Fault Protection and Inspection Flashcards
(30 cards)
Short duration overloads should operate a circuit breaker immediately - True or False?
False
A protective device operates quickly to limit the let-through energy before damage occurs, in response to what condition?
Short circuit
Short Circuit Current is a product of the Full Load Current and what?
100 / % impedance of transformer
Full Load Current is determined by the transformer KVA rating x 1000, divided by what?
root3 x V(line)
Total prospective fault current per phase is determined by the phase of supply, divided by what?
Fault per phase (Z1) and impedance of consumer mains (Z2)
Supply phase impedance is determined by dividing the phase voltage by what?
Calculated short circuit current.
Consumer mains impedance is determined how?
Table 35 of the AS/NZS 3008, then:
L x Rc x 2 / 1000
Disconnection time for a protective device on a final subcircuit with socket outlets is to be maximum ___.
0.4 seconds
What two conditions must be met in order to use SELV and PELV to provide basic and fault protection?
1 - Nominal voltage not to exceed 50 VAC or 120 VDC (ripple free) and source of supply not to exceed these values.
2 - Circuits to be electrically isolated from each other and from circuits of higher voltages.
Before commencing the _____ of an electrical installation, you must consider the nature of the current, nature of conductors, and voltage tolerances.
Design
What setting on a megger is used to test insulation resistance on a low voltage transformer test?
500V
What should ensured to be disconnected before performing an insulation resistance test?
Any electronics.
What does Out Of Range indicate during a continuity test on a transformer?
Open circuit in the winding.
What instrument is used to perform a continuity test?
Ohmmeter
What type of contact switch is used on a current transformer (CT) to avoid a voltage spike in the windings, and why?
Make-before-break.
Ensures transformer is not open-circuited while equipment is connected or disconnected.
Why is the secondary circuit of a transformer earthed?
To protect secondary from reaching hazardous potentials.
Why is the secondary circuit of a transformer well insulated from the primary?
To protect against shock hazard.
The voltage between the feet of a person standing near a live grounded object is referred to as…
Step potential
The voltage between an energized object and the feet or hands of someone in contact with the object is referred to as…
Touch potential
According to the AS/NZS 3000, a 230V circuit may be supplied from a 400V autotransformer as long as it is ensured to only be connected to the 230V tap. True or false?
False. Autotransformers shall not be used to supply electrical equipment having a voltage rating less than the highest I/O voltage rating of the TX.
Why does a person experience electric shock during direct contact with a live conductor?
They become part of the fault path.
If contact is made with equipment that has become live under a fault condition, this is referred to as…
Indirect contact.
Three factors that may effect the severity of shock to a person are…
1 - Area of the electrical contact.
2 - Frequency of the supply
3 - Magnitude of the voltage
If a person comes into contact with a live conductor, what happens around 15-17 mA rms and what is this referred to as?
It becomes difficult to release the conductor due to involuntary muscle contraction. Referred to as grip current.
