One of the key requirements for any critical real-time automation signals is the overall time for the decision of one function to be taken into use by the operation of another function. Protection systems sometimes have extremely fast Critical Fault Clearance Time requirements. As an example, the Australian National Electricity Rules define the total fault clearance time limits as follows: (click image to enlarge)  
Taking into account original fault detection, protection operation time, time to detect failed CB, time to CBF element to operate and finally time for the upstream CB to operate. Hence the time for messages to be passed from one function to another can be critical and must take into account total network latency IED message sending and receiving processing. Whilst the Standard Part 5 refers to times "in the order of a quarter cycle", it is not really tied to the 20 ms or 16.67 ms period of a 50 Hz 60Hz cycle. It is to do with ensuring that the performance of the LAN based signals is faster than wire based connections. For wire based systems, IEDs with a physical relay output take 3-10 ms to operate the actual output relay. IEDs with opto inputs have de-bounce filters etc which can also have 3-10 ms delay. We just never bothered to identify these times when evaluating relay overall performance. Bearing in mind that the same signal being sent from a Logical Node to another Logical Node now has to go through the outgoing stack and the incoming stack at the other end, plus through several LAN switches in between, it is important to maintain that level of performance or better - as function operation t function operation stacks and network included - than the wire based schemes.  
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