Basic Electrical
Explain the different ways to check fuses and what conditions must be met to do so.
Some fuses can be visually inspected but for most you will need to de-energize the circuit, remove the fuse if possible, check resistance across the fuse – resistance should be low
Explain the differences between complete, open, and short circuits
Complete circuits allow the flow of current from source, through the load, and back to the source – open circuits have a “gap” that disallows flow of current – a short circuit is an unintended path for the current
What are common voltages you should see in your business unit? List as many as you can.
480V in the MCC – 240/208V for motors – 120V for outlets etc – 24V for control circuits
How can you tell if your equipment is properly bonded and grounded? List more than one way with your meter’s features.
Check for voltage difference from chassis of equipment to nearest ground (building steel) – check for continuity between pieces of equipment
Explain how an E-Stop must be set up in a circuit to work correctly.
The E-Stop must be set up as a normally closed switch that bypasses anything that would allow the equipment to run. It cannot be a momentary switch and most send an alarm signal to the control equipment.
Explain why circuit breakers or fuses open and when it is ok to reset or replace them.
Circuit breakers and fuses are designed to protect equipment and will trip only under certain conditions. Some will trip as soon as an overload/overheat/etc is detected, others will have a short delay. They should only be reset/replaced after the cause of the trip has been identified and corrected.
Explain why overloads trip and how to reset them.
Thermal overloads trip due to heat generated by overcurrent/overload conditions. They will need to cool down before resetting.
What are the hazards associated with an open neutral?
An open neutral can develop a large potential difference and present a shock hazard.
What is Ohm’s Law? Define the components of Ohm’s Law.
V=IR – Voltage ( energy potential measured in volts) equals Current (flow of electricity measured in amperes) multiplied by Resistance (opposition to electrical flow measured in ohms (Ω))
What is the Power formula? Define it’s components.
P=IR – Power (amount of work the energy could do measured in watts) equals Current (flow of electricity measured in amperes) multiplied by Resistance (opposition to electrical flow measured in ohms (Ω))
Why are transformers rated in kVA instead of kW?
Wattage is dependent on resistance. Transformers work with the voltage and amperage as the resistance is unaffected.
In a 3-phase system the line voltage is equal to the_______.
Not sure
Why do we use the square root of 3?
The square root of three is used to calculate voltage on one phase of a three phase system.
What is the difference between AC power and DC power?
AC is alternating current – the polarity (or flow direction) alternates
DC is direct current – the electricity flows only in one direction; there are no changes in polarity
What is Control Power? How is it different from the other power in your business unit?
Control Power is the lower voltage power used to run control systems. Many of our control systems can be run on only 24V.
Motors, Starts, Test Equipment
Explain how to size fuses for a 3-phase motor.
The fuse should be 125% of the rated FLA of the equipment being protected.
Explain how to size overload heaters for a 3-phase motor.
Not certain but I believe they’re also 125% of the FLA rating….
Explain what each component of the motor starter does:
Disconnect – isolates equipment from power
Fuses – protect equipment by opening in an overload condition
Control Transformer – reduces “noise”
Starting Contacts –
Overloads – protect equipment by opening in an overheat condition
E-Stop – protect people by producing an immediate “hard stop” of the equipment; manually operated
Stop Button – produces a “soft stop” of the equipment; equipment shuts down in a known position
Start Button – allows current to flow starting the equipment
Holding Coil – energized when momentary start button is press; works with holding contact to keep circuit closed when start button has been released
Holding/Maintaining Contacts – works with holding coil as described above.
Explain how you can trouble shoot a motor starter circuit.
Start with the obvious. Examine the current condition; what was happening when the failure occurred. Try a quick restart so you can observe the failure first hand. Assuming that doesn’t tell you what the problem was, start at the motor and test what can be tested without de-energizing. Check voltage at each “work point”. De-energize the system. Check breakers. Check resistance at each protective device and “work point”.
How can you tell if a motor is running within its allowable current range? Why is this important?
Check the allowable current range on the motor nameplate then use your meter to check actual current flowing through the motor. This will help you spot problems before they occur preventing overload and overheat conditions that could damage the motor.
What types of motor started are in your business area.
I don’t know. What are the different types of motor starters?
(referring to diagram) If everything is good, what voltage would you expect from L1 to ground? L2 to ground? L3 to ground?
…not sure, need to review but if memory serves… from each line to ground, I would expect 277V.
Are the numbers from number seven the same or different? Why?
Same
If a motor is running and fuse F2 opens, will the motor stay running? Why?
Yes, it still has current coming from the other two lines.
If a motor is running and fuse F6 opens, will the motor stay running? Why?
I don’t see a fuse F6 on the diagram. I’m guessing it was meant to be on the high side of the transformer in which case, no. There would be no induction happening to power to control circuit.
If the motor is running and everything is good, what voltage can you expect from A to ground?
120V
If the motor is running and everything is good, what voltage can you expect from A to ground?
120V
If fuse F4 is blown, what voltage can you expect testing each side of the fuse? Why?
I would expect 480V on one side and 0V on the other.
What does your multimeter measure and when would you use it?
The Fluke T8 1000 can measure voltage, current, and resistance. The most important time to use the volt-meter function is to verify that a system has been de-energized. All three functions are used in troubleshooting. I would NOT use it to test insulation on a motor. For that, we need to use a megger.
What does a clamp on ammeter measure?
Amperage in a line. A common mistake I’ve been warned against is trying to measure voltage with a clamp-on. It would always read 0V since there is no potential difference.
What would be the difference in measuring one cable vs clamping all three cables in your 3-phase system?
It has been emphasized that we want to do one cable at a time (usually). I don’t remember the difference.
What is a megger used to measure?
Resistance of insulation in a motor
What safety concerns are associated with a megger?
A megger imparts a high voltage
And 20. I have seen the term DLRO but truly know nothing about it.