Circuit breakers and fuses

In the most common design, a fail-passive arrangement reduces the system to its lowest energy level. The system will not operate again until corrective action is taken. Circuit breakers and fuses for protection of electrical devices are examples of this type of fail-safe device. Solenoid valves (see Figure 11-3), such as this one on a steam control valve which is configured fail close shuts off instrument air, are another example. 

Fire/Explosion Circuit Breakers and Fuses Safety Analysis/USQ Issues... 

Panelboards provide for local protection and control of apparatns and lighting. They are usually composed of integrated groups of circuit breakers and fused switches providing protection for circuits terminating in their immediate area. 

Circuit breakers and fuses are installed in circuits to operate in the event of excess current arising from overload conditions and faults. The most common type of fault is an earth fault, but it is frequently the case that the current flowing due to earth faults is too low to operate the overcurrent protection devices. In addition, the overcurrent protective devices will not operate in the event of somebody making direct contact with a live conductor the current which flows through the body to earth will be too low to operate the devices but will often be high enough to cause fatal electric shocks. These two problems can be obviated by the use of earth leakage protection devices. 

Ensure that circuit breakers and fuses are adequately rated for the potential fault level and can safely and rapidly interrupt the short circuit current. 

Moulded case circuit breakers and moulded case switches up to 1000 V a.c. Rating more than 5000 A Moulded ease circuit breakers and their application, up to 1000 V a.c. Rating 5000 A and more Low-voliage cartridge fuses... 

Capacitors, circuit breakers and HRC fuses must be selected with care for use with capacitor circuits, with contractors chosen on the basis that the capacitor current can rise by 25 per cent above nominal line current. Equally, HRC fuses for capacitor applications should be de-rated by a factor of 1.5. 

Service entrance equipment should be installed in an indoor electrical room whenever possible. The equipment should include one to six main service entrance power circuit breakers or load break disconnect switches and feeder circuit breakers for loads within the plant. Circuit breakers and disconnect switches for 480/277 V systems that are rated at 1000 A or more must include groimd fault protection. Feeder circuit breakers will typically be molded case. Integrally fused circuit breakers are available with very high short circuit lAC ratings for both power and molded case circuit breakers. Fuses and current limiters for integrally fused circuit breakers must be stored within the electrical room. 

Fuses should not be used to protect secondary voltage feeders. The time current characteristics of fuses above 100 A will not coordinate with the groimd fault pickup currents and time delays of the main overcurrent protection (circuit breaker or fused disconnect switch) ground fault protection. A main load break disconnect switch can be equipped with current-limiting fuses to reduce the available short-circuit current from the utility and should have a three-phase voltage relay for single-phase protection. 

Switching devices for the power circuits that need to be operated frequently are usually circuit breakers and contactors. Manually operated load break switches, fuse-switch combinations and molded case circuit breakers are used for feeder circuits that are infrequently operated. 

Euses or moulded case circuit breakers have known current-time functions and for practical purposes these functions can be simply converted into their /-squared- characteristics by using the above method on as many sample points as can be conveniently transcribed. Figures 7.7 and 7.8 show the 1-t and the corresponding derived /-squared- characteristics for 32 A and 125 A fuses, 32 A and 125 A moulded case circuit breakers, and appropriate cables for the circuit. 

Electrical entrance package- The entry point of the electrical power including (1) the strike or location where the overhead or underground electrical lines connect to the house, (2) The meter which measures how much power is used and (3) The panel or circuit breaker box (or fuse box ) where the power can be shut off and where overload devices such a fuses or circuit breakers and located. 

Electrical equipment—Contact switches, fuses, circuit breakers, and so on can discharge spaik energy greater than MIE, thereby causing explosion in an environment containing flammable dust or vapor. It is necessary to make sure that such electrical equipment does not come in contact with dust or flammable vapor. 

Simple devices such as fuses, circuit breakers, and ground fault interrupters are available to cut off equipment when they overload or short out or an inbalance develops between the input and output current from a device or circuit. More sophisticated devices can also be used to determine a problem, such as a redundant heat detector used to deactivate a circuit serving a still, condenser, or heat bath should the temperature become too high. 

The object of this test is to verify that all fuses, circuit-breakers and switches are connected in the line or live conductor only, that all socket outlets are correctly wired and that Edison screw-type lamp holders have the centre contact connected to the live conductor. It is important to make a polarity test on the installation since a visual inspection will only indicate conductor identification. 

Using an approved voltmeter or test lamp and probes which comply with the HSE Guidance Note GS38, again carry out a polarity test to verify that all fuses, circuit breakers and switches are connected in the live conductor. Test from the common terminal of switches to earth, the live pin of each socket outlet to earth and the centre pin of any Edison screw lampholders to earth. In each case the voltmeter or test lamp should indicate the supply voltage for a satisfactory result. 

Identify outlets and fixtures for each circuit breaker or fuse. 

Circuit protective devices, such as fuses, circuit breakers, and GFCIs, automatically limit or shut off current flow in the event of a ground-fault, overload, or short circuit in a wiring system. 

Over-current protective devices (OCPDs) such as fuses, circuit breakers and protective relays, if properly adjusted and maintained, can detect an arcing condition almost instantaneously and clear the fault quickly. Old components that have not been well-maintained result in a slower reaction and elevated safety hazard. A requirement to maintain electrical distribution equipment was added to NFPA 70E in 2009. 

Prevent the build-up. Example Utilize fuses, circuit breakers, and... 

Control measures wMeh reduce the risk of fire and explosions involve the use of fuses, current circuit breakers and earthing devices. Equipment insulation is also recognized as an essential control measure. 

Electrical parts, including fuse housings, circuit breakers, and household articles CRITICAL FACTORS AFFECTING PRODUCT PERFORMANCE Material temperature of 300°C should not be exceeded during molding. 

Fuses, circuit breakers, and ground-fault circuit interrupters are three well known-... 

For motors with greater than fractional horsepower ratings, devices such as fuses, circuit breakers, and self-protected combination controllers must be installed ahead of the motor control apparatus to protect branch-circuit conductors, motor control apparatus, personnel, and the motor itself against fault conditions that may be the result of short circuits or grounds. 

When a fault occurs, the overcurrent device must safely open and interrupt the fault. Overcurrent devices such as fuses, circuit breakers, and self-protected combination controllers, which interrupt fault currents, must have an interrupting rating equal to or greater than the available short-circuit current at their line-side terminals. Control devices, such as motor starters and overload relays, must have a short-circuit current rating equal to or greater than the available short-circuit current. That is to say, they must be able to withstand the fault current for the time it takes the overcurrent device to interrupt the fault. In practice, the actual current which flows during a fault is less than the available short-circuit current, and it is this current which the control device must withstand. 

Type 1 protection. When Type 1 protected, the equipment is damaged (i.e., SCRs are shorted) and needs to be repaired, bnt the wiring and enclosure are intact. Type 1 protection is usually given by circuit breakers and class H, K, R, and RK-5 fuses. 

The common enclosure design (Fig. 5.141) and the use of combination starters offer both economy and ease of installation in multiple motor control installations. In addition, motor control centers (MCCs) provide proper coordination between short-circuit protective devices and the controller. Since MCCs are engineered systems, the components are closely coordinated to work together, and the unit is rated for a particular value of short-circuit interrupting duty at the point of its installation. MCCs may contain a molded-case circuit breaker and starter, or a fused switch and a starter. 

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