Monday, March 17, 2014
Wednesday, March 12, 2014
Selection of an instantaneous, or short-time-delay, tripping threshold
Tripping unit Applications
Low setting
type B
§ Sources producing low short-circuit- current levels (standby generators)
§ Long lengths of line or cable
Standard setting
type C
§ Protection of circuits: general case
High setting
type D or K
§ Protection of circuits having high initial transient current levels (e.g. motors, transformers, resistive loads)
12 In
type MA
§ Protection of motors in association with discontactors (contactors with overload protection)
Copyrights :
http://www.electrical-installation.org/enwiki/Selection_of_a_circuit-breaker
Choice of a circuit-breaker
Choice of a circuit-breaker
The choice of a CB is made in terms of:
· Electrical characteristics of the installation for which the CB is intended
· Its eventual environment: ambient temperature, in a kiosk or switchboard enclosure, climatic conditions, etc.
· Short-circuit current breaking and making requirements
· Operational specifications: discriminative tripping, requirements (or not) for remote control and indication and related auxiliary contacts, auxiliary tripping coils, connection
· Installation regulations; in particular: protection of persons
Low Voltage Circuit Breakers
Based on IEC 60947-2 (LV switchgear and controlgear - Part 2: Circuit breakers):
In (Rated current)
The rated continuous / uninterrupted current that the circuit breaker can carry.
Icm (Rated short-circuit making current)
The short-circuit current that the circuit breaker can withstand as it is closing where the act of closing initiates the fault.
Icu (Rated ultimate short-circuit current)
The maximum symmetrical short-circuit current the circuit breaker can interrupt.
Ics (Rated service short-circuit current)
The breaking capacity such that the circuit breaker is tested to carry its current continuously. The test sequence verifies that the breaker can be returned to service. Ics is the maximum current the breaker can interrupt multiple times and be returned to service without being damaged and is expressed as a % of Icu.
Icw (Rated short time withstand current)
This is the steady state symmetrical fault current the breaker has to be able to carry for a duration of 0.05s to 3s without exceeding its thermal integrity.
Tuesday, March 11, 2014
Enclosure Powder coating RAL codes & chart
RAL is a colour matching system used in Europe. In colloquial speech RAL refers to the RAL Classic system, mainly used for varnish and powder coating but nowadays there are reference panels for plastics as well. Approved RAL products are provided with a hologram as of early 2013 to make unauthorised versions difficult to produce. Imitations may show different hue and colour when observed under various light sources.
Tuesday, March 4, 2014
Medium Voltage Design Guide (Schneider Electric)
DOWNLOAD :
http://www.schneider-electric.ie/documents/panelbuilders/en/shared/technical-ressources/mv_design_guide.pdf
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.
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