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Monday, April 15, 2019

11:15 AM

DISTRIBUTION BOARD | DISTRIBUTION FUSE BOARDS


DISTRIBUTION BOARD | DISTRIBUTION FUSE BOARDS



DISTRIBUTION BOARD | DISTRIBUTION FUSE BOARDS


DISTRIBUTION FUSE BOARDS:


In the industries or in very big buildings, where a number of circuits are to be wired, the distribution fuse boards become a necessity.
They usually ironclad and are designed to provide a large space for wiring and splitting the circuits.
The fuse bank in the distribution board can easily be removed. Since there are a number of wires making connection to the various fuse at top and bottom, to facilitate this, the fuse bank can be titled.
Moreover, for further convenience of wiring, the cover plate, top and bottom plates can be removed easily. In the top place, knockouts are provided for fixing the conduits.
NEUTRAL LINKS:
With the help of a neutral link, the normal three-phase fuse board can be converted into 3-phase 4-wire circuits.
Such neutral terminal consists of a terminal for incoming neutral main and a three-way going circuit terminal, both are connected by link which can be detached for test purpose.
The metal components are mounted on high grade vitreous porcelain base.

MINIATURE CIRCUIT BREAKER:
 

The modern day practical it to use miniature circuit breakers which are available in sailing varying from 0.5 amps to 100 amps.
These can be available in sailing varying from 0.5 amps to 100 amps.
These can be single pole, double poles or three poles.

Miniature circuit breaker commonly known as MCB are final sub circuit protective device as it comprises of thermal overload and magnetic short circuit tripping devices.
The MCB can carry rated continuously but sustained overloading of 150% or above.
Can be detected by thermal overload device which will trip the MCB.
The thermal elevate prevents rapid resoling of a circuit while the overload still persists.

 PLANNING THE INSTALLATION: 


Until recently a building used to be designed and built before even the electrical services were considered
They were an afterthought and no one contemplated asking an electrical engineer for his opinion at the design stage.
Nowadays with the tremendous demand for electricity, the architect has to design a building with the main points to consider when planning the electrical installation of a building is given, together with practical advice and some typical layouts.

STEP TO CONSIDER AT DESIGN STAGE:


The first and foremost question when planning an installation is whether there is a supply of electricity available.
If there is , how near is it to the proposed building site, is it just across the road or a mile away?
If it is a long distance away, what are the charges to bring it to the site and how longest will take the Supply Company to make the connection?
Although most supplies are now standardized at 440 volts 3 phase a.c., and 250 volts single phase a.c., it is wise to check this.
Before approaching the Electricity Supply Company, the engineer should know the approximate loading of the installation, probable future requirements, and the times when the maximum power can be supplied all the time, or whether there will not be sufficient at peak periods etc.

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Wednesday, April 10, 2019

12:42 AM

JOBS IN PAKISTAN AERONAUTICAL COMPLEX, KAMRA

JOBS IN PAKISTAN AERONAUTICAL COMPLEX, KAMRA      


SITUATIONS VACANT 

Logistic / Accounts / Admin / Medical / IT / Software Officers and Technicians posts are vacant for employment on contract basis in PAKISTAN AERONAUTICAL COMPLEX KAMRA

Candidates will have to apply online, only on PAC website latest by 13 April 2019.

Apply online:www.pac.org.pk
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Tuesday, April 9, 2019

