Downstream Protective devices

Moulded case circuit breakers are also available for incoming and busbar section purposes, with ratings up to 6000 A and service voltages between 220 V and 660 V. (At 415 V a 4000 A circuit breaker would satisfy the duty of a 2500 kVA feeder transformer with about 15% spare capacity.) These are also available as 4-pole units. Circuit breakers having ratings of 800 A and above are often provided with several adjustments that widely modify the shape of the complete protection curve, as described in Chapter 12. This enables the curve to coordinate with almost any other protective device or equipment that is immediately upstream or downstream of the circuit breaker. Some circuit breakers with the higher rated currents are also provided with integral earth fault protection facilities. 

When fuses or moulded case circuit breakers are applied to a circuit it is necessary to ensure that their /-squared-t characteristics coordinate properly with the thermal capabilities of the downstream equipment, especially the cables. In order to determine the /-squared-t characteristics of a protective device it is assumed that the current in the device suddenly changes from a normal load value to the fault value in a very short period of time, i.e. similar to a step change in a control system. Hence for each value of current along the x-axis of the device s time-current characteristic the value of the current squared multiplied by the corresponding time can be plotted. For cables and busbars the /-squared-t function equals a constant (k) for each cross-sectional area of conductor, as explained... 

Generally, in gas-fueled pulse combustors separate gas and air valves are situated in their respective inlet pipes thus set back from the combustion chamber. In the case of pulse combustors that burn liquid fuels, fuel valves are generally not present the liquid fuel is admitted downstream of the air valve. In some systems, however, the fuel is premixed with air and in such instances a thermal-protection device is required between the valve and the combustion chamber. 

The 1997 edition of the API RP 521 extends the two-thirds rule to include the upstream and downstream system. At a minimum, the inlet and outlet piping up to and including isolation valves must be designed for the two-thirds rule to be able to block in the exchanger. If the upstream and downstream equipment is not designed for the two-thirds rule, relief devices may be required on both the inlet and outlet piping to protect the piping and adjaeent equipment. 

As long as pressure, level, and temperature control devices are operating correctly, the safety system is not needed. If the control system malfunctions, then pressure, level, and temperature safety switches sense the problem so the inflow can be shut off. If the control system fails and the safety switches don t work, then relief valves are needed to protect against overpressure. Relief valves are essential because safety switches do fail or can be bypassed for operational reasons. Also, even when safety switches operate correctly, shutdown valves take time to operate, and there may be pressure stored in upstream vessels that can overpressure downstream equipment while the system is shutting down. Relief valves are an essential element in the facility safety system. 

There are numerous potential locations for filters in an HVAC system. The most common filter placement location is in the mixed airstream within an airhandling unit. Using that location results in filtration of both outside air and return air, and it protects coils and other downstream components from fouling. Filters could also be located directly in the outside airstream, at supply to individual spaces, on the return from individual spaces, in the common return, and on exhaust air (if there is concern about the consequences of contaminated exhaust). Stand-alone filtration devices that recirculate and clean air within a single zone might also be desirable. 

An unrestricted earth fault relay that is connected in the star-point earth circuit of the equipment being protected. The characteristic is time dependent so that time coordination is achieved with the 50 N devices downstream. 

Realization of a flow sensor depends on its specific application. Overall, the spatial and transient resolution and the compatibility of the sensor within the desired device are of major concern [6]. In addition, the protection of the fluids and components demands a reduction in the thermal crossover from the flow sensor. The microflow sensors are usually automatically integrated with the microchannel during the fabrication process. The sensing element should be a resistor that has a resistance with high temperature sensitivity [2, 4, 9]. The heater of the sensor is often fabricated from a platinum or polysilicon resistor and acts as a microheater while the upstream and downstream temperature sensors are made from either polysilicon resistors or thermopiles. Such materials have excellent chemical resistance, hiocompatihUity, and high TCR [9]. 

We recommend that you install a pressure-relief valve that releases pressures above 2000 psig (140 bar) on each main gas line, to protect downstream equipment from high-pressure failures. The best location for the device is after the regulator shutoff valve (Figure 10.17). Pressure-relief valves on nonflammable gas lines need not be vented to a hood, but be sure to direct the vents downward (away from operator). 

Residual current devices - detect earth faults and cut off the supply to the circuit. Their use should be considered a requirement on all temporary power circuits, because they will offer improved protection where, for example, connectors and cables downstream have been damaged and not detected. 

As the basic component of medical textile materials, the structures and properties of the constituent polymers have a significant effect on the biodegradability, biocompatibility, absorbency, antimicrobial property, and other functional performances of the final medical textile products. Functional modifications of polymers have far-reaching effects on the fibers, yams, fabrics, and textile materials that are processed in a series of downstream operations. In order to generate the desired product performance characteristics for their diverse applications such as hygiene, protection, therapeutic, nonimplantable or implantable materials, extracorporeal devices, etc., the chemical and physical structures of the relevant polymers should be engineered to suit their required specifications. 

Smaller size SWRO desalination plants with open intakes use strainers instead of bar screens to protect the downstream intake pumps and pretreatment systems. For plants that have conventional granular media pretreatment systems, 500 to 900-p.m strainers are usually adequate prescreening device. Plants equipped with membrane pretreatment filters would require the use of 80 to 120-p.m strainers. 

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