Open & Short Circuit Faults
In this lesson we will define an open circuit and a short circuit, and desscribe how to determine if a circuit has an open or short circuit fault.
The Open Circuit
A closed circuit has loads, a voltage supply, and wires which complete the circuit.
An open fault in that closed circuit will cause the current to stop flowing because an open fault offers an infinitely high path of resistance.
The components in series with the open will no longer have a voltage drop across them, while the open component will have a larger than normal voltage drop across it.
An open is often caused when a period of over-current has occurred in the circuit, causing the wire or component to overheat and burn up/melt down
Let's explore this further.
This circuit is not working but it is not immediately obvious why.
We will need to use a multimeter (set to measure volts) to determine the source of the open.
Measuring the voltage drop across each lamp will let us know whether or not it is burnt out.
There is no voltage drop detected across the lamp so we can conclude that it is not the source of the open.
This test shows that there is no voltage drop across Lamp 2, proving that Lamp 2 is not the source of the open either.
The voltage drop is 24V; same as the source.
We have found an open.
This test shows that there is no voltage drop across Lamp 4.
Lamp 4 is not the source of the open.
We can prove the loop voltage law,
"The voltages around a loop equal zero."
Starting at TP1 go around the loop:
+0V +0V +0V +24V +0V -12V -12V = 0V
Here's Another Method
The black lead (COM) is rarely moved when testing. Skilled technicians always connect the black lead (COM) to a ground point which will make all measurements referenced to ground.
Let's test a new circuit, with the black lead fixed on the ground.
Testing the supply indicates 24V.
Testing from TP3 to ground also shows 24V.
This proves that lamp 1 is good!
Testing from TP4 to ground shows 0V.
Therefore, the voltage drop across Lamp 2 is 24V.
Lamp 2 is the open in the circuit.
The Short Circuit
A short circuit is an unintentional path of zero resistance.
For example, if a piece of wire or a screwdriver were to be dropped into electrical equipment and somehow created a connection across a supply or load, a short circuit would be created!
A short circuit has the potential to be far more dangerous and destructive than an open circuit.
Using Ohm's Law we know that a decrease in resistance will cause an increase in amperes.
The lower the resistance, the greater the amperage.
And as the resistance approaches 0 the amperes head toward infinity!
Consider what would happen if a wire were accidentally to drop across the Lamp 1 terminals.
There would be a tremendous current flow through the wire, and no current flowing through the lamps.
The fuse would blow to prevent any damage.
Once again we have a circuit that is not working but it is not immediately obvious why.
We know the fuse was blown so a short circuit somewhere is suspected.
The only way to determine if a component is causing a fault is to ensure that the component being tested is the only path for the ohmmeter current.
This means that you have to remove a wire.
Lamp 1 reads 13 Ω, no short there.
Testing Lamp 2 also reads 13 Ω
The resistance across Lamp 3 has dropped to 0 Ω. This is the short!
Review
A short circuit...
It's time to break out the ohmmeter! What will the reading be?
Recap
- An open circuit has a path of infinitely high resistance.
- A short circuit fault is an unintentional path of zero resistance.