Electrical troubleshooting sample question

Use the wiring diagram shown to the questions below

1) Wire number 7 between TB40 and TB6 has become disconnected where it enters TB6, what would the following voltages be while SW3 is closed?

   A) Between TB6 and TB39

Voltage would read approximately 24 volts.

   B) Between TB6 and TB40 

Voltage would read 24 volts. The open circuit between the wire and terminal block 6 would result in the full voltage being dropped across the open circuit since the resistance of lamp PL13 is so small compared to that of an open circuit.

2) Assuming the circuit is energized and SW3 is closed, what would the voltage across PB9 be, before and after initially pushing the momentary, normally open pushbutton PB9?

Before the start button is pushed the voltage across PB9 would be 24 volts. After PB9 is pushed CR1 would energize closing contact CR1-1a, resulting in 0v across the contact, and therefore 0v across PB9. This is OFTEN referred to as a relay LATCH or SEAL IN circuit.

3) Assuming the circuit is energized and SW3 is closed, and SW12 is in the left (open) position, and speed control rheostat is set to max speed position, what would the voltage be across CR2-1a contacts (pin 1-3 of CR2)?

If CR2's coil is burned out, the magnetic field will not pull the relay in. Therefore, the normally closed CR2 contacts would allow current flow and the normally open contacts would remain open. This would result in reading of 24v across CR2-1a, since it would be effectively measuring the voltage across M18.