Protection device characteristics

In order to overcome (1) it is often necessary to introduce a form of assisted starting, which can also help in overcoming problem (2). However, modification of the overload protection device characteristics may also be necessary. 

Pressure Relief Devices The most common method of overpressure protection is through the use of safety rehef valves and/or rupture disks which discharge into a containment vessel, a disposal system, or directly to the atmosphere (Fig. 26-13). Table 26-8 summarizes some of the device characteristics and the advantages. 

When there is no dedicated transformer and these circuits are connected on the system bus directly a large inductor will be essential at the incoming of the static circuits, sufficient to absorb the trapped charge within the transformer and the interconnecting cables up to the converter unit. The size of the inductor can be calculated depending on the size (kVA) of the distribution transformer, its fault level and the characteristics of its current limiting protective device. An inductor sufficient to absorb //, L of the transformer and the cables may be provided at the incoming of the sialic circuits. 

The duration, however, is no criterion for a current limiting type protecting device, and a protected equipment, device or component can have a short-time rating commensurate with the tripping characteristics of the protecting interrupter. Accordingly these two types of tripping characteristics are explained below. 

Low voltage contactors are usually fitted with purpose-made protection devices for gnarding against overloading and single-phase operation. These devices are used individually or in combination and operate on magnetic, thermal or electronic principles. Electronic static devices offer the widest range of time-current characteristics. 

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... 

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. 

These logarithmic scales are shown in the graphs of Figs 3.29 and 3.30. From Fig. 3.29 it can be seen that the particular protective device represented by this characteristic will take 8s to disconnect a fault current of 50 A and 0.08s to clear a fault current of 1000A. 

Figure 3.30(a) shows the time/current characteristics for a Type B MCB to BS EN 60898. This graph shows that a fault current of 4000A will trip the protective device in 20ms. Since this is quicker than 82.66ms, the SOAType B MCB is suitable and will clear the fault current before the temperature of the cable is raised to a dangerous level. 

Figure 3.29 Time/current characteristic of an overcurrent protective device.

Appendix 3 of the lET Regulations gives the time/cun ent characteristics and specific values of prospective short-circuit current for a number of protective devices. 

A TN supply feeds a domestic immersion heater wired in 2.5 mm PVC insulated copper cable and incorporates a 1.5 mm CPC. The circuit is correctly protected with a 15 A semi-enclosed fuse to BS 3036. Establish by calculation that the CPC is of an adequate size to meet the requirements of lET Regulation 543.1.3. The characteristics of the protective device are given in lET Regulation Fig 3A2(a) of Appendix 3. 

In the event of a fault occurring on an electrical installation only the protective device nearest to the fault should operate, leaving other healthy circuits unaffected. A circuit designed in this way would he considered to have effective discrimination. Effective discrimination can he achieved hy graded protection since the speed of operation of the protective device increases as the rating decreases. This can he seen in Fig. 12.5(h). A fault current of 200A will cause a 15 A semi-enclosed fuse to operate in about 0.1 s, a 30A semi-enclosed fuse in about 0.4 s and a 60 A semi-enclosed fuse in about 5.0 s. If a circuit is arranged as shown in Fig. 12.6 and a fault occurs on the appliance, effective discrimination will be achieved because the 15 A fuse will operate more quickly than the other protective devices if they were all semi-enclosed types fuses with the characteristics shown in Fig. 12.5(b). 

Ear-plugs are only generally effective up to noise levels of 100-105dBA while ear-muffs can provide protection at higher noise levels to meet a 90 dBA criterion, for noise received by the wearer. Comparative attenuation characteristics for various personal hearing protection devices are shown in Figure 20.15. 

This practice is built around the idea that, if hydrocarbons can remain contained in the system of pipes and vessels, then a serious process-related accident is unlikely to occur. This goal is achieved by identifying those process hazards that could lead to a release, and then installing two independent protective devices for each detectable event. The two levels of protection should be independent of, and in addition to, the control devices used in normal process operation. In general, the two levels should be provided by functionally different types of safety devices for a wider spectrum of coverage. Two identical devices would have the same characteristics and might have the same inherent weaknesses. 

FIGURE 3.35 Typical rain-protection devices. Rain protection characteristics of these caps are superior to a deflecting cap located 0.75D from top of stack. The length of the upper stack is related to rain protection. Excessive additional distance may cause blowout of effluent at the gap between upper and lower sections. From Industrial Ventilation—A Manual of Recommended Practice. 23d ed.. Committee on Industrial Ventilation, American Conference of Governmental Industrial Hygienists, copyright1998, pp. 5-67.)... 

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