2012
Prevention of Fire arising from
Short Circuit in Wiring
Installations
Subhodeep Kar
Electrical Engineering , University
of Calcutta
(Duration- 4th June 2012 to 14th July 2012)
Sponsored by- CESC Limited
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
THIS IS TO CERTIFY THAT MR. SUBHODEEP KAR, FINAL
YEAR STUDENT OF ELECTRICAL ENGINEERING,
DEPARTMENT OF APPLIED PHYSICS, UNIVERSITY OF
CALCUTTA,
HAS
COMPLETED
THE
PROJECT
“PREVENTION OF FIRE ARISING FROM SHORT CIRCUIT
IN WIRING INSTALLATIONS” UNDER MY GUIDANCE
DURING THE PERIOD 04.06.2012- 13.07.2012
__________________
DATE: 13/07/2012
1
(SOUMYEN CHATTERJEE)
(SENIOR MANAGER, CENTRAL)
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Executive Summary
The Kolkatans watched In the television with horror the devastating fire that
took place at Nandaram Market in January,2008. It made an international news
and peaple from all over the world were astonished to notice how it took the Fire
Service Department of West Bengal to douse the flames after battling for 3 days.
Although there were no casualties but property worth several crores were
gutted.
As we couldnot learn from the mistakes of the fire incident at Nandaram Market
another catastrophic such fire incident occurred within 2 years from the above
date at Stephen Court, Park Street that claimed 43 innocent lives apart from
causing severe damage to the building.
The natural question therefore arises whether fire that is regarded as one of the
greatest inventions of mankind, is a boon or curse to us. Therefore it should be
our earnest endeavour to fight the menace of fire qand take all possible measures
to stop recurrence of such grave dangers arising from fire.
It is observed from newspaper reports and other reports from electronic media
that several committees has been formed to investigate the detailed causes that
leads to such collosal inferno. These committees look at curbing fire from
administrative point of view. However, the detailed understanding or root cause
analysis that lead to fire have not been done in a major way.
This project is an attempt to find out the causes of fire, especially that occur from
electrical short circuit in wiring installations of the households or the business
establishments.
The major causes of short circuits in electrical wiring and its consequent fire has
been dealt with from practical as well as theoretical electrical engineering point
of view. It has also been specifically addressed why air conditioners and
fluorescent lights gets overheated and catch fire.
On ascertaining the specific causes of electrical short circuit the project enlightens
about the measures to be adopted from electrical point of view to prevent short
circuit, the relative advantage and disadvantage of fuses and why Miniature
Circuit Breakers(MCBs) are preferred to fuses as an improved protective device.
The next section deals with the technology involved in volved in the construction
and operation of MCBs and how it generally helps to protect the electrical
circuits.
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
The MCBs have the limitation and can be used up to a certain quantum of load
above which MCCBs are to be used which are very sophisticated and are
improved protective devices. The construction, principle of operation has been
discussed in detail.
Wires play the most important part in the electrical installations. It has been
shown how and why we should move from normal PVC wires to FRLS and
ZHFR wires/cables to minimize the risk of short circuit and the aftereffects by
reducing the hazards of smoke when a fire occurs.
The project further deep dives into what should be the correct earthing practice,
what are the deviations that usually take place and why RCCBs should be
mandetorily used for specialized circuits for preventing hazards from electrical
shocks.
Modern electrical practices in the developed countries of the world indicates
further improvement in protection by the use of GFCI and AFCI, the principles
of which has been discussed in detail.
The project also makes an endeavour to ascertain the cost of making a fire prone
building completely safe from electrical fires. This would give the readers an
idea about the total investment required for carrying out the above work.
Finally,
the
project
has
been
concluded
by
suggestions/recommendations to help minimize electrical fires.
3
providing
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Acknowledgement
The special thanks goes to my helpful project guide, Mr. Soumyen Chatterjee.
The supervision and support that he gave truly helped the progression and
smoothness of the project. The co-operation is much indeed appreciated.
My grateful thanks also go to Mr. Dipanjan Sarkar, Mr. Ramkrishna Saha, Mr.
Rajdeep Chandra, Mr. Pratap Pal and Mr. Soumyadeep Mitra. All of them were
of great support and help during my internship. Besides, this the internship
program gave me a chance to visit and get acquainted with the work culture of
an industry. So, thanks to CESC Limited for giving such an exposure. Not to
forget, a great appreciation goes to the rest of CESC’s staff who helped me from
time to time during the project.
Also, I would like to thank all the respected professors of The Department Of
Applied Physics, University Of Calcutta who were also of great help whenever I
needed.
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
CONTENTS
TOPICS
List of abbreviations
List of pictures
List of tables
Introduction
1.Common causes of fire
2.Causes of short circuit
3.Causes of fire in fluorescent lights
3.1 Why is fluorescent light ballast so hot!
3.2 How to avoid heat of the ballast from damaging house
4.Cause of fire in an air conditioner
4.1 Possible remedy
5. Measures to prevent short circuit
5.1 How to prevent persistent short circuit
6.Fuses
6.1 How a fuse works
6.2 Advantages of fuses
6.3 Disadvantages of fuses
6.4 Why MCB?
7. Miniature Circuit Breaker(MCB)
7.1 What is it?
7.2 Overload protection
7.3 Shortcircuit protection
7.4 Arc quenching
7.5 MCB types
7.6 MCB specification
8. Moulded Case Circuit Breaker(MCCB)
8.1 MCCB ratings
9. Wires
9.1 Selection of rated voltage and conductor cross-section
9.2 Current carrying capacity of PVC wires
9.3 Ageing of PVC wires
9.4 Advantages of FRLS or ZHFR wires/cables over PVC
wires/cables
10. Earthing practice
11. Deviation from ideal practice
5
PG.NO.
7
8
9
10
11
12
13
13
14
14
14
15
15
16
16
17
17
17
18
18
19
19
22
24
25
28
30
31
31
32
33
36
40
41
Prevention of Fire arising from Short Circuit in Wiring Installations
12. Residual-Current Circuit Breaker(RCCB)
12.1 Purpose and operation
12.2 Limitation
12.3 Types of Ground Fault Circuit Interrupters – GFCI
12.4 Technical Characteristics
13. Arc Fault Circuit Interrupter(AFCI)
13.1 Why AFCI?
13.2 Arcing fault hazards
13.3 How it works?
13.4 Types of Arc Fault Circuit Interrupters (AFCI)
13.5 Advantage of AFCI
13.6 Typical wiring details for AFCI circuit breakers
14. Making a fire prone building safe from electrical fires
15. Conclusion
References
6
2012
42
43
45
46
47
51
51
52
53
53
54
55
56
57
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Prevention of Fire arising from Short Circuit in Wiring Installations
List of Abbreviations
AFCI – Arc Fault Circuit Interrupter
DB – Distribution Box
FRLS – Fire Resistant Low Smoke
GFCI – Ground Fault Circuit Interrupter
HCL – Hydrogen Chloride
ISI – Indian Standards Institute
MCB – Miniature Circuit Breaker
MCCB – Moulded Case circuit Breaker
OCPD - Overcurrent Protective Device
PCB – Polychlorinated Biphenyl
PVC – Polyvinyl Chloride
RCCB – Residual Current Circuit Breaker
RCD – Resisidual Current Device
RMS – Root Mean Square
ZHFR – Zero Halogen Fire Resistant
A/Amp – Ampere
V - Volt
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
7
2012
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
List of Pictures
Picture no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
8
Title
Jumbled up and undressed wiring
Fuse and main switch
Miniature Circuit Breakers(MCBs)
Labelled diagram of different parts in an MCB
ON state and OFF state of MCB
Path of current in an MCB
Electric arc
Arc quenching by arc chutes
Time characteristics of MCB
Moulded case circuit breaker(MCCB)
RMS and peak values of current
Current vs Time curve
Schematic diagram showing the installation of MCCB
and MCB
Residual Current Device(RCD)
Schematic diagram depicting the operation of RCD
Schematic diagram depicting the operation of RCD
Schematic diagram depicting the operation of RCD
Schematic diagram depicting the operation of RCD
Schematic diagram depicting the operation of RCD
Schematic diagram depicting the operation of RCD
Connection diagram for installation of MCBs and RCDs
Time-Current characteristrics of a circuit breaker
Series arc current
Series arc voltage and current
Wiring details of AFCI circuit breakers
Damaged tubelight
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
List of Tables
Table No.
