim it. Now, if he drives that screw through the insulation of the cable that feeds the socket outlet... he's in for a rather nasty surprise... right?
How can we prevent someone from getting hurt if something goes wrong in an electrical installation? Better still, can we prevent a fire?
Well, we have to devise something that would switch off or disconnect the electricity to the affected part or even the complete installation. "Ah! A fuse," you say, "a fuse or a circuit breaker will solve the problem."
It may solve the problem... but it's not as easy as you may think... But why? It is such a simple principle. Something goes wrong - you simply automatically switch off - that's it!
I repeat: At face value, it could sound simple but it's not... So, without any further debate about the simplicity or otherwise, let's have a look at what is involved.
A good place to start would be to ‘classify' or ‘group' the type of faults that can occur. "Oh my hat!" I hear some of you sigh... "Not again! This classification thing really gets me down." But, to be able to address this problem, we unfortunately have to go that route.
Let me explain: When you touch the bare ends of two live conductors there is a small explosion and the visual indication of a problem is clearly visible. But, what if those same two conductors come into contact but have some kind of resistance in between... just enough to stop the ‘sparks and fireworks', but the current flow is still enough to kill you? What then?
Or even worse... What if one of those live conductors touches the metal of a light switch or a distribution board that is somehow isolated from earth (in other words, not earthed)? There will be no indication that something is wrong until you touch that metal part (read conductive part) and complete the circuit to earth. Unfortunately, you would have a slim chance of surviving to tell the story... Are you willing to take that gamble?
So, categorise or group the type of fault (current) and be done with it.
Definition 3.22 in SANS 10142 starts the ‘classification' like this:
Current: ‘flow of electric charge through a conductor'
(Now we all know that a conductor is not necessarily a piece of cable... my last two columns defined conductors in detail.)
3.22.1 - conditional short-circuit current: ‘the value of prospective current, which the equipment (when protected by a short-circuit protective device, as specified by the manufacturer) can withstand for the operating time of the device, under specified test conditions'.
Essentially a motor manufacturer, for instance, may state that if the protective device (fuse or circuit breaker) takes 0.25 seconds to clear the fault (short circuit), the motor will not ‘pack up' and could go on to work another day.
3.22.2 - earth fault current: ‘fault current that flows to earth'.
Of course, it stands to reason there must be some form of earth path - earth continuity conductor, earth electrode or whatever.
3.22.3 - earth leakage current: ‘current that flows to earth in an electrically undamaged circuit'.
The key word here is ‘undamaged'. This type of fault is common in circuits where you have elements, such as the element in a stove, for instance. The plate or oven will begin to get warm, but before long it will trip the Earth Leakage Unit. Now, if this fault is allowed to escalate and we lose the insulation altogether (the element ‘blows'), we get to the next stage, namely...
3.22.4 - fault current: ‘current that results from an insulation failure or from the bridging of insulation'.
In the example above this will show itself as a hole in the side of the element. Basically, it indicates that the heating element itself has managed to ‘push through' the insulating white powder and touched the outer metal jacket and shorted to earth.
3.22.5 - Overcurrent: ‘current that exceeds the rated current'.
In a socket outlet circuit for instance, this will be more than 125% of the rating of a 16A socket.
NOTE 1: Depending on its magnitude and duration, an overcurrent might or might not be harmful.
This happens when a motor starts up. We all know that a motor starts up at a much higher Amperage than its actual running current. Even though there is up to eight times more current in the circuit at start up, the duration is short and it does not cause any damage.
NOTE 2: In the case of conductors, the current-carrying capacity is deemed to be the rated current.
In this case, a piece of 2.5mm2 housewire has a current rating of 25A under particular circumstances; anything higher than 25A under those circumstances would then be an ‘overcurrent'.
3.22.6 - overload current: 'overcurrent that occurs in an electrically undamaged circuit'.
This happens when you use a handful of ‘double adaptors' in one socket outlet, thereby ‘overloading' the socket outlet.
3.22.7 - prospective short-circuit current: ‘value of overcurrent at a given point in a circuit, which results from a fault of negligible impedance between live conductors that have a difference of potential under normal operating conditions, or between a live conductor and an exposed conductive part'.
Prospective short-circuit current is a value that can be calculated when you have a ‘dead short'... the expression everyone knows to mean ‘a fault of negligible impedance'.
Till next time.