WEB LINKS

http://www.slideshare.net/prernasharma10297701/element-of-switching-system?utm_source=slideshow02&utm_medium=ssemail&utm_campaign=share_slideshow_loggedout - Elements of switching system

Google ebooks:

1)https://books.google.co.in/books?id=btVRxi1-JLMC&printsec=frontcover&source=gbs_vpt_reviews#v=onepage&q=manual%20switch%20board&f=false

2)https://books.google.co.in/books?id=ggyDBgAAQBAJ&printsec=frontcover#v=onepage&q=manual%20switch%20board&f=false