Selective coordination with fuses can be achieved by adhering to ?.
a. current-limiting fuses
b. electrical monitoring systems
c. emergency standby systems
d. selectivity ratios
The correct answer and explanation is:
Correct Answer: d. selectivity ratios
Selective coordination with fuses can be achieved by adhering to selectivity ratios. Selective coordination refers to the ability of an electrical system to isolate only the portion of the circuit that experiences a fault, allowing the rest of the system to remain operational. This is important for maintaining continuity of service, especially in critical applications like hospitals, data centers, and industrial processes.
When using overcurrent protection devices like fuses, proper coordination is necessary to ensure that only the fuse closest to the fault blows, rather than upstream devices. This is done by selecting fuses with time-current characteristics that are coordinated based on their ratings and their response to overcurrent conditions. The method used to achieve this is by following selectivity ratios, which are guidelines or tables provided by fuse manufacturers.
A selectivity ratio is the minimum ratio of the ampere ratings of two fuses—one upstream and one downstream—needed to ensure proper coordination. For example, if a downstream fuse is rated at 100 amperes, and the selectivity ratio with an upstream fuse is 2:1, then the upstream fuse must be rated at least 200 amperes to ensure selective coordination.
Fuse manufacturers often provide coordination tables and time-current characteristic curves to help electrical engineers and designers apply these principles effectively. These resources show how fuses will behave under various fault conditions and help ensure that the correct fuse operates at the right time.
By following selectivity ratios and using properly rated and coordinated fuses, designers can create systems that improve reliability, minimize downtime, and enhance safety. Options like current-limiting fuses help reduce damage but do not, by themselves, ensure selective coordination unless used according to selectivity ratios. Therefore, answer d is the most accurate.