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1. What is a Short Circuit and or Relay Coordination Study?

A. Short Circuit Study is an electrical safety analysis of a power system, this study determines the faults occurred in an electrical system. Differentiate these calculations against the ratings of the equipment. It is the first step to ensuring that the electrical system is safely protected. After completion of knowing the short circuit faults, then determine the characteristics of the protective devices in the power system.

2. What is a breaker coordination study?

A. Breaker coordination study is also known as Protective Device Coordination study. This study includes checking of upstream and downstream protective device coordination to avoid the nuisance, to stop disturbances of tripping and avoid the plant shutdown. This study helps to the electricians to identify the causes of faults and make corrections.

3. What is protection and coordination?

A. In Protective Device Coordination studies, the term protection states that to protect the power system by stopping the tripping and plant shutdown. At the same time in electrical power distribution system, the coordination study is mandatory to analyze the characteristic curves of upstream and downstream breakers to avoid the tripping of fuses. These studies depend on present electrical safety standards and regulations like NFPA 70E, NEC, IEEE 1584 and OSHA.

4. What is a coordination study for?

A. As per the Electrical safety standards and Regulations, Overcurrent Protective Coordination study is very important to protect the downstream and upstream breakers to avoid the faults in a circuit breakers. This coordination study is mandatory to minimize the power system faults and successful fault isolation in a sequential manner.

5. What is coordination of protective devices?

A. In Electrical Distribution system or Transmission system commonly use circuit breakers and fuses to protect the equipment like Motors, Transformers, Conductors and other electrical equipment. If any kind of faults can occur within this components, this study can help to protect some portion of the system and minimize the failures within this system.

6. How to do relay coordination calculation?

A. As per the IEC 60255 report, the equipment safety requirements for the calculation of relays and protective devices ratings as shown below:
For A.C voltage up to 1000v with a rated frequency up to 65HZ and For D.C voltage up to 1500v
Especially, this standard requires electrical safety to minimize the risks of flashes and Hazards. The calculations of Over current relay coordination is based on the equations (Relay Characteristic Equation as per IEC 60255 standard) as follows:
For Standard Inverse Relay Characteristic equation (SI) 𝑡 = TMS× 0.14 I𝑟 0.02 – 1
For Very Inverse Relay Characteristic Equation (VI) 𝑡 = TMS × 13.5 I𝑟 – 1
For Extremely Inverse Relay Characteristic Equation (EI) 𝑡 = TMS × 80 𝐼𝑟 2 − 1
The above Relay coordination characteristics equations are used to do relay coordination calculation.

7. What is fuse coordination?

A. Fuse Coordination is also known as selective coordination. It is an important system to identify the faults. This study describes to draw the single line diagram of a power system in upstream protection system near to the fault area and clear the faults without affecting the fuses and also circuit breakers (protective Devices).

8. What is the purpose of selective coordination?

A. The purpose of selective coordination is to identify the faults in upstream fuses and breakers. It also used to minimize the effects by using single line diagram. In this study involves Electronic-trip and ANSI breakers, Molded-case breakers and Ground fault protection to clear the trappings of the entire upstream system.

9. What is the need of relay coordination?

A. In any distribution system, the relay coordination study is used to isolate the faults in the protective system within the short time to stop the fault causing minimum causing of a distribution system. Mainly protective study protects the upstream protective devices of the power system.

10. What is the importance of extremely inverse characteristic relay?

A. In Extremely Inverse Relay the Current Transformer saturation later stage as compared with Inverse Relay. Hence the characteristic remains inverse up to maximum range of fault current. The Extremely Inverse Relay equation describes


  I=Operating current
  t=Time Operation of the relay

11. What is protection coordination study?

A. Protective Relay coordination study is used isolate the faults and fault clearance sequence. This study required to select the protective devices like fuses and circuit breakers to specify the necessary settings to fix the faults that's why the intended target will be achieved. There is so many software to perform Protective Coordination functions in personal computers.

12. What is protective relay?

A. The relay which protects the upstream protective devices like fuses and breakers in the Electrical Distribution system is known as protective relays. The main purpose of the protective relay is designed to trip a breaker in the circuit to detect the fault. Which minimize the fault occurring time to save overall distribution system.

13. What is protective relay testing?

A. The Protective Relay Testing depends on the type of relay and determines by the recommendations of manufacturers and updated application considerations. First, the testing is starting to inspect of mechanical problems and recovery on each control element and timing curve of three points, Overcurrent protection and other control systems and final testing of the overall functioning system.

14. What is the best practice for testing protective relays?

A. Best practice for testing protective relays is based on the basic results of a protective device coordination analysis are the time-current coordination graphs that are plotted to illustrate the degree of selective coordination achieved in the system. Adjacent to each Single line diagram TCC curves are plotted. TCC curves show the device number, description, and settings for each device.

15. How to do relay coordination calculation?

A. Basics of Relay Coordination is as follows:
a) How the protective devices are working so well with each other.
b) The amount of power can be lost during a fault condition.
c) Power system equipment is protected or not.
d) The ratings and settings as we chosen cause nuisance trips or not.