9:50 PM

EARTHING SYSTEM

EARTHING SYSTEM


EARTHING SYSTEM

 EARTHING SYSTEM
The meaning of earthing or grounding is to connect the electrical equipment to the general mass of earth by wire of negligible resistance.
This brings the body of the electrical equipment to zero potential and thus will avoid the sock to the operator. The neutral of the supply system is also solidly earthed to ensure equal to zero.
WHY IS EARTHING NECESSARY?
If all electrical equipment produced was totally insulated, then there would be no need for earthing.
Unfortunately up to now, insulated covers or casings have been expensive to produce and so metal has been used. The use of metal means that if a fault occurred, the metal could become alive at anything up to mains voltage.
HOW DOES EARTHING GIVE PROTECTION?
The usual method of earthing is to join to exposed metal work to earth to earth via an earth continuity conductor connected to an electrode burried in the earth.
In conjunction with a fuse, or other sillar device, this then forms a protective system. Thus if a live conductor accidentally comes into contact with any exposed metal, it is effectively connected to earth.
WHY IS EARTHING SO IMPORTANT?
Human life is involved where electrical apparatus is used, and an electric shock can be fatal.
Also a violent short circuit can easily start a disastrous fire on a premises.
These are well known facts that are often forgotten when electricity is being used every day.
A can be now, tomorrow, five years time, or possible even never.
It is because an emergency might never occur that an efficient earthing system is often neither installed nor maintained.
Many installations in pakistan have totally inadequate earthin systems, but this is not realized since a fault has not yet occurred.
It is only after someone has been electrocuted or a building is gutted by fire, that it is realized that little or no protection had been given by the protective system.
COMPONENTS / PARTS OF EARTHINGS SYSTEM:
(1) Earthing Continuity Conductor
(2) Earthing Lead or Earth Conductor
(3) Earth Electro ac
EARTH CONTINUITY CONDUCTOR
This is the part of the earthing system, which joinjs or bends together all the metal parts of an insulation.
Conduits, ducts, boxes, metal casings of switches fuse distribution  boards, regulating and controlling apparatus, exposed metal work machines and any metal frame work on which electrical apparatus is mounted.
EARTH LEAD OR EARTH CONDUCTOR:
The earth conductor is the conductor, which provides connection between earth electrode and earth connecting point (Main Earth Point). The earthing conductor must be short and straight with the minimum number of joints. Two forms of earthing conductors, depending upon the load, are commonly used.
these are copper wire and copper strip. Copper strip is used on very large installations.
The usual practice in Pakistan is use to hard drawn copper wire, In case of copper wire duplicating earthing conductor should be run up to the earth electrode in order to increase the protection of installation. If there are two earth plates there should be four earthing conductors.
The area of each of the earthing conductor should not be less than half the area of the largest current path.
When copper wire is installed, it should be enclosed in the G.I Pipes. This gives protection against mechanical damage and corrosion and also can used to guide water to the plate in order to keep the plate and surrounding artificially damped.
EARTH ELECTRODE:
An earth electrode provides connection between the installation metal work and general mass of earth. For domestic installation main water pipe was used as earth electrode. Big installation (over 21 kW) has to have its own earth electrode.
The most common type of electrode is plate of either galvanized iron or copper, Cast iron pipes can also be used in place but are not as efficient as plates.
The main purpose of an electrode is to provide good conductivity with earth at all time.
The way is to install the electrode well below water level. The plate should be placed in an upright position and surrounded by a bed of at least on ft.
REQUIREMENTS FOR EARTHING:
A complete protective system consists of two parts a protective device connected in the supply conductor (such as re-wireable fuse, or overload circuit breaker.
And  a low resistance earth return path bonded to all exposed metal work and earth.
A protective system must be able to deal efficiently with either one or all of the possible circuit faults .
METHODS OF EARTHING:
There are three methods of earthing.
 1.  EARTHING THROUGH A WATER MAIN
 2.  PIPE EARTHING
 3.  PLATE EARTHING:
EARTHING THROUGH A WATER MAIN
Before making an earthing connection to the water main, it must be ascertained that throughout G.I Pipes have been used, otherwise of the cement concrete popes have been used, the earthing will not be effective.
When making an earthing connection, care must be taken to limit the contact resistance to the minimum.
For that purpose properly designed earthing clamp should be used. The stranded copper lead is fanned out and is soldered to make it solid.
Then the lead strip is bent round the pipe so that it may be seated properly over the pipe.
The surface of the pipe is cleaned properly, and all traces or grease are removed and then over it is placed the clamp.
This method is however not popular as water means are of concrete or cement.

 PIPE EARTHING:
If the water-pipe cannot be used as an earth, a galvanized iron pipe of approved length and diameter can be sued. The size of the pipe depends upon (a) the current to be, carried (b) the type of soil.
According to I.S.I.  Standard No. 732-1963 the galvanized iron pipe shall not be less than 38.1 mm.
Diameter and 2m. Long for ordinary soil but if the soil is dry and rocky, the length of the pipe should be increased to 2.75m.