1
2
2A
3
4
5
6
7
8
9
10
11
12
12A
13
13A
9
Title
MCB ratings for different loads
A sample price list of MCBs
A sample price list of MCBs (contd.)
Current rating of wires and protective fuses for ambient
temperature up to 25°C
Permissible loading (for different ambient temperature) in
% of the values given in table – 3
Conversion factors (for bunched cables) of load capacity
for various air temperature
Life untill deterioration against conductor core
temperture
Causes leading to failure or ignition of PVC – insulated
electromechanical products
Comparison of PVC, FRLS, and ZHFR compounds
Comparison of current carrying capacity of FRLS and
ZHFR wires
A sample comparative price list of PVC,FRLS and ZHFR
wires
A sample price list of ZHFR wires
A sample price list of RCCB – ‘AC’ Type
A sample price list of RCCB – ‘A’ Type
A sample price list of RCBO – ‘A’ Type(SPN-2M)
A sample price list of RCBO – ‘A’ Type(TPN-4M)
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Introduction
The hazards of electrical fires is becoming very common in the city of kolkata.
The city is comprised of many old buildings, old markets with unmaintained
electrical installations which eventually leads to such mishaps. This takes away
lives, properties and affects daily life. Below is listed some of the cases of
electrical fires in the recent past –
o Fire hazard at Nandaram market – In the year 2008
o Fire hazard at stephen court – In the year 2010
o Fire hazard in a multistoried building at south Kolkata’s Golpark area –
March 30, 2012
o Fire at Food Plaza of Howrah Station – June 02, 2012
o Fire hazard at Karnani Estate – June 16, 2012
Since fire can be caused at a building also from sources other than electricity
outlets, a holistic view should be taken in this regard with particular reference to
the construction of the building, provision of fire protection arrangement and the
electrical installations.
A committee has been formed with representatives from Kolkata Police, KMC,
West Bengal Fire and Emergency Services, CESC Limited and Directorate of
Electricity, Government of West Bengal.
Several joint inspections were carried out in various fire prone buildings. And,
during inspections we have seen that in most of the cases the electrical
installations are no way satisfactory and are very much prone to electrical fire
hazards. So, both short term and long term recommendations were forwarded to
the owners/occupiers of the buildings were advised to comply with short term
and long term recommendations within 15 days and 6 months respectively.
This project deals with the recent developments in the electrical field, which if
adopted in the consumer’s electrical installations,it certainly can reduce the
electrical fire hazard upto a great extent.
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Prevention of Fire arising from Short Circuit in Wiring Installations
1.
2012
Common Causes of Fire
Monitoring the trends related to the common causes of fire provides invaluable
information that helps focus fire prevention efforts.
Below is listed some of the common causes leading to fire hazards.
Open Flames
Examples of such unsafe conditions are as follows:
•
•
•
•
negligence in conducting hot work, such as welding, gas cutting or
grinding;
improper use of candles
improper handling of flammable or combustible liquids or flammable
gases in near-to-potential ignition sources; and
matches and cigarettes that are improperly disposed of, or left unattended
near combustibles.
Electrical
Examples of such unsafe conditions are as follows:
•
•
•
•
•
damaged electrical conductors, plug wires or extension cords;
use of faulty, modified or unapproved electrical equipment;
insufficient space or clearance between electrical heating equipment and
combustibles;
short circuits or overloaded circuits;
loose electrical connections
Cooking
Examples of possible unsafe conditions are as follows:
•
•
•
11
deep frying in pots or pans on stove tops;
unattended cooking appliances; and
combustibles located dangerously close to cooking equipment.
Prevention of Fire arising from Short Circuit in Wiring Installations
2.
2012
Causes of short circuit in electrical wiring
The touching of two electrical live parts coming from any potential power
source will cause a short circuit. Short circuit relates to the quickest path back to
the source with out any resistance to the current flow.
The two live parts may come in contact because of the following reasonso Breakdown of insulation due to ageing.
o Cramping of wires in undersized box may cause them to come loose and
contact each other thereby creating short circuit.
o Jumbled up wires may result in short circuit at points of loose connection
or damaged insulation.
o Wires if not properly bushed may result in a short circuit.
o Wire or distribution boxes becoming exposed to moisture and/or
dust/dirt.
Pic-1 (showing jumbled up and undressed wiring)
Source- Photograph taken by self
Apart from these sometime electrical fires also occurs due to the bursting of the
ballast of a tube light or malfunctioning of an unmaintained air conditioner.
These cases are discuss in the next page.
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Prevention of Fire arising from Short Circuit in Wiring Installations
3.
2012
Causes of fire in fluorescent lights-
A factor that worries many users of fluorescent lights is the ballast getting very
hot after a few minutes of use. The temperature of the ballast can reach a level so
hot that you cannot even touch it with your naked hands. Thus, lots of house
owners worry that the heat generated from the fluorescent light ballast might
cause a fire, damaging their homes. This is especially true when the lights are
fitted closely against a wooden ceiling. The heat generated by the fluorescent
light ballast can actually roast the wooden structure after prolonged use.
3.1 Why Is The Fluorescent Light Ballast So Hot!
Pic-26
Source- www.louyeh.com
The ballast gets heated up due the process where the ballast has to generate
power to make the light tube lit up. Unlike the incandescent light bulb which
gets its light from heating up the filament in the bulb, the light from the
fluorescent light tubes comes from the gas that is filled inside the tube. These
gases has to be electrically charged to emit the light.
This electrical charges comes from the electrodes present at both ends of the
fluorescent tube which is in turn connected to the ballast. The ballast will then
have to generate power to electrically charge up the electrodes in the tube. This
generation of power is what causes the ballast to heat up and become so hot.
Also the fluorescent lamp has to match the installed ballast. This means for
example, that you should not install 40 watt lamp in a fixture running for 34 watt
lamp. Moreover, the lamps actually have a life expectancy. The standard lamp
manufactured today is designed to last approximately 3000hours. After this time
13
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
a burnout occurs, called cathode evaporation. This shows up as browning on the
ends of lamps. At this point, the visual lumens emitted by the lamp drop by 4050%. The lamp then needs to be changed or else it will begin to stress and
eventually ruin the ballast.
After a stressed ballast has been trying to work for an hour or more, it gets
extremely hot, the tar inside (loaded with PCBs) becomes molten, shorts out the
circuit and the fixture no longer works. After a while, the ballast cools off enough
for the molten tar to gel and the fixture starts to work again. But since the ballast
is no longer balancing, the voltage remains in the start mode or very high. This
cycle repeats itself over and over until one of the two things happen. Either the
temperatures in the ceiling, aided by the heat build up finally reach the point that
the ceiling bursts into flames, or the ballast itself will burst open in flames,
spilling the molten flaming tar down onto the floor.
