Transcript Elements Of Electrical Engineering_3 - GTU E

OM INSTITUTE OF
TECHNOLOGY
Elements of Electrical Engineering
PRESENTED BY –
• GUIDED BY- Mr. Bipin saxena
•
Giri Pankajkumar Shyamkant
ENROLLMENT NO.- 131030119013
•
Patel Harsh Mukesh bhai
ENROLLMENT NO.- 131030119037
•
Sevak jigar Ketanbhai
ENROLLMENT NO.- 131030119053
Earthing AND
ELECTRICAL SAFETY
Earthing
The process of connecting metallic bodies of
all the electrical apparatus and equipment to
huge mass of earth by a wire having negligible
resistance is called Earthing.
Qualities Of Good Earthing
• Must be of low electrical resistance
• Must be of good corrosion resistance
• Must be able to dissipate high fault current
repeatedly
ELECTRICAL SAFETY
Effects of Amount of AC Current
• 3 ma- painful shock which cause
indirect accidents
• 10ma- muscle contraction...”no let
go” danger
• 30ma- lung paralysis- usually
temporary
• 50ma- possible ventricular
fibrillation (heart dysfunction,
usually fatal)
• 100 ma- certain ventricular
fibrillation, fatal
• 4 amps- heart paralysis, severe
burns
Electrical Shock
Received when current passes
through the body.
Severity of the shock depends on:
• Path of current through the
body
• Amount of current flowing
through the body
• Length of time the body is in
the circuit
LOW VOLTAGE DOES NOT MEAN
LOW HAZARD!
Protective devices
TYPES OF ELECTRICAL PROTECTIVE
DEVICES
TYPES OF FUSE
DIFFERENT TYPES OF
MCB(MINIATURE
CIRCUIT BREAKER)
MCCB (MOLDED
CASE CIRCUIT
BREAKER)
Characteristics of MCB
Rated current not more than 100 A.
Trip Characteristics normally not adjustable.
Thermal or thermal-magnetic operation.
Characteristics of MCB
Rated current up to 1000 A.
Trip current may be adjustable.
Thermal or thermal-magnetic operation.
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The first characteristic is the overload which is intended to prevent
the accidental overloading of the cable in a no fault situation. The
speed of the MCB tripping will vary with the degree of the
overload. This is usually achieved by the use of a thermal device
in the MCB.
The second characteristic is the magnetic fault protection, which is
intended to operate when the fault reaches a predetermined level
and to trip the MCB within one tenth of a second. The level of this
magnetic trip gives the MCB its type characteristic as follows:
TYPE
TRIPING CURRENT
Type B
TRIPPING TIME
0.04 To 13 Sec
3 To 5 time full load current
Type C
5 To 10 times full load current
0.04 To 5 Sec
10 To 20 times full load current
0.04 To 3 Sec
Type D
The third characteristic is the short circuit
protection, which is intended to protect against
heavy faults maybe in thousands of amps
caused by short circuit faults.
The capability of the MCB to operate under
these conditions gives its short circuit rating in
Kilo amps (KA). In general for consumer units a
6KA fault level is adequate whereas for
industrial boards 10KA fault capabilities or
above may be required.
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Fuses and MCBs are rated in amps. The amp rating given on the fuse or MCB body is the
amount of current it will pass continuously. This is normally called the rated current or nominal
current.
Many people think that if the current exceeds the nominal current, the device will trip,
instantly. So if the rating is 30 amps, a current of 30.00001 amps will trip it, right? This is not
true.
The fuse and the MCB, even though their nominal currents are similar, have very
different properties.
For example, For 32Amp MCB and 30 Amp Fuse, to be sure of tripping in 0.1 seconds, the
MCB requires a current of 128 amps, while the fuse requires 300 amps.
The fuse clearly requires more current to blow it in that time, but notice how much
bigger both these currents are than the ’30 amps’ marked current rating.
There is a small likelihood that in the course of, say, a month, a 30-amp fuse will trip when
carrying 30 amps. If the fuse has had a couple of overloads before (which may not even have
been noticed) this is much more likely. This explains why fuses can sometimes ‘blow’ for no
obvious reason
If the fuse is marked ’30 amps’, but it will actually stand 40 amps for over an hour, how can
we justify calling it a ’30 amp’ fuse? The answer is that the overload characteristics of fuses
are designed to match the properties of modern cables. For example, a modern PVCinsulated cable will stand a 50% overload for an hour, so it seems reasonable that the fuse
should as well.
ELCB (Earth Leakage Circuit Breaker)
Characteristics
Phase (line), Neutral and Earth wire
connected through ELCB.
ELCB is working based on Earth leakage
current.
Operating Time of ELCB:
The safest limit of Current which Human Body
can withstand is 30ma sec.
Suppose Human Body Resistance is 500Ω and
Voltage to ground is 230 Volt.
The Body current will be 500/230=460mA.
Hence ELCB must be operated
in 30maSec/460mA = 0.65msec