18th Edition Part 5 b

A circuit is installed with 10 mm² 70°C thermoplastic multicore cable which has a protective conductor made of copper. The maximum fault current that can occur in the circuit is 600 amps, which will disconnect the circuit in 0.1 seconds, what is the minimum acceptable cross-sectional area of the cpc? Mark one answer

• 1.0 mm²

• 1.5 mm²

• 2.5 mm²

• 4.0 mm²

Answer explanation

Amendment 2 (Brown Book) Reference: Page 200, table 54.3
• Amendment 1 (Blue Book) Reference: Page 197 - Table 54.3 - Linked with 543.1.3 (Page 196) - The first thing you have to do is to obtain the k value from the Tables on, page 197. You are informed the cpc i.e. protective conductor is made of 'Copper' with 70℃ thermoplastic insulation, and is part of a multicore cable. This means you must use Table 54.3, the other tables are for when the protective conductor is either 'Single-core (T 54.2) or 'Armoured' (T 54.4) or 'Steel conduit, ducting, or trunking' is used as the protective conductor (T 54.5)

Cross-referencing Table 54.3 you will find for 'Copper' conductors with 70℃ thermoplastic insulation the k factor is 115 (103* refers to conductors above 300 mm²)

You will need to use the adiabatic equation given in regulation 543.1.3 on, page 196 as shown below.



Where:

S = The cross-sectional area of the protective conductor you need to calculate

I = The value of the fault current

t = The operating time of the protective device in seconds

k = The factor value which takes into account resistivity, temperature coefficient, and the heat capacity of the conductor material, linked with the initial and final conductor temperature.

We now have the following information

k value = 115 (found in table 54.3)

I = 600 amps (given in the question)

t = 0.1 seconds (given in the question)

We place these values in the adiabatic equation as follows



The minimum acceptable size of protective conductor would be 1.649 mm² however since we are not given this value in the four possible answers the next size up 2.5 mm² is the correct answer

What is the minimum allowable size of copper cable used for a main protective bonding conductor?

• 1 mm²

• 25 mm²

• 35 mm²

• 6 mm²

Answer explanation

Amendment 2 (Brown Book) Reference: Page 205, section 544.1.1

Where a private source for a safety service can operate in parallel with the public supply systems, permission to operate the source must be obtained from?

• The distribution network operator (DNO)

• The duty-holder

• The health and safety executive

• The installation designer

Answer explanation

Amendment 2 (Brown Book) Reference: Page 227, section 560.6.9 Note 1

Three circuits can share the same ducting system, provided?

• Every cable is insulated to the highest voltage present

• Only plastic ducting is used

• The cables are separated by 500 mm

• The ducting has partitions

Answer explanation

Amendment 2 (Brown Book) Reference: Page 151, Section 528.1

An installation includes a d.c. power circuit with a grey coloured cable, which terminal does this cable connect to? Mark one answer

• A Earth

• B Mid-wire

• C Negative

• D Positive

Answer explanation

Amendment 2 (Brown Book) Reference: Page 134, table 51

A powertrack system shall be installed in accordance with the manufactures instructions, taking into account?

• The external influences

• The maximum prospective fault current of the conductors

• The maximum temperature rating of the track

• The number of bunched circuits

Answer explanation

Amendment 2 (Brown Book) Reference: Page 139, section 521.4

An RCD is to be used to provide protection against electric shock, the RCD shall be capable of disconnecting?

• The cpc

• The line conductors

• The live conductors

• The neutral conductor

Answer explanation

Amendment 2 (Brown Book) Reference: Page 156, section 531.3.1

It can be considered that the neutral conductor may carry a significant electric current in a three-phase system when a third harmonic current is present with a harmonic distortion greater than?

• 0.01

• 0.05

• 0.1

• 0.15

Answer explanation

Amendment 2 (Brown Book) Reference: Page 145, section 523.6.1

What is the minimum cross-sectional area of an unscreened multi-core copper conductor used for data transfer?

• 0.1 mm²

• 0.2 mm²

• 0.4 mm²

• 0.5 mm²

Answer explanation

Amendment 2 (Brown Book) Reference: Page 218, table 55.2

Where does it state they should be red?

• 511.2

• 537.3.3.5

• 541.3

• 560.9

Answer explanation

Amendment 2 (Brown Book) Reference: Page 187, section 537.3.3.5