What is the time interval between a high-set (instantaneous) unit and IDMT-relay? I know time between relay-relay is 0.3 - 0.4 seconds, is it the same between high-set and relay or not?
Since an Inst relay will pickup with no intended time delay, it is not realistic to consider separation by a time interval. Obviously, the only separation possible is by current settings.
In general terms, the main point to remember is that you should try to avoid two devices tripping for the same fault by their Inst trips. This is done by limiting Main device Inst settings to the maximum fault value seen at the bus upon which they are located, or by not having Inst trips on the Main device at all.
The time delay separation should be at the end of the inverse current curves(high current, low time), not at the Inst curves, which should be separated by current pickup values.
for:
7.5.4 Low set stage time multiplier/time delay
This is normally selected to give sufficient time delay between upstream and downstream operating
times to allow for breaker clearance and measurement errors etc. This is usually taken as 0.3 - 0.4
seconds