Protective devices

Experimental circuit and results for PD characteristics (a) experimental circuit for a PD characteristic, (b) measured results of V-t characteristic, and (c) measured results of V-I characteristic. 

Simulation circuit for a PD characteristic (a) simulation circuit and (b) PD model. 

Experiment and simulation results of PD voltages (a) applied voltage 400 V with the wavefront of 50 ns and (b) applied voltage 600 V with the wavefront of 50 ns. 

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NMR can be carried out over a wide range of temperatures, although there is a time and often a resolution penalty in using temperatures other than ambient. An effective lower limit of - 150 °C is set by the lack of solvents that are liquid below this. Temperatures above 130 °C require special thennal protection devices, although measurements have even been made on molten silicates. 

AH operations producing dust require the usual measures to prevent dust in the atmosphere exceeding the allowable daily concentration. If this is not feasible, personal protection devices should be used. Especially when hydroquinone is present as a powder, adequate eye protection should be provided. 

Underwriters Laboratories 333 Pfingsten Road Northbrook, Id. 60062 Standards for Safety is ahst of more than 200 standards that provide specifications and requirements for constmetion and performance under test and in actual use of a broad range of electrical apparatus and equipment, including household appHances, fire-extinguishing and fire protection devices and equipment, and many other nongenerady classifiable items, eg, ladders, sweeping compounds, waste cans, and roof jacks for trader coaches. 

Nickel carbonyl should be used in totally enclosed systems or under good local exhaust. Plants and laboratories where nickel carbonyl is used should make use of air-monitoring devices, alarms should be present in case of accidental leakage, and appropriate personal respiratory protective devices should be readily available for emergency uses. Monitoring of urinary nickel levels is useful to help determine the severity of exposure and identify appropriate treatment measures. Some large-scale users of nickel carbonyl maintain a supply of sodium diethyldithiocarbamate, or Antabuse, a therapeutic agent, on hand for use in case of overexposure. 

NIOSH/MSHA-approved respiratory protection devices should be used. For skin protection, use of protective garments and gloves of Viton, Nitrile, or PVA constmction should be made. The acute effects of overexposure to styrene are shown in Table 6 (74). 

Finally, it must be pointed out that all devices suitable for use in explosion isolation—or, quite generally, all explosion protection devices used in practice—may be used only when their pressure rating, ignition breakthrough protection, and reliability have been proven in suitable investigations by competent bodies. 

Motor Protection Money spent lor motor-protective devices can be compared to insurance, in which premiums depend on the protected value vvFen the protected value is the cost ol the motor, the cost ol anticipated repairs, or the cost ol downtime, lost production, and, in some cases, contingent damage to other equipment. 

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. 

Operating conditions that may overload a machine and raise its temperature beyond permissible limits may be called unfavourable. This overheating, however, will be gradual (exponential), unlike rapid (adiabatic) heating as caused during a locked rotor condition. The machine now follows its own thermal curve and therefore a conventional thermal protection device can be used to protect it from such conditions. These conditions may arise due to one or more of the following ... 

Excessive harmonics may also make the protective devices behave erratically or render them inoperative. Filter circuits would suppress the harmonics and eliminate these effects. 

These are conditions in which overheating of the machine may not trace back to its own thermal curves as in the first case. The temperature rise may now be adiabatic (linear) and not exponential and hence rapid. Now a normal thermal protection device may not be able to respond as in the previous case. Some conditions causing overheating may not necessarily be fault conditions. Nevertheless, they may require fast tripping, and hence are classilled in this category for more clarity. Such conditions may be one or more of the following ... 

Coordination of fuses with an overcurrent relay or any other overcurrcnt protective device The selection of the fuses should be such that ... 

They do not operate against overloads as these are taken eare of by the overcurrent relay or any other overcurrent protective device. 

A separate single-phasing protection device is available in two versions ... 

Thermal capacity to perform the required switching duties and sustain the fault conditions, at least up to the cut-off time of the short-circuit protective device, say, the HRC fuses. 

These assemblies are fitted with switching devices (breakers, switches, fuse switches and contactors etc.) and control and measuring instruments, indicating, regtilating and protective devices etc. to transform the assemblies into composite units, called control centres to perform a number of functions in the field of distribution and control of electrical power. Some of these functions may be one or more of the following ... 

The bus system of a switchgear assembly, its interconnecting links and wires are the protected type components, whereas an interrupter (breaker, switch or a fuse) may be a protecting or protected type, depending upon their application and location in the circuit. A contactor and an OCR are therefore protected devices in the same context, for they provide no short-time protection. A protecting device may become protected when it is also provided with a back-up protection. 

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. 

Since will be too low (< 5 ms for a 50 Hz system) 1. will be much less than 7% of 1 in all situations. To assign a short-time rating to the protected devices and components in such cases is therefore of little relevance. As noted above, current is the cause of heat, for which is assigned the thermal duty of a current-carrying device, component or part. Also note the following ... 

Examples are HRC fuses (both LT and HT) and MCCBs and MCBs (LT only), which are available with current limiting features and are in extensive use. The tripping time of these devices is extremely low and much less than one half of a cycle of the current wave. They therefore do not allow the fatilt ctirrent to rise to its prospective peak. The protected devices and components can thus be selected based on the let-out energy of such devices on fault, which is extremely low, than the fault level of the system. If... 

Role of an OCR this is only an overcurrent protection device and does not provide short-circuit protection. 

Similarly, a contactor is a simple circuit making and breaking device, and is protected by other protective devices used in association with it. It has no role in the decision making for the current rating of other... 

Normally only a 1-second system is in use. The 3-second system is severe, for which protective devices in certain ratings may not be possible or may become prohibitively costlier to prvivluce. The 3-sceond system may. however, be used for a generator circuit to protect the generating. source from a fallout on a fault elsewhere in the system. 

It is advisable that all the protective devices provided in the assembly be set to their minimum values to ensure fast tripping. They can be adjusted for proper settings when putting the assembly into service. 

Testing of the main circuits (with short-circuit protective devices)... 

This class may be selected for protective devices that operate on the basis of phase relationship between the voltage and the current phasors, such as in a directional overcurreni protection, reverse power or directional distance protection. 

For lower voltages (< 33 kV), any fault on the VT will be detected by the protective devices installed in the main circuit. 

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