DOWNLOAD :
http://www.schneider-electric.ie/documents/panelbuilders/en/shared/technical-ressources/mv_design_guide.pdf
Tuesday, March 4, 2014
Determining Temperature Rise
The temperature rise inside a sealed cabinet without forced ventilation can be approximated as follows.
First calculate the surface area of the enclosure and, from the expected heat load and the surface area, determine the heat input power in
watts/ft.2
Then the expected temperature rise can be read from the Sealed Enclosure Temperature Rise graph. Find where the input power
intersects the line for the enclosure material and read the approximate expected temperature rise at the left.
example:
What is the temperature rise that can be expected from a 48 x 36 x 16 in. painted steel enclosure with 300 W of heat dissipated within it?
solution:
Surface Area = 2[(48 x 36) + (48 x 16) + (36 x 16)] ÷ 144 = 42 ft.2
Input Power = 300 ÷ 42= 7.1 W/ft.2
From the Sealed Enclosure Temperature Rise graph:
Temperature Rise = approximately 30 F (16.7
Heat Dissipation in sealed electrical enclosures
heaT DissipaTion in sealeD elecTrical enclosures
The accumulation of heat in an enclosure is potentially damaging to electrical and electronic devices. Overheating can shorten the life
expectancy of costly electrical components or lead to catastrophic failure.
enclosure maTerials
The following discussion applies to gasketed and unventilated enclosures. Higher temperature rises can be expected with unfinished
aluminum and unfinished stainless steel enclosures due to their material's less efficient radiant heat transfer. Non-metallic enclosures
have similar heat transfer characteristics to painted metallic enclosures, so the graph can be used directly despite the difference in
material.
enclosure surface area
The physical size of the enclosure is the primary factor in determining its ability to dissipate heat. The larger the surface area of the
enclosure, the lower the temperature rise due to the heat generated within it.
To determine the surface area of an enclosure in square feet, use the following equation:
Surface Area = 2[(A x B) + (A x C) + (B x C)] ÷ 144
where the enclosure size is A x B x C in inches.
This equation includes all six surfaces of the enclosure. If any surface is not available for transferring heat (for example, an enclosure
surface mounted against a wall), that surface's area should be subtracted. Note: Enclosure volume cannot be used in place of surface area.
enclosure heaT inpuT
For any temperature rise calculation, the heat generated within the enclosure must be known. This information can be obtained from the
supplier of the components mounted in the enclosure.
enclosure TemperaTure rise (ΔT)
research has shown for every 18 f (10 c) rise above normal room
temperature 72 - 75 f (22 - 24 c), the reliability of electronic
components is cut in half.
The temperature rise illustrated by the curves in the Sealed
Enclosure Temperature Rise graph is the temperature difference
between the air inside a non-ventilated and non-cooled enclosure
and the ambient air outside the enclosure. This value is described
in the graph as a function of input power in watts per square
foot. In order to predict the temperature inside the enclosure,
the temperature rise indicated in the graph must be added to the
ambient temperature where the enclosure is located
Thursday, February 27, 2014
Tuesday, February 25, 2014
Terms AC1 and AC3
AC-1 - This category applies to all AC loads where the power factor is more than 0.95. These are primarily non-inductive or slightly inductive loads, such as heating. Breaking the arc remains easy with minimal arcing and contact wear.
AC-3 - This category applies to squirrel cage motors with breaking during normal running of the motor.
On closing, the contactor makes the inrush current, which is about 5 to 7 times the rated full load current of the motor.
On opening, the contactor breaks the rated full load current of the motor.
Monday, February 24, 2014
Cross Reference (Approximate) NEMA, UL, CSA, vs. IEC Enclosure Type
(Cannot be used to convert IEC Classifications to NEMA Type numbers)
Thursday, February 20, 2014
Understanding Type 2 Coordinated Protection in Motor Branch Circuits
The new IEC (International Electrotechnical Commission) standard, publication
947 “Low Voltage Switchgear and Control, Part 4-1: Contactors and Motor Starters,”
has been recognized by UL (Underwriters Laboratories) and is becoming
widely accepted by designers and users of motor control in the U.S. This standard
addresses coordination between the branch circuit protective device and the motor
starter. It also provides a method to measure performance of these devices if a short
circuit occurs. This standard defines two levels of component protection in the
event of a short circuit: Type 1 and Type 2 coordination.