3.2 How to avoid heat of the ballast from damaging a houseo Use of spacer while installing a fluorescent light
o Use of suspension rods while installing the fluorescent light
o Regular inspection
4.
Cause of fire in an Air-Conditioner-
Fires caused by air conditioner units most often happen at the cord. The reason is
that the cords overheat due to unit drawing too much current. High current
draw is most often caused by a dirty condenser (outdoor coils). As dirt is blown
through the coils with the air that cools them it slowly builds up on the coils.
Eventually the build up clogs the coils more and more. When this happens the
pressure inside the system rises causing the compressor to work harder to do its
job. In working harder the current draw increases until it exceeds the current
rating for the cord being used. The copper wire will then act like a heating
element getting hotter and hotter. The easiest way to tell if this is happening is to
feel the cord. If it feels warm unplug and inspect the prongs. If the insulation
around the prongs look burned then discontinue using it until you get the
condenser cleaned and have the cord replaced. Most of the newer window units
today come with breakers built into the cords. If you have one and it is tripping
then check to see if the a/c needs cleaning.
4.1 Possible remedyProduct maintenance, routine inspection, and the installation of smoke alarms
can aid in the prevention or reduction of air conditioning fires.
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Prevention of Fire arising from Short Circuit in Wiring Installations
5.
2012
Measures to prevent short circuit
1. Always use good quality ISI cables.
2. Make sure that the electrical outlets are designed to handle the appliance
loads.
3. Avoid joints in wiring (taping of wires). Instead use extension box with
fuse or else go for soldering and proper mechanical joints.
4. Always renew the wiring after ageing (discussed in the wires section)
5. Wirings should not be jumbled up.
6. Wirings used should be of about 20% higher capacity compared to the
load it is supplying because the capacity of wiring degrades with ageing.
7. Wiring should be properly dressed and clamped.
8. Use of separate breakers for high power pumps.
9. Use one socket for one appliance.
10. There should not be any uncovered or un-insulated wiring portions.
11. All dead ends of the wiring should be properly bushed.
5.1 How to prevent persistent short circuit –
o Use of MCBs and MCCBs of proper rating as discussed in the respective
sections.
o Proper earthing system should be employed as described later.
o In no case the breakers or the fusing system should be bypassed.
During inspections to fire prone buildings in Kolkata we have seen that many
consumers are still using fuses as the protective device for their electrical
installations. Protection given by fuses are not sufficient to minimize electrical
fires.
We will be discussing about the fuse protection from the next page.
15
Prevention of Fire arising from Short Circuit in Wiring Installations
6.
6.1
2012
FUSES
How a fuse works
When an over-current or short circuit condition occurs, which is the same
as saying that the magnitude of current flowing in the circuit becomes
higher than the rating of the fuse employed, its internal "fuse wire" gets
heated up to such an extent that it melts and breaks apart, which cuts off
the supply of current to the circuit downstream. When this happens the
fuse is said to have "blown".
After the cause of the over-current or short circuit condition in the circuit
has been repaired, a blown fuse must either be replaced with a new one
or, if it is the type of fuse which can be rewired, it must be rewired using
the correct size of special "fuse wire".
A new fuse should then remain in place un-blown until another overcurrent or short circuit condition occurs in the circuit it is protecting,
which would then cause the new fuse wire to blow.
1. Fuses 2. Main switch
Pic-2
Source- www.ultimatehandyman.co.uk/old_fuse_box
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
6.2 Advantages of fuses
To give the same over-current protection, fuses are generally cheaper to make
and smaller in size than circuit breakers.
6.3 Disadvantages of fuses
•
•
•
Some fuses are rewireable, meaning they can be repaired if they blow.
This must only be done using new fuse wire of the correct size. It is
dangerous to use fuse wire which is thicker than the size marked on the
rewireable fuse's body.
Other fuses, known as "one-time" or "cartridge" fuses, cannot be repaired
when they blow. If that happens they must be thrown away and replaced
by a new fuse of the correct size.
An ordinary fuse cannot blow as quickly as a circuit breaker can trip.
6.4 Why MCB?
So, it is evident that the protection given by fuses is not up to the desired level.
Also, the main switch installed in the consumer’s premises doesnot have short
circuit sensing feature and it is manually operated. So in case if there is a fire in
the meter board then operating the main switch is going to be a problem.
Moreover , as mentioned above if a blown fuse is rewired using wire of larger
cross-section then it will not serve the purpose at all because this wire will melt
for much higher current than it was desired.
To overcome the above drawbacks we need some device which can sense
overloading and fault and thereby interrupts the flow of current automatically.
Automatic operation will help to eliminate the errors or problems arising out of
manual operation.
Miniature circuit breaker (MCB) is a device incorporated with the above
mentioned features.
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Prevention of Fire arising from Short Circuit in Wiring Installations
7.
2012
Miniature Circuit Breaker
7.1 What is it?
MCBs or Miniature Circuit Breakers are electromechanical devices which
protect an electrical circuit from an overcurrent. The overcurrent, in an electrical
circuit, may result from short circuit, overload. An MCB is a better alternative to
a Fuse since it does not require replacement once an overload is detected. Unlike
fuse, an MCB can be easily reset and thus offers improved operational safety and
greater convenience without incurring large operating cost.
Pic-3
Source- www.engineersgarage.com
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
The principle of operation is simple. An MCB functions by interrupting the
continuity of electrical flow through the circuit once a fault is detected. In simple
terms MCB is a switch which automatically turns off when the current flowing
through it passes the maximum allowable limit. Generally MCB are designed to
protect against overcurrent.
7.2 Overload protection To protect against overcurrent from overload a bi-metallic strip is used. When
overcurrent occurs the temperature of the bimetallic strip rises and due to the
unequal expansion co-efficient of the two materials in the bimetallic strip it bends
and thereby breaks the current flow. In case of overloading it may take 2 seconds
to 2 minutes for the MCB to trip.
7.3 Short circuit protection There are two contacts one is fixed and the other moveable. When there is a short
circuit in the downstream circuit then current exceeds the predefined limit (eg. for b class- 3 to 5 times) a solenoid forces the moveable contact to open (i.e.,
disconnect from the fixed contact) and the MCB turns off thereby stopping the
current to flow in the circuit. In order to restart the flow of current the MCB is
manually turned on. This mechanism is used to protect from the faults arising
due to over current or over load. MCBs are generally designed to trip within 2.5
milliseconds when a short circuit occurs.The following image in the next page shows
the different internal parts of an MCB with top casing removed.
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Pic-4, Source- www.engineersgarage.com
The following image illustrates the tripping mechanism of the MCB. The circuit
breaker contacts and the position of the knob can easily be seen in both, ON and
OFF, state. In the ON state the moveable contact touches the fixed contact as
shown in the image.
Pic-5, Source- www.engineersgarage.com
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
The path of the current in the ON state is shown in the image below. The live
wire is screwed at the input terminal. From the input terminal the current flows
to the fixed contact which is in electrical contact with the input terminal. The
current then moves to the moving contact (since in the ON state it is touching the
fixed contact). The moving contact is connected to the electromagnetic coil of the
solenoid by means of a thick wire. The current from the moving contacts enters
the electromagnetic coil. The current from the coil then goes to the bimetallic
strip by means of another thick wire and finally to the output terminal from
which load is drawn.
