Electricity circuit breaker fuses

All electrical protective equipment (e.g., circuit breakers, fuses, bus bars, and motor starters) is rated for maximum short circuit currents by NEMA standards. Proper selection of equipment must be based on available short circuit currents. 

Our home circuit breakers, or fuses, are, of course, trips to prevent overheating electric circuits or electric motors. The only difference is that at work, our electric circuit breakers have a built-in time delay. This is needed to allow the motor driver to overcome the starting torque inherent in most large pieces of rotating process equipment. 

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. 

Fail-safe devices may be fail-passive, fail-active, or fail-operational. A fail-passive device, such as electrical circuit breakers or fuses, wdl render a system inoperative or de-energized until corrective action is taken. A fail-active device will keep a system energized but in a safe mode until there are corrective actions. A fail-operational device allows a system to function safely, even when the device fails. 

Zilnder Zilndschnur electr (circuit breaker) Sicherung fuse vb... 

Electrical Equipment (cont d) Circuit Breakere. Fuses... 

Safe electrical design (circuit breakers, fuses, physical separation of wires, etc.)... 

All electrical systems must be provided with protection equipment, the purpose of which is to isolate faulty electrical equipment from the electrical supply system as rapidly as possible. This can be achieved by use of devices which respond directly to the current flowing (e.g. fuses), or by protective relays which respond to fault current flow and are used to initiate the tripping of other devices (e.g. circuit breakers). 

Safe Work Practices. Individuals working with electrical equipment must use safe work practices. All must know how to shut off power to a piece of equipment by using either the power switch on the equipment or the circuit breaker. Circuit breakers or switch boxes must be secure but readily accessible. Each circuit must be clearly labeled as to the equipment or area that it activates. Indii iduals need to know how to use tlie master electrical switch under emergency conditions. (Tlie circuits sliould be checked to ensure that room lighting is on a separate box.) Individuals should be aware of the "left-hand rule. Anytime tliey prepare to turn tlie master switch back on, especially after cliangiiig a fuse, tliey sliould stand to tlie side, face tlie wall instead of the box. and use their left lituid to push tlie switch back on. In this way. if the box explodes when power is restored, they are less likely to suffer severe burns to the face or ei eii death. 

For example, why will a short circuit blow a fuse or circuit breaker When a short circuit occurs, most of the electrical resistance in the circuit is bypassed In effect, a new circuit with a very low resistance is created. So, according to Ohm s law if the resistance is very low the current must be very high. Fuses and circuit breakers are designed to protect the circuit by blowing when the current becomes too high. Hence, the short circuit will produce a current high enough to blow the fuse. As another application, electronic devices often have resistors placed in the circuit to increase the resistance and therefore Umit the current. 

When an earth fault occurs at the far end of a cable it is possible that the armouring, cable gland and the frame of the consumer equipment can be raised to a dangerous potential with respect to electric shock exposure to human operators. This subject has been given considerable attention over the last 20 years, and is well documented in for example IEC60364. The international documentation concentrates on low voltage fixed and portable equipment protected by fuses and miniature circuit breakers. See also Chapter 13. 

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. 

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. 

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. 

In the UK, the electricity supply is connected to earth. It is this system that enables earth faults on electrical equipment to be detected and the electrical supply to be cut off automatically. This automatic cut-off is performed by fuses or automatic circuit breakers if a fault occurs the fuse will blow and break the circuit. Although they do not eliminate the risk of electric shock, danger may be reduced by the use of a residual current device (RCD) designed to operate rapidly at small leakage currents. RCDs should only be considered as providing a second line of defence. It is essential to regularly operate the test trip button to maintain their effectiveness. 

To prevent people from receiving an electric shock accidentally, all circuits contain protective devices. All exposed metal is earthed fuses and miniature circuit-breakers (MCBs) are designed to trip under fault conditions, and residual current devices (RCDs) are designed to trip below the fatal level as described in Chapter 4. 

Protection against electric shock, both basic protection and fault protection , is provided by insulating and placing live parts out of reach in suitable enclosures, earthing and bonding metalwork and providing fuses or circuit-breakers so that the supply is automatically disconnected under fault conditions. 

Circuits are controlled by switchgear which is assembled so that the circuit may be operated safely under normal conditions, isolated automatically under fault conditions, or isolated manually for safe maintenance. These requirements are met by good workmanship carried out by electrically skilled persons and the installation of approved British Standard materials such as switches, isolators, fuses or circuit-breakers (IET Regulation 131.1.1). The switchgear must also be non-combustible or housed in a non-combustible cabinet or enclosure (IET Regulation 421.1.200). The equipment belonging to the supply authority is sealed to prevent unauthorized entry, because if connection were made to... 

The single-storage heater is heated by an electric element embedded in bricks and rated between 6 and 15kW depending upon its thermal capacity. A radiator of this capacity must be supplied on its own circuit, in cable capable of carrying the maximum current demand and protected by a fuse or miniature circuit-breaker (MCB) of 30, 45 or 60 A as appropriate. At the heater position, a DP switch must be installed to terminate the fixed heater wiring. The flexible cables used for the final connection to the heaters must be of the heat-resistant type. 

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