Contact design for relays and contactors: low-level vs power switching

A relay and a contactor are not scaled versions

A relay is an electromechanical switch. It may carry a signal at milliamps or switch a small load at a few amps. Its contact system is optimised for reliable low-level behaviour, low bounce and long mechanical life. The coil and the magnetic circuit dominate the package, and the contacts are small.

A contactor is a power switch. It starts and stops motors, heaters and other heavy loads. Its contact system is optimised for arc control, erosion resistance and the ability to carry and break a high current. The mechanism is larger, the force is higher and the arc chamber is part of the design.

The two products share a principle but not a design rule. A contact that works in a relay may weld in a contactor. A contactor contact may never make a reliable low-level circuit because its force and wipe are set for power, not for film breakdown.

The low-level zone

At low voltage and current, the contact does not have enough energy to break down a surface film. Oxide, sulfide, tarnish and organic contamination can insulate the interface. The contact may read open or unstable even though the metal is sound.

The design answer is mechanical, not just metallurgical. A high force over a small area, a wipe that scratches through the film, and a contact material that resists tarnish all help. Gold plating is common for very low level signals because it does not form a thick oxide. Silver is used where some film breakdown is possible and cost matters.

The load also matters. A dry circuit at a few millivolts cannot clean a film. A slightly wet circuit at a few volts and milliamps can break it down. The datasheet rating should state the load range, not just a maximum.

The power zone

At power level, the contact draws an arc at every break. The arc melts a small volume of metal, and the molten bridge can transfer material or weld the faces. Erosion and welding become the limiting failure modes, and the design has to manage the arc energy.

A contactor uses a larger face, a higher force and an arc chamber to move the root away from the contact. The material is chosen for erosion and welding resistance, often silver tin oxide or a tungsten containing grade. The spring and the magnetic circuit are sized to close against the inrush and to open against the arc.

The contactor rating is tied to a utilisation category. A category for resistive loads is less severe than one for motor starting. The same contactor may have different ratings for different categories, and the contact life depends on which one applies.

Force, wipe and bounce

Force sets the real contact area and the film breakdown. In a relay, the force is often a compromise between contact resistance and the magnetic pull available. Too little force and the contact is unstable. Too much and the coil has to be larger or the return spring stiffer.

Wipe is the sliding motion at make. It breaks films and can improve low-level reliability. It also wears the plating and moves the arc root. A small, controlled wipe is usually better than a large one. The wipe should be set by the mechanism geometry, not left to the tolerance stack.

Bounce is the rebound after impact. It produces a series of short arcs at make, which erode the face and can weld it. In a relay, bounce also produces electrical noise that can disturb a logic input. In a contactor, bounce adds erosion and mechanical wear. The spring, the armature and the damping have to be designed together.

Materials by zone

Gold and gold alloys are used for very low level and dry circuit contacts. Gold does not form a thick oxide, so the contact stays conductive at low voltage. It is soft, wears faster and is expensive, so it is usually a thin plating over a harder base.

Fine silver and silver nickel are used for signal and small power contacts. Silver can tarnish, but the film is soft and can be broken by wipe or by a slightly higher voltage. Silver nickel adds hardness and resistance to erosion at a small cost in resistance.

Silver tin oxide is the general power contact material. It resists erosion and welding and is cadmium free. Silver tungsten and silver tungsten carbide are used for high energy and DC duties. They are harder, need more force and are usually brazed.

The contact material is only part of the system. The plating, the carrier, the joint and the atmosphere all affect performance. A gold plated contact on a contaminated carrier can still fail at low level.

The magnetic circuit and the contact

The coil and the magnetic circuit set the force available at the contact. A relay with a weak magnetic pull may use a lighter contact force, which limits its low-level stability. A contactor with a large magnetic circuit can supply a high force, but the impact energy at make is also higher.

Dropout behaviour matters. When the coil is de-energised, the return spring opens the contact. If the spring is too weak, the contact may open slowly and arc for longer. If it is too strong, the coil has to work harder to hold the contact closed. The balance is part of the contact design.

Chatter at the end of the stroke can produce multiple short arcs and rapid erosion. Damping, shading rings in AC magnets and a controlled air gap help. The contact and the magnet should be tested together, not as separate components.

Load types that change the rating

A resistive load is the easiest to switch. A motor load has a high starting current and an inductive kick at break. A lamp load has a high inrush when the filament is cold. A capacitive load has a very high inrush at make. Each of these produces a different arc and a different contact duty.

A relay rated for a resistive load may not be suitable for a lamp or a motor load at the same current. The datasheet should state the load type and the life at that load. If it only gives a maximum current, ask for the test conditions.

DC loads are more severe than AC at the same current because the arc does not self-extinguish. A relay or contactor rated for AC may need a different contact material, a larger gap or a blowout for DC. Check the DC rating separately.

Testing in the right zone

For a low-level relay, test at the actual signal level, including the open circuit voltage and the test current. A dry circuit test at a few millivolts is the right screening for a dry circuit application. Add a storage and thermal cycling test if tarnish is a risk.

For a contactor, test at the rated utilisation category with the real load. A motor test should include the starting current and the inductive break. A lamp test should include the inrush. Measure the arc duration, the bounce and the face erosion.

In both cases, inspect the contact face at intervals and keep the material lot and the test conditions. A life number without the load conditions is not a specification.