Thursday, July 17, 2014
Friday, June 27, 2014
Hot dip galvanizing and corrosion categories
In which corrosion categories could galvanized steel be used? This question is frequently
asked, when corrosion protection only is specified to a certain corrosion category. Such a
specification is not enough, since corrosion categories give quite wide ranges for the
environmental exposure of the steel structure without specifying corrosion protection or life
time. More information is needed as basis for a good decision.
Corrosion in different environments
Atmospheric exposure
The corrosion rate of a zinc coating is affected by the time for which it is exposed to wetness, air
pollution and contamination of the surface, but the corrosion rates are much slower than for steel and
often decrease with time. General information on the atmospheric corrosion rate for zinc is given in
ISO 9224.
Table 1 gives basic groups of environments (related to ISO 9223). Where the relative humidity is
below 60 %, the corrosion rate of iron and steel is negligible and they may not require zinc
coating, e.g. inside many buildings. When the relative humidity is higher than 60 % or where they
are exposed to wet or immersed conditions or prolonged condensation then, like most metals, iron
and steel are subject to more serious corrosion. Contaminants deposited on the surface, notably
chlorides and sulphates, accelerate the attack. Substances that deposit on the surface of the iron
and steel increase corrosion if they absorb moisture or go into solution on the surface of the iron
and steel. The temperature also influences the corrosion rate of unprotected iron and steel and
temperature fluctuations have a stronger effect than the average temperature value.
The micro-environment, i.e. the conditions prevailing around the structure, is also important because it
allows a more precise assessment of the likely conditions than study of the basic climate alone. It is
not always known at the planning stage of a project. Every effort should be made to identify it
accurately, however, because it is an important factor in the total environment against which corrosion
protection is required. An example of a micro-climate is the underside of a bridge (particularly over
water).
The corrosion of steelwork inside buildings is dependent upon the internal environment but in “normal”
atmospheres, e.g. dry and heated, it is insignificant. Steelwork in the perimeter walls of buildings is
influenced by the configuration within the perimeter wall, e.g. steelwork without direct contact with the
outer leaf of a wall comprising two parts separated by an air space is at less risk of corrosion than
steelwork in contact with or embedded in the outer leaf. Buildings containing industrial processes,
chemical environments, wet or contaminated environments should be given special consideration.
Steelwork which is partially sheltered, e.g. farm barns and aircraft hangars, should be considered as
being subject to the exterior environment.
Table 1 sets out an indication of the likely range of corrosion rates which are applicable to zinc
coatings exposed to the different types of corrosively category dealt with in ISO 9223.
Table 2 indicates the life to first maintenance for hot galvanized steel exposed to the corrosively
categories C3 to CX. For C1 and C2 hot dip galvanizing normally gives very long life times, and could
be used without further analyze.
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Tuesday, June 24, 2014
Monday, June 23, 2014
Saturday, June 21, 2014
Friday, June 20, 2014
Thursday, June 19, 2014
Flame retardant meterial
Flame retardants are compounds added to manufactured materials, such as plastics and textiles, and surface finishes and coatings that inhibit, suppress, or delay the production of flames to prevent the spread of fire. They may be mixed with the base material (additive flame retardants) or chemically bonded to it (reactive flame retardants. Mineral flame retardants are typically additive while organohalogen and organophosphorus compounds can be either reactive or additive.
Friday, June 6, 2014
Wednesday, May 28, 2014
IEC Contactor Application Ratings (Cuttler Hammer Technical Data )
DOWNLOAD :
http://support.automationdirect.com/docs/iec_contactors.pdf
Friday, May 23, 2014
MOTOR CONTROLS / Electrical Symbols and Line Diagrams
Download :
MOTOR CONTROLS
http://css.wsu.edu/wp-content/uploads/2012/09/Motor-Controllers1.pdf
Electrical Symbols and Line Diagrams
http://abe.ufl.edu/tburks/Presentations/ABE5152/Electrical%20Symbols%20and%20Line%20Diagrams.pdf
Thursday, May 8, 2014
Industrial Control Wiring Guide
Industrial Control Wiring Guide
DOWNLOAD :
Wednesday, May 7, 2014
Monday, May 5, 2014
Enclosure design tips handbook
DOWNLOAD :
http://www.digikey.com/Web%20Export/Supplier%20Content/BUD_377/PDF/Bud_handbook.pdf?redirected=1
Wednesday, April 23, 2014
Monday, April 21, 2014
Hot rolling & Cold rolling steel
Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products, such as slabs, blooms, and billets. If these products came from a continuous casting operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces and for smaller workpieces induction heating is used. As the material is worked the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor a finishing temperature is defined above the recrystallization temperature; this is usually 50 to 100 °C (90 to 180 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling.
