You need to create new linestyles for your diff. types of lines if you want to beable to turn them off separately.
Site lines are a good example,
I typ create
Site-Thin
Site-Med
Site-Wide
That way I can turn off just the site lines and not the other lines.
Same thing with your situation, create new linestyles, ie
plumbing- thin,med, wide
elec.-thin,med,wide.