Spring strap contacts in high-voltage equipment

In the power transmission and distribution industry, an electrical connection must be safe and stable. A spring contact achieves this through very low heat generation, very low contact resistance and a stable, uniform contact force that ensures a good electrical connection. Based on design requirements and field experience, this article introduces the spring strap contact finger and its use in high-voltage electrical appliances.

Common problems and their causes

The main failure mechanism of an electrical connection is poor contact, which appears as increased or unstable contact resistance. Poor contact causes abnormal heat generation that can exceed the allowable temperature rise specified in the relevant standards, degrading the insulation and potentially leading to a short circuit.

Contact theory

An electrical connection transfers electrical energy between conductors through mechanical force. The contact surface over which this force acts is not equal to the useful contact area carrying the load current. A contamination layer on the metal surface, made up of an oxide layer, oil film and metal compounds, makes the useful contact area far smaller than the apparent surface area, and the current channel is concentrated at single contact points, raising the contact resistance. What appears to be a connection between surfaces is in fact a connection between points.

Spring strap contact fingers

Common contact types on the market include spring strap contacts, spring fingers, plum fingers and self-actuating fingers. Spring strap contacts have been available since the early 1960s and have been in successful service for more than half a century. They are an excellent choice for demanding applications: they operate safely where temperature, oscillation and shock are critical, they carry high current and suit high-load high-voltage switches, and their high number of mating cycles fully meets switch life-test requirements.

Compared with conventional spring fingers, spring strap contacts offer three main advantages: a small slot size, low contact resistance and high stability.

Applications and trend

Miniaturization is the direction of GIS development, and spring strap contact technology is moving towards more compact designs in step with this change. As an electrical contact, spring strap fingers are widely used in electrical appliances, circuit breakers, transformers, switchgear and gas-insulated busbars, where their very low heat generation, very low contact resistance and stable, uniform contact force ensure a good electrical connection and bring long-term benefits to the power system.