Years ago, Siemens put out a series of training courses on-line that were very good for learning the basics. Siemens no longer publishes them because some of the data is now dated with regards to products they sell (and because this was not written in Germany...). But the concepts are the important thing and so some distributors have them archived. Here's one.
EandM provides industrial automation software and hardware for manufacturing data acquisition, plant intelligence, resource tracking, inventory and SCADA control systems.
www.eandm.com
You can skip down to the ones on distribution equipment, but it's designed to step you through the process by building on knowledge. so if you are not familiar with how circuit breakers work, you might want to start there.