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I2T: Intelligent Interface Technology
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∙ 11y ago
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Explain the significance of let through energy when assessing discrimination between major and minor fuses in a system of feeder protection?
I2t is imoprtant to know if a fuse that protects a whole branch is going to blow before one of its ramifications. So it is important that the fuse protecting the branch has bigger I2t than the branch. Another important fact is that the fuse protecting the branch has to be 1.6 bigger than the max nominal value of the current in the branch European Rule Of Thumb
How can you have a ground fault protection in a 3 phase 3W system?
3 phase ground fault detection on a 3 phase system requires that the vector summation of all three phase currents be measured. The vector sum of these currents should equal zero, or nearly zero for a balanced system. If a ground fault exists, then the vector sum will not equal zero. This condition must be validated using i2t techniques in order to eliminate false trips.
What is the dimension of resistance?
Dimension of resistance can be worked out by using Ohm's lawYou know V = IR or R = V/I....(1)The dimensional formalism of equation (1) can be written as follows[R] = [V]/[I] = [W/q]/[I] { since potential refers to work done(W) per unit charge }So [R] = [w]/[qI] = [W]/[I2T] { [I] = [q]/[T] where T stands for time }thus [R] = [ML2T-2]/[A2T] { dimension of current is taken as [A] }= [M1L2T1A-2] (ans)
What are the possible short circuit current rating of cables for different high voltages?
The short-circuit current rating of a cable is the thermal energy it withstands in the short time (normally berween 0,2 and 3 seconds) a short-circuit holds out, till it is limited by the opening of a circuit breaker or the melting of a fuse. This thermal energy strain is characterized in electrical engineering by the product I2t. It order to make this rating independent of the cross section A of the cable, we prefer to replace I by the normalized intensity strain I/A. The modified thermal withstand becomes now(I/A)2t. In this form it depends only on the conductor material, inversely on the initial temperature of conductor before the short circuit and directly on the final temperature of conductor after the short circuit. The last two temperatures depend on the material of the cable's insulation and on the normal operating current of the cable. All these are embodied in a factor K so that (I/A)2t=K. We can find values of K in form of tables or formulas in technical literature. According to the international standards, the coefficient K takes on the following values where K [(A/mm2)s1/2]:K=115 for copper conductors with PVC insulationK=135 for copper conductors with insulation XPLE or EPRK=74 for aluminum conductors with PVC insulationK=115 for the connections of copper conductors with tin solder The short-circuit rating of a cable is useful because we must compare this rating with the specific let-through energy I2t of the overcurrent protective device in correspondence with the maximum short-circuit current of the installation. If it is smaller, it is quite possible under short-circuit conditions that the cable will be damaged, although the protective device will open the faulted circuit without damage to itself.
