What contact types does a timing relay use? Most electronic timing relays ship with one or two change-over (SPDT/DPDT) output contacts, rated a few amps under the AC15/AC12 utilization categories defined in IEC 60947-5-1, switched only after the set delay elapses. Pick the wrong contact rating or configuration and the timer either can't drive the downstream contactor coil directly, or the contact welds shut and fails silently well before the electronics do. This article covers change-over configurations, resistive vs inductive load ratings, minimum switching current, plug-in socket pinouts, and how to derate a contact for inductive inrush.
How Many Output Contacts Does a Timing Relay Have?
Single-function DIN-rail timers such as Schneider's Zelio Time RE17 or ABB's CT-ERD and CT-AHD carry one change-over contact: one common, one normally-open, one normally-closed pole. Multifunction and star-delta units — Zelio RE22R, ABB CT-VWD, CT-YDD — carry two, either two independent change-over contacts or a shared-actuator block. Star-delta timers need the second contact by design: one output drives the star contactor, the second drives delta, both switching off the same internal timing sequence.
Plug-in (socket-mounted) timers, the Zelio Time REXL series among them, typically use an 8-pin or 11-pin octal socket matching general-purpose plug-in relay pinouts, so a panel builder can swap a timer into an existing socket bank without rewiring the base.
Resistive vs Inductive Load: Why the AC15/AC12 Rating Matters
Contact ratings on a timer's datasheet are almost always split by utilization category, per IEC 60947-5-1. AC12 covers resistive or lightly inductive loads — indicator lamps, PLC inputs — at the full nameplate current. AC15 covers switching an electromagnet load, a contactor coil, where inrush at pickup runs several times the sealed current and the break arc is harder on the contact material. A timer contact rated 5 A AC12 may carry only a fraction of that figure under AC15.
This is not a footnote. Wire a timer contact straight into a contactor coil sized off the AC12 figure and the contact arcs on every operation until it welds shut — the timer keeps timing correctly, the panel just stops responding because the output contact never opens again.
Minimum Load and Low-Signal (Gold-Flash) Contacts
Standard silver-alloy contacts need a minimum current to self-clean the contact surface. Too little load over years of cycling and oxide film builds up, raising contact resistance until the signal drops out intermittently. Where a timer output feeds a PLC digital input or another low-current control circuit, some ranges offer a gold-flash or bifurcated contact option rated for milliamp signal levels instead of the standard few-amp minimum. Mixing the two — a low-signal load on a standard power contact — is a common source of intermittent faults that read fine on a multimeter under test current but fail once installed.
Star-Delta Timers: Two Outputs, One Sequence
A star-delta timing relay times three intervals across two contacts: star contactor closed for the run-up time, both contactors open for the transition dead-time, then delta contactor closed. The transition contact has to open fast enough that the mechanical interlock on the contactors — not the timer alone — backs up the electronic dead-time in preventing any overlap. Losing either one risks a line-to-line short across the motor windings during changeover.
Contact Configuration Codes: SPDT, DPDT and 1 c/o, 2 c/o
Catalogs mostly use IEC-style "c/o" notation rather than the SPDT/DPDT pair common in North American documentation. 1 c/o equals one change-over contact (SPDT); 2 c/o equals two independent change-over contacts (DPDT). A few multifunction ranges add a fixed NO or NC auxiliary alongside the timed change-over pole — read the contact diagram on the datasheet, not just the pole count in the title, since two ranges can share a "2 c/o" headline while wiring the second contact differently.
| Criteria | 1 c/o (SPDT) | 2 c/o (DPDT) |
|---|---|---|
| Typical application | Single on/off delay, indicator or single contactor coil | Star-delta, dual-load or interlocked sequencing |
| Panel footprint | Narrower module, often 17.5 mm | Wider module, often 22.5 mm |
| Wiring complexity | Lower, one load circuit | Higher, two independent load circuits, verify each pole's rating |
Contact Life and Derating for Inductive Loads
Datasheets list two figures: mechanical life, contact operations with no load, sometimes 10 million or more, and electrical life, operations at rated load, typically two to three orders of magnitude lower. The mechanical figure reflects wear on the actuator and contact spring. The electrical figure reflects arc erosion of the contact material itself. A timer switching a lightly loaded PLC input approaches its mechanical life; the same timer switching a contactor coil near its AC15 limit wears out at the electrical figure instead, often well before the electronics show any sign of drift.
