2391 : 1#Exam Sample Question (Section B) - Electricians Forum | Electrical Forum | An electrical forum for electricians in the UK

**LukeScotty** · 12-10-2007

An automobile repair workshop and showroom which is six years old is of a steel frame
construction with metallic water and oil services entering in the workshop.
The 400/230 V supply forms part of a TN-C-S system, with a Ze of 0.023Ω and a single-phase
PSCC of 10 kA.
The distribution board for the workshop and showroom is a TP and N metal-clad board fitted
with BS EN 60898 circuit breakers. Spare capacity sufficient for the proposed additions exists,
supplied via a 300 A TP and N metal-clad isolator, both of which are located in the workshop.
An existing distribution circuit connects the workshop distribution board and a single-phase,
metal-clad distribution board supplying and located in the offices, all having suitable spare
capacity.
The office contains five personal computers used by staff installed on a single ring final circuit
and the showroom has 13A socket outlets supplied via three radial circuits.
The workshop contains a three-phase, 400 V electrical hoist, electronic diagnostic
instruments, three-phase socket outlets for electric welders and an inspection pit.
The lighting in the car park is provided by six high-pressure sodium vapour lamps controlled
by a photo-cell located on the roof of the premises. The car park lighting has been added
since the building was originally constructed.
The building is fitted with a lightning protection system and the electrode for this is located in
the workshop yard and car parking area, as is the oil storage tank.
An electrical contractor has been requested to carry out the installation of some new circuits
to the office and workshop area, and alterations to the existing office lighting. The second
inspection and test of the original installation is to be undertaken at the same time as this
additional work.

21

a) Excluding diagrams, charts and tables, list four items of documentation that should
be made available to the electrical contractor before work on the installation
commences. (4 marks)

b) List the details that will need to be confirmed with the client prior to the
commencement of the inspection and test on the existing installation. (2 marks)

c) State all the forms of documentation which will be produced on completion of the
electrical work, detailing the extent of the work recorded on each form. (7 marks)

d) State two statutory documents which relate to the electrical inspection and test
activity concerning both the process and the activities of those engaged in the
work. (2 marks)

22 In relation to the existing car park lighting circuit

a) list, in correct sequence, the first four tests to be carried, together with the
appropriate test instrument and scale for each test (8 marks)

b) list any electrical items which may require special consideration before carrying out
the test for insulation resistance and identify any measures that would need to be
taken (4 marks)

c) state at which point(s) on the circuit the test for earth fault loop impedance would
be carried out and which test result(s) would be recorded for the circuit. (3 marks)

23

When carrying out the earth fault loop impedance tests on the socket outlets in the car
showroom the following values are obtained.
BS 7671 Maximum tabulated Values Measured Value
3.00 Ω 2.1 Ω
2.40 Ω 1.9 Ω
1.92 Ω 1.38 Ω
1.50 Ω 1.2 Ω

a) Showing all calculations determine whether the measured values are acceptable (8 marks)

b) Explain the reasons for calculation in a) above. (4 marks)

c) State three methods by which compliance could be achieved, for any circuit(s)
found that may be unacceptable. (3 marks)

24

A new ring final circuit has been installed in plastic trunking, using single-core
thermoplastic (pvc) conductors with 2.5mm2 live and 1.5mm2 cpc copper conductors,
within the office area to supply additional equipment.

a) Describe, in detail, the procedure for carrying out the continuity of ring final circuit
conductors test on the new office ring circuit, prior to final connection to the local
distribution board. (9marks)

b) When carrying out the above test it is found that the reading obtained at each
socket outlet increases significantly with the distance from the distribution board.
Explain two possible reasons for this effect. (6 marks)

25 a) List the extraneous parts where main equipotential bonding conductors need to be
connected if the installation is to be compliant with BS7671. (4 marks)

b) Explain the actions to be taken in respect to the main equipotential bonding
conductors when carrying out the tests to establish
i) Ze for the installation
ii) Zs for the socket outlet circuits (4 marks)

c) Describe two methods of establishing continuity of the main equipotential bonding
conductors for this installation. (2 marks)

d) With the aid of a labelled diagram, show the earthing arrangement for this
installation from the electricity suppliers intake equipment to the earthing terminal
in the main distribution board show all conductors, but exclude the outgoing circuit
cpcs. (5 marks)

26

a) With the aid of a fully labelled diagram show the complete earth fault loop
impedance path for the ring final circuit installed in the offices. (9 marks)

b) Explain how the prospective fault current at the origin of the installation may
established when the instruments available are designed to test at voltages up to
240 V. (6 marks)

EasyFox · 15-10-2007

I did this paper for 2391 when it was first released.