If the reset timer reaches the value set by Reset delay time, the operation timer resets andthe PICKUP output is deactivated.Timer calculates the start duration PICKUP_DUR value, which indicates the percentageratio of the start situation and the set operating time. The value is available in theMonitored data view.The activation of the BLOCK input resets Timer and deactivates the PICKUP and TRIPoutputs.Blocking logicThere are three operation modes in the blocking functionality. The operation modes arecontrolled by the BLOCK input and the global setting Configuration/System/Blockingmode which selects the blocking mode. The BLOCK input can be controlled by a binaryinput, a horizontal communication input or an internal signal of the protection relay'sprogram. The influence of the BLOCK signal activation is preselected with the globalsetting Blocking mode.The Blocking mode setting has three blocking methods. In the "Freeze timers" mode, theoperation timer is frozen to the prevailing value. In the "Block all" mode, the wholefunction is blocked and the timers are reset. In the "Block TRIP output" mode, the functionoperates normally but the TRIP output is not activated.4.3.4.5 Application87A, 87B, 87C provides a secure and dependable protection scheme against all types offaults. The high-impedance principle is used for differential protection due to itscapability to manage the through-faults also with the heavy current transformer (CT)saturation.For current transformer recommendations, see the Requirements formeasurement transformers section in this manual.High-impedance principleThe phase currents are measured from both the incoming and the outgoing feeder sides ofthe busbar. The secondary of the current transformer in each phase is connected in parallelwith a protection relay measuring branch. Hence, the relay measures only the differenceof the currents. In an ideal situation, there is a differential current to operate the relay onlyif there is a fault between the CTs, that is, inside the protected zone.If there is a fault outside the zone, a high current, known as the through-fault current, cango through the protected object. This can cause partial saturation in the CTs. The relayoperation is avoided with a stabilizing resistor (Rs) in the protection relay measuring1MAC059074-MB A Section 4Protection functions615 series ANSI 553Technical Manual
