Westinghouse made a design change to the DB circuit breaker line about mid-way through production with the addition of a "test position" for the circuit breaker while it is in the switchgear. The original design, now designated "single position" only had a provision for the breaker to be secured in the cell while it was engaged to the primary buss stabs. Once withdrawn from the buss stabs the breaker would simply slide out of the cell on its rails. The new improved "3 position" breaker and switchgear had a provision for the breaker to be engaged on the buss stabs, withdrawn from the buss stabs but in a stable locked "test" position on the rails or disengaged to be withdrawn. This new test position allowed for safer lockouts during service among other things.
One of the most common questions we ask people when they are looking for a DB vintage circuit breakers and parts is, "is it a single or 3 position breaker?" If a photo of the breaker isn't available, and because it isn't listed on the nameplate, there are a series of questions we've developed to help identify the breaker style, assuming they have seen the breakers.
DB Single Position
DB 3 Position
The most obvious question is "does it have a movable or fixed escutcheon (faceplate)?". The 3 position breakers all had movable escutcheons so the switchgear door could be closed with the breaker in the test position.
If that doesn't help, the next question is "how does the breaker close and trip?". This works for about 99.5% of DB-15, DB-25 and DB-50 breakers. Single position breakers close by turning the handle clockwise a quarter turn and trip by turning the handle counter clockwise. 3 position breakers close by turning the handle clockwise and trip by pressing a rectangular trip button directly above the handle.
For larger frame DB-75 and DB-100 breakers, which are all electrically operated, you need to look for the movable escutcheon plate vs. a flat front, or when looking at the cell, the window in the door is much smaller on a 3 position, approximately 5" x 7" whereas the single position is wider and taller to expose the whole front faceplate of the breaker.
EXCEPTIONS TO THE RULE!
Stationary/fixed mount breakers are always single position.
When someone needed a replacement breaker for a single position during the "3 position breaker era", Westinghouse would modify a 3 position breaker to what we call a "new style single position", it used the same handle as a 3 position, but they added a round trip button at about 2 o'clock above the handle. This accounts for the other 0.5%.
Other things to know...
There was an option for 3 position breakers to have a "spring assist" mechanism. This spring greatly aided the speed at which the breaker would close. This is available for DB-15, DB-25 and DB-50 only. Non-spring assist 3 position breakers are interchangeable. DBL(fused) breakers are always 3 position.
Field breakers designated DBF-6 and DBF-16 are usually 3 position, but it's worth double checking. Single position field breakers would normally be designated DB-25F and DB-50F.
Field modifications by end-users are always a possibility. I saw one line-up of single position breakers that an engineer had modified by removing the series overcurrent trips and bolting fuses in their place. Without single phase protection, the breaker had no way to trip and the fault on one phase cascaded up to the main, but by then it had also caught on fire! When in doubt, pictures are the best solution.