I've come across a proof for star-delta transformation which goes like this.(Refer to the diagram for notation. Pardon me for my bad drawing skills.) In the delta, he found the effective resistance between two vertices ( say a and c, which can found easily). Then he found the effective resistance between the same two vertices (a and c) in the star. According to him Rac in the star(which denotes the resistance between a and c) is equal to Ra + Rc. What about Rb? It might not be connected here but it may in a circuit. It is like this(refer to second attachment). The resistance about ac in the left diagram of second attachment is 2[tex]\Omega[/tex](drawing analogy from the "faulty" proof) but link it to another circuit (shown in right diagram), the resistance about ac is different. So just because it isn't connected, it doesn't mean that you can ignore it. Here's the surprise. The proof gives the CORRECT EQUATIONS. Work it out. You'll end up with the right thing!Can anybody make things clear for me?