When you have completed this chapter, you should be able to define the basic principle of current differential protection. This includes a clear understanding of the concept of bias, which is a crucial principle in current differential protection. Identifying the need for bias and being able to derive both biases and operate quantities is a key skill that will make you feel more skilled and proficient. You will also be able to assess the advantages and disadvantages of different types of current differential protection schemes and understand the requirement for a sound, reliable communications system in differential protection.