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Saturday, April 6, 2019

11:50 PM

ELECTRICAL PROTECTIVE DEVICES

ELECTRICAL PROTECTIVE DEVICES


ELECTRICAL PROTECTIVE DEVICES


ELECTRICAL PROTECTIVE DEVICES

In an electrical power station, when any thing becomes abnormal, it becomes predetermined time delay.

The action may be automatic and selective. The function can be fulfilled by using the protective devices like fuses or relays in the circuit from the damage to be occurred due to short circuit or overloads etc.

FUSE:
Fuse is a device used for protecting the cable in a circuit agiant damage from an excessive current.

FUSE ELEMENT OF FUSE WIRE:
It is the part of the fuse, which melts when an excessive current flows in the circuit , and thus isolates the device from the supply mains.

MAIN FEATURES OF GOOD PROTECTIVE DEVICES:
(i) Sensitivity. The protective system should be sensitive that it should operate for low values of fault current.

(ii) Selectivity. The protective system should select correctly the faulty part of the power system and disconnect the same without disturbing the rest of the system.

(iii) Reliability. The protective system should operate definitely under predetermined condition.

(iv) Quickness. The protective system should be such that it should respond quickly in order to improve quality of service, increase safety & life of equipment and increase stability.

FUSE:
Fuse is a wire of short length or thin strip of material having low melting point and is insertede in an electric circuit as protective device to the flow of an  excessive current through the circuit (i.e current greater than the maximum current allowed to flow through the circuit in normal condition).

MATERIAL:
Tin, lead zinc and allow of lead and tin, silver copper, copper, aluminum etc.
material can be used for fuse wire.

WORKING:
Under normal working conditons the current  flowing through the circuit is within safe limits but when some faults, such as short circuit occurs or when load more than limiting value the wire gets heated up, melts and breaks the circuit.
It thus protects a machine or apparatus from being damaged due to excessive current.

TYPES OF FUSE:

Following are the types of fuse, such as:
 1.  Kit-cat type or Rewirable fuse
 2.  Cartridge type fuse
 3. H.R.C (High Ruptuting Capacity) fuse.

KIT-CAT TYPE OR REWIRABLE FUSE:

This is commonly used in house wiring and power wiring up to 440 volts. It is made of porcelain in two parts  i.e base and fuse carrier.
The base is fixed with switchboard. The phase or incoming  wire is connected to its one terminal. The connection is taken from the other terminal is known as outgoing terminal. The two fuse wire can easily be easily be replaced if fuse blown out.

PARTS OF REWIRABLE FUSE:

1. Fuse carrier 
 2. Fuse carrier contact 
 3. Fuse element 
 4. Asbestos tube
 5. Fuse base contact 
 6. Fuse base

ADVANTAGES OF REWIREABLE FUSE:

1. It is the cheapest from of fuse.
 2. It requires minimum time to replace.
 3. It require no maintenance
 4. It has current limiting effect.

DISADVANTAGES:

Rewireable fuses suffer from the following disadvanatages:

 1. Oxidation of fuse wire and consequent thinning of wire section with lapse of time.
 2.  Loose connection causes the local heating.
 3. Heating radiating devices used in the circuit.
 4. Single phasing of three phase induction motors when one of the fuses is blown off.

CARTRIDGE TYPE FUSE:

This is usually enclosed type fuse. The fuse element or wire is enclosed in an 
insulating  container of tube shape. 
This is filled with power and sealed at its ends with metallic caps known as cartridge. It is not possible to rewire it.

It is available for voltage up to 660 volts and current rating 800 amperes. The powder provides good insulating path and helps to extinguish the arc at the time of blowing up of fuse. It is not very economical.

PARTS OF CARTDGE TYPE FUSE:

 1.  Fuse up 
 2. Cartige 
 3. Adaptor ring 
 4. Fuse base

H.R.C (HIGH RUPTURING CAPACITY) FUSE:

It modern power station, extermely heavy would flow due to the fault .
Therefore, HRC fuse units are used for such duties. Theses fuses are specially designed for extremely rapid operation.

PARTS OF H.R.C FUSE:

 1.  Filling powder 
 2. Bi-metal thermal control
 3. Fuse element 
 4. Fuse link contact 
 5. Brass end cap
 6. Cartridge 
 7. Outer element

It has ceramic body with metallic end caps. Fuse element is placed inside and welded to end caps.
Powder is filled in it, which helps in extinguish the fire.

