Electromechanical Relays, Contactors & Reed Switches

An electromechanical relay lets a low-power coil command one or more galvanically isolated contact circuits. A contactor uses the same physical principle but is built for repeated power switching, with arc control, larger clearances, replaceable or inspectable power contacts in some families, and coordinated overload accessories. A reed switch removes the armature linkage: a magnetic field directly bends sealed ferromagnetic reeds until they touch.

Device roles are not interchangeable #

The word relay describes a control function, not a universal contact. A contact rated for a resistive DC load may weld or erode rapidly on a motor, solenoid, incandescent lamp, or capacitor because the making current, breaking voltage, and arc energy are different.

Coil, armature, and contact state #

De-energized state is the reference used for contact names and schematic symbols. A normally open (NO, make) contact is open with the coil de-energized; a normally closed (NC, break) contact is closed. A changeover contact moves a common terminal between NC and NO. The names do not describe the state during normal machine operation.

The operating sketch is useful for state reasoning. In a multi-contact relay, one armature and return system transfer several insulated spring stacks. Contact pressure, overtravel, alignment, and the compliance of each spring therefore matter independently even though the contacts share one coil.

A reed switch replaces that linkage with two ferromagnetic contact members sealed inside a glass capsule. An external coil or permanent magnet can actuate normally open, normally closed, or changeover constructions. Operate/release ampere-turns and switching life must come from the chosen manufacturer's datasheet.

Energizing the coil raises flux and attraction until magnetic force exceeds spring preload, friction, and load. The armature then accelerates and impacts a stop; the contacts can bounce before settling. Removing coil power starts a different transient because stored magnetic energy must decay before the spring can release the mechanism. Pull-in voltage, dropout voltage, operating time, release time, bounce, and coil temperature therefore describe different limits.

Contact ratings and failure modes #

Contact current alone is insufficient. Select against the manufacturer's rating for the actual voltage, AC/DC type, utilization category or load class, duty cycle, ambient temperature, and required electrical life. Check both making and breaking duty.

• An inductive load raises contact voltage when current is interrupted; suppression belongs at the load when practical.
• A capacitive input or lamp can have a large inrush current even when steady current is small.
• Very small dry-circuit signals need contact materials and minimum wetting conditions intended for that service.
• Contact bounce can create multiple digital edges; filtering or state qualification is still required.
• Welding, erosion, contamination, mechanical wear, broken springs, and coil open circuits are credible failures; galvanic isolation does not imply a safe failure state.

Why an arc persists #

Opening contacts force current through a rapidly shrinking metal bridge and then an ionized gap. An inductive load contributes $L\,di/dt$ voltage in the direction required to keep current flowing; a DC arc has no natural current zero, so breaking the same current at DC is often much harder than at AC. Contact separation speed, gap, material, magnetic blowout, arc chutes, external suppression, and source impedance all change whether the arc extinguishes or transfers destructive energy into the contacts.

Suppression is a system tradeoff. A diode across a DC coil sharply reduces contact overvoltage but slows current decay and mechanical release. A TVS or RC network permits a higher clamp voltage and faster release at the cost of more electrical stress or loss. Put the network at the load when practical so cable inductance is inside the protected loop, and verify the relay manufacturer's life data with the actual network.

Utilization category is part of the rating #

IEC 60947-4-1 covers electromechanical contactors and motor starters up to 1,000 V AC or 1,500 V DC and defines application-dependent duties. The category is not a marketing suffix; it tells the test duty the device survived.

Use the current manufacturer's coordination table for the exact contactor, overload device, short-circuit protective device, voltage, and prospective fault current. A frame size or thermal ampere value alone does not establish an IEC utilization category or short-circuit coordination.

Direct-on-line motor starter #

A three-wire direct-on-line (DOL) starter uses one momentary START command, one maintained auxiliary contact, and series stop/trip contacts. The main contactor connects all motor phases at full line voltage; the control circuit determines when the coil is allowed to remain energized.

The sequence is structural:

1. With STOP and overload contacts closed, pressing START energizes coil KM.
2. KM closes its three main contacts and auxiliary NO contact 13-14.
3. Releasing START leaves 13-14 as the parallel holding, or seal-in, path.
4. Pressing STOP, an overload trip opening 95-96, or loss of control power de-energizes KM.
5. When control power returns, 13-14 is open because the coil dropped out, so the motor does not restart until START is pressed again.

This no-voltage-release behavior is why a momentary three-wire circuit is materially different from driving the coil with a maintained two-wire switch. Automatic restart may be correct in a deliberately engineered application, but it must not appear accidentally through the wrong control topology.

ISO 13850 defines the emergency-stop function independently of the control-energy technology; it is a complementary protective measure, not a substitute for guards or inherent risk reduction. ISO 13849-1 supplies a methodology for designing and integrating safety-related control-system parts at the required performance level. The practical consequence is that “use an NC contact” is not a safety argument: architecture, fault detection, component reliability, common-cause measures, reset, validation, and the resulting machine state all have to support the risk reduction claimed.

Overload and short-circuit protection are separate #

The overload relay measures motor current or its thermal equivalent and opens its auxiliary NC trip contact when sustained loading exceeds the setting. Its job is motor overload protection. High fault current must be cleared by a coordinated upstream fuse, circuit breaker, or motor-protection device with adequate interrupting rating.

Coordination also includes the contactor: after a short circuit, the allowed damage and whether the starter may remain serviceable depend on the protective combination and declared coordination. Do not assemble a protection chain by matching only nominal amperes.

Reversing and interlocking #

A reversing starter uses two contactors and swaps two motor phases. Electrical NC auxiliary interlocks prevent either coil from energizing while the other is active; a mechanical interlock prevents simultaneous main-contact closure if an electrical fault or welded auxiliary contact defeats the logic. The interlock is a physical invariant, not merely a PLC condition.

The same principle applies to mutually exclusive actuators: implement software sequencing for normal operation, but place the necessary hard interlock in the energy path when simultaneous actuation would be hazardous.

Reed switches and reed relays #

Reed contacts are sealed in glass, so the contact environment can be cleaner and more repeatable than an exposed mechanism. A permanent magnet can operate a bare reed switch for position sensing, or a coil can turn it into a reed relay with galvanic isolation. Magnetic sensitivity also creates system constraints: orientation, nearby coils, ferromagnetic parts, and external fields change operate and release points.

Reed switches are not contactors in miniature. Their strengths are low leakage, isolation, and signal switching; their limitations are contact energy, mechanical shock, bounce, and magnetic cross-coupling. Use a manufacturer load-life curve and protection network for the actual load.

Application review #

Before releasing a relay or contactor design, verify coil voltage across temperature and wiring drop, pull-in and dropout margins, suppression and release time, contact load class, making and breaking current, electrical and mechanical life, overload setting, upstream fault protection, terminal numbering, interlocks, control-power loss behavior, enclosure, and the safe state after each credible failure. Then test representative hardware with the real load: a schematic symbol cannot reveal bounce, arc energy, acoustic chatter, thermal rise, or welded-contact behavior.