Test Procedure Working Groups » IEC 61850 TPWG » Test Procedure Issues » Server

Instead of using a percentage it's better to use number of significant digits. The maximum number of significant digits for FLOAT32 is 6 to 7 digits (depending on the value). Not every device / data point may support the maximum number of significant digits. We allow a lower number of significant digits when specified/documented in the MICS (and/or PIXIT). sMdl15 allows differences in the non-significant digits.
For example:
- SCL value = 5.1234567
- the number of significant digits is 5 => the significant value becomes = 5.1235
- sMdl15 does allow all (MMS) values from the device between 5.1234501 and 5.1235499 because the significant value is 5.1235

The IEC 61850-7-2 defines BasicType FLOAT32 as the IEEE 754 single-precision floating point (61850-7-2 Table 2 - BasicTypes).

The IEEE 754 standard specifies this format as having: a sign bit, 8 bits of exponent and explicitly stored 23 significant bits of fraction (in sum occupying 32 bits). The single-precision floating point format covers a large range of values -3.4E+38 to+3.4E+38, but it has limited precision. The format gives 6 to 9 significant decimal digits precision. When converting decimal values to values of the Float32 data type, depending on the value, a rounding error might occur from the seventh decimal place on. From technical point of view the float32 encoded values have guaranteed only 6 significant digits.

In my opinion, if a DUT does not state explicitly - in MICS, for BasicType FLOAT32 - its limitation in the internal float precision, then the expected result are 6 significant digits. As well by decimal values read from SCL as mapped to/from MMS or GOOSE.