ARTCC CTLR MISJUDGED SPACING WHILE DESCENDING AN SMT THROUGH THE ALT OF ANOTHER SMT AND HAD LESS THAN STANDARD SEPARATION.

Narrative

BOTH ACFT WERE IN STRAIGHT AND LEVEL FLT BEFORE THE ERROR. SMT X AT 7000' WAS DSNDED TO 4000' TO MEET THE MSP APCH RESTRICTION. THE ACFT WERE TO PASS LATERALLY; BUT NOT 5 MI OR MORE. MY LACK OF EXPERIENCE IN THE LOW ALT SECTORS CAUSED ME TO MISJUDGE THE RANGE BEING USED AT THE TIME. I DO BELIEVE THAT MANY OF THESE TYPE OF ERRORS ARE TAKING PLACE ACROSS THE COUNTRY BECAUSE OF THE HI/LOW CONCEPT. THERE IS USUALLY A CAUSE WRITTEN DOWN; BUT THE UNWRITTEN CAUSE CAN BE WORKING HIGH ALT OR VICE VERSA MOST OF THE TIME. I HAVE YET TO MEET A CTLR WHO CAN WORK HIGH AND LOW AND BE GOOD AT BOTH. USUALLY; A PERSON IS COMFORTABLE IN ONE OR THE OTHER; BUT NOT BOTH. HI/LOW HAS CAUSED A DECREASE IN THE SVC WE PROVIDE TO THE FLYING PUBLIC AND THE CLOSER TO THE MAJOR ARPTS A PERSON WORKS; THE MORE THIS BECOMES A PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: DURING CALLBACK TYPE ACFT WAS GIVEN. RPTR WAS WORKING A COMBINED POS OPERATION WITH LIGHT TFC VOL. TFC ADVISORY WAS NOT GIVEN NOR DID THE ACFT INDICATE THAT THEY SAW EACH OTHER. ARTCC HAS BEEN TRANSITIONING TO THE HIGH/LOW CONCEPT FOR THE LAST 2 YRS. RPTR DOES NOT LIKE THIS CONFIGN; EVEN THOUGH MOST OF THE ARTCC HAVE CONVERTED TO IT. RPTR OLD TIMER WHO HAS NOT BEEN AT THIS FAC TOO LONG AND FAC TRANSFERRED FROM HAD NOT YET GONE TO THIS CONFIGN.