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Which equipment has minimum spark over voltage during insulation coordination?
Lightning arrester
Transformer
Circuit breaker
Bus insulator
Lightning arrester
In insulation coordination, the lightning arrester (surge arrester) is designed to have the lowest spark-over (or discharge) voltage among all protective devices and protected equipment. This ensures that the arrester conducts surge currents to the ground before the voltage reaches a level that could cause dielectric breakdown in expensive equipment like transformers.
In insulation coordination, the lightning arrester (surge arrester) is designed to have the lowest spark-over (or discharge) voltage among all protective devices and protected equipment. This ensures that the arrester conducts surge currents to the ground before the voltage reaches a level that could cause dielectric breakdown in expensive equipment like transformers.
VSAтАЛ<VBILтАЛ тАФ The Spark-over voltage of the surge arrester must be lower than the Basic Insulation Level of the transformer.
I=RVтИТVdischargeтАЛтАЛ тАФ Current through the arrester during a transient surge.
The lightning arrester acts as a non-linear voltage-dependent resistor. Under normal operating conditions, it behaves as an insulator (high impedance). When a surge voltage exceeding its protective level appears, the arrester's internal material (typically Zinc Oxide) becomes highly conductive (low impedance), providing a low-resistance path to the ground for the transient energy.
The lightning arrester serves as the 'weakest link' in terms of voltage, but the 'strongest' in terms of survival.
It limits the transient overvoltage to a level known as the 'Protective Margin'.
Insulation coordination involves selecting insulation strengths that are statistically coordinated with the protective characteristics of the arresters.
Prevents catastrophic failure of power transformers
Reduces the cost of insulation requirements for electrical equipment
May require replacement after extreme surges
Increases system complexity if not correctly coordinated
Substation incoming lines
Transformer bushings
Cable terminations
The lightning arrester does not protect against power frequency overvoltages; it is specifically for switching and lightning surges.
Option B (Transformer) is the equipment that MUST be protected, hence its BIL must be higher than the arrester spark-over voltage.
A is correct тАФ The lightning arrester is deliberately engineered with the lowest spark-over voltage in the system to divert surges safely to the ground.
Remember the 'Protective Margin' rule: Always ensure that the withstand voltage of the protected equipment is significantly higher than the protective level of the arrester.