Touch and step potential when a fault occurs,the flow of current to ground results in voltage gradient on the ground surface in the vicinity of grounding system.This voltage gradient may affect a person in two ways through step or foot-to-foot contact and hand to both feet or touch contact. Resistance of the human body is … Read more
Soil Resistivity Definition Before the design of protective earthing,it is necessary to understand the nature of the resistance of ground and the various factors which govern it.When an electrode is driven into the ground,the region around the electrode is known as resistance area or potential gradient area and the fault current tries to flow away … Read more
Measurement of Earth Resistance Potential fall method is used for determining resistance between the earthing plate and the surrounding ground in distribution system.The resistance area of an earth electrode is the area of soil around the electrode within which a voltage gradient measurable with commercial instruments exists. E = earth electrode under test,and A = … Read more
Resonant grounding or arc suppression coil grounding In the arc suppression coil method of grounding,the arcing-ground danger has been eliminated and the system is approximated to the isolated neutral system,in which one or two healthy phases continue to supply power and complete shutdown on the system is avoided till the fault was located and isolated. … Read more
Resistance grounding This type of grounding is used to limit the earth fault current.In this type of neutral grounding a resistor is inserted between the neutral and earth to limit the fault current to a safe value.The resistor used for the neutral grounding is usually a metallic resistance mounted on insulators in a metallic frame.For … Read more
Grounding Transformer The neutral or star point is usually available at every voltage level form generator or transformer neutral .However if no such point is available due to delta connections or if neutral point is desired on bus-bars,a Zig-Zag transformer is most commonly used .Zig-Zag transformer has no secondary .Zig-Zag transformer is core type transformer … Read more
An ordinary dc series motor ,when connected to single-phase ac supply,operates,bu not very satisfactorily.The reasons for this are as follows: The torque developed is not of constant magnitude,as in dc series motors,but pulsates between zero and maximum values each half cycle because the field and rotor currents both reverse every half cycle. The alternating filed … Read more
Isolator Isolator is a mechanical switch which isolates apart of circuit from system as required. Electrical isolators separate a part of the system from rest for safe maintenance works. Isolator is a manually operated mechanical switch which separates a part of the electrical power system at off load condition. Isolator in a switchyard Types of … Read more
DOL(Direct online starter) At the starting instant,a 3-phase induction motor acts as a poly phase Transformer having an air gap with a short-circuited secondary.If the motor of higher capacity(larger than 2 kW) is connected directly to the supply mains,it will draw heavy current(about 5-8 times the rated full-load current) at the starting instant which is offensive … Read more
Circuit breaker testing CB testing All electrical equipment’s need to be tested for assurance of their satisfactory performance. However, testing of circuit breakers is more difficult than the testing of other electrical equipment’s because the short circuit currents are very high. Testing of circuit breaker can be classified into two main groups. • Type test … Read more