But they are counted separately. The upstream and downstream are configured independently.
But the MTBE threshold will be much lower on the upstream than it is on the downstream
No its not - its exactly the same. They are just counted and configured independently of each other.
If your MTBE for the upstream = 30 (or 60 for standard) then you go red and the DLM makes a change to the upstream only.
Theres no reason for it to be lower for the upstream. An error is an error and once you are getting more than 'x' then the DLM will take appropriate action. Thats why they use MTBE for the calculation rather than 'x' per day.
and don't know how the DLM reacts when the US errored second has reached it's limit
Then it applies changes to the upstream
only - as long as the downstream hasnt also reached its limit.
As proof of this you only have to look at my own line last October when I had some sort of line fault that affected only my upstream. My upstream MTBE was less than 30 and so the ILQ went red the DLM applied Interleaving the next day. My downstream was completely untouched. It went through the same changes & process as it would if the problem was on downstream. First it applied Interleaving (& error correction) , then it changed level of INP (which in turn increased the depth of interleaving & ErrCorrection) , then it banded the sync.