The short answer
Control Relays
Control relays in a panel interpose between the controller and the loads it commands, so that a small controller output drives a contact rated for the starter coil, the solenoid, or the 120 V circuit, and so that a fault on the load side stops at the relay instead of the output module. They are chosen by coil voltage, contact rating for the actual load type, and form factor, fitted with coil suppression to protect the output that drives them, and mounted on sockets that can be labeled and replaced. Their contacts wear and their coils fail, which is why they are on sockets and why the schematic gives each one a tag.
Key points
- A controller output drives a relay coil; the relay contact drives the load. The output module is protected and the load can be any voltage.
- Rate the contact for the load type. An inductive load at 120 V AC needs far more than the resistive rating suggests.
- Suppress the coil: a diode on DC coils, an RC or varistor on AC coils. The output that drives it lives longer.
- Sockets with retention clips, labels on the socket and the relay, and a spare of every type in the panel.
- A relay is the first suspect when a circuit is dead. Test the coil and each contact before anything else.
Why interpose
A controller output module switches a small current at a fixed voltage: a few hundred milliamps at 24 V DC on a transistor output, or an amp or two on a relay output. A starter coil at 120 V AC draws an inrush of an amp or more and is inductive; a solenoid valve is worse; a pilot light circuit may be on a different voltage entirely. Driving those loads directly from the output module either exceeds its rating or exposes it to the transients the load produces. An interposing relay solves both: the module drives the relay coil, a small, clean, resistive-looking load with suppression; the relay contact drives whatever the load is, at whatever voltage, with a rating chosen for it. When the load side fails, shorts, or takes a surge, the relay is what is damaged, and a relay is a five-dollar plug-in part.
Relays also do logic that belongs outside the controller: the float backup path that must work when the controller is dead, the safety circuit that must not depend on software, the alternation in a station that has no controller. Those relays are on the schematic as logic, and the rules for them are the same.
Types
| Type | Description | Where it fits | Note |
|---|---|---|---|
| General purpose plug-in (ice cube) | A cube relay in a socket, with two to four changeover contacts rated around 10 A resistive, coils in every common voltage | Interposing to starters and solenoids; panel logic | The workhorse; inexpensive, visible, replaceable in seconds |
| Terminal block relay (slim) | A single-contact relay in a 6 mm terminal block footprint, often with an LED and a suppression diode built in | Interposing for many controller outputs in a dense panel | Saves space; contact ratings lower, typically 6 A resistive; the whole block is replaced |
| Solid-state relay | A semiconductor switch with no moving contacts, optically isolated | High cycle counts, fast switching, no contact wear | Leaks a small current when off, so a light load may not turn off fully; needs a heat sink at higher currents; fails shorted |
| Power relay or contactor | Larger contacts rated for motor and heater loads | Loads beyond the ice cube rating | Enters the power circuit and the SCCR calculation |
| Safety relay | A force-guided contact relay or a safety module with monitored contacts | Emergency stop and safety circuits | Chosen by the safety function; not a general purpose relay with a different label |
| Timing and specialty relays | On-delay, off-delay, alternating, latching, phase monitor | Functions kept outside the controller | Each has a configuration that is documented on the schematic |
Contact ratings
A relay contact is rated for a current at a voltage for a load type, and the load type changes everything. A 10 A resistive rating on a general purpose relay may correspond to 3 or 4 A for an inductive AC load, less for a motor, and much less for a DC inductive load, which arcs on break because the current does not cross zero. The data sheet gives the ratings by category; the load is matched to the right one. A relay that is switching a starter coil with a 10 A resistive contact is fine; one switching a 2 A DC solenoid with the same contact is near its limit and will pit.
- Resistive
- Heaters, incandescent lamps after their inrush; the rating printed largest on the relay.
- Inductive AC
- Starter and contactor coils, AC solenoids; the current is lower and the break is manageable because the current crosses zero.
- Inductive DC
- DC solenoids and coils; the arc on break is the hardest duty a contact sees; suppression at the load and a derated contact.
