Fuse-switch combinations

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. 

The outgoing switching device in a high current, high fault level, switchboard will usually be a power circuit breaker if it feeds more that about 400 amps to the load. Below 400 amps the circuit could have a fuse-contactor combination, see sub-section 7.3.2 for comments on contactors and Chapter 8 on fuses. Therefore if the outgoing device must be a circuit breaker then the comments and discussion in sub-section 7.2.3 above apply. Low voltage switchboards often use moulded case circuit breakers... 

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. 

Line Starters and Combination Starters A line starter consists of a contactor (motor-starting switch) and motor-overload relays, (Contactors are capable of canving and interrupting normal motor-starting and -running currents they are not, hovv ever, normally capable of interrupting short-circuit currents. They must be backed up by fuses or a circuit breaker for this function,... 

Low voltage switchgear and conlrolgern. Switches, disconnectors, switch-disconnectors and fuse-combination units 13947-3/1993 BSEN 60947-3/1992 ... 

Third-order susceptibilities of the PAV cast films were evaluated with the third-harmonic generation (THG) measurement [31,32]. The THG measurement was carried out at fundamental wavelength of 1064 nm and between 1500 nm and 2100 nm using difference-frequency generation combined with a Q-switched Nd YAG laser and a tunable dye laser. From the ratio of third-harmonic intensities I3m from the PAV films and a fused quartz plate ( 1 thick) as a standard, the value of x(3) was estimated according to the following equation derived by Kajzar et al. [33] ... 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

When a disconnect switch, circuit breaker, or set of fuses is included in the same enclosure as the contactor, the starter is then called a combination starter. In addition to the fault-current-interrupting function, the breaker or fuses serve as the disconnecting device. Figure 29-5 illustrates schematically combination starters of various types. The latch is arranged to open the disconnect before the door can be swung open. There are also provisions for padlocking the disconnect open with the door closed so that maintenance wonc on the motor may proceed in safety. 

FIG. 29-5 Simplified schematic diagram of a combination line starter with a circuit breaker as the fault interrupter and disconnect. Alternative fuses and disconnect switch are shown as substitutes for the circuit breaker. 

Low-voltage air-break switches, air-break disconnectors and fuse combination nnits. 

Circuit protection devices, such as fuses, breakers, or fault-interrupters, may be required in case of excessive current draws as a result of a short circuit, overcurrent, or earth fault. Several options exist with circuit breakers preferred over fuses. In some of the newer devices several functions are combined into one, thereby reducing the total number of components. An example of this is a combined power-switch/circuit-breaker. The switch/breaker is used as a switch and breaker and senses each line and opens all lines, except the grounded line, simultaneously when a fault current is present. Also, the switch/breaker looks and functions as a standard power ON/OFF rocker switch. In some standards simultaneous interruption of the phase and neutral is required, thereby, precluding the use of fuses. 

Combination starter A magnetic starter having a manually operated disconnecting means built into the same enclosure. The disconnect may be a motor-circuit switch (with or without fuses) or a circuit breaker. 

A disconnect switch or a nonautomatic circuit interrupter can be used to provide an electrical circuit disconnect function. Manual motor controllers marked Suitable as Motor Disconnect may also be used. Short-circuit protection is provided for these devices when fuses are added to the disconnect switch or thermal and/or magnetic trip units are added to the nonautomatic circuit interrupter. Self-protected combination controllers may be used to provide both the disconnect function and the short-circuit protection for motor circuits. 

A combination starter (Fig. 5.139) combines a disconnect switch with or without fuses or a circuit breaker together with the starter and its accessories into one compact package. It may be provided as an enclosed unit or as an open unit which must be mounted in the appropriate enclosure. 

As an example, in TN-C-S supplies to domestic premises a break in the combined neutral/earth (CNE) conductor of an overhead service line would cause the metalwork in the premises to become live at or about the supply voltage if any apparatus, such as a dishwasher, were to be connected and switched on. To prevent this type of hazard, the integrity of the CNE conductor, throughout the system, has to be maintained, so all joints have to be properly made and reliable, and fuses and solid state devices are prohibited in these conductors. 

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