Random Questions Flashcards
Swer?
Single wire earth return
Luminous flux? And unit
The quantity of light emitted by a source in any direction measured in lumens
Mins elements fixed wire appliances or damp situation IRT reading should be?
10,000 ohms
What tester would you determine short circuit capacity rating On protective devices?
Earth impedance tester
Why are transformer cores iron laminated?
To reduce eddy currents
Define peak inverse voltage as applied to a semiconductor?
The max voltage a semi conductor can withstand in reverse bias
Inverse time current characteristics?
An increase of current through a protective device beyond its normal rating causes a decrease in operating time
How is the voltage regulation of a power transformer achieved with varying loads?
Use of a tap charger
An increase of resistance in the protective earth conductor would effect the operation of the circuit protection device how?
It may not operate due to reduced fault current
Three phase half wave rectifier frequency?
150 hz full wave is 300 hz
This motor has low starting torque and poor efficiency, commonly found in small appliances such as small fans, coffee grinders. Non reversible
Shaded pole motor
A low power motor, series wound that gets its name from its ability to operate from DC OR AC, for reversal it cannot be achieved by swapping supply connections over. What motor is this and How is it reversed?
Universal/series motor. Reversed by interchanging either armature or field leads but not both.
In this motor both windings are identical and therefore have the same resistance and number of turns. Found in electric lawnmowers. What is the motor and how is it reversed?
Permanently split capacitor motor. Reversal comes from connecting a reversing switch to enable the capacitor to be shifted from one winding to the other.
How to reverse 3 phase motor?
Switch two phases
Loss caused by the alternating of flux in the core molecules. Energy is consumed in this process and the energy loss is called?
Hysteresis
Transformer used for variable load?
Distribution transformers
Luminous intensity? And unit
Means the luminous flux per unit solid angle in a specified direction measure in candela (CD)
Voltage and torque relationship at induction motor terminals?
Torque is proportional to speed
The current transformer must be shorted out before removal why?
Magnetic field saturated, burnt out, shock hazard
Smallest and largest your main earthing conductor can be?
4mm and 120 mm
How are zener diode connected?
In reverse bias
Close excess current protection?
Will operate 1.5x rating within 4 hours
Ladder ratio?
1:4
What rcd is voltage dependant?
Portable rcd
Relationship between avaliable torque at a given speed and voltage at an induction motor terminal?
Torque is proportional to voltage squared
Define clearance time?
Total time for any fuse to operate and clear a fault
In what form is the voltage output from a PWM converter?
A series of rectangular pulses
Hi
Dude
Why are induction motor rotor bars normally skewed?
To give smoother quieter running and more uniform torque
Three phase 50hz output ripple frequency of a full wave rectifier
300 hz
Pull out torque as it applies to an AC induction motor?
That’s the max torque it can produce
Main purpose of a fault loop impedance test at the main switchboard of a low voltage installation?
Confirm that the protective device can safely interrupt the installation pscc
Rated current of property RCD?
300 ma
Personal protection rcd and medical rcd ma?
Personal 30 ma medical 10 ma
What is burden?
The max load in VA which may be applied to the secondary terminals in a voltage or current transformer
Course excess current protection?
When it may not operate within 4 hours at 1.5x current
Define discrimination?
To install in such a way that the fault in a circuit will trip the closest protective device first
Medical grade RCD ms time at 1x and 5x current
40 ms and 40 ms
Person protection ms at 1x and 5x current rating
300 ms and 40 ms
Property protected RCD ms time at 1x and 5x current
500 ms and 150 ms
What replaced the term fusing factors?
Utilisation category
What do all course excess protection devices need?
Overload protection
In regards to fuses what is the 100 a?
Max current fuse can carry without deterioration
With regards to fuses what is the ACka
Max fault current the fuse can safely interrupt
With regards to fuses what is GG
Fusing factor
Testing checklist? With numbers 1-7
1) visual check
2) earth continuity
3) IRT
4) polarity
5) correct circuit connection
6)earth impedance
7) operation of RCDs
Medical grade MS 1x and 5x PROPER QUESTION
40 ma and 40 ms
This motors start winding has fewer turns and is of finer wire than the run winding (main winding). Disconnection comes from a centrifugal switch. Used in fans and places where you don’t need high torque. What is the motor and how is it reversed?
Standard split phase motor. Reversed by reversing the start or run winding but not both.
Cap start motor how is it reversed?
Reversing start or run winding but not both. Disconnection comes from centrifugal switch
Cap start/ cap run motor and how it is reversed?
Used for concrete mixers/ large air con that needs high start and run torque. Disconnection comes from centrifugal switch, reversal comes from fitting an reversal switch
Lux for offices?
500 or 300-400 probably 500
