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Electricity and Electronics
Troubleshooting Electrical Devices
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Question 1
A normally open (NO) pushbutton is connected to a control relay in a 24 volt DC circuit.
The circuit fault is a broken wire between the pushbutton and the relay coil. What voltages should be measured at each lettered point?
Reveal answerA = 24 V
B = 24 V
C (button unpressed) = 0 V
C (button pressed) = 24 V
D = 0 V
E = 0 V
F = 0 V
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Question 2
A normally open (NO) pushbutton is connected to a control relay in a 24 volt DC circuit.
The circuit fault is a broken DC power supply. What voltages should be measured at each lettered point?
Reveal answerA = 0 V
B = 0 V
C (button unpressed) = 0 V
C (button pressed) = 0 V
D = 0 V
E = 0 V
F = 0 V
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Question 3
A normally open (NO) pushbutton is connected to a control relay in a 24 volt DC circuit.
The circuit fault is a broken spring return in the pushbutton - always closed. What voltages should be measured at each lettered point?
Reveal answerA = 24 V
B = 24 V
C (button unpressed) = 24 V
C (button pressed) = 24 V
D (button unpressed) = 24 VD (button pressed) = 24 V
E = 0 VF = 0 V
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Question 4
A normally open (NO) pushbutton is connected to a control relay in a 24 volt DC circuit.
The circuit fault is an incorrectly chosen 120 VAC relay coil. The relay refuses to energize. What voltages should be measured at each lettered point?
Reveal answerA = 24 V
B = 24 V
C (button unpressed) = 0 V
C (button pressed) = 24 V
D (button unpressed) = 0 VD (button pressed) = 24 V
E = 0 VF = 0 V
Notes:The voltages are actually all correct in this case - the relay isn’t working because there is not enough voltage from the 24 V power supply!
When the voltages are correct, but the system isn’t working, be prepared to think outside the box for potential problems.
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Question 5
A normally open (NO) pushbutton is connected to a control relay in a 24 volt DC circuit.
The circuit fault is a burned-out, shorted relay coil (overvoltage at some point in its life). What voltages should be measured at each lettered point?
Reveal answerA = 0 V
B = 0 V
C (button unpressed) = 0 V
C (button pressed) = 0 V
D (button unpressed) = 0 VD (button pressed) = 0 V
E = 0 VF = 0 V
Notes:This is a bit of a trick question; typical troubleshooting would have us believe that the voltages at A & B should be 24 V when the button is unpressed.
If only 0 volts values are indeed the measured values, and the power supply still appears to be functional, the problem is most likely a blown fuse or open breaker on the branch line - look before point A.
In a real-world circuit, a shorted coil would break the fuse or breaker and all values would be 0 V.
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Question 6
A normally open (NO) pushbutton is connected to a control relay in a 24 volt DC circuit.
The circuit fault is an incorrect wiring installation where wire end C is accidentally screwed into an open TB point. What voltages should be measured at each lettered point?
Reveal answerA = 24 V
B = 24 V
C (button unpressed) = 0 V
C (button pressed) = 0 V
D (button unpressed) = 0 VD (button pressed) = 0 V
E = 0 VF = 0 V
Notes:Point C here is somewhat ambiguous - it could refer to the contact block of the switch, or it could be the end of the coil connection wire (C-D).
To verify the problem, measure both the contact block of the button AND the end of the wire at point C. If they do not register the same voltage, this is where the open wire fault has occurred.
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Question 7
A normally open (NO) pushbutton is connected to a control relay in a 24 volt DC circuit.
The circuit fault is a broken wire between points E and F. What voltages should be measured at each lettered point?
Reveal answerA = 24 V
B = 24 V
C (button unpressed) = unknown V
C (button pressed) = 24 V
D (button unpressed) = unknown VD (button pressed) = 24 V
E (button unpressed) = unknown VE (button pressed) = 24 V
F = 0 V
Notes:Why unknown?
This is called a floating voltage. Neither end of the wire is connected to the + or - of the power supply when the button is unpressed.With a floating voltage, the measured value may be very low, but is often non-zero due to electrical noise induced on the wire. It can be as high as a few volts, and it might even be a negative value!
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Question 8
A normally open (NO) pushbutton is connected to a control relay, which is energizing a motor contactor through the normally open CR-1 contact in this 24 volt DC circuit.
The fault appears to be a non-energizing M contactor. List all possible reasons for which this fault may be occurring (ignore likelihood, just brainstorm all options).
Reveal answerSome options include but are not limited to:
Open contactor coil
Open wire at any point in the circuits
Failed (open) relay contact CR-1
Shorted relay or contactor coil (leading to blown fuse or breaker)
Push button stuck open
Failed power supply
Wire shorted to ground at A-B, C-D, G-H, or I-J
Wire shorted to 24 V at E-F or K-L
Incorrectly selected contactor (120 or 230 v AC coil)
Incorrectly selected relay (120 v AC coil)
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Question 9
A normally open (NO) pushbutton is connected to a control relay, which is energizing a motor contactor through the normally open CR-1 contact in this 24 volt DC circuit.
The fault appears to be a non-energizing M contactor.
A further investigation of symptoms concludes:
The power supply is working properly.
The circuit was working properly yesterday and no components were replaced since that time.
The voltage at point C is 0 volts when the button is unpressed and 24 volts when the button is pressed.
The voltage at point I is 0 volts when the button is unpressed and 0.25 volts when the button is pressed.
The voltage at point J is 0 volts when the button is unpressed and 0 volts when the button is pressed.
Based on these symptoms and measurements, what is the most likely fault, and where?
Reveal answerOpen (broken or disconnected) wire between points I and J.
Notes:Check all terminal strips in that circuit branch (I to J) to ensure clean, tight connections, and then trace all conduits and cable trays for signs of recent damage. The circuit was working properly just yesterday.
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Question 10
A normally open (NO) pushbutton is connected to a control relay, which is energizing a motor contactor through the normally open CR-1 contact in this 24 volt DC circuit.
The circuit fault is a poorly tightened terminal at point H which has come disconnected. What voltages would be measured at each lettered point?
Reveal answerA = 24 V
B = 24 V
C (button unpressed) = 0 V
C (button pressed) = 24 V
D (button unpressed) = 0 VD (button pressed) = 24 V
E = 0 V
F = 0 V
G = 24 V
H (at the wire end) = 24 VH (at the terminal) = 0 V
I (button unpressed) = 0 VI (button pressed) = 0 V
J (button unpressed) = 0 VJ (button pressed) = 0 V
K = 0 V
L = 0 V
Notes:Always use caution in measuring voltages - in this scenario, there is a free-hanging wire at point H with live 24 volts. If it touches any grounded metal, it will cause an arc, a short-circuit, and a fuse or power supply failure and risk further system damage.
