LVQ Panel Questions - Low Voltage Fuses

Define the Primary Purpose of a Fuse and explain how it is different from a circuit breaker

Protects electrical circuits, equipment, and potentially individuals by interrupting the flow of electricity in a current when it becomes at an unsafe level

Differences - Fuses are not resettable, Fuses open much quicker than circuit breakers, aprox. 1/8th of a sine-wave cycle (1/8 of 1/60th of a second), Fuses do not have mechanical parts, circuit breakers do

Explain Components of Fuse and How it functions

Element, Filler, Body, Inner and Outer Caps

A fuse is a carrier of electrical current which has a special function to interrupt the passage of current flow whenever there is an overcurrent in its path, whether that be a sustained overload, fault, or short circuit in the wiring, equipment, or electrical devices.

When the fuse blows, heat from the arc causes the boric acid to evolve large volumes of gases. The associated high pressure (often greater than 100 atmospheres) and cooling gases rapidly extinguish (quench) the resulting arc. The hot gases are then explosively expelled out of the end(s) of the fuse.

Explain Differences between the Classes of Fuses

Explain Characteristics of a Duel-Element Fuse

See Print Out Chart

Explain the Purpose and function of a “rejection” fuse

To prevent the wrong type/ fuse to be installed in the fuse holder

Explain the “Interrupting Rating” of a fuse and why important

Specifies the maximum amount of current a protective device can cut off safely without harm to personnel or resulting damage to equipment, the premises or the device itself. For example, a circuit breaker that trips "safely" successfully interrupts the fault, can be reset and will function properly afterward.

• You don’t want them to blow up.

Explain the term "Current Limiting" with respect to fuses and why important.

A current-limiting fuse is one that opens and clears the current in less than ½ of a cycle (0.0083 sec.).

If a current-limiting fuse is used, the maximum instantaneous peak current is limited to a small fraction of the peak current that could possibly flow if the fuse were not used.

Explain what "single phasing" in a three phase system is.

The term “single-phasing” is the term used when one phase of a three-phase system opens.( Fuse could be blown) On the primary side or secondary side of a distribution transformer. Three phase motors, when not individually protected by three time-delay, dual-element fuses, or three overload devices, are subject to damaging over currents caused by primary single-phasing or secondary single-phasing.

Using appropriate PPE, demonstrate the ability to safely check a motor branch circuit fuse.

Energized work permit, PPE, Verify meter on known live circuit, test circuit, re-verify meter or remove fuse and check resistance (ohms).

Demonstrate the ability to properly respond to a blown fuse in a motor branch circuit.

Determine the cause of the overcurrent condition. Shut off power. Only need to replace the blown fuse

Explain the purpose of overcurrent device coordination.

Overcurrent protection schemes are generally designed with a primary means of clearing a fault, as well as one or more backup methods. Where possible, it is preferred that instantaneous methods of detecting overcurrent be used as the primary protection method on all of the major equipment associated with the power system. Instantaneous clearing of a fault is desirable:

• To minimize the damage due to the fault,

• To minimize the potential exposure of personnel to the hazards of an arc flash

• To avoid long clearing times that could result in the entire system becoming unstable, resulting in a complete loss of power to the system. Instantaneous methods of relaying generally include differential, pilot wire, and impedance relays. Backup protection is generally accomplished with time overcurrent relays and impedance relays with a time delay.

Good objectives for clearing times are:

• That the primary protection clear a fault in less than 0.25 seconds for the maximum available fault current and

• That the backup protection clear a fault in less than 1.00 seconds for the maximum available fault current.

Faster clearing times are desirable whenever it is possible for the three reasons given above.

Explain the difference between Type 1 and Type 2 motor branch circuit overcurrent protection.

Coordinated protection of the branch circuit protective device and the motor starter is necessary to insure that there will be no permanent damage or danger to either the starter or the surrounding equipment.

Type 1. Considerable damage to the contactor and overload relay is acceptable. Replacement of components or a completely new starter may be needed. There must be no discharge of parts beyond the enclosure. In addition, the enclosure must not become energized and discharge of parts from the enclosure is not permitted. See figure below.

Type 2. No damage is allowed to either the contactor or overload relay. Light contact welding is allowed, but must be easily separable. Only extremely current-limiting devices have been able to provide the current-limitation necessary to provide verified Type 2 protection. In most cases, Class J, Class RK1, or Class CC fuses are required to provide Type 2 protection.