ADVANTAGES OF H.R.C FUSE:

The H.R.C fuses posses the following advantages.

 1.  Simple and easy installation.
 2. No maintenance except periodic tightening of connections.
 3. High breaking capacity.
 4. Low initial cost compared to circuit breaker.
 5. Extreme reliability.
 6. Non-deterioration for long periods.
 7. Coordination and discrimination to a high degree.

DISADVANTAGES OF H.R.C FUSES:

 1.  After each operation, its replacement is required.
 2. Interlocking is not possible in these types of fuses.
 3. The lack relays incomplete discrimination.

SELECTION OF FUSE:

The following considerations may be kept in view while selecting a fuse.

 1.  There must be security agiant fire risk when the fuse blows.
 2. there should be good insulaing path for the fuse.
 3. Sufficient clearance between terminals.
 4. It should provide easy replacement of fuse wire.
 5. It should not operarte under conditons of mementary over loads and wiring switching surges which are experienced when starting a motor and switching on transformers, capacitors etc.
 6.  It must opearte when sustained overload or short-circuit occurs.

MINIATURE CIRCUIT BREKER:

The miniature circuit breaker, although a somewhat complicated piece of mechanism is gradually occupying the place of the fuse.
IT is being fitted in increasing numbres in distribution boards and consumer control units. Its action is based on the principle as fitted to control large industrial circuits, whereby a predetermined excess of current operates an electromagnet.
This in turn, through a switch mechanism, automatically permits the breaking current to be lower than an equivalently reated rewirable.

 1.  It can be made to give a close degree of small excess of current protection. This is indirect contrast to the rewireable fuse which may be wrongly wired with a heavier gauge fuse wire, so that a larger current will flow before the wire actually blows.
 2. If necessary for certain purposes the mechanism may be fitted with a time delay action.
 3. After a fault in the circuit has been cleared, the supply can very easily be restored by the simple toggle switch.

PARTS OF MINITURE CIRCUIT BRESKER:

 1.  Circuit breaker 
 2. Auxiliaty switch 
 3. Current transformers
 4. Battery 
 5. Realay 
 6. Trip coil

COMPARISION OF H.R.C FUSE AND CIRCUIT BREAKER:

(i) H.R.C fuse posses higher  rupturing capacity compared to circut breaker in the medium voltage range. Fuse with rupturing capacity of 75 MVA (breaking  current app. 100 KA) are available breakers with corresponding capacity will be too bulky.

(ii) H.R.C fuses are cheaper than the circuit breakers.

(iii) H.R.C fuse take less space and are simpler to install and maintain.

(iv) Circuit breaker are however, more advantageous where by the nature of location, enviroment and duty, the the fuses are likely to blow quite often, as replacement cost of fuses is high.

(v) It takes some time to replace fuses but a circuit breaker can be quickly operated.

TERMS GENERALLY USED:

The following are the definition of a few terms mostly used in the study of fuse.

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Tuesday, April 2, 2019

1:32 AM

Temp

Temp

Temp

Temp:
Condition for installation of cables in Pakistan are often quite different from those in Europe and America.
Thus wiring regulations used in these countries do not give much information or advice on installation of general wiring cables in extreme conditions.
Also the current derating factors used for cables operating at higher than normal temperatures unfortunately differ from country to country thereby causing confusion.
In order to produce a practical guide for given, based on temperature rise, which combine both the English and American recommendations.
The current rating for cables insulated in the four temperature ranges most common in Pakistan are listed in Tables 1 to 20.
From the Tables 1 to 20 it can be seen that hotter the conditions of installation, the less the current carrying of a cables. To understand this effect, two definitions must be given:

Ambient Temp: The temperature of air or material surrounding a cables.

Conductor Temp: The temperature of the conductors of a cables when
current is passed through it.

Thus conductor temp = ambient temperature + temperature rise to current.

All conductor have maximum continuous operating temperature above which the insulation starts to deteriorate, and the increased resistance of the heated conductor causes excessive volt drop.
If a cable has rated full load current passing continuously through it and the imbient temperature is raised, allowable value.
Also, if a fault occurs, resulting in a fault current not quite large enough to operate the protective device, then again the cable will heat up to beyond its maximum temperature.
For these resons the rated current of a cable has to be decreased if the ambinet temperature is high.

