• The maximum error of the last used synchronization message• The time since the last used synchronization message• The rate accuracy of the internal clock in the function.17.2.8.2 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-timeclock that will provide 35 ppm accuracy for 5 days. This means that if the power is off,the time in the IED may drift with 3 seconds per day, during 5 days, and after this timethe 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) givesan accurate time to the IED. The IED is brought into a safe state and the time is set tothe correct value. After the initial setting of the clock, one of three things happens witheach of the coming synchronization messages, configured as “fine”:• If the synchronization message, which is similar to the other messages, from itsorigin has an offset compared to the internal time in the IED, the message is useddirectly for synchronization, that is, for adjusting the internal clock to obtain zerooffset 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 in thesynchronization message is compared to a threshold that defaults to 500milliseconds. If the offset is more than the threshold, the IED is brought into a safestate 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. Withan adjustment of 10 000 ppm, it takes 50 seconds to remove an offset of 500milliseconds.Synchronization messages configured as coarse are only used for initial setting of thetime. After this has been done, the messages are checked against the internal time andonly an offset of more than 10 seconds resets the time.Section 17 1MRK 502 043-UUS BBasic IED functions550Technical Manual
