Book at a Glance

PART III- Voltage Surges, Over- voltages, Circuit Interrupters and Grounding Practices

Chapter 18. Surge arresters: applications and selection

Surge arresters
Electrical characteristics of a ZnO surge arrester
Basic insulation level (BIL)
Protective margins
Protective level of a surge arrester
Selection of gapless surge arrester
Classification of arresters
Surge protection of motors
Pressure relief facility
Assessing the condition of an arrester

Surge arresters
When surge protection is considered necessary, surge arresters* may be installed on or near the equipment being protected. This is a device that limits the high TVs (transient voltages) generated during a system disturbance by diverting the excessive part of it to the ground and reducing the amplitude of the transient voltage wave across the equipment to a permissible safe value less than the impulse withstand level of the equipment (Tables 11.6, 14.1, 32.1(a), 13.2, and 13.3). The rate of rise of transient voltage remains the same. A surge arrester does not tame the steepness of the surge, i.e.(curves oa1 and oa2 of Figure 17.21)
thus shielding the connected equipment from dangerous voltage surges. This is achieved by providing a con ducting path of relatively low surge impedance between the line and the ground to the arriving surge. The discharge current to the ground through the surge impedance limits the residual voltage across the arrester hence the equipment and the system connected to it. During normal service this impedance is high enough to provide a near-open circuit. It remains so until a surge voltage occurs and is restored immediately after discharge of the excess surge voltage.

An arrester can be considered a replica of an HRC fuse. What a fuse is to a fault current, arrester is to a voltage surge, both limit, their severity. While a fuse is a current limiting device and protects the connected equipment by limiting the prospective peak fault currents, Isc (Figure 12.18), an arrester is a voltage limiting device and protects the connected equipment by limiting the prospective peak surge voltage, Vt (curve oa2, Figure 17.21).

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