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
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.
Starting at TP1 go around the loop:
+0V +0V +0V +24V +0V -12V -12V = 0V
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.
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