NCT Controller described a loss of B757 separation event on final to SFO when he/she failed to note the 'BAT'; (an electronic RADAR display separation device); was set at 3 miles instead of the required 4 miles in this instance.

Narrative

A 3 mile 'BAT' had already been placed on the B737. [I was] briefed that the B737 was reduced from 180 KTS to 160 KTS; as was the preceding aircraft; a B757; until 5.3 mile final. The ground speed showed 190 KTS; just as other aircraft on approach had indicated 190 KTS ground speed when at 180 KTS. Watching the replay; this speed was assigned approximately 17 miles from airport and ground speed remained constant at 190 KTS for almost 8 miles. At this point I had just taken sector and issued 160 KTS to the B737 approximately 10 miles from airport and it was acknowledged. I was basing my separation on the 3 mile 'BAT' and sufficient distance remained for more than 3 mile spacing at the threshold. I changed the B737 frequency to SFO Tower approximately 6 miles from the airport and they immediately issued 'go-around' instructions. At this point I saw the spacing was adequate for 3 miles but saw that 4 miles was needed. I was fully aware the preceding aircraft was a B757 and 4 miles was required. I was not aware; and did not notice; that the 'BAT' was incorrectly applied by the previous controller. Again; I was basing separation with the incorrect 3 mile 'BAT'. As I received the hand-offs at this sector I determine the spacing needed by applying the 'BAT'. Only 2 'BATs' were displayed as I took the sector. Obviously; I should have checked the previous controller's use of the 'BAT'. The previous controller had issued the approach clearance. I just worked the aircraft for about 4 or 5 miles; watching the speeds closely; before changing the frequency. It is obvious now that the required distance was not maintained.