CMH Controller described a TCAS RA event between IFR traffic level at 5;000 FT and VFR traffic reported/observed at 4;500 FT; the reporter questioned TCAS software vs. ATC separation standards.

Narrative

C172 was on an assigned heading of 240 to have him pass 15 miles East of CMH at 45; so he would be below the glide slope for the 28L ILS which was the only one in use as Runway 28R was closed. EMB-145 was on an assigned heading of 310 to intercept the ILS 28L localizer and level at 50. I exchanged traffic at approximately 10 miles separation and both pilots acknowledged they would look for traffic but said it was hazy. I issued traffic again at approximately 2 miles and neither one reported the other in sight. C172's Mode C altitude had indicated 46 when I first issued traffic at 10 miles and I gave him the altimeter setting and he verified he was level at 45. The required separation standard between IFR and VFR aircraft in Class C outer area airspace is 500 feet. EMB-145 advised that his TCAS had issued a resolution advisory and that they were climbing when the aircraft were within 1/2 mile of each other. I advised them that there was no other traffic and asked them to advise when they were able to return to 50. EMB-145 appeared to climb to 57 before returning to 50 about 1 1/2 miles past C172. At that point; I cleared EMB-145 for the ILS 28L. I advised them that they appeared to be above the glide slope; but they said they wished to continue with the approach. I acknowledged them and told them to contact CMH tower. Recommendation; I probably should have turned C172 so that EMB-145 could have intercepted the glide slope without being the minimum altitude above him. I was trying to best accommodate both aircraft while using minimum separation standards. TCAS doesn't seem to agree with our standards.