Hot rolled metals generally have little directionality in their mechanical properties and deformation induced residual stresses. However, in certain instances non-metallic inclusions will impart some directionality and workpieces less than 20 mm (0.79 in) thick often have some directional properties. Also, non-uniformed cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as I-beams. While the finished product is of good quality, the surface is covered in mill scale, which is an oxide that forms at high-temperatures. It is usually removed via pickling or the smooth clean surface process, which reveals a smooth surface. Dimensional tolerances are usually 2 to 5% of the overall dimension.
Hot rolled mild steel seems to have a wider tolerance for amount of included carbon than cold rolled, making it a bit more problematic to use as a blacksmith. Also for similar metals, hot rolled seems to typically be less costly.
Hot rolling is used mainly to produce sheet metal or simple cross sections, such as rail tracks. Other typical uses for hot rolled metal includes truck frames, automotive wheels, pipe and tubular, water heaters, agriculture equipment, strappings, stampings, compressor shells, truck frames, railcar components, wheel rims, metal buildings, railroad-hopper cars, doors, shelving, discs, guard rails, pipe and tubular, automotive clutch plates.
Cold rolling
Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used.[2] Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.
Cold-rolled sheets and strips come in various conditions: full-hard, half-hard, quarter-hard, and skin-rolled. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction.Skin-rolling, also known as a skin-pass, involves the least amount of reduction: 0.5-1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield point phenomenon (by preventing Lüders bands from forming in later processing). It locks dislocations at the surface and thereby reduces the possibility of formation of Lüders bands. To avoid the formation of Lüders bands it is necessary to create substantial density of unpinned dislocations in ferrite matrix. It is also used to breakup the spangles in galvanized steel. Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required.
Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small. Cold rolling shapes requires a series of shaping operations, usually along the lines of sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing.
If processed by a blacksmith, the smoother, more consistent, and lower levels of carbon encapsulated in the steel makes it easier to process, but at the cost of being more expensive.
Typical uses for cold rolled steel include metal furniture, desks, filing cabinets, shelves, tables, chairs, motorcycle exhaust pipes, computer cabinet and hardware, all home appliances and components, shelving, lighting fixtures, hinges, tubing, steel drums, lawn mowers, electronic cabinetry, lighting fixtures, water heaters, metal containers, and a variety of construction related products.
Wednesday, April 9, 2014
Tuesday, April 8, 2014
EMC Environment
Assemblies can emit and the must be immune to external
electromagnetic disturbances. IEC defines two categories
a) Environment A - relates to low-voltage non-public or industrial
networks / locations / installations including highly disturbing sources.
b) Environment B - relates to low-voltage public networks such as
domestic commercial and light industrial locations / installations.
This environment does not cover highly disturbing sources such as arc welders.
The specifier should detail a requirement for either Environment A or B.
In exceptional applications, for example, some rail applications, it is
necessary to specify a higher level of immunity.
Monday, April 7, 2014
Internal Arc inside Switchboard
Arc Proof metal-clad switchgear and controlgear is designed
and manufactured to prevent the occurrence of internal faults.
If the switchgear and controlgear is installed, operated and
maintained following the instructions of the manufacturer, there
should be little probability that an internal arc occurs during its
entire service life, but it cannot be completely disregarded for
hazard risk assessment.
An internal arc fault, which constitutes a hazard, if operators
are present, though extremely rare might occur due to reasons
such as failure of insulation, contacts due to ageing, overvoltages
in system because of switching or lightning surges, pollution
due to environmental conditions, mal-operation or insufficient
maintenance.
Engineers and site managers have a legislated “duty of care” to
make proper equipment selection, operating procedures and
service conditions. The effectiveness of the selection, at
providing the prescribed level of protection of operators in case
of an internal arc, can be verified by type testing.
Designs which have been successfully type tested qualify as
IAC classified. This classification is intended to offer a tested
level of protection to operators in the vicinity of the equipment
in normal operating conditions and with the switchgear and
controlgear in normal service position, in the event of internal
arc.
Other enhanced measures may be adopted to provide the
highest possible level of protection to operators in case of an
internal arc. These best practices measures are aimed to limit
the external consequences of internal arc;
a) Rapid fault-clearance times initiated by arc detect sensors or
by a busbar protection.
b) Application of fault current limiting fuses to limit the let-through
current and fault duration.
c) Fast elimination of arc by diverting it to metallic short circuit
by means of fast sensing and fast closing devices (arc terminator).
d) Remote control to allow operators to stay outside arc flash
boundary.
e) Pressure relief device.
f) All operations behind type tested arc proof doors.
Normal operating conditions means the conditions of metalclad
switchgear and controlgear required to carry out operations
such as opening or closing HV switching devices,
connecting and disconnecting withdrawable parts, reading of
measuring instruments and monitoring equipment, etc. Therefore,
if to perform any of such operations any cover has to be
removed and/or any door has to be opened, the test shall be
carried out with the cover and/or door removed.
