Contact design for switches and sockets: force, wipe and arc

Two products, two duties

A switch and a socket both make an electrical connection, but they do different work. A switch is operated occasionally and has to break a load, so the contact sees an arc at every operation. A socket is loaded once and then left alone, so the contact has to hold a plug firmly, carry current for hours and survive a limited number of insertions.

That difference decides the design. The switch contact needs a fast break, a defined gap and a material that resists arc erosion. The socket contact needs a spring that does not relax, a shape that wipes the plug pin and a temperature rise that stays low at full load. A design that treats them as the same problem usually gets one of them wrong.

The product standard sets the test that matters. A switch is tested for the number of operations at a defined load, with the contact temperature and the dielectric strength checked. A socket is tested for temperature rise, insertion and withdrawal force and endurance. The design should start from that test, not from the shape of the part.

Contact force and wipe

Contact force keeps the contact spot closed and breaks through surface films. Too little force and the resistance rises, the joint runs hot and the contact may weld. Too much force and the operating effort rises, the mechanism wears and the contact material erodes faster. The window is set by the material, the current and the expected life.

Wipe is the small sliding movement as the contact closes. It cleans the contact spot and moves it away from the pit that the previous arc left. A contact with no wipe switches on the same spot every time and fails earlier. A contact with too much wipe wears the plating and generates debris.

In a switch, the wipe comes from the geometry of the blade and the stop. In a socket, it comes from the shape of the spring and the lead-in of the pin. Both need to be designed, not left to the tolerance of a bend. State the force at the closed position and the wipe distance on the drawing, and test them on the assembly.

Material and finish

Silver and silver alloys are the default for switch contacts because they combine low resistance with resistance to arc erosion. Silver nickel and silver tungsten are common where the switching duty is heavier. Silver tin oxide is used where cadmium free materials are required and the duty is moderate to heavy. The choice depends on the load, the number of operations and the environment.

A socket contact often uses a copper alloy spring with a silver or tin finish. The spring has to hold its force at temperature, so the alloy and the temper matter as much as the coating. A brass spring that is cheap and easy to form may relax at a high ambient, and the contact force falls over the life of the product.

The finish has to survive the wipe. A thin tin finish on a socket contact is common and works well at moderate current, but tin is soft and can wear through at the contact spot. Silver has better resistance and wear behaviour at a higher cost. The choice should follow the current, the insertion count and the environment, not the price of the coating alone.

The arc and the break

An arc forms when the contact opens and the current keeps flowing through the ionised air. The arc heats the contact face, erodes the material and can weld the contacts if the current is high enough. The design has to break the arc quickly and move the contact spot away from the damage.

A fast break helps, and so does a wider gap. Magnetic blowout can move the arc away from the contact face in a larger device. In a small wall switch, the geometry and the speed of the mechanism do most of the work. The contact material has to tolerate the arc energy that remains.

The make is gentler than the break in most switches, but not always. A capacitive load can produce a high inrush at make, and a motor load can produce a high current at both. The load type belongs in the specification, because a contact that is fine for a resistive load can fail on a motor or a lamp load.

Life testing

The life test is the only proof that the contact design works. It has to use the rated current, the rated voltage, the load type and the operating rate. A test at a lower current or a resistive load only proves that the contact can carry current, not that it can break it.

Record the contact resistance, the temperature rise and the contact gap through the test. A contact that passes the operations but ends with a resistance four times the initial value has used up its margin. A contact that welds once has failed, even if it separates afterwards.

For a socket, the life test is an insertion and withdrawal test plus a temperature rise test at full load. Measure the force at the start, the middle and the end. A spring that loses force early will overheat later, and the temperature rise is the symptom that matters to the user.

The socket contact and the plug

A socket contact is a spring in a housing. It has to accept a pin with a defined force, hold it against a pull and carry the current without overheating. The contact shape, the material and the housing all contribute. A contact that grips well but cannot dissipate heat will still run hot.

The lead-in matters as much as the contact area. A sharp lead-in damages the pin and the plating, and a wide lead-in reduces the contact area. The contact should wipe the pin as it enters and settle on a defined surface. The insertion force and the withdrawal force are separate requirements, and both belong on the drawing.

The housing holds the contact in position and provides the creepage and clearance. A contact that moves in the housing changes the force and the alignment. The housing material has to hold its dimensions at the operating temperature, and the contact retention has to survive the pull test without moving.

What the drawing has to say

The table below lists the calls that decide a switch or socket contact. A drawing that states only the material and the shape leaves the function to the supplier.