This Product Data Bulletin describes:
_ How to conformto the new standard using motor controls built to meet
NEMA and IEC standards
_ Related benefits associated with Type 2 coordination
The IEC standard for motor starters and contactors, 947-4-1, defines two levels of
protection/coordination for the motor starter (contactor and overload relay) under
short circuit conditions. Each level of protection is achieved by using a specific
combination of motor starter and short circuit protective device.
_ Type 1 Coordination
Under short circuit conditions, the contactor or starter shall cause no danger
to persons or installation and may not be suitable for further service
without repair and replacement of parts.
_ Type 2 Coordination
Under short circuit conditions, the contactor or starter shall cause no danger
to persons or installation and shall be suitable for further use. The risk
of contact welding is recognized, in which case the manufacturer shall indicate
the measures to be taken in regards to equipment maintenance.
Faults in electrical systems are most likely to be of a low level, which are handled
well by motor controllers built to meet Type 1 coordination standards. After the
fault is cleared, the only action necessary is to reset the circuit breaker or replace
the fuses. In situations where available fault currents are high and any period of
maintenance downtime is crucial, a higher degree of coordinated protection may
be desirable.
Many industries are dependent upon the continuous operation of a critical manufacturing
process. In these conditions, it is especially important to understand that
Type 1 protection may not prevent damage to the motor starter components. In order
to ensure that high level fault or short circuit does not interrupt a critical process,
it may be prudent to consider implementation of Type 2 coordination in the
selection and application of low voltage motor controllers.
Type 2 coordination, which has no equivalent U.S. standard, does not permit damage
to the starter beyond light contactwelding, easily separated by a screwdriver or several
coil operations. Type 2 coordination does not allowreplacement of parts (except fus-
es) and requires that all parts remain in service. Beyond providing basic electrical and
fire protection, it also minimizes lost production, reduced productivity and unscheduled
disruptions resulting fromdowntime needed to replace or repair a starter.
SQUARE D Product Data Bulletin
Why Are Copper Bus Bars Plated?
Even though copper is the most popular choice for use in bus bars, and used very often in other electrical applications because it is more resistant to rust and corrosion than other metals, this doesn’t mean that it won’t oxidize over time.
When metals oxidize, the resistance in the conductive metal will increase, requiring more power to be used to carry current along the surface. When the copper oxidizes beyond a certain point, the metal can begin to flake and fall apart.
Many metals are plated in order to help them retain their positive qualities and attributes. When it comes to copper bus bars, plating is an important factor in longevity as well as maintaining the integrity of the conductive surface. When copper bus bars are not plated, over time the surface will oxidize. When that occurs, then more power is required to push electricity along the surface because the oxidized surface simply doesn’t conduct as well as a smooth, plated surface.
Plating, using tin or silver acts as a coating over the surface of the copper, helps to protect the copper from oxidizing. While this will not completely prevent oxidizing over a long period of time, it will dramatically reduce the effects of such oxidization. The reason why tin and silver is commonly used in the plating technique for copper is that both metals are considered soft metals, easier to work with when plating, and more importantly they don’t offer a great deal of resistance to electrical conductivity.
Which is better? Tin or Silver?
Throughout the industry there are different thoughts about which metal is better for plating copper, tin or silver. 10 microns of tin will outperform 1 micron of silver. With the price of silver climbing, tin becomes more economical, even though ten times the amount of tin will be required to do the same job.
When using silver to plate copper bus bars, a minimum of 3 microns should be used, and preferable 6 microns. On top of that, an anti-tarnish would need to be applied as well to protect the finish. In most fixed bus bar applications, tin is recommended. Silver should be used for moving bus bar parts in which arcing may be a concern.
For both tin and silver plating, anti-tarnish is important to keep the surface clean and conductive. When working with copper bus bars, plating is essential not only for longevity, but also integrity and safety.
Copyright :
http://blog.prv-engineering.co.uk/2012/05/why-are-copper-bus-bars-plated/
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