IEC/IEEE 61850-9-3 (Power Utility Profile) or PUP is an international standard for precise time distribution and clock synchronization in electrical grids with an accuracy of 1 μs. It supports precise time stamping of voltage and current measurement for differential protection, wide area monitoring and protection, busbar protection and event recording. It can be used to ensure deterministic operation of critical functions in the automation system. It belongs to the IEC 61850 standard suite for communication networks and systems for power utility automation. IEC/IEEE 61850-9-3 is a profile (subset) of IEEE Std 1588 Precision Time Protocol (PTP) when clocks are singly attached. IEC/IEEE 61850-9-3 provides seamless fault tolerance by attaching clocks to duplicated networks paths and by support of simultaneously active redundant master clocks. For this case, the extensions to PTP defined in IEC 62439-3 Annex A apply. Main features IEC/IEEE 61850-9-3 uses the following IEEE Std 1588 options: uses the PTP timescale based on TAI International Atomic Time, also delivers UTC Coordinated Universal Time communicates on Layer 2 over Ethernet (IEEE 802.3) links, either in a Local Area Network or in a Wide Area Network (Metro-Ethernet) measures the link delay by peer-to-peer (Pdelay) message exchange transmits the clock correction indifferently in 1-step (preferred) or in 2-step operates with the default best master clock algorithm, performed by master and by slave clocks Performance IEC/IEEE 61850-9-3 aims at an accuracy of better than 1 μs after crossing 15 bridges with transparent clocks. It assumes that all network elements (bridges, routers, media converters, links) support PTP with a given performance: Grandmaster (GC): 250 ns maximum inaccuracy Transparent Clocks (TC): 50 ns maximum inaccuracy Boundary Clocks (BC): 200 ns maximum inaccuracy Media Converters: 50 ns maximum jitter and 25 ns maximum asymmetry Link asymmetry: 25 ns maximum asymmetry By relying on these guaranteed values, the network engineer can calculate the time inaccuracy at different nodes of the network and place the clocks, especially the grandmaster clocks suitably. IEC TR 61850-90-4 (Network engineering guidelines) gives advice on the use of IEC/IEEE 61850-9-3. IEEE 1588 settings IEC/IEEE 61850-9-3 restricts the parameters of IEEE Std 1588 to the following values: domainNumber: 0 (default), 93 (when conflict exist with another PTP time distribution) Announce interval: 1 s (fixed) Sync interval: 1 s (fixed) Pdelay interval: 1 s (fixed) Announce receipt time-out: 3 s (fixed) priority1: 255 for slave-only priority2: 255 for slave-only transparent clock primary syntonization domain: 0