Example: 12½ h.p three phase squirrel cage induction motor of continuous rating Full load current stated on nameplate = 18 amps:
Declared supply voltage 440 volts.

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1:30 AM

WOODEN CASING AND CAPPING WIRING

WOODEN CASING AND CAPPING WIRING


WOODEN CASING AND CAPPING WIRING

WOODEN CASING AND CAPPING WIRING

The cables used in this type of wiring is eihter VIR or PVC or any other oppproved insulated cables.

The cables are carried through the wooden casing enclosures. The casing consists of V-shaped grooves (usually two to hold the cables of opposite polarity in different grooves) and is covered at the top by means of rectagular strip of wood, known as capping as capping of same width as that of casing.
The capping is screwed to the casing by means of 13 mm x 4 wooden screws fixed at every 15 cm on the center fillet.
To protect the casing against white ants first class seasoned teak wood varinshed by shellace varnish is employed.
Two or three cables of same polarity (either all phases or all neutrals) may be run in one groove.
The casing is usually placed 3.2 mm apart from the wall or ceiling by means of porcelain distance pieces of thickness not than 6.5 mm in order to keep the casing dry at the back.
The wooden gutties on which the casing is screwed by means of 32 mm x 8 wooden screws are fitted into walls or ceiling at intervals not exeeding 90 cm for sizes of caisng capping up to 64 mm exceeding 60 cm for sizes more than 64  mm. The sizes of the casing to be used depends upons the number and size of cables to be accommodated in any particular length of run.
The table II showing the number of cables of differnt sizes, which can be accommodated in each groove of differnt sizes of casing capping are given.
The length in which casing capping is available varies from 2.5 meters to 3 meters.

Advantages 
 1.  Chpeap in cost as compared to lead sheathed and conduit wiring system.
 2. Easy to instal and rewire.
 3. It provides good installation as conductors are good distance apart.
 4. Free from trouble of consdensation so advantageous in tropical countries where there might be trouble from condensation of moisture in steel conduit.
 5. Easy to inspect be opening the capping.

Disadvantages
 1.  This type of wiring even being coasted with paint or shellace varnish, is not damp proof so can not be used in damp places.
 2. Since there is a risk of fire,  so it can not be used where there is a possibility of fire hazard.
 3. This type of wiring can be used only on surface and can not be concealed in plaster.
Fields of application:
This type Of wiring of suitable for low voltage domestic installations in dry places and where there is no risk of fire hazard.

Precautions to be taken

 1.  The casing-capping used should be of first class seasoned striaght, smooth and hard teak wood.
 2. In no case the cables of opposite polarity should be run in one groove.
 3. The casing must be kept at least 3.2 mm apart from the walls or ceiling by means of porecelain distance pieces of thickness ont less than 6.5 mm in order to keep the casing dry at the back.
 4. The wooden gutties on which the casing is screwed must be fitted into walls or ceillings at an intervals not exceedings 90 cm for sizes of casing capping up to 64 mm and not excceding 60 cm. For sizes more than 64 mm.
 5. The casing should be well fixed fixed to its supporting structure i.e., walls or ceilings.
 6. Gaps should be avoided.
 7. At bends the grooves must be rounded off in order to save installation from damage.
 8. The joint whenever required should be made with good workmanship and in approved manner.
 9. All cappings, should be attached to the casings (after all insulated wirs are laid inside grooves) by round head screws (perferably of brass) fixed on edges and screwed to the outer walls of the casing at an interval not exceeding 15 cm cross-wise (i.e., 30 cm between successive screws one each side) for all sizes upto 64 mm casing and capping. For size above  64 mm similar additional round-head screws should be fixed on the center wall (or alternative walls in case of 3 grooves) at an interval of 45 cm.
10. While screwing capping over the casing precautions should be taken that screws go into the dividing wall of the casing otherwise they will damager the cable instalation.
11. The casing and capping should be painted with the paint as specified in I.S.732 before erection. It is also to be painted or varnished to give desired finish after erection.

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1:29 AM

INTRODUCTION TO CLEAT WIRING

INTRODUCTION TO CLEAT WIRING

INTRODUCTION TO CLEAT WIRING
INTRODUCTION TO CLEAT WIRING

INTRODUCTION TO CLEAT WIRING

In this arrangement of inner wiring the links utilized are either VIR or PVC type.

