Snag 7 The absence of a legible diagram, chart or table may result in danger or nuisance, such as if someone inadvertently switches off the supply to an important service, for example a lift.
Apprentice Corner answers
Solution
1.
The information required at a distribution board or consumer unit should indicate the particulars of the installation. For example, simply labelling a protective device as 'socket-outlets' would indicate the purpose of the circuit but it would not be sufficient to provide information required by Regulation 514.9.1. A convenient way of providing such information for a simple installation such as a domestic installation is by means of a separate copy of the completed Schedule of Circuit Details for the Installation (pictured below), which forms part of the Electrical Installation Certificate and Domestic Electrical Installation Certificate.
Correct option is (d) For a typical perforated cable tray having at least 30 % of the base occupied by holes, the reference method is E or F. However, where the whole percentage is less than 30 %, the reference method is taken to be C. Paragraph 7.2 of Appendix 4 of BS 7671 refers.
2. Correct option is (a) There are numerous ways in which this can be solved, one such method is: Impedance: Z = √(R2 + X2 ) = √( 1.92 + 0.142 = 1.905 mΩ/m Total impedance = 1.905 × 10-3 × 120=0.229 Ω
3. Correct option is (c) There are three trig ratios that can be used to answer this question. However, since R and X was given it would be sensible to use; tan φ =
ICN4/
(To the person ordering the work)
This certificate is not valid if the serial number has been defaced or altered
SCHEDULE OF CIRCUIT DETAILS FOR THE INSTALLATION TO BE COMPLETED IN EVERY CASE
Supply to distribution board is from:
Type: BS(EN)
V
Associated RCD (if any): BS(EN)
Overcurrent protective device for the distribution circuit: Distribution board designation:
Nominal voltage:
No of phases:
Original
TO BE COMPLETED ONLY IF THE DISTRIBUTION BOARD IS NOT CONNECTED DIRECTLY TO THE ORIGIN OF THE INSTALLATION*
Location of distribution board:
Rating:
A
RCD No of poles:
I⌬n
mA
Maximum Zs permitted by BS 7671
(s)
Short-circuit capacity
(mm2)
Operating current, I⌬n
(mm2)
(kA)
(mA)
(Ω)
RCD
Overcurrent protective devices BS (EN) Rating
cpc
Type
Live
Max. disconnection time permitted by BS 7671
➺ Reference method
Number of points served
Type of wiring (see code below)
Circuit number and line
Circuit conductors: csa
The phase angle: φ = tan-1 0.0737 = 4.2°. The power factor: cos φ = 4.2 = 0.997 lagging
4. Correct option is (b) Design current: Ib =
P
100×103
=
√3×UL×p f
= 180 Amps
√3×400×0.8
As there are no correction factors and since the motor starter will provide protection against overload,
CIRCUIT DETAILS Circuit designation
X 0.14 = 0.0737 = R 1.9
(A)
It ≥ Ib = 180 A. (Regulation 433.3.1 and clause 5.2 of Appendix 4 refers.) Using Table 4J4A select a 70 mm2 cable (columns 1 and 5).
First floor lighting except bathroom
5. Correct option is (d) As the power factor is known, equation 2 can be used: if cos φ = 0.8 then cos-1 0.8 = 36.8699° 7
so sinφ = sin 36.899 = 0.6.
First floor socket-outlets New shower
Using Table 4J4B for 70 mm2 conductors; r = 0.96
40
10.0 4.0
and x = 0.135 Voltage drop =
0.8×0.96+0.6×0.135 ×180×80=12.23 V. 1 000
Cable complies with Table 4Ab.
6. Correct option is (a) Using Table 4B1, the correction factor for 35 °C is 0.96. The tabulated current: It =
Ib
=
180
Ca 0.96
= 187 Amps
Using Table 4J4A: select 95 mm2 conductors; ➺
Using Table 4J4B: r = 0.71 and x = 0.13. The correction factor for temperature is only applied to
See Table 4A2 of Appendix 4 of BS 7671 CODES FOR TYPE OF WIRING A
B
C
D
E
F
G
Thermoplastic Thermoplastic Thermoplastic Thermoplastic Thermoplastic Thermoplastic Thermosetting/ cables insulated/ cables cables cables /SWA SWA in metallic in non-metallic in metallic in non-metallic sheathed cables cables conduit cables trunking conduit trunking
H
O (Other - please state)
Mineralinsulated cables
(mV/A/m)r (Paragraph 6.3 of Appendix 4 refers). Page 5 of
5
Voltage drop =
* In such cases, details of the distribution (sub-main) circuit(s), together with the test results for the circuit(s), must also be provided on continuation schedules.
This certificate is based on the model forms shown in Appendix 6 of BS 7671 Published by Certsure LLP. Certsure LLP operates the ELECSA & NICEIC brands. © Copyright Certsure LLP (January 2015)
See next page for Schedule of Test Results
0.96×0.8×0.71+0.6×0.13 1 000
×180×80=8.98 V.
ICN4/9
63 AU T U MN 2 017
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