DMMs: the importance of choosing the correct lead configurations

The outcome of a particular measurement can be adversely affected if the wrong terminals, connecting leads or measurement configuration have been used. DMM (Digital Multi-Meter) terminals are usually chosen by the equipment manufacturer to provide the best performance for a given sensitivity and typical operating environment. The connecting leads are often made by the user and may be inappropriate in terms of materials and length. Finally, connecting line-powered instruments together can result in unwanted circulating currents between them. This paper explores the reasons why particular materials and interconnection methods are used for d.c. and a.c. voltage measurements and suggests ways of avoiding problems. Three separate problems will be examined from the point of view of connections: thermal e.m.f.s, connections to an a.c. source, guarding