The links are held by porcelain spikes around 6 mm over the dividers or roofs.

The spikes are made in two parts, one base and the other top. The base is groved to suit the links and the top put over it and entire of it at that point screwed on wooden attachments (gutties) already comented into the divider and roof.

Along these lines the links are immovably grumbled between the two hales of the spikes and verified to the supporting divider and verified to the supporting divider and roof. The screws utilized are of size 38 mm.

The cross-area of the wooden gutties in 38 mm x 38 mm enormous end 25 mm x 25 mm at little and length is aabout 6.5 cm. The spikes utilized are diverse seizes and distinctive sorts so as to accomodate links of different sizes and diverse quantities of links separately.

The spikes are of three sorts one score, two notches and three forests to suit one, two and three links individually.

For low voltage (ap to V) establishments, spikes will be of such measurement that links will not be under 2.5 cm sport for branch circuits and under 4 cm separated for submains. So as to guarantee longer life to projection wiring frameworks, spikes ought to be normally utilized at interims of 30 cm and for no situation at additional

than 60 cm.

ADVANTAGES;

1. It is the least expensive arrangement of inner wiring.

2. Its establishment and disassembly is simple and fast.

3. Material is recoverable after the disassembly.

4. Assessment, shifts and increments can be effectively made.

5. Ability required is pretty much nothing.

DISADVANTAGES;

1. It isn't attractive.

2. It is very impermanent and perishes rapidly.

3. The wires are presented to mechanical damage.

4. The protection gets moistness from the environment and a typical salt like substance shows up on the protection which brings down the protection obstruction and cause spillage. Consequently this arrangement of wiring can not be utilized in soggy spots safety measures in an affirmed way are embraced to successfully keep clamminess from weakening the protection of transmitters.

5. Oil and smoke are wounds to VIR protection.

FIELDS OF APPLICATIONS: 

The wiring of this sort is entirely reasonable for transitory establishments in dry spots. This is likewise acknowledged where appearance isn't so imperative and efficiency is the principle thought. This framework isn't reasonable for use in local premises.

PRECAUTIONS TO BE OBSERVED:

1. The wooden gutties ought to be embedded in dividers with their more noteworthy x-segment towards the divider and ought to be separated at an interim not surpassing 60 cm vertically or evenly.

2. Appropriate sort of spikes ought to be utilized for example one way spikes ought to be utilized to convey one wire, two-way spikes ought to be utilized to convey two wires, etc.

3. For no situation two wires ought to be accomodated in one depression of porcelain cleasts.

4. For low voltage (upto 250 V) establishments, cleasts utilized ought to be of such measurements that links will not be under 2.5 cm separated in branch circuits and 4 cm separated in sub-mains.

5. The links must be laid extended between the spikes so that there is a contact with the divider or roof.

6. The links ought not be kept running close water funnels, gas channels and structureral work.

7. In fitting wiring, joint patterns or breaker patterns ought not be utilized for any reason. In the event that joints become unavoidable wooden intersection boxes with porcelain conductors inside might be utilized.

8. Every single wooden fitting, for example, sheets, squares and so forth. Ought to be of all around prepared teak wood or of some other endorsed protecting material and ought to be of twofold sort for example separate base and best, the wooden sheets ought to be all around varnshed on all sides (both inside and outside) and might be mounted with reasonable porcelain insulatores behind the sheets whenever wanted.

9. The links should nor be packed nor left free.

10. While drawing links, the protection ought not get harmed in light of undue winding or bowing.

11. Sharp curves ought to be stayed away from and the separating between the spikes under the twists must be diminished.

12. At the point when the links are to go through floors, dividers, allotments, ceillings or other hid areas, they should be helped through channels. While going through floors the channel ought to be proceeded to a tallness at least 1.5 meter over the floors. The channel ought to be appropriately earthed to evade any peril.

13. Wooden busings ought to be utilized at end of the courses.

14. A projection must be fixed near each finish of the courses and fittings.

15. Where links cross one another, they ought to be isolated by a protecting extension pipe piece, which will keep up a separation of at 1.3 cm between the links.

16. After fulfillment of work, two coatings of varnish ought to be given before puting it to utilize.l
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