I have a customer that has asked that R class fuses be used on LV building service distribution and lighting transformer primaries. They would also be inclined to extend that requirement to other transformers, such as what I use for step down to servo power. Their reason for this is to eliminate "nuisance tripping". My suspicion is that this arises from the customer previously having an improperly sized J (with an overburdened transformer), put an R in place, and "solved" the problem. So: I'm wondering, given that my fuse catalogue and other information indicates that a class J is the best choice for a transformer primary, what advantage a class R would give? The Gould catalogue I have in my library even goes as far as saying that class R's are generally not used on new installations. The differences that I can see between the J and R is that the R has a longer trip curve, and is physically much larger. Anthony Kerstens P.Eng.