ABSTRACT
This chapter presents a comprehensive discussion of guarding and shielding techniques which are available in electronic measuring systems in order to avoid any kind of interference. Suppose two instruments are required, both of which need shielding; more than one shielding cabinet is involved and it is recommended that only one earth connection is used for the two cabinets. Inside the instrument shielding techniques too can give a large improvement of the common-mode rejection ratio. Most instruments use a mains supply transformer to deliver the required electric power. In normal circumstances the instruments are connected to the mains via a three-core cable, of which one lead carries the power voltage and the other is the neutral or null lead. This null lead is connected to earth at the power distribution station where the starpoint is always earthed at the secondary coil of the step-down transformer.
