20.6.3.2 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 LON, SNTP and GPS)gives an accurate time to the IED. After the initial setting of the clock, one of the threefollowing conditions happens with each of the coming synchronization messagesconfigured as FineSynchSource:• If the synchronization message, which is similar to the other messages, has anoffset compared to the internal time in the IED, the message is used directly forsynchronization, which means, for adjusting the internal clock to obtain zerooffset at the next coming time message.• If the synchronization message has a large offset compared to the other messages,a spike-filter in the IED removes this time-message.• If the synchronization message has a large offset and the following message alsohas a large offset, the spike filter does not act and the offset in the synchronizationmessage is compared to a threshold that defaults to 500 milliseconds. If the offsetis more than the threshold, the clock jumps a whole number of seconds so theremaining offset is less than 500ms. The remaining offset is then slowly adjustedwith 1000 ppm until the offset is removed. With an adjustment of 1000ppm ittakes 500 seconds to remove an offset of 500 milliseconds.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.Rate accuracyThe Rate accuracy is the accuracy of the internal clock while the external time signalis lost (e.g. GPS lost). In the IED, the rate accuracy at cold start is 100 ppm but if theIED is synchronized for a while, the rate accuracy is approximately 1 ppm if thesurrounding temperature is constant. It can take up to one hour after a cold start of theIED to reach to a stable rate due to the normal warming-up procedure of the IED.Time-out on synchronization sourcesAll synchronization interfaces have a time-out, and a configured interface mustreceive time synchronization messages regularly in order not to give an error signal(TSYNCERR) that is reported from the INTERRSIG function block. The time-out is1MRK502052-UEN B Section 20Basic IED functions1111Technical manual