Pic-6, Source- www.engineersgarage.com
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
7.4 Arc quenching
Another important point to be considered in the design of circuit breakers is Arc
quenching. To understand how MCB carries out the arc quenching, it is
important to know about the electric arc and how is it produced. When an
overcurrent is interrupted by the circuit breaker by opening its contacts, current
tries to bridge the gap. In an attempt to maintain the circuit, the air heats up gets
ionised and becomes a conductor. As a result an arc forms. The following image
shows how an electric arc looks like.
Pic-7, Source- www.engineersgarage.com
The heat from an uncontrolled arc in a circuit breaker can cause a rapid and
violent expansion of the nearby air and could severely damage the circuit
breaker. Therefore, besides separating the electrical contacts, a circuit breaker
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
also has to quickly extinguish the arc. A number of methods can be employed for
quenching the arc, such as, speed, distance, dielectric strength, cooling etc.
a)
Speed : When the contacts separate rapidly, there is less time for the arc to
form and maintain itself.
b)
Distance : When the distance between opened contacts is more, the arc has
to stretch more to maintain the current flow which requires more voltage.
c)
Cooling : When the arc is forced against a cold material, it absorbs and
dissipates the heat.
d)
Dielectric Strength : When the arc is submerged in a medium with higher
dielectric strength than air (sulfur hexafluoride, SF6), the insulating nature of the
medium helps in quenching the arc.
In an MCB, arc chutes or arc dividers are used for arc quenching. When the
contacts of an MCB separate, generating an electrical arc between them through
air, the arc is moved into the arch chute where it is divided into small segments.
The overall energy level of the arc gets split up which is not sufficient to sustain
the arc and therefore it gets dissipated.
Pic-8, Source- www.engineersgarage.com
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
7.5 MCB Types
Based on the type of application MCB are classified as B,C,D curve type.
Specifically, these curves denotes the instantaneous tripping current, if fault
current crosses the designated mark
B class : 3-5 times the rated current.
C class: 5-10 "
D class: 10-15 "
"
"
"
"
"
"
o C- curves type are used for heavy currents in normal working.(e.g.
Heaters, Geysers, Irons, air conditioners)
o D-curves types are used for heavy inrush currents.(Motors etc)
I(single
Preferred
KW
phase)in
MCB rating in
0.5
2.558
3
1.0
1.5
2
5.115
7.673
amps
6
10
Amps
.849
1.698
2.547
Preferred
MCB rating in
Amps
3
3
3
10.23
16
3.396
6
3
15.345
20
5.094
6
4
20.46
25
6.792
10
25.575
32
8.49
10
2.5
3.5
4.5
5
24
Amps
I(3 phase) in
12.788
17.902
23.018
16
20
25
4.25
5.94
7.64
TableTable-1
-MCB ratings for different loads-
6
10
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Prevention of Fire arising from Short Circuit in Wiring Installations
PicPic-9(source9(source-www.google.com)
7.6 MCB specification
Poles available: 1,2,3,4
Tripping characteristics: B,C,D
Nominal voltage 1 pole: 230V-400V ac
Nominal voltage multipole: 400V ac
Nominal frequency: 50Hz-60Hz
Maximum voltage: 400V ac
Minimum voltage: 12V ac
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Short circuit capacity: 4.5KA, 6KA, 10KA ( dependent on current rating or type)
Storage temperature: -40 to 70 degree Celsius
Operating temperature: -25 to 55 degree Celsius
Table-2
o A sample price list of MCBs
26
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Table-2A
0 A sample price list of MCBs (continued)
Merely installing MCBs in a consumer’s electrical installation will not serve the
purpose at all. MCBs of proper ratings are to be installed. Table – 1 given earlier
gives an idea about the ratings of MCBs for different loads. The problem which
may arise due to using MCBs of improper rating is discussed later on in this
project.
27
Prevention of Fire arising from Short Circuit in Wiring Installations
8.
2012
Moulded case circuit breaker(MCCB)
MCCB is used for protecting of low-voltage electrical wires and connected
equipment, against overloads and short-circuits. It is generally installed on
secondary side of transformer stations (main switch or for protecting individual
branches). In industry we can use it in switchgears or for protecting the motors
as well we have big requirements in shipbuilding. For building applications you
can find it as main protection switch.
Pic-10, Source- www.google.com
Now days we can find two version of MCCBs on the market, one is with
thermalmagnetic meanwhile other is with microprocessor release.
Thermal-magnetic is working on the basis of bimetal and electro-magnetic
trigger. When you have overload current running through MCCB the trigger is using
bimetal’s nature and start to bend till circuit breaking.The bimetal trigger is calibrated at
45°C and for lower or higher temperatures of ambient we can expect longer or
shorter delay at breaking. For precise results it’s necessary to consider
compensation factor, which is provided from manufacturer. At short circuit we
have big electro magnetic field, which is a cause for reaction of electro-magnetic
trigger.
Microprocessor or electronic release is working on monitoring of current true
R.M.S value. It is simulated and calculated from peak values, which installed
microprocessor, can detect.
pic-11
Source-www. Google.com
28
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Electronic trip systems have allowed extremely fast recognition of high fault
current events, surpassing electro-mechanical trip systems for speed. Using
current sensors in each phase of a three pole circuit breaker, electronic trip
systems derive both their control power and current magnitude from these
sensors. Employing a microprocessor with embedded software, electronic trip
systems monitor the current magnitude for overload, short circuit and high
energy trip conditions. It is the third condition, high energy trips, that provide
the current limiting performance. With the ability to sense the rate-of-rise of the
current, di/dt as it is called; electronic trip systems allow end users to customize
the current limiting performance of the circuit breaker according to fault current
rate-of-rise.
Pic-12
Source- www.05abb.com
In Figure above, two trip currents are shown; curve 1 is a high energy fault
current, curve 3 is a lower level fault current. For the circuit breaker to respond
within the first half cycle, the rate of- rise, di/dt1 is calculated. This is compared to
the programming of the trip system, if the rate-of-rise meets the current limiting
trip criteria, the circuit breaker flux shifter is energized to trip the opening
mechanism.
The other fault condition, signified by curve 3 would not meet the criteria for
high speed instantaneous tripping; its rate-of-rise di/dt2 is much lower. Curve 2
represents the ‘letthrough’ current allowed by the current limiting circuit breaker
prior to complete interruption. Obviously the I2t, the area under that curve, is
29
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
much less than that of curve 1. The potential damage that could have resulted in
downstream devices served by a current limiting circuit breaker is, therefore,
significantly reduced. That capability plays a key role in the short circuit current
ratings of industrial control panels.
8.1 MCCB Ratings
a). Voltage Ratings
• Ue = Rated operational voltage
• Ui = Rated insulation voltage
•U imp = Rated impulse withstand voltage
b).Current ratings
• In = Rated current
• Ith = Conventional free –air thermal current
• Ithe = Conventional enclosed thermal current
c). Short Circuit ratings
• Icm = Rated short circuit making capacity
• Icu = Rated ultimate short circuit breaking capacity
• Ics = Rated service short circuit breaking capacity
• Icw = Rated short time withstand current
d). Trip unit ratings
• Ir = Adjustable overload setting current
• 1.05xIr = Conventional non –tripping current
• 1.30xIr = Conventional tripping current
• Isd = Short time delay tripping setting current
• Ii = Instantaneous tripping setting current
30
Prevention of Fire arising from Short Circuit in Wiring Installations
9.