- Lamp and capacitive
- LED drivers, power supplies, long cable runs; a high inrush at make that welds a light contact.
- Low level
- Signals into a controller input, milliamps at 24 V; needs a contact that stays clean, often gold-flashed, because a power contact that never carries enough current to clean itself oxidizes and goes open.
Installation
- Sockets on the DIN rail with retention clips, so vibration and a technician pulling a neighbor do not unseat the relay.
- A tag on the socket and on the relay, matching the schematic, so a replaced relay goes back in the right socket.
- Coils and contacts on the schematic with cross-references, and the coil voltage on the drawing.
- Relays grouped by function and by voltage, with the 120 V side segregated from the 24 V side per the panel wiring practice.
- LED indicators on the relays or the sockets, so the state is visible with the door open.
- A spare of each relay type in a labeled bag in the panel pocket.
Failure and diagnosis
Relays fail in a few ways: a contact welds closed from an inrush or a short, a contact burns open from arcing, a coil opens from age or overvoltage, a coil shorts from moisture, and a socket contact loosens. Each has a signature. A welded contact is a load that will not turn off when the coil is de-energized. A burned contact is a load that will not turn on with the coil pulled in: the LED is lit, the armature has moved, and the contact resistance is open. An open coil is a relay that never pulls in with voltage at the coil terminals. The diagnosis is a meter at the coil terminals with the output commanded, then across each contact pair with the relay in each state, and it takes a minute. A relay that has failed once in a socket that has failed several times is telling you about the load or the socket, not the relay.
Frequently asked questions
- Can I drive a starter coil directly from a relay output module?
- Some relay output modules are rated for it, at 120 V AC and a couple of amps, and panels are built that way. The cost is that a shorted coil or a surge on the coil circuit damages the module, which is far more expensive and slower to replace than an interposing relay. Most panel standards interpose everything that leaves the panel or exceeds a small load.
- Why did my controller output fail after a year?
- An unsuppressed relay coil is the usual reason: the inductive kick each time the output opens degrades the transistor until it fails. Add suppression at every coil, and replace the module. The second reason is a load beyond the output rating, which the same fix addresses by interposing.
- Solid-state or mechanical for a load that cycles constantly?
- Solid-state, because a mechanical relay has a contact life of hundreds of thousands to a few million operations and a load that cycles every few seconds reaches that in months. Check the leakage current against the load: a small pilot light or a controller input can stay on from the leakage of a solid-state relay, and a bleed resistor or a mechanical relay is needed there.
- Should relay logic still be used with a PLC in the panel?
- For the functions that must work when the PLC does not: the float backup, the safety circuit, the hardwired high-level call. Those stay in relays by design. Everything else belongs in the program, where it is visible, documented, and changeable without rewiring.
Related topics
- Component LayoutArranging a panel so it is safe to work in, cool enough to run, quiet enough for its signals, and possible to modify: power and control separation, heat, wireways, terminals, and the working space rules.
- Lift Station Backup ControlHow a lift station keeps pumping when the transmitter, the PLC, or the SCADA link is gone: float backup logic, the control transfer relay, redundant-off protection, and how to test it with the controller actually dead.
- Outputs Not EnergizingThe program says the output is on and the equipment is not running. Where the signal gets lost between the output image and the field device, and how to find the point in a few measurements.
- Control SchematicsThe ladder-style drawing of every control circuit in a panel: how a schematic is organized by line number, the conventions for contacts, coils, and cross-references, what belongs on a schematic and what belongs on a wiring diagram, and reading one under pressure.
- Hand-Off-Auto SelectorsWhat each position of an HOA selector does, how it is wired into the starter circuit, which protections must survive Hand, what the controller should see, and the mistakes that let a pump run with nobody watching.
- PLC I/O SystemsDiscrete and analog input and output modules, sinking and sourcing, relay and transistor outputs, wiring conventions, and the module diagnostics that find field faults from the panel.
Direct contact
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