ELECTRICAL EASY

As we know, both Snubber Circuit and Crowbar Circuit is used for protection against Overvoltage using Thyristor but they way they protect the Thyristor has some difference. Crowbar Circuit: Power supplies are normally reliable, but if they fail then they can cause significant damage to the circuitry they supply. To protect against this it is possible to add a simple SCR overvoltage crowbar protection circuit. Crowbar protection is a fail-safe protection mechanism which short circuits the output of a power supply under failure conditions such as over voltage. Here Fail-Safe means, in case of failure the system will go to safe condition. Crowbar protection can also refer to a circuit which has its sole purpose to cause a fuse to blow by subjecting it to high current. Figure below shows a typical Crowbar Circuit. Above Crowbar Circuit uses an 8V supply and has its overvoltage protection set at 9.1V but this overvoltage setting can be adjusted by changing to the zene...

Step potential is the voltage between the feet i.e. one step of a person standing near an energized grounded object. Touch potential is the touch voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object and a point some distance away. Now, the most important thing is to understand the physical meaning and significance of Step Potential and Touch Potential. For theoretical considerations, True Earth is a conductor deep in the ground with virtually no resistance. With this definition of True Earth , ground resistance can be thought of as the resistance between, a ground mat or substation grid and true earth. By this definition, the surface then is not true earth. Current can and will flow along the surface, but has a tendency to go deep. In the figure below, Ground Network is the ground Mat. Now consider a line to ground fault on a Transmission Tower in the Switchyar...

SCR / Thyristor is a four layer three junction device. It mainly consists of interleaved layers of P and N layers. It can block Voltage of either polarity but conducts current only in one direction that is from Anode to Cathode under normal working of forward biasing. Basically to forward bias an SCR we need to make Anode more positive than Cathode. So we can say that SCR is said to be forward biased. Even if it is forward biased without a suitable gate pulse the device does not turn on. We need a positive gate pulse to turn on SCR. On forward biased the junctions J1 and J3 are forward biased while J3 is reverse biased. To initiate the conduction at forward biased junction J2 we need a positive gate pulse to introduce charge carriers into junction J2. Under reverse biased condition the junction J1 and J3 are reverse biased and J2 is forward biased. The first N layer from Anode is actually an N layer with less concentration of charge carriers, so it will block about ...