The time delay for ROV2PTOV (59N) is seldom critical, since residual voltage is relatedto ground faults in a high impedance grounded system, and enough time must normally begiven for the primary protection to clear the fault. In some more specific situations, wherethe single overvoltage protection is used to protect some specific equipment, the timedelay is shorter.Some applications and related setting guidelines for the residual voltage level are givenbelow.7.3.3.1 Power supply qualityThe setting must be above the highest occurring "normal" residual voltage and below thehighest acceptable residual voltage, due to regulation, good practice or other agreements.7.3.3.2 High impedance grounded systemsIn high impedance grounded systems, ground faults cause a neutral voltage in the feedingtransformer neutral. Two step residual overvoltage protection ROV2PTOV (59N) is usedto trip the transformer, as a backup protection for the feeder ground fault protection, andas a backup for the transformer primary ground fault protection. The setting must be abovethe highest occurring "normal" residual voltage, and below the lowest occurring residualvoltage during the faults under consideration. A metallic single-phase ground fault causesa transformer neutral to reach a voltage equal to the nominal phase-to-ground voltage.The voltage transformers measuring the phase-to-ground voltages measure zero voltagein the faulty phase. The two healthy phases will measure full phase-to-phase voltage, asthe ground is available on the faulty phase and the neutral has a full phase-to-groundvoltage. The residual overvoltage will be three times the phase-to-ground voltage. SeeFigure 64.1MRK 511 286-UUS A Section 7Voltage protection159Application manual
