MARKING GUIDELINE

NATIONAL CERTIFICATE

ELECTROTECHNICS N4

12 April 2021

This marking guideline consists of 9 pages.

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MARKING GUIDELINE -2-
ELECTROTECHNICS N4

QUESTION 1: PRINCIPLES OF ELECTRICITY

1.1 1.1.1 True

1.1.2 True
1.1.3 False
1.1.4 True
1.1.5 False
1.1.6 False
1.1.7 False

a
1.1.8 True
1.1.9 True

.z
1.1.10 True
(10 × 1) (10)

1.2 Given:

o
R1  7
R2  5

.c
E1  1,5V
r1  1, 2 s
E2  1,1V
er
r2  0, 7

1.2.1 R1 xR2 (7 x5)

RT    2,92 
R1  R2 (7  5)
ap

rT  r1  r2  1, 2  0, 7  1,9 
ET  E1  E2  1,5  1,1  2, 6V 

E  I ( RT  rT )
tp

2, 6  I (2,92  1,9)
I  0,539 A 
e

ALTERNATE SOLUTION:
tv


= 4,817Ω 



Current through both cells is equal (I = 0,54 A), since it’s a series
connection. (4)

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 1.2.2 ET = V + IT Rt But Rp = Rt
V = 2,6 – 0,536 (1,9)
= 1,582 V 

Or

V p  I p R p  0,539 x 2,92  1,574V 

Vp 1,574

a
I R1    0, 225 A 
R1 7

.z
Vp 1,574
I R2    0,315 A 
R2 5 (3)

o
1.2.3 E1  V1  Ir1

.c
1,5  V1  (0,539 x1, 2)
V1  0,853V 

E2  V2  Ir2
1,1  V2  (0,539 x0, 7)
s
er
V2  0, 723V  (2)

1.3 Given:
ap

I1 = 4.1 A
t = 60⁰
ᴓ = 20⁰
V = 230 V
α0 = 0,004/⁰C
tp

Rt = Rᴓ [1 + α0 (t - ᴓ)]
e


tv

Rt = 

= 65,074 Ω 

 (5)

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1.4 Given:
C1 = 15µF
V = 60 V
C2 = 5 µF

1.4.1 Q1 = VC1 = 60 x 15 = 900 μC 

QT = Q1 + Q2 = 900 + 0 = 900 μC 

a
CT = C1 + C2 = 15 + 5 = 20 μF 

.z
 OR QT = Q1 + Q2

o
QT = VC1 + VC2
QT = V(C1 +C2)
900 = V(20)

.c
V = 45 V  (4)

1.4.2 Q1 = VC1 = 45 (15) = 675 μC 

s
er
Q2 = VC2 = 45 (5) = 225 μC 
(2)
[30]
ap
e tp
tv

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QUESTION 2: DC MACHINES

2.1 Armature reaction:

Is the distortion of the main flux entering and leaving the armature, caused by
the flux set up by armature conductors when they carry current. (2)

2.2 Given:
p=3
c = 2p
IL = 50 A

a
V = 240 V/50Hz

.z
Z = 864 with Rsh = 0,1 ohms
  50 x103Wb

o
Rsh  120

.c
V 240
I sh    2A 
Rsh 120

I a  I L  I sh  50  2  52 A 
s
er
 240  (52 x0,1) 
 245, 2V 
ap

60cE 60 x 2 x3x 245, 2

tp

N   
2Zp 2 x864 x3 x50 x103

N = 340 r/m  (7)

e
tv

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2.3 Given:
IL = Ise = 65 A
Rsh = 220 Ω
V = 400 V
Rse = 0,1 Ω
Ra = 0,15 Ω
E = 420 V

a


.z


o


.c

s
er

 (8)

2.4 Given:
V  500V
ap

Ra  0, 2
Rst  ?

I a ( start )  125%of 100  125 A  OR Ia(start) = 1,25 X100 = 125 A 

tp

V
I a ( start ) 
Ra  Rst
e

V
 Rst   Ra
I a ( st )
tv

500
  0, 2 
125
 3,8 
(3)
[20]

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QUESTION 3: AC THEORY

3.1 Effective value of an alternating current:

Is that value of alternating current, which produces the same amount of heat
energy, at the same rate as a direct current would, if passed through an
identical resistance. (2)

3.2

3.2 3.2.1 For coil A:

XLA = = 62,832 Ω 

a
Xc 

.z
ZA 

o
Zcap 

.c
ZA + Zcap 

s Ω 

For coil B:
er
XLB 

ZB 
ap



 (12)
tp

3.2.2 VcoilA = ItZp

= =152,203<-20,967⁰ V 
e

VcoilB=ItZB
 (4)
tv

3.2.3 IA 

Icap A (2)
[20]

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QUESTION 4: TRANSFORMERS

4.1 a) Core-type with concentric windings

b) Shell-type with sandwich windings (2)

4.2 Given:
f  50Hz
N1  20turns
N2  273turns
A  400cm2  (400x102 )2 m2  400x104 m2

a
E1  220V

.z
4.2.1 E  4, 44m fN
220  4, 44m x50 x 20 

o
m  0, 0495Wb 

.c
m
Bm 
A
0, 0495
  s
400 x104
 1, 238T 
er
(4)

4.2.2 V1 N1

V2 N 2
ap

220 20
 
V2 273
V2  3003V  (2)
tp

4.2.3
 (2)
[10]
e

QUESTION 5: AC MACHINES
tv

5.1 The end plates of a motor

 are mounted at each end of the stator
 house the bearings
 hold the rotor in position relatively to the stator (3)

5.2 a) Resistance-start induction motor

b) Capacitor-start induction motor
b) Capacitor-start capacitor-run induction motor (3)

5.3 Starting winding:

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  Has thinner conductors.
 Located on top of the running winding.
 Has high resistance and low inductance.

Running winding:
 Has thicker conductors.
 Located at the bottom of the slots.
 Has low resistance and high inductance.
Any two from each winding (2 × 2) (4)
[10]

a
QUESTION 6: GENERATION AND SUPPLY OF AC POWER AND MEASURING

.z
INSTRUMENTS

6.1 a) Coal power stations and

b) Nuclear power stations (2)

o
6.2 a) To reduce transport costs.

.c
b) To reduce serious traffic problems on the roads.
c) To keep pollution away from densely populated areas. (3)

6.3 Given: s
er
ap




tp

 (5)
e

[10]
tv

TOTAL: 100

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