This guide covers the design@ installation@ and monitoring of time synchronization systems in power utility substations. This includes time sources such as Global Positioning Satellite (GPS)@ and time distribution systems such as Inter-Range Instrumentation Group Format B (IRIG-B)@ Network Time Protocol (NTP) or Simple Network Time Protocol (SNTP)@ and IEEE Std C37.238? plus IEC/IEEE 61850-9-3. This guide assumes that the time source and the intelligent electronic device (IED) have accurate clocks. Purpose Time synchronization is now critical to the smart grid@ with substations using legacy systems and new standards for network time synchronization@ a guide is needed to help ensure adequate system design is undertaken to support the applications relying on accurate and reliable time.