Its actually 24-pins and 8-pins connectors. The 24-pins connector supplies power to the motherboard, and the 8-pins connector supplies power to the CPU.
If you have a red switch at the back of the PSU, it is for selecting 220/110 Volts input. Whatever you did, if the wires (insulation) on the 8-pins connector melted, it means that there is a short-circuit somewhere that caused it.
DO NOT re-connect anything until the source of the short-circuit is determined and corrected.