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Electricity is either connected in single phase or three phase. In single phase, wire is
connected in phase to neutral with supplying 230 or 240 volts of electricity. Single phase
power is commonly used in households. While, in three phase, wire connected in phase to phase with supplying 400 volts of electricity. hree phase power is commonly used in
commercial premises.
In single phase! " # $Icos Ɵ
In three phase! " #$I cos Ɵ
Electrical wiring composes of electrical e%uipment such as ca&les, switch &oards, mainswitches, miniature circuit &rea'ers ()*+ or fuses, residual current devices (-*,
lighting points, power points, lightning arrestors, etc.. E/ample of a single phase
consumer electrical wiring is as shown in 1igure 3.2
1rom figure 3.2, we can see that 00m E*+ is connected to lighting general power
circuits, such as lamp, fan, motor (.. 5 irect n ine, alarm and air5conditioner.
While, 30m E*+ is connected to soc'et outlets.
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irect n inehe irect n ine )otor Starter ( consist a )*+ or circuit +rea'er, contactor
and an overload relay for protection. Electromagnetic contactor which can &e opened &y
the thermal overload relay under fault conditions. has several &uilt5in protectivefeatures
(a Short5circuit protection
(& verload protection(c 6nder5votage protection
as !5
)ain circuit7"ower circuit
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2 *ontrol *ircuit
verload protection for an electric motor is necessary to prevent &urnout and to ensure
ma/imum opeer rating life. 6nder any condition of overload, a motor draws e/cessive
current that causes overheating. Since motor winding insulation deteriorates due to
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overheating, there are esta&lished limits on motor operating temperatures to protect a
motor from overheating. verload relays are employed on a motor control to limit the
amount of current drawn.
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1or 8+ 39
oad )a/imum *urrent Estimate *urrent
1ilament ight (mount of the oad / "ower
-e%uired &y the oad / .: 7(230 / 0.:;
<<= of its ma/imum current
ight +ul& (mount of the oad / "ower
-e%uired &y the oad 7 230
<<= of its ma/imum current
Soc'et utlet
(S..>
(2 S..> / "ower -e%uired &y
each oad / ?alf the mountof the oad 7 230
00= / (2 S..> / "ower
-e%uired &y each oad 7 230@
40= / A)a/. *urrent5 00= /
(2 S..> / "ower -e%uired &y
each oadB 7 230
ir *onditioner (mount of the oad / "ower-e%uired &y the oad 7 (230 /
0.:;
Same as its ma/imum current
Water ?eater (mount of the oad / "ower
-e%uired &y the oad 7 230
(00= / )a/. *urrent 7
mount of the oad@
(00= / )a/. *urrent 7
mount of the oad
@(2;= / )a/. *urrent 7 mount
of the oad
*oo'er *ontrol 6nit (mount of the oad / "ower
-e%uired &y the oad 7 230
0 @ 30= / ()a/. *urrent 5
0
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oad )a/imum *urrent Estimate *urrent
Soc'et utlet
(S..>
(2 S..> / "ower -e%uired &y
each oad / ?alf the mountof the oad 7 230
00= / (2 S..> / "ower
-e%uired &y each oad 7 230@
40= / A)a/. *urrent5 00= /(2 S..> / "ower -e%uired &yeach oadB 7 230
ir *onditioner C
)otor (3 phase
(mount of the oad /
"ower -e%uired &y
the oad 7 ( / 400 / 0.:;
00= / st ?ighest )a/.*urrent for each 6nit
@
:0= / 2nd ?ighest )a/.*urrent for each 6nit
@
<0= / Aotal )a/. *urrent of 3
phase oad 5 (00= / st?ighest )a/. *urrent for each
6nit 5 (:0= / 2nd ?ighest
)a/. *urrent for each 6nitB
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