This document tries to clear out some details of transmission lines and cable inductance. This document is only a brief introduction to those topics. If you expect to work much with transmission lines, coaxial or otherwise, then it will be worth your while to get a book on that subject. The ideal book depends on your background in phsics or electrical engineering, and in mathematics.
Current loops are a standard which are used widely in process automation.
20 mA are wirely used for transmitting serial communication data to programmable process controlling devices. Other widely used standard is 4-20mA current loop, which is used for transmitting analogue measurement signals between the sensor and measurement device. A potentially dangerous ground fault is any amount of current above the level that may deliver a dangerous shock. Color code of AC power cords Desciption of color codes in AC power cords.