Removing or replacing active components (for example, HV
fuses or any other removable component) are not considered
to be normal operations, neither those required to carry out
maintenance works.
The Internal Arc Classification IAC makes allowance for internal
overpressure acting on covers, doors, inspection windows,
ventilation openings, etc. It also takes into consideration the
thermal effects of the arc or its roots on the enclosure and of
ejected hot gases and glowing particles, but not damage to
internal partition and shutters not being accessible in normal
operating conditions.
Saturday, March 29, 2014
Current carrying capacity of busbars
The current-carrying capacity of a busbar is usually determined by the maximum temperature at which the bar is permitted to operate, as defined by national and international standards such as British Standard BS 159, American Standard ANSI C37.20, etc. These standards give maximum temperature rises as well as maximum ambient temperatures.
BS 159 stipulates a maximum temperature rise of 50°C above a 24 hour mean ambient temperature of up to 35°C, and a peak ambient temperature of 40°C.
ANSI C37.20 alternatively permits a temperature rise of 65°C above a maximum ambient of 40°C, provided that silver-plated (or acceptable alternative) bolted terminations are used. If not, a temperature rise of 30°C is allowed.
A very approximate method of estimating the current carrying capacity of a copper busbar is to assume a current density of 2 A/mm2 (1250 A/in2) in still air. This method should only be used to estimate a likely size of busbar, the final size being chosen after consideration has been given to the calculation methods. Refer catalogue of manufacturers.
The more popular thumb rule being followed in India is to assume current density of 1.0 Amps / Sq.mm for Aluminium and 1.6 Amps for Copper for any standard rectangular conductor profile.
Current carrying capacity of Cu busbar
| Size in mm | Area sqmm | Weight/ km | current carrying capacity in amp ( copper ) at 35 deg.C | |||||||
| AC ( no. of bus) | DC ( no. of bus) | |||||||||
| I | II | III | II II | I | II | III | II II | |||
| 12X2 | 24 | 0.209 | 110 | 200 | 115 | 205 | ||||
| 15X2 | 30 | 0.262 | 140 | 200 | 145 | 245 | ||||
| 15X3 | 75 | 0.396 | 170 | 300 | 175 | 305 | ||||
| 20X2 | 40 | 0.351 | 185 | 315 | 190 | 325 | ||||
| 20X3 | 60 | 0.529 | 220 | 380 | 225 | 390 | ||||
| 20X5 | 100 | 0.882 | 295 | 500 | 300 | 510 | ||||
| 25X3 | 75 | 0.663 | 270 | 460 | 275 | 470 | ||||
| 25X5 | 125 | 1.11 | 350 | 600 | 355 | 610 | ||||
| 30X3 | 90 | 0.796 | 315 | 540 | 320 | 560 | ||||
| 30X5 | 150 | 1.33 | 400 | 700 | 410 | 720 | ||||
| 40X3 | 120 | 1.06 | 420 | 710 | 430 | 740 | ||||
| 40X5 | 200 | 1.77 | 520 | 900 | 530 | 930 | ||||
| 40X10 | 400 | 3.55 | 760 | 1350 | 1850 | 2500 | 770 | 1400 | 2000 | |
| 50X5 | 250 | 2.22 | 630 | 1100 | 1650 | 2100 | 650 | 1150 | 1750 | |
| 50X10 | 500 | 4.44 | 920 | 1600 | 2250 | 3000 | 960 | 1700 | 2500 | |
| 60X5 | 300 | 2.66 | 760 | 1250 | 1760 | 2400 | 780 | 1300 | 1900 | 2500 |
| 60X10 | 600 | 5.33 | 1060 | 1900 | 2600 | 3500 | 1100 | 2000 | 2800 | 3600 |
| 80X5 | 400 | 3.55 | 970 | 1700 | 2300 | 3000 | 1000 | 1800 | 2500 | 3200 |
| 80X10 | 800 | 7.11 | 1380 | 2300 | 3100 | 4200 | 1450 | 2600 | 3700 | 4800 |
| 100X5 | 500 | 4.44 | 1200 | 2050 | 2850 | 3500 | 1250 | 2250 | 3150 | 4050 |
| 100X10 | 1000 | 8.89 | 1700 | 2800 | 3650 | 5000 | 1800 | 3200 | 4500 | 5800 |
| 120X10 | 1200 | 10.7 | 2000 | 3100 | 4100 | 5700 | 2150 | 3700 | 5200 | 6700 |
| 160X10 | 1600 | 14.2 | 2500 | 3900 | 5300 | 7300 | 2800 | 4800 | 6900 | 9000 |
| 200X10 | 2000 | 17.8 | 3000 | 4750 | 6350 | 8800 | 3400 | 6000 | 8500 | 10000 |
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