Last week I wrote about short-circuit currents hopefully in a way that could be easily understood by a non-engineer. I was besieged by e-mails thanking me for putting the topic in an easy-to-understand format. Several of the respondents asked me to please try and give the same treatment to over-load currents that I did for short-circuit currents (there were also several questions on other topics as well... hopefully these topics will be addressed over the next few weeks).
Okay...remember for a moment that large short-circuit currents are generated when the "load" in a circuit is bypassed with a lower resistance path. The current then takes this path of least resistance and big short-circuit currents are created.
Now let’s talk about over-load currents...
Below is the illustration of Ohm’s law that I discussed last week
The current above travels from the power source through the wires and load and back to the power source, thus completing the loop. The load of 12 ohm, in this instance, draws a current of 10 amps from the power source rated at 120 volts.
In a over-load situation, more load is placed on the power source. This happens when someone perhaps plugs too many devices into the same outlet as illustrated below:
Load 1 might be a computer, load 2 might be a big screen TV and load 3 might be a vacuum cleaner all plugged into the same outlet.
If each load (computer, TV and vacuum cleaner) each require 10 amps to operate, then the power source will deliver a total of 30 amps to the circuit.
The real problem in this type of overcurrent situation is that the wires carrying this additional current are not typically big enough to carry this amount of current. For instance, a #16 AWG insulated wire can safely carry 15 amps indefinitely but will begin to heat up if 30 amps flow through the wire. Eventually the heat will cause the plastic insulation on the wire to melt, exposing the bare wires to a potential short-circuit.
In order to prevent the wire from over-heating (and eventually failing), an overcurrent protective device (fuse or circuit breaker) should open before the wire gets too hot and permanent damage occurs.
Typically overloads are not potentially lethal like short-circuits, however they can cause considerable amounts of damage (especially if you need to replace wires in inaccessible places - like the walls of your house).
Every circuit requires overload protection and short-circuit protection in order to be truly protected.
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