A Masterpact MP breaker with an STR58U trip unit is nuisance tripping at low currents
MASTERPACT MP, MC, M series circuit breakers (CURRENTLY OBSOLETE)
The trip unit on a breaker tripping with low current levels should be replaced.
These are sold to Schneider Services only and they must do the replacement.
See Supplemental Digest 177, page 11-37 for catalog numbers.
The alternative is to replace the breaker with a Masterpact NW. Retrofit work is required.