Sage-Tips

Why fuses can interrupt heavy short circuit current successfully?

A fuse will restrict fault currents to such low values that a high degree of protection is given to circuit components against even very high short-circuit currents. They permit breakers with lower interrupting ratings to be used.

What is discrimination of fuses?

Discrimination occurs when the protective device nearest to the fault operates, leaving all other circuits working normally. Fig 3.26 System layout to explain discrimination. {Figure 3.26} shows an installation with a 100 A main fuse and a 30 A submain fuse feeding a distribution board containing 10 A fuses.

Why fuse is not reliable protection?

1. Consistent performance. One downside of fuses is that their performance can degrade due to aging, even causing them to open during normal conditions. In addition, there is no way to test a fuse, so you will never know the exact current value that will cause it to open.

Why are fuses current-limiting?

Fuses that are current-limiting open severe short-circuits within the first half-cycle (180 electrical degrees or 0.00833 seconds) after the fault occurs. Current-limiting fuses also reduce the peak current of the available fault current to a value less than would occur without the fuse.

What is the purpose of fuse in a circuit?

The fuse breaks the circuit if a fault in an appliance causes too much current to flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily.

How does the fuse protect the circuit?

The fuse breaks the circuit if a fault in an appliance causes too much current to flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

What is circuit discrimination?

Circuit breaker discrimination (also known as selectivity) is the ability of the local circuit breaker to operate before any upstream circuit breakers under fault conditions. The intention is to limit the service interruption to the minimum amount for any fault, whether this fault is overload or short-circuit.

What is discrimination in circuit protection?

Discrimination is the coordination between the operating characteristics of circuit breakers placed in series. When the fault occurs in system only the circuit breaker placed immediately upstream of the fault will trip.

Why are time delay Class J fuses not recommended for short circuit protection?

The Time Delay Class J fuses do not provide adequate short-circuit protection as the available short-circuit current may not be high enough to clear the fuse within one-half cycle and therefore should not be used in this application.

What is the purpose of a fuse in a short circuit?

The primary role of the fuse is to prevent equipment fire, not to protect the equipment itself from damage. The available short circuit current at a given point in the electrical system can be determined by reviewing the components which are upstream from the electrical supply point.

What is the maximum fuse size allowed in a circuit?

According to the National Electric Code (NFPA-70 or CSA 22.1), per Article 450-3 (B) of NFPA-70 (similar statements can be found in CSA 22.1), the maximum fuse size is defined in table 450.3 (B) with a rating not greater than 125\% of the rated secondary current.

How does a short circuit protect a system from over current?

short circuit makes the short path voltage zero and huge amount of current flows through that path. Fuse can protect a system from over current . Short circuit is a significant cause of over current in the system .