How the proliferation of upstream devices has made them necessary (News)

FOR A MOTOR, "RATED CURrent" means only a couple of clearly defined numbers: full-load amperes, and locked-rotor amperes. Except under certain transient conditions, those are the only two values of importance to the application. With upstream switching, control, and protection devices, however, it's not so simple.

A motor is an end-of-the-line energy sink. The only current flowing to it is what results from the applied voltage and motor internal impedance. In contrast, a circuit breaker, fuse, or switch -as well as the conductors themselves-is subjected to whatever a downstream load (or a circuit fault) may demand. Hence, such devices can have several different current ratings depending upon the nature of that load.

The basic rating is continuous current, the value of amperes intended to flow through the device. (This is sometimes described as "operational current" or "thermal current.") When we speak of a "100 ampere fuse," for example, we mean that a load drawing 100 amperes continuous current is the maximum for which that fuse is intended to provide fault protection. How long is "continuous"? "More than three hours," according to one definition.



Although we tend to think of "inductive" and "resistive" circuits as two separate entities, the distinction is only one of degree. Whether a-c or d-c, all real circuits contain both resistance and inductance. Opening a current-carrying circuit will therefore always result in an arc across the opening contacts. How intense-and therefore how damaging-that arc may be depends upon the relationship between resistance and inductance, expressed by the circuit power factor. (Rating News)

As long as the current (and therefore the arc intensity) does not exceed a design level, a fuse or circuit breaker will snuff out the arc before too much damage is done. Current at that point is the interrupting capability of the device. Sometimes called the breaking current, it will far exceed the basic "continuous current rating."



It's often assumed that this is the amount of current that will actually cause the fuse or breaker to open the circuit. That's not the case. As the National Electrical Code explains, in Article 100, this rating is "The highest current at rated voltage that a device is intended to interrupt under standard test conditions." Those test conditions are outlined in UL standards. Suppose a voltage source can theoretically supply 50,000 amperes to a short-circuit. The complete circuit including that fault must have an impedance that includes the connecting wire from the power source to the breaker being tested, and that impedance will reduce the actual current the breaker sees during the test to something less than 50,000 amperes. Thus, there may be no contradiction in applying a circuit breaker tested to "only" 40,000 amperes, for example, in a power system capable of supplying 50,000 short-circuit amperes (see Figure 1 for typical relationship).



Two different interrupting ratings may be assigned. In a medium-voltage power circuit breaker, for example, the full magnitude of electromagnetic stress exists immediately upon initiation of a fault. That imposes a short-time or momentary current limit, such as 60,000 amperes for a 1200 ampere unit. The thermal stress accumulates with time, leading to a lower interrupting current limit of 35,000 amperes. (Low-voltage breakers operate so quickly that this duality does not exist.)



Whenever abnormally high current flows through a circuit component, that component is subject to two kinds of damage. The first is the combination of thermal stress and mechanical force created by the flow of current. Depending upon the device and the applicable standard (usually UL or NEMA), a withstand rating exists to define the maximum amperes that can flow without causing such damage to the device itself. Any switch or contactor not intended to clear a fault must have a withstand rating with which upstream protective device behavior must be coordinated.



Interestingly, if you look up withstand rating in the index to the National Electrical Code, in the IEEE Dictionary, in power system design literature, or in industry standards for various devices, you won't find the term listed. One definition of this rating is: "the ability ... to withstand the let through energy of a short circuit protective device under fault conditions. . . ." Some documents do mention surge voltage withstand, which has nothing to do with current.



Adding confusion to all this is the wording of one UL control equipment standard in which a table of "interrupting current" values is referred to in the text as "withstand values."



A second source of damage to any switching device (and its surroundings) is contact arcing. This applies only to devices intended to open the circuit either when a fault exists or when a control function dictates either opening a closed current-carrying contact, or closing an open one in a "live" circuit.

(Peace News) 

Tata, the Indian conglomerate and one-time MG Rover partner, is developing plans for what will be the world's cheapest car. (News)

To be launched in 2008, the 'people's car' will cost just 100,000 rupees (pounds 1,300) and is set to become the Model T Ford for the 21st century.

The vehicle will be aimed at the Indian market where car ownership is low. Tata hopes to price the car cheaply enough to tempt people off their bicycles and motorcycles and on to four wheels.

The project is the brainchild of Ratan Tata, chairman of India's largest conglomerate. He plans to defy his many critics, who claim that a car cannot be mass-produced so cheaply.

While the car will hardly be sporty " with an engine expected to be around the 600cc mark " Mr Tata has told his designers that the vehicle must not be seen as a stripped-down version of a normal car. (Car News)

To keep costs to a minimum, many of the body parts of what is known as the '1 Lakh Car' will be made out of plastic. And Tata is exploring the possibility of selling the car in kit form, with final assembly of the vehicles to be completed in rural workshops in India.

The designs for the car remain on the drawing board. A Tata spokesman said: 'The car is still in development. Because car ownership is so minimal in India, it will be aimed at this market.'

Paul Newton, automotive expert at the research group, Global Insight, said: 'Tata's strategy is to get people off their Honda bikes and into cars. India is a unique market as the market for cars under $10,000 [pounds 5,600] accounted for 70 per cent of the growth last year.'

Last year there were more than one million cars sold in India. The growth in sales has been fuelled by rising disposable incomes and the willingness of India's banks to lend money for car purchases.



While small, cheap cars are the big driver in the Indian market, luxury car makers such as Mercedes-Benz are also targeting the country's increasingly affluent middle class.



Tata's ambitions to make an ultra-cheap car have been called into question by its rivals. One of the doubters is India's largest car maker, Maruti Udyog, which currently makes the subcontinent's cheapest and best-selling car, the M800, selling for around 200,000 rupees.



Korea's Hyundai has also raised questions about Tata's plans.



The spokesman for Tata said that the company had no plans to sell the 1 Lakh Car in the UK.



Nevertheless, it is understood that it is considering launching new models in the UK on top of its limited range of mainly commercial and 4x4 vehicles.



In 2003, Tata formed a partnership with MG Rover, which saw an Indian- built car rebadged as a CityRover. But the car was panned by its critics for being unsophisticated and, as a result, it failed to sell in large volumes.



Tata does, however, have a major stake in the UK consumer market as the owner of Tetley Tea, which it bought five years ago for pounds 271m.



Copyright 2005 Independent Newspapers UK Limited  (Online News)

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