16.3.8.2 Real-time clock (RTC) operationThe IED has a built-in real-time clock (RTC) with a resolution of one second. Theclock has a built-in calendar that handles leap years through 2038.Real-time clock at power offDuring power off, the system time in the IED is kept by a capacitor-backed real-time clock that will provide 35 ppm accuracy for 5 days. This means that if thepower is off, the time in the IED may drift with 3 seconds per day, during 5 days,and after this time the time will be lost completely.Real-time clock at startupTime synchronization startup procedureThe first message that contains the full time (as for instance SNTP and IRIG-B)gives an accurate time to the IED. The IED is brought into a safe state and the timeis set to the correct value. After the initial setting of the clock, one of three thingshappens with each of the coming synchronization messages, configured as “fine”:• If the synchronization message, which is similar to the other messages, fromits origin has an offset compared to the internal time in the IED, the message isused directly for synchronization, that is, for adjusting the internal clock toobtain zero offset at the next coming time message.• If the synchronization message has an offset that is large compared to the othermessages, a spike-filter in the IED removes this time-message.• If the synchronization message has an offset that is large, and the followingmessage also has a large offset, the spike filter does not act and the offset inthe synchronization message is compared to a threshold that defaults to 500milliseconds. If the offset is more than the threshold, the IED is brought into asafe state and the clock is set to the correct time. If the offset is lower than thethreshold, the clock is adjusted with 10 000 ppm until the offset is removed.With an adjustment of 10 000 ppm, it takes 50 seconds to remove an offset of500 milliseconds.Synchronization messages configured as coarse are only used for initial setting ofthe time. After this has been done, the messages are checked against the internaltime and only an offset of more than 10 seconds resets the time.Rate accuracyIn the IED, the rate accuracy at cold start is 100 ppm but if the IED is synchronizedfor a while, the rate accuracy is approximately 1 ppm if the surroundingtemperature is constant. Normally, it takes 20 minutes to reach full accuracy.Time-out on synchronization sourcesAll synchronization interfaces has a time-out and a configured interface mustreceive time-messages regularly in order not to give an error signal (TSYNCERR).Normally, the time-out is set so that one message can be lost without getting aTSYNCERR, but if more than one message is lost, a TSYNCERR is given.1MRK 502 034-UEN - Section 16Basic IED functions493Technical Manual
