Principles of Troubleshooting

If the open occurs in the lamp branch of a series-parallel circuit:

Lamp does not light, rest of circuit functions

In the example provided, three resistors of 30 Ω each are connected in parallel. What is the total resistance?

10 Ω

Why can't a voltmeter reliably detect an open resistor in a parallel circuit?

It measures the voltage drop of another branch

Which instrument is most effective for detecting a shorted resistor in a parallel circuit?

Ohmmeter

A voltmeter reading of full source voltage between points B and D in Fig. 12-190 indicates:

R1 is open

When a voltmeter is connected across a shorted resistor in a parallel circuit, what will it read?

Zero volts

When using an ammeter in a parallel circuit with an open resistor, the total current is:

Less than calculated

How can an ammeter identify which branch is open in a parallel circuit?

By reading zero current in the open branch

In a series-parallel circuit, what happens when the open occurs in the series portion?

Entire circuit stops functioning

In a parallel circuit, a shorted resistor effectively acts as:

A direct wire across the source

What must be done before using an ohmmeter to detect a shorted resistor?

Remove one end of the resistor from the circuit

If the lamp is burning in the series-parallel test circuit, what is known?

Lamp branch is functioning

Why can't a voltage or current check usually be performed after a resistor shorts in a parallel circuit?

The fuse opens immediately

When measuring between points A and E, what fault is being checked?

Open conductor or fuse

Why does an ohmmeter give a false continuity reading across an open resistor in a parallel circuit?

Current bypasses through parallel resistors

In the same parallel example, how much current flows through each 30 Ω branch when all are functioning?

1 A

To accurately test a resistor suspected of being open in a parallel circuit, the technician must:

Disconnect one end of the resistor from the circuit

What is the immediate effect of a short in a parallel branch?

Fuse opens

What would the ammeter reading be if one branch in the example circuit is open?

2 A

When connecting a voltmeter between points A and D in Fig. 12-190, what is being checked?

Continuity of the battery and switch

With a source voltage of 30 V and a total resistance of 10 Ω, what is the total current in the circuit?

3 A

What resistance will the ohmmeter display if an open resistor remains paralleled by two equal 30 Ω resistors?

15 Ω

In the series-parallel circuit example, when the open occurs in the parallel portion, what happens?

Lamp burns dimmer

If one resistor in a three-branch parallel circuit is open, why might the circuit appear to function normally?

Current continues to flow in the remaining branches

Why might troubleshooting the parallel portion of a series-parallel circuit be more difficult?

Voltage readings can be misleading