Circuit protection, electrical

In other designs, a diffused siUcon sensor is mounted in a meter body that is designed to permit caUbration, convenient installation in pressure systems and electrical circuits, protection against overload, protection from weather, isolation from corrosive or conductive process fluids, and in some cases to meet standards requirements, eg, of Factory Mutual. A typical process pressure meter body is shown in Figure 10. Pressure measurement from 0—746 Pa (0—3 in. H2O) to 0—69 MPa (0—10,000 psi) is available for process temperatures in the range —40 to 125°C. Differential pressure- and absolute pressure-measuring meter bodies are also available. As transmitters, the output of these devices is typically 4—20 m A dc with 25-V-dc supply voltage. 

A form of electrical circuit protection that comprises a strip of metal of such size to melt at a predetermined value of curreut flow. It is placed in the electrical circuit, and upon melting, due to excessive current flow, prevents the flow of electricity supply to the circuit. 

There are two basic types of electrical circuit protection devices fuses and circuit breakers, and ground fault interrupt (GFI) devices. A circuit breaker is more or less like a reusable fuse. Many people are unfamiliar with how electrical safety devices work, even though these devices are found in all homes. We assume that circuit breakers and GFI devices are... 

Patent laws provide for several stages in the life of an application for a patent on an invention. The pattern followed by patent laws in effect in most industrialized countries during the nineteenth and early twentieth centuries, and still in effect in the United States in 1995, calls for the examination of all patent appHcations to certify that the claimed invention meets the national standards for novelty, usehilness, and inventiveness. The owner of the technology to be patented files appHcation papers that include a specification containing a description of the invention to be patented (called the disclosure) and claims defining the limits of the invention to be protected by the patent, a formal request for the issuance of a patent, and fees. Drawings of devices and apparatuses, electrical circuits, flow charts, etc, are an important part of the disclosures of most nonchemical and many chemical patents. 

The environment plays several roles in corrosion. It acts to complete the electrical circuit, ie, suppHes the ionic conduction path provide reactants for the cathodic process remove soluble reaction products from the metal surface and/or destabili2e or break down protective reaction products such as oxide films that are formed on the metal. Some important environmental factors include the oxygen concentration the pH of the electrolyte the temperature and the concentration of anions. 

The electrical distribution system design and equipment selection must consider requirements of the utility company for protection and metering. Available short circuit currents from the utility distribution network to the primary of the facility s main transfoiTner must be considered in selecting circuit protection devices for the facility distribution system. 

For the corrosion process to proceed, the corrosion cell must contain an anode, a cathode, an electrolyte and an electronic conductor. When a properly prepared and conditioned mud is used, it causes preferential oil wetting on the metal. As the metal is completely enveloped and wet by an oil environment that is electrically nonconductive, corrosion does not occur. This is because the electric circuit of the corrosion cell is interrupted by the absence of an electrolyte. Excess calcium hydroxide [Ca(OH)j] is added as it reacts with hydrogen sulfide and carbon dioxide if they are present. The protective layer of oil film on the metal is not readily removed by the oil-wet solids as the fluid circulates through the hole. 

First, once designed, evaluated, and installed, the safety of the system cannot easily be degraded because the safety is in the design, not protection added afterward. In fact, the intrinsically safe electrical circuit will cease to fulfill the function for which it was designed long before it can become a hazard. This is due to the consideration which must be given to fault conditions. The only possible way for the circuit to become hazardous is if an unapproved or unauthorized component is substituted into the circuit. 

Fourthly, intrinsically safe electrical circuits are the easiest to maintain. Since intrinsically safe circuits by their nature are incapable of causing ignition, they can be maintained without regard to shutting down operations, nor are hot permits required, or is lengthly disassembly, assembly and recertification of added protection required. 

In principle, it is not a big step from a system protected against all malfunctions to a fully automatic, program-controlled plant, though the complexity of the electrical circuits, of course, increases significantly. 

The electrons pumped into the corrodible metal have come, in the above method, from the dissolution of a scarificial auxiliary metal. Instead, they can come from an external current source (i.e., an electrical power supply). The electrical circuit, however, has to be completed, and toward this end, an auxiliary inert electrode can be immersed in the corrosive electrolyte to provide a return path for the electron current (Fig. 12.38). The external source can then be adjusted so that the potential difference between the corrodible metal and its environment becomes negative with respect to its equilibrium potential. Under these circumstances, the whole of the metal to be protected against corrosion will function as an electron source for the electronation reaction, and the second electrode will serve as an electron sink for some deelectronation reaction (Hoar). 

Only in the field of data transmission, communications, remote controlling and monitoring, has the very small energy amount, when switching electric circuits, formed a very useful basis for a special type of explosion protection, the so-called intrinsic safety as given in IEC 60079-11 and EN 50020. 

This chapter will seek to answer the question Why do standards ask for specified temperatures of windings, for limited gap and joint values and for restricted voltages and currents in electric circuits The following intends to clarify the physical background of the standards technical content, to explain the methods of explosion protection in electrical engineering, and demonstrate modem explosion protected apparatus to give an appreciation of the bandwidth of the different types of protection. 

Figure 6.10 shows a typical example for a q-apparatus. It is a power supply for intrinsically safe electric circuits to be installed in zone 1 (and zone 2). The power input (at 24 V AC/DC level) is fed via flameproof plug-and-socket connectors to the apparatus. So, the power supply can be replaced under load without conflicting with the requirements of explosion protection. 

A very generalized principle of an intrinsically safe electric circuit is shown in Fig. 6.169. Power source, voltage and current limitation are located in a safe area or shall be explosion protected (e.g. in a flameproof enclosure) if located in a hazardous environment. The electric circuit entering the hazardous area as an intrinsically safe circuit is not capable of producing ignitable sparks at make or break. 

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