Protective relays Undervoltage

Tb relaying is based on either undervoltage reduced power flow or reversed power flow to the motors plus normal motor protection relaying. 

Undervoltage and overvoltage relays are often used on the generators to protect against prolonged overload, seen as undervoltage and excessive stress on insulation, seen as overvoltage. These relays are usually chosen with adjustable definite time delays. 

Undervoltage Protection. Undervoltage relays and three-wire control are employed to initiate the opening of a motor circuit and maintain it open on a reduction or failure of voltage. Undervoltage protection generally is applied where uncontrolled motor starting could result in potential hazards. 

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