Spring contact fingers: principle and benefits

A spring contact finger is an elastic contact installed between two rigid input and output electrical contacts. Once it is compressed by a defined amount, it generates elastic force that ensures reliable contact. The operating principle is straightforward: each spring coil acts as a contact point, so the spring behaves as many circuits in parallel. The more contact points and turns, the smaller the contact resistance, the greater the shunting, and the smaller the current flowing through each turn.

How the design performs

With many turns, current distribution is even and heat dissipation is good. The relatively large, stable contact-force range ensures that contact resistance remains stable under vibration. Contact fingers can be produced in different forms, including single-surface, double-surface and round-surface contacts.

The contact resistance of a single-turn spring contact finger is 4-5 microohms. With, for example, 50 turns in contact, the combined resistance is about 0.1 microohm, so resistive heat loss is very small.

Material and contact wear

Beryllium copper offers high hardness and strength, while the strength of zirconium copper is a closer fit for some applications. Contact friction is low, more than half lower than that of a plum-blossom (multilam) contact. This matters for high-current applications: a 3150/4000 A contact with 82 fingers has high contact pressure, so racking the circuit breaker in and out requires a large torque and is difficult, whereas the spring contact finger does not require a large operating force.

Why a silver-graphene coating is used

Plum-blossom contacts are silver-plated, but because contact pressure is high and the contacts are frequently plugged and withdrawn, the silver layer wears and can peel. This exposes copper, which then corrodes rapidly as an anode and loses its conductive function, threatening the safety of power-grid operation. The newer approach is to prepare a silver-graphene composite plating on the copper contact substrate, which improves the corrosion resistance and wear resistance of the contact material. The coating has high hardness and excellent wear and corrosion resistance, but its cost is high.

Operational advantages and shapes

A spring contact finger is selected because its low contact pressure and many contact points produce small contact wear, and it can be racked in and out 10,000 times. Its small outer diameter, high current-carrying capacity and strong short-circuit resistance give a uniform electric field in the contact system.

Alignment is a common concern with spring contact fingers in draw-out circuit breakers. After improving machining accuracy, the positioning pin is located in advance, which fully meets the plus-or-minus 1 mm alignment requirement. Spring contacts do not have to be round: other shapes can be made for different uses, such as rectangular contacts for frame circuit breakers and single-sided contacts for low-voltage drawers.