Facing the complex relay products, how to choose, the right to use, should be fully studied the actual use of the system conditions and technical specifications, duly propose a selected relay product technical performance requirements that must be met. Selection of components and applications. First according to the degree of importance of the whole system, reliability and environmental conditions for use and cost demands comprehensive consideration and selection. Specifically, according to article-by-article analysis of the following elements, call to confirm the level and magnitude. Choices must pay attention to the following requirements. All in all at the time of selection and application of relay, depending on the specific environmental conditions, facilitate comprehensive consideration of the costs and maintenance. The selected Relay must meet the technical requirements. While serious analysis of relay technique and its application in production, suitable, practical and reliable relay to contribute to the production.