2012
Wires
PVC is a plastic which is very widely used for LT insulations. Its poor
performance at elevated temperatures must be taken into account while thinking
reducing electrical fire hazards. Commercial PVC wire insulation is calcium
carbonate filled and this has a unique failure mode in modestly elevated
temperatures (110°c) can induce wet tracking.
Well-known factors leading to the ignition of PVC wiring include:
(a) manufacturing defects; (b) grossly excessive current; (c) over-insulation,
sometimes augmented by overcurrent; (d) localized heating due to strand
breakage; (e) localized heating due to mechanical strand severing by staples or
nails; and (f) localized heating due to failed terminations. Other failure modes
are known but have received only limited study. These include (i) excessive force
and creep; (ii) chemical interaction effects; and (iii) breakdown under voltage
surge conditions.
When PVC is used for wire/cable insulation, it is not used as a pure polymer.
Since intrinsically PVC is a rigid material and wire/cable insulation must be
flexible, there has to be a significant loading of a plasticizer.
A typical wire/cable formulations contain 52 –63% PVC resin, 25 – 29%
plasticizer, around 16% filler (but occasionally as low as5%), 2 – 4% stabilizer, 0.2
– 0.3% wax, and small amounts of lubricants and colorants; occasionally an FR
agent is also included. Antioxidants are also often included in small amounts
(less than 0.1%). The plasticizer is typically either a phthalate (e.g., diisodecyl
phthalate, ditridecyl phthalate) or a trimellitate, e.g., tris(2-ethylhexyl)
trimellitate, while CaCO3 and kaolin are common fillers. In recent years,
formulations also commonly include a costabilizer, typically zeolite (the
hydrated, sodium Type A variety) which also functions as an HCl absorber,
taking over that role from conventional fillers.
9.1 Selection of rated voltage and conductor cross-section:
Rated voltage is the voltage to which the insulation of a wire or cable is rated
according to defined operational characteristics.
Service voltage is the voltage which exists between two conductors at the
terminals of an installation.
Under steady state conditions the service voltage must not exceed the rated
voltage by more than 15%.
31
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Cables can therefore be used in three systems where the service voltage is not
higher than 1.15 times the rated voltage of the cable. In single phase and DC
systems with symmetrical voltage distribution, where the service voltage is not
higher than 1.15 times the rated voltage of the cable. In single – pole earthed
single phase or DC systems, where the voltage to earth during operation doesnot
exceed 1⁄√ 3х1.15=0.66 times the rated voltage of the cable
9.2 Current carrying capacity of PVC wires:
For ambient temperature up to 25°c the permissible current carrying capacity
and the rating of line protective fuses or line protective circuit breakers are given
in the following table –
Rated cross-section of
copper conductors( in
sq.mm)
0.75
1
1.5
2.5
4
6
10
16
25
35
50
70
Load capacity (A)
Fuse rated current (A)
16
16
20
20
25
25
34
35
45
50
57
63
78
80
104
100
137
125
168
160
210
200
260
250
Table-3
Where it is anticipated that the cable will be loaded for long periods at loads
higher than the tabulated values, a fuse or circuit breaker should be selected
having a lower rating than that shown in the table above.
The current carrying capacities given above do vary with the ambient
temperature as in indicated in the table below.
Ambient temperature in °C
5
10
15
20
25
30
35
40
45
50
32
Permissible loading in %
120
115
110
105
100
94
88
82
75
67
Prevention of Fire arising from Short Circuit in Wiring Installations
55
60
65
2012
58
47
33
Table-4
o Permissible loading in % of the values of the previous table
If several wires are bunched or placed close to other cables, the load capacity is
reduced relative to the ambient conditions. The permissible load capacity can
then be calculated using the conversion factors given in the following table.
Air temperature in °C
15
20
25
30
35
40
45
50
55
60
o
Conversion factors
1.17
1.12
1.06
1.0
0.94
0.87
0.79
0.71
0.61
0.50
Table-5
Conversion factors for various air temperatures
9.3 Ageing of PVC wires:
Estimating the life of a cable can only be approximate because of the obvious
difficulties in accumulating data. There is a general understanding that p.v.c.
cables with a continuous conductor operating temperature of 70°C have a life of
20 years. There is also a rough guide that for each 8 °C increase in core conductor
continuous operating temperature above 70 °C the life of the cable will be
halved. A p.v.c. cable running with an overload such that its core conductor
temperature is 78 °C will last for 10 years.
The general equation for ageing is:
Loge t = A/t + A'
where:
t = time in hours
T = absolute temperature K (273 + °C)
A = a constant 15 028 for PVC
A'= a constant –31.6 for PVC
The table in the next page provides further guidance -
33
Prevention of Fire arising from Short Circuit in Wiring Installations
o
2012
Table-6
Life until deterioration against conductor core temperature
Notes: 1. The values indicated are only orders of magnitude due to the different
types of materials and the great dispersion of the complex ageing phenomena of
these materials.
2 The temperature referred to is that of the cable conductor resulting from the
ambient temperature and its own temperature rise.
3 Permanent rating – load/temperature maintained 24 hours a day
4 Normal rating – load/temperature maintained 8 hours a day
5 PVC-polyvinyl chloride, EPR-ethylene/propylene rubber, PRC - chemically
reticulated polyethylene
Cable loadings are rarely constant, estimates can be made of the combined affects
of different loadings by the use of the formulae below:
where:
L1, L2 and L3 = lives at specific temperature
a, b, c, etc. = hours in day at these temperatures
1/L = (a/L1 +b/L2 + c/L3)
34
Prevention of Fire arising from Short Circuit in Wiring Installations
o
35
2012
Table-7
Causes leading to failure or ignition of PVC-insulated electrotechnical products
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
9.4 Advantages of FRLS or ZHFR
wires/cables over PVC wires/cables:
During a fire, ordinary PVC insulated wires give out thick black smoke and toxic
fumes of hydrochloric acid gas. This impairs visibility and hampers rescue
operations.
FRLS/ZHFR Cables not only emits very little smoke and toxic gases, but also
retards the spread of the fire. It is thus ideal for concealed and conduit wiring in
multi-storied high rise buildings such as hotels, banks, hospitals, factories,
commercial and residential complexes etc.
Electrical Safety is a function of five characteristics viz. smoke, hazardours gas
generation, rate of heat release, flame spread and rate of burning. In case of fire
in a closed space, trapped people are unable to find the exit due to emission of
thick black smoke and lose conciousness due to the inhalation of toxic fumes
before they can be evacuated to safety. The advantage of low smoke and low acid
gas generation are additional and critical features available with FRLS/ZHFR
wires in comparison with FR (Flame Resistant) wires which do not provide these
properties.
In case of FRLS wires the insulation & in case of LT 1.1KV grade cables the outer
sheath is specially formulated flame retardant low smoke type PVC Compound
having high oxygen & temp. index, with reduced emission of halogen & smoke.
The table in the next page shows a comparartive study of FRLS, ZHFR and PVC
compounds.
