Over-current performance of movable springs of a latched-type relay used in Watt-hour meters

Latched-type relays used in smart Watt-hour meters need to carry a current some ten times higher than its rated value when a short circuit happens in its testing region. The Holm's repelling force and the static welding are two major challenges faced to the contact system leading by the short circuit current. Movable springs of those relays are usually metal stacks in “V” shape for providing not only additional force to overcome the Holm's repelling force but also tangential motion to break the static welding. Transient electro-thermal coupling analysis was carried out to a typical contact system under a variety of short currents with different lasting time to evaluate the highest temperature rise for the metal stack. Then, thermo-structural model was used to predict, firstly the expansion and elongation degree of the metal stack when the contacting point was set to be free (without welding) and secondly the tangential force exerted on the contacting point when it was set to be fixed (with welding).