The methodology to compute the time accuracy from the SNTP Root Dispersion attribute value explained as "The unadjusted DELTA to the SNTP Root Dispersion attribute" is not clear.
The extract from RFC 5905 discusses EPSILON and LAMBDA and DELTA but does not explain which of these is used to compute TimeAccuracy.

Proposal

Explain exact calculation, for example TimeQuality.TimeAccuracy =
16-LOGbase2(SNTP.RootDispersion+(0.5*SNTP.RootDelay)) where SNTP.x are the 32-bit (integer) values at octet offset 4 and 8 in the SNTP response from th server. Note that if the calcuation results in negative values then ClockNotSynchronized shall be set.

Discussion

Created

Status

Not Applicable

06 Dec 21

Not Applicable

Refused. The method of timesync is out of scope for 61850 and therefore the formula for calculation of the error is a local issue.

27 Nov 20

Approval (N/A)

Time accuracy is the estimated error on the time stamps on an IED, which encompasses all sources of error, NTP being one of them.
RFC 5905 is vague about the calculation of "precision exponent", the other parameters are not well defined. One cannot use directly Rootdispersion for this purpose, and the client can compute itself the variance, which gives a more conservative estimate.
IEC 61850-8-1 should not prescribe how to interpret RFC 5905, the IED manufacturer is requested to provide a conservative value, especially when the client clock is in holdover.