In the old NEMA style OL relays with melting alloy elements it was not necessary to use all 3 elements when protecting a 1 phase motor. What changed is that when IEC (European) designed starters hit the US market years ago, they brought with them the IEC style OL relay which has what is called a "differential bar" in the trip mechanism. This was a cheap way of attaining current imbalance protection, because if any phase had more or less current running through it than the others, the differential bar would toggle partially and allow the other two phases to trip faster. While that worked OK on imbalances, a complete loss of phase current could be a nuisance if it wasn't a 3 phase system (where you WOULD want to detect a complete loss of phase). In Europe they don't use a lot of 1 phase motors outside of household appliances, so the IEC manufacturers didn't bother to make 1 phase OL relays. Their answer was to just loop one phase back through the 3rd unused OL element so that they all saw the same current under normal circumstances. Cutler-Hammer, in an attempt to try and market their product world wide, went to the IEC OL relay style years ago after they discontinued the old Citation starter line, so now you MUST loop through C-H OLRs to make them work on 1 phase applications.
[This message has been edited by jraef (edited 07-29-2006).]