Fault Protection and Inspection

Question 1

Short duration overloads should operate a circuit breaker immediately - True or False?

Answer 1

False

Question 2

A protective device operates quickly to limit the let-through energy before damage occurs, in response to what condition?

Answer 2

Short circuit

Question 3

Short Circuit Current is a product of the Full Load Current and what?

Answer 3

100 / % impedance of transformer

Question 4

Full Load Current is determined by the transformer KVA rating x 1000, divided by what?

Answer 4

root3 x V(line)

Question 5

Total prospective fault current per phase is determined by the phase of supply, divided by what?

Answer 5

Fault per phase (Z1) and impedance of consumer mains (Z2)

Question 6

Supply phase impedance is determined by dividing the phase voltage by what?

Answer 6

Calculated short circuit current.

Question 7

Consumer mains impedance is determined how?

Answer 7

Table 35 of the AS/NZS 3008, then:

L x Rc x 2 / 1000

Question 8

Disconnection time for a protective device on a final subcircuit with socket outlets is to be maximum ___.

Answer 8

0.4 seconds

Question 9

What two conditions must be met in order to use SELV and PELV to provide basic and fault protection?

Answer 9

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.

Question 10

Before commencing the _____ of an electrical installation, you must consider the nature of the current, nature of conductors, and voltage tolerances.

Answer 10

Design

Question 11

What setting on a megger is used to test insulation resistance on a low voltage transformer test?

Answer 11

500V

Question 12

What should ensured to be disconnected before performing an insulation resistance test?

Answer 12

Any electronics.

Question 13

What does Out Of Range indicate during a continuity test on a transformer?

Answer 13

Open circuit in the winding.

Question 14

What instrument is used to perform a continuity test?

Answer 14

Ohmmeter

Question 15

What type of contact switch is used on a current transformer (CT) to avoid a voltage spike in the windings, and why?

Answer 15

Make-before-break.

Ensures transformer is not open-circuited while equipment is connected or disconnected.

Question 16

Why is the secondary circuit of a transformer earthed?

Answer 16

To protect secondary from reaching hazardous potentials.

Question 17

Why is the secondary circuit of a transformer well insulated from the primary?

Answer 17

To protect against shock hazard.

Question 18

The voltage between the feet of a person standing near a live grounded object is referred to as…

Answer 18

Step potential

Question 19

The voltage between an energized object and the feet or hands of someone in contact with the object is referred to as…

Answer 19

Touch potential

Question 20

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?

Answer 20

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.

Question 21

Why does a person experience electric shock during direct contact with a live conductor?

Answer 21

They become part of the fault path.

Question 22

If contact is made with equipment that has become live under a fault condition, this is referred to as…

Answer 22

Indirect contact.

Question 23

Three factors that may effect the severity of shock to a person are…

Answer 23

1 - Area of the electrical contact.

2 - Frequency of the supply

3 - Magnitude of the voltage

Question 24

If a person comes into contact with a live conductor, what happens around 15-17 mA rms and what is this referred to as?

Answer 24

It becomes difficult to release the conductor due to involuntary muscle contraction. Referred to as grip current.

Question 25

Two checks an electrician would carry out during an inspection of a wiring system in a low voltage electrical installation are…

Answer 25

1 - Conductor size

2 - Identification of cable cores

Question 26

Isolation/switching devices and connection/support/fixing are two checks an electrician would carry out during an inspection of the _____ in a low voltage electrical installation.

Answer 26

Electrical equipment

Question 27

Verifying the impedance required for automatic supply disconnection is done after the electrical installation is connected to the Network Operator’s supply, True or False?

Answer 27

False. It is mandatory to verify before the installation can be deemed safe for connection.

Question 28

To test earth fault-loop resistance of a final subcircuit, where should an active + protective earth conductor be connected and where should the ohmmeter be connected?

Answer 28

Active + PEC connected at origin of circuit (where protective device is fitted) and ohmmeter to be connected at farthest point of branch.

Question 29

A 230V single phase installation must have insulation resistance to earth tested at what voltage?

Answer 29

500 VDC

Question 30

RPHE values can be referenced using which tables in the AS/NZS 3000?

Answer 30

Tables 8.1 and 8.2