36
Prevention of Fire arising from Short Circuit in Wiring Installations
TEST
FUNCTION
TYPICAL
VALUES
OF
FRLS
COMPOUND
CRITICAL OXYGEN
Percentage of oxygen
INDEX
required
for
TYPICAL
OF
VALUES
HALOGEN
FREE COMPOUND
2012
TYPICAL
VALUES
OF ORDINARY PVC
COMPOUND
More
29%
than More
29%
than 23%
More
2500°c
than More
2500°c
than 1500°c
More
40%
than More
80%
than 10-15%
supporting
combustion
of
insulating material at
room temperature
TEMPERATURE
To determine at what
INDEX
temperature
oxygen
normal
content
of
21% in air support
combustion
of
insulating material
SMOKE
DENSITY
RATING
(Light
Transmission)
ACID
GENERATION
To
determine
the
visibility under fire of
insulating material
GAS
To
ascertain
amount
the
Less than 20%
of
Less
0.5%
than 45-50%
hydrochloric acid gas
evolved
from
insulation
of
cable
under fire
TABLE-8
o
Comparison of PVC, FRLS and ZHFR compounds
So we see from the above table that use of ZHFR (zero halogen fire retardant) or
FRLS(fire retardant low smoke) wires for electrical installations in fire prone
areas is a good idea to deal with electrical fire hazards.
37
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Nominal
Cross
Sectional
Area of
Conductor
No. / Nom. Dia.
Of Cond.
mm
Thickness
of
insulation
mm
Approx.
Overall
Diameter
mm
FRLS
ZHFR
Resistance
Max.
Ω / Km. @ 200
C.
0.50
16 / 0.20
0.6
2.20
4
5
39.00
0.75
24 / 0.20
0.6
2.45
7
8
26.00
1.00
32 / 0.20
0.6
2.50
11
13
19.50
1.50
30 / 0.25
0.6
2.90
13
16
13.30
2.50
50 / 0.25
0.7
3.55
18
20
7.98
4.00
56 / 0.30
0.8
4.30
24
26
4.95
6.00
84 / 0.30
0.8
5.20
31
36
3.30
10
140 / 0.30
1.0
6.70
46
50
1.91
16
224 / 0.30
1.0
7.80
62
66
1.21
25
350 / 0.30
1.2
9.70
80
84
0.780
35
490 / 0.30
1.2
10.90
102
105
0.554
50
396 / 0.40
1.4
13.20
138
141
0.386
70
360 / 0.50
1.40
15.3
214
218
0.272
95
475 / 0.50
1.60
17.9
260
265
0.206
120
608 / 0.50
1.60
19.4
305
310
0.161
150
750 / 0.50
1.80
21.9
355
368
0.129
185
925 / 0.50
2.00
24.5
415
420
0.106
240
1221/ 0.50
2.20
28.0
500
504
0.0801
Current Carrying
Capacity.
Table-9
o
38
Comparison of current carrying capacity of FRLS and ZHFR
Prevention of Fire arising from Short Circuit in Wiring Installations
Table- 10
o
A sample comparative price list of PVC, FRLS and ZHFR wires
Source-www.capitalcables.com
Table-11
o A sample price list of ZHFR wires of another manufacturer
39
2012
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
10. Earthing Practice
Circuits can be grounded for several different reasons. The most common is to
prevent having metal parts of an item coming into contact with electricity in the
event that insulation doesn’t work or fails.
There are two kinds of grounds required by Design Safety Standards for
Electrical Systems. One of these is called the "service or system ground." In this
instance, one wire-called "the neutral conductor" is grounded at the generator or
transformer . This type of ground is primarily designed to protect machines,
tools, and insulation against damage.
To offer enhanced protection to the workers themselves, an additional ground,
called the "equipment ground," must be furnished by providing another path
from the tool or machine through which the current can flow to the ground. This
additional ground safeguards the electric equipment operator in the event that a
malfunction causes the metal frame of the tool to become accidentally energized.
The resulting heavy surge of current will then activate the circuit protection
devices and open the circuit.
In electrical installations the following components must be earthed:
1)The frames,tanks, and enclosures of electrical machines, transformers,
apparatus, lighting fittings, and other items of equipments.
2)The frame works of switch boards, control boards, individual panel boards,
cubicles
40
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
11. Deviation from the ideal practice
Nowadays consumers are installing MCBs in their premises. But due to lack of
awareness or carelessness they sometime do install MCBs of some higher ratings
than prescribed. In doing so the purpose of overloading protection is not served
as desired.
So due to overloading the insulation of wires in the downstream degrades over
time. And if this continues then the insulation may fail resulting in a short
circuit.
Let us consider the situation shown below.
pic-13
Here all the consumers are having rated load of 20A. And consumer no.1 has got
a MCB of say 40A installed in his system. Now if a situation arises when
consumer no. 2 – 5 is not taking any supply and the consumer no.1 is drawing a
load of say 36 A . In this situation the main MCCB of 100A rating will not trip
and the consumer no.1 will continue to draw overload and in turn will damage
the insulation of the wires downstream which may further result in a short
circuit.
The current carrying capacity of wires and its insulation degrades over time due
to ageing. So the wiring installations should be of 20%-25% higher current
carrying capacity than the load demand.
41
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
12. Residual-current circuit breakers
A ResidualResidual-Current Circuit Breaker (RCCB) is a device that disconnects a circuit
whenever it detects that the current is not balanced between the energized
conductor(live conductor) and the return neutral conductor. Such an imbalance may
indicate current leakage through the body of a person who is grounded and accidentally
touching the energized part of the circuit. A lethal shock can result from these conditions.
Also if the leakage current exceeds 300mA-500mA then it may result in a spark which
may cause fire hazard. RCCBs are designed to disconnect quickly the affected circuit.
They are not intended to provide protection against overcurrent (overload) or short circuit
condition.
In the United States and Canada, a residual current device is most commonly known as
a ground fault circuit interrupter (GFCI), ground
ground fault interrupter (GFI) or an appliance
leakage current interrupter (ALCI). In Australia they are sometimes known as "safety
switches" or simply "RCD" and in the United Kingdom, along with circuit breakers, they
can be referred to as "trips" or "trip switches".
A two pole residual current device
Pic-14,Source-wikipedia
42
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
12.1 Purpose and operation
RCDs are designed to prevent electrocution by detecting the leakage current,
which can be far smaller (typically 5
5–30
30 milliamperes) than the currents needed
to operate conventional circuit breakers or fuses (several amperes). RCDs are
intended to operate within
ithin 25
25-40
40 milliseconds, before electric shock can drive the
heart into ventricular fibrillation, the most common cause of death through
electric shock.
1. Electromagnet with help electronics
2. Current transformer secondary winding
3. Transformer core
4. Test switch
L live conductor
N neutral conductor.
PicPic-15; (Source(Source- wikipedia)
Rcds operate by measuring the current difference between the live(hot) and
neutral conductor using a differential current transformer. If this doesnot sums up
to zero then there
ere is a leakage of current to somewhere else (to earth/ground, or
to another circuit), and the device will open its contacts.
When Alternating Current flows in a conductor, a magnetic field builds and
collapses around the conductor . . . the mo
more
re current flows, the stronger the
magnetic field.
43
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Pic-16 ; Source-www.google.com
A current transformer is a coil of wire, and when a conductor that carries an
Alternating Current is routed through the coil, the magnetic field around the
conductor induces a voltage in the coil. The stronger the current in the
conductor, the higher the induced voltage in the coil. A voltmeter can be
connected to the CT to measure the induced voltage.
Pic-17; Source-www.google.com
When both conductors of an AC circuit pass through a CT, the magnetic fields
around the conductors cancel each other, and no voltage is induced in the coil.
pic-18 ; Source-www.google.com
If one of the conductors of the circuit goes to ground, part of the current supplied
44
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
by the AC source will pass through one conductor, but not the other. This causes
an imbalance in the strength of the magnetic fields, they no longer cancel each
other, and a voltage is induced in the current transformer coil.