Duty cycle matters as much as load type. A star-delta timer on a pump that starts twice a shift accumulates operations slowly. The same timer on a conveyor that starts and stops every few minutes reaches its electrical life figure in a fraction of the calendar time the slower-cycling application would take. Where cycling is frequent, oversizing the contact rating relative to the load — leaving headroom under the AC15 figure rather than running at it — extends practical service life more than any other single choice made at the specification stage.
Where the datasheet states only the AC12 figure and the coil inrush current is known, an approximate switching current can still be checked against a general derating factor before wiring the timer directly into a contactor coil circuit.
Formula: Inductive load derating — Source: IEC 60947-5-1, utilization category guidance
IAC15 = IAC12 × k
| Symbol | Description | Unit |
|---|---|---|
| IAC15 | Permissible current for an electromagnet (coil) load | A |
| IAC12 | Rated current for a resistive/PLC-input load | A |
| k | Manufacturer derating factor for AC15 duty, typically well under 1; confirm the exact figure on the specific timer's datasheet | - |
Treat k as a sanity check, not a substitute for the datasheet. Manufacturers publish an explicit AC15 figure precisely because the derating factor is not constant across ranges. When coil inrush is unusually high — a large contactor, or several coils paralleled off one timer contact — route the load through an interposing relay sized for that duty instead of the timer's own contact.
Choosing and Wiring the Right Contact for the Job
Contact selection is one line item inside the broader timing relay selection process, alongside function and timing range. Once the rating is confirmed, wiring a timing relay correctly still depends on matching the load — coil, lamp, PLC input — to the pole rated for it, particularly on 2 c/o units where one contact may see a resistive load and the other an inductive one. What we see in the field: panel builders sometimes wire the heavier contactor coil onto whichever pole sits closer on the terminal block, not the one better suited to inductive duty. Both poles carry the same catalog rating on paper, so it works until the pole degrades faster under years of inrush cycling.
On multifunction ranges such as Schneider's Zelio Time RE22 or ABB's CT-MFD, covered further in the multifunction and multi-range timing relay guide, the contact rating stays fixed across every function the rotary selector offers. Changing the timing function does not change what the output pole can switch.
Frequently Asked Questions
Can a timing relay contact drive a contactor coil directly?
Usually yes, within the AC15 rating printed on the datasheet, not the AC12 resistive figure. Check the coil's inrush current against that AC15 rating before wiring it directly; if the coil inrush is close to or above it, use an interposing relay instead.
What does "1 c/o" mean on a timing relay datasheet?
One change-over contact — the IEC notation equivalent of a single-pole double-throw (SPDT) output, with one common terminal, one normally-open and one normally-closed contact.
Why does a star-delta timer need two contacts?
One contact drives the star contactor coil, the other drives the delta contactor coil, both sequenced off the same internal timing chain with a transition dead-time between them.
What is a low-signal or gold-flash contact used for?
Switching low-current loads such as a PLC digital input, where a standard silver contact may not carry enough current to self-clean and can develop intermittent high resistance over time.
Does contact rating change with the selected function on a multifunction timer?
No. On most multifunction ranges the output contact rating is fixed regardless of which function the rotary selector is set to — only the timing behavior changes.
Conclusion
Contact type and rating decide whether a timing relay survives its panel or fails quietly behind a timer that keeps timing correctly. Match the contact count to the function — one change-over for a single on/off delay, two for star-delta or dual-output multifunction use — confirm the AC15 rating against the actual coil inrush, and route anything close to the limit through an interposing relay rather than the timer's own contact. The rest of the specification, covered in the timing relay engineering guide and the broader timing and control relays range, only matters once the contact side is right.