The DT mode is enabled when the Operating curve type setting is selected either as "ANSI Def. Time" or "IEC Def. Time". In the DT mode, the OPERATE output of the function is activated when the time calculation exceeds the set Operate delay time .
The user can determine the reset in the DT mode with the Reset delay time setting, which provides the delayed reset property when needed.
The purpose of the delayed reset is to enable fast clearance of intermittent faults, for example self-sealing insulation faults, and severe faults which may produce high asymmetrical fault currents that partially saturate the current transformers. It is typical for an intermittent fault that the fault current contains so called drop-off periods, during which the fault current falls below the set start current, including hysteresis. Without the delayed reset function, the operate timer would reset when the current drops off. In the same way, an apparent drop-off period of the secondary current of the saturated current transformer can also reset the operate timer.
In case 1, the reset is delayed with the Reset delay time setting and in case 2, the counter is reset immediately, because the Reset delay time setting is set to zero.
When the drop-off period is longer than the set Reset delay time, as described in Figure 2, the input signal for the definite timer (here: timer input) is active, provided that the current is above the set Start value. The input signal is inactive when the current is below the set Start value and the set hysteresis region. The timer input rises when a fault current is detected. The definite timer activates the START output and the operate timer starts elapsing. The reset (drop-off) timer starts when the timer input falls, that is, the fault disappears. When the reset (drop-off) timer elapses, the operate timer is reset. Since this happens before another start occurs, the OPERATE output is not activated.
When the drop-off period is shorter than the set Reset delay time, as described in Figure 3, the input signal for the definite timer (here: timer input) is active, provided that the current is above the set Start value. The input signal is inactive when the current is below the set Start value and the set hysteresis region. The timer input rises when a fault current is detected. The definite timer activates the START output and the operate timer starts elapsing. The Reset (drop-off) timer starts when the timer input falls, that is, the fault disappears. Another fault situation occurs before the reset (drop-off) timer has elapsed. This causes the activation of the OPERATE output, since the operate timer already has elapsed.
If the BLOCK input is activated when the operate timer is running, as described in Figure 4, the timer is frozen during the time BLOCK remains active. If the timer input is not active longer than specified by the Reset delay time setting, the operate timer is reset in the same way as described in Figure 2, regardless of the BLOCK input .