Pic-19;Source-www.google.com
A GFCI device uses a very fast acting, voltage sensitive switch to disconnect the
source voltage from the circuit, and prevent current from flowing in the
unintentional fault path.
Pic-20;Source –www.google.com
The use of GFCI devices for personnel protection against shock is required in
many residential locations including outside receptacles, basement and garage
receptacles, and bath and kitchen outlets with-in six feet of a sink.
12.2 Limitation:
Rcds cannot detect overload or short circuit condition and hence will not operate
unless the short circuit is from live to ground.
45
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Residual current devices with overload protection are devices which combines
RCDs and miniature circuit breakers which enables in detecting ground fault as
well as overload or short circuit condition.
Residual current and overcurrent protection may be combined in one device for
installation into the service panel; this device is known as a GFCI breaker
(Ground Fault Circuit Interrupter) in USA/Canada and as an RCBO (Residual
current circuit breaker with overload protection) in Europe.
12.3 Types of Ground Fault
Circuit Interrupters - GFCI:
Receptacle GFCI: This GFCI is used in place of a regular wall outlet or "duplex
receptacle". This GFCI is normally found throughout the house in places like
bathrooms, kitchens, garages, outdoor areas and other locations where damp
conditions do or could exist. The receptacle GFCI fits into the standard outlet box
and protects you against ground faults when an electrical product is connected to
the GFCI protected outlet. Modern homes use receptacle-type GFCls that protect
other electrical outlets connected on the branch circuit. Picture a bathroom outlet
upstairs not working because something tripped the GFCI in the downstairs
bathroom.
Temporary/Portable GFCI: When permanent GFCls are not practical, temporary
GFCls are used. Temporary GFCIs contain the GFCI circuitry in an enclosure
with plug prongs in the back and receptacle plugs in the front. It can be plugged
into an unprotected outlet, then, the electrical appliance/device is plugged into
the temporary GFCI. Portable GFCIs is simply an extension cord combined with
a GFCI. It adds flexibility in using receptacles that are not protected by GFCls.
Extension cords with GFCI protection incorporated are great for use when
permanent
or
portable
GFCI
protection
is
unavailable.
Circuit Breaker GFCI: Residences equipped with circuit breakers can have
circuit breaker GFCI protection installed in the panel box to give protection for
specific circuits The circuit breaker GFCI serves two functions. The circuit
breaker GFCI will shut off power to the circuit in the instance of a ground fault
plus the GFCI protected circuit breaker will turn power off if a short circuit or
overload is detected.
46
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
12.4 Technical characteristics
Number of poles
RCDs may comprise two poles for use on single phase supplies (two current
paths), three poles for use on three phase supplies (three current paths) or four
poles for use on three phase & neutral supplies (four current paths).
Rated current
The rated current of an RCD is chosen according to the maximum sustained load
current it will carry (if the RCD is connected in series with, and downstream of a
circuit-breaker, the rated current of both items shall be the same).
Sensitivity
RCD sensitivity is expressed as the rated residual operating current, noted IΔn.
IΔn
Preferred values have been defined by the IEC, thus making it possible to divide
RCDs into three groups according to their IΔn value.
•
•
•
High sensitivity (HS): 6 – 10 – 30 mA (for direct-contact / life injury protection)
Medium sensitivity (MS): 100 – 300 – 500 – 1000 mA (for fire protection)
Low sensitivity (LS): 3 – 10 – 30 A (typically for protection of machine)
Type
Standard IEC 60755 (General requirements for residual current operated
protective devices) defines three types of RCD depending on the characteristics
of the fault current.
•
•
•
47
Type AC: RCD for which tripping is ensured for residual sinusoidal alternating
currents
Type A: RCD for which tripping is ensured
o for residual sinusoidal alternating currents
o for residual pulsating direct currents
o for residual pulsating direct currents superimposed by a smooth direct
current of 0.006 A, with or without phase-angle control, independent of
the polarity
Type B: RCD for which tripping is ensured
o as for type A
Prevention of Fire arising from Short Circuit in Wiring Installations
o
o
o
o
2012
for residual sinusoidal currents up to 1000 Hz
for residual sinusoidal currents superposed by a pure direct current
for pulsating direct currents superposed by a pure direct current
for residual currents which may result from rectifying circuits
three pulse star connection or six pulse bridge connection
two pulse bridge connection line-to-line with or without phaseangle monitoring, independently of the polarity
Break time
There are two groups of devices:
•
•
G (general use) for instantaneous RCDs (i.e. without a time delay).
o Minimum break time: immediate
o Maximum break time: 200 ms for 1x I∆n, 150 ms for 2x I∆n, and 40 ms
for 5x I∆n
S (selective) or T (time delayed) for RCDs with a short time delay (typically used
in circuits containing surge suppressors)
o Minimum break time: 130 ms for 1x I∆n, 60 ms for 2x I∆n, and 50 ms for
5x I∆n
o Maximum break time: 500 ms for 1x I∆n, 200 ms for 2x I∆n, and 150 ms
for 5x I∆n
Let us consider the connection diagram shown below which will give us an idea about the
way of installation of RCDs combined with MCBs in consumer’s premises.
Pic-21, Source- www.engineersgarage.com
48
Prevention of Fire arising from Short Circuit in Wiring Installations
RCCB - ‘AC’ Type
Table-12
o A sample price list of RCCB – ‘AC’ Type
RCCB - ‘A’ Type
Table-12A
o
A sample price list of RCCB – ‘A-Type’
RCBO - A Type (SPN - 2M)
Table-13
o
49
A sample price list of RCBO- A Type(SPN-2M)
2012
Prevention of Fire arising from Short Circuit in Wiring Installations
RCBO - A Type (TPN - 4M)
Table-13A
A sample price list of RCBO – A Type(TPN – 4M)
50
2012
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
13. Arc Fault Circuit Interrupter (AFCI)
13.1 Why AFCI?
The AFCI was developed in order to eliminate certain unwanted arcs as
potential, electrical fire causes. Electric arcs operate at several thousand degrees
Celsius at their center. They also generate a pressure wave that will blow molten
metal or burning material from their center onto ignitable materials. Either the
high temperature or the materials discharged from the center of the arc can cause
a fire. The intent of the AFCI is to detect hazardous arcing and turn off the circuit
in order to greatly reduce the potential of fire from an arc.
Pic-22,Source- IAEI Blog.htm
Since an overcurrent protective device (OCPD), a circuit breaker or fuse, will
detect and interrupt an arc above the OCPD characteristic curve, circuits are
already protected against these higher current arcs. The AFCI addresses arcs
below and to the left of the characteristic curve of an overcurrent protective
device. The blue colored area in figure 1 represents the unprotected area in
which the AFCI detects potentially harmful arcs.
51
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
13.2 Arcing Fault Hazards:
We are all aware that arcs occur in useful circuits today. For example, arcs are
found in universal motors, light switches, and thermostatically controlled
appliances. Also, we find circuit signals that may look much like some arc
characteristics in other equipment and appliances. Computers, audio equipment,
light dimmers and many other devices with electronic controls generate “noisy”
or unusual signals that could appear to be an arc to some form of detection
devices. We wish for the circuit to continue to supply energy to these useful
items, but we want the circuit to be interrupted when a potentially hazardous arc
emerges. The AFCI must distinguish between useful conditions and hazardous
arcing. Hazardous arcing faults may occur in any of three configurations:
•
•
•
Line-to-neutral,
Series (such as in a broken or frayed wire or at a loose connection),
Line-to-ground.
So far, we know that the AFCI must detect a hazardous arc with current levels
up to those of an overcurrent protective device characteristic. It must be capable
of opening the circuit when one is detected. It must distinguish between
hazardous arcs and normal circuit conditions involving arcs or signals that look
like an arc.
Arc Recognition
The information in the figure
A shown alongside tells us
that voltage and current
waveforms will exhibit some
unique characteristics
including the following:
•
•
Flat “shoulders” in the
current around current
zero
Arcing current lower
than ideal current
Pic-23, Source- IAEI magazine
•
•
•
52
Voltage across the arc approaching a square wave
Voltage spikes each half cycle as the arc ignites and extinguishes
High frequency “noise”
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
13.3 How it works?
Pic-24,Source-IAEI magazine
The AFCI unit will use the circuit information to perform an analysis to
determine whether an arc is present and whether it is an arc that should be
interrupted. It will very likely be looking for several simultaneous indications of
arc presence and persistence in order to verify that the signal is from a hazardous
arc. On determining that a hazardous arc is in the circuit, the unit signals circuit’s
interruption.
With several experiments on various load devices and arc fault conditions a few
sample signature waveforms are developed which are different from normal
switch transients produced by many electrical devices. The AFCI operates with
microprocessor based technology.
Sensors are used to feed the current waveforms and these are compared with the
sample waveforms. Once the present waveform is identified with the sampled
arc waveforms, the circuit is interrupted.
13.4 Types of Arc Fault Circuit Interrupters – AFCI:
Branch/Feeder AFCI:
AFCI A device installed at the origin of a branch circuit or feeder,
such as at a panel board, to provide protection of the branch circuit wiring, feeder
wiring, or both, against unwanted effects of arcing. This device also provides
limited protection to branch-circuit extension wiring. It may be a circuit-breaker
type device or a device in its own enclosure mounted at or near a panel board.
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
Outlet Circuit AFCI:
AFCI A device installed at a branch circuit outlet, such as at an
outlet box, to provide protection of cord sets and power-supply cords connected
to it (when provided with receptacle outlets) against the unwanted effects of
arcing. This device may provide feed-through protection of the cord sets and
power-supply cords connected to downstream receptacles.
Combination AFCI:
AFCI An AFCI which complies with the requirements for both
branch/feeder and outlet circuit AFCIs. It is intended to protect downstream
branch-circuit wiring, cord sets and power-supply cords.
Portable AFCI:
AFCI A plug-in device intended to be connected to a receptacle outlet
and provided with one or more outlets. It is intended to provide protection to
connected cord sets and power-supply cords against the unwanted effects of
arcing.
Cord AFCI: A plug-in device connected to a receptacle outlet, to provide
protection to the power-supply cord connected to it against the unwanted effects
of arcing. The cord may be part of the device. The device has no additional
outlets.
13.5 Advantage of AFCI The arc fault circuit interrupter (AFCI), a circuit breaker designed to prevent fires,
is designed to open on intermittent resistive short circuits. For example, a normal
15 A breaker is designed to open circuit quickly if loaded well beyond the 15 A
rating, more slowly a little beyond the rating. While this protects against direct
shorts and several seconds of overload, respectively, it does not protect against
arcs– similar to arc -welding. An arc is a highly variable load, repetitively
peaking at over 70 A, open circuiting with alternating current zero-crossings.
Though, the average current is not enough to trip a standard breaker, it is
enough to start a fire. This arc could be created by a metallic short circuit which
burns the metal open, leaving a resistive sputtering plasma of ionized gases.
The AFCI contains electronic circuitry to sense this intermittent resistive short
circuit. It protects against both hot to neutral and hot to ground arcs. The AFCI
does not protect against personal shock hazards like a GFCI does.
Older homes with aging and deteriorating wiring systems can especially benefit
from the added protection of AFCIs. AFCIs should also be considered whenever
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Prevention of Fire arising from Short Circuit in Wiring Installations
adding or upgrading
circuit conductors.
a
panel
box
while
using
existing
2012
branch
13.6 Typical wiring details for AFCI circuit breakers
Pic-25 ; Source –www.inspectapedia.com
The light green arrow points to the AFCI device - you'll notice that it is much
longer than conventional circuit breakers in the panel. Each AFCI breaker
involves three electrical connections:
1. The red arrow indicates the "hot" wire connection at the AFCI breaker.
2. The yellow arrow indicates the circuit neutral wire connection at the
AFCI, and
3. the white arrow shows that the multi-strand coiled wire from the AFCI
device is connected to the neutral bus in the electrical panel.
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Prevention of Fire arising from Short Circuit in Wiring Installations
2012
14. Making a fire prone building
safe from electrical fires:
During my project period, I went through the consumption pattern of an old
multistoried building (7 storied). And after visiting the place I tried to chalk out a
rough idea about the amount of FRLS wire and the rating of MCBs required.
Below is given the steps followedo According to the maximum demand the rating of the main MCB is
chosen
o A rough idea of the length and cross-section of the main wiring is
taken
o The amount of load in each room of a particular consumer is noted
and the rating of MCB for that room is chosen accordingly
o Separate MCB of proper ratings are chosen for air conditioners,
geysers if present
o A rough idea of the length and cross-section of the sub circuit
wiring is taken
(The cross-section of the wires chosen is more than required at present keeping
in mind the possibilities of load augmentation in future)
The length of wires required and the number of MCBs required with proper
rating along with their cost is given in the annexure given later.
Total no.
of
consumers
Total
no. of
MCBs
Cost in
Rs.
Total
No.of
RCCBs
Cost
in
Rs.
Total
no. of
coils
of 6
sq.mm
wire
37
393
1,25,030
4
8570
10
56
Cost
in Rs.
63,213
Total
no. of
coils
of 4
sq.mm
wire
56
Cost in
Rs.
3,58,929
Total
no. of
coils
of 2.5
sq.mm
wire
148
Cost in
Rs.
Total labour
cost in
Rs.(Approx.)
3,73,428
3,00,000
Prevention of Fire arising from Short Circuit in Wiring Installations
2012
15. Conclusion
We see that electrical fire hazards can be minimized to a great extent by
using the several safety devices and also maintaining sincerity regarding
the use of electricity.
During inspections in the fire prone buildings we have seen that the
electrical installations are not provided proper maintenance. Here are
listed some common problems observed in the fire prone buildings.
o Jumbled up and undressed wirings
o Fuse protection still prevailing
o If MCBs are installed, then also some are of over rating
Below is listed a few suggestions which can help minimize electrical fire
upto a great extent –
o Get rid of fuse protective system
o Use MCBs instead of fuse of proper rating by taking suggestions from
a professional
o Use FRLS wires in new installations
o For new installations use RCBs along with MCBs for a better level of
protection
o Branch circuits and plug – and – socket devices must not be inserted
between the beginning of a cable circuit and the overcurrent protective
device
o Avoid jumbled up wirings
o Always maintain and dress the undressed parts of wiring
o Raise public awareness
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Prevention of Fire arising from Short Circuit in Wiring Installations
References
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o
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o
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o
o
o
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o
58
www.wikipedia.org
www.engineersgarage.com
www.louyeh.com
www.inspectapedia.com
www.doctorfire.com
www.nfphampden.com
www.lccables.com
Siemens Electrical Installations Handbook
2012