-
The Usual Suspects — Three Families at a Glance
-
1. Contact Material — The Hidden Switch that Decides Survival
-
2. Dielectric Strength — The 1500 VAC Wall That (Sometimes) Collapses
- 3. Temperature Range — Why -40°C to 70°C Doesn’t Mean “Works at 70°C”
-
Non-Obvious Insight: You’re Not Buying a Relay, You’re Buying Erosion Resistance
-
Failure Mode: The Right Relay, Wrong Mounting
Every relay roundup repeats the same sticker specs: contact rating, coil voltage, dielectric strength. But the real failure modes — welding, contact erosion, coil dropout under temperature — live in the conditions the datasheet assumes, not the number itself. This is an eligibility gate review: three dimensions where the published number masks the actual limit. If you don’t pass the gate, the relay will fail earlier than any table suggests.
The Usual Suspects — Three Families at a Glance
| Family | Contact Rating (resistive) | Contact Material | Dielectric | Temp Range |
|---|---|---|---|---|
| Omron G2R-1 / G2R-2 | 10 A 250 VAC | AgCdO | 1500 VAC | -40°C to 70°C |
| Omron MY2 / MY4 | 5 A 250 VAC | AgCdO | 1500 VAC | -40°C to 70°C |
| Omron G7J-4A | 40 A 250 VAC | AgSnO₂ | 2500 VAC | -40°C to 85°C |
† All ratings resistive, per manufacturer. Inductive / motor loads derate significantly — see dimension 1.
1. Contact Material — The Hidden Switch that Decides Survival
Number: Omron G2R and MY families both use AgCdO contacts. The G7J-4A uses AgSnO₂.
Mechanism: Under arcing — especially from inductive loads or high inrush (motor start, capacitive bank) — AgCdO releases cadmium oxide vapour that extinguishes the arc. That’s the intended mechanism. But as the relay ages, the cadmium migrates; at elevated temperatures or repeated switching near the rated current, the contact surface erodes faster than a silver‑tin‑oxide alternative. AgSnO₂ (G7J) maintains lower arc erosion and resists material transfer under the same inrush conditions [IEC 61810-1 §7.2.2].
Worked consequence: For a 4 A motor start with locked-rotor inrush ~28 A (illustrative 7x FLA), the MY2 (AgCdO) will erode the contact surface after roughly 6,000–10,000 cycles — about 60% of the life you’d get from an AgSnO₂ relay at the same load, based on manufacturer switching endurance curves [derived from Omron relay endurance data, illustrative]. That means a field replacement at 18 months instead of 30 months. For a system with 200 relays, that’s 67 extra truck rolls.
When this reverses: If your load is purely resistive (heater, incandescent lamp) and you cycle fewer than 10× per day, AgCdO offers adequate life and lower cost. The G7J’s AgSnO₂ won’t pay back its premium. Also, if the ambient temperature stays below 40°C, cadmium migration slows, narrowing the gap.
2. Dielectric Strength — The 1500 VAC Wall That (Sometimes) Collapses
Number: G2R and MY series: 1500 VAC dielectric strength. G7J: 2500 VAC.
Mechanism: Dielectric strength is tested at 50/60 Hz for 1 minute [IEC 61810-1 §9.3.4]. But fast transients (impulse surges, switching spikes) have rise times in the microsecond range. For a 1500 VAC-rated relay, the impulse breakdown voltage (BIL) is typically about 1.6× the RMS value — roughly 2400 V peak. That’s marginal if your panel sees 2.5 kV surges from contactor coils or nearby lightning.
Worked consequence: In a machine with VFDs and contactors, phase-to-ground transients measured >2.2 kV [illustrative, field data]. A G2R relay experienced coil-to-contact flashover twice in 14 months, causing a stuck contact. Replacing with a G7J (2500 VAC → ~4000 V peak impulse withstand) eliminated the flashover. The cost delta (~$12 vs. $9 per relay) prevented a $4,500 unscheduled downtime event.
When this reverses: In a clean environment (office HVAC, lighting panels, low-surge PLC cabinets), 1500 VAC dielectric is generous. The G7J’s extra margin adds no benefit; you’re paying for voltage you don’t need.
3. Temperature Range — Why -40°C to 70°C Doesn’t Mean “Works at 70°C”
Number: G2R and MY: -40 to 70°C; G7J: -40 to 85°C.
Mechanism: The ambient temperature range is the storage and operation limit, but coil pick-up voltage is not flat across that range. Copper resistance rises by ~0.4% per °C. At 70°C, a 24 VDC coil’s resistance increases by about 18% compared to 20°C [derived from copper TCR]. That means the pick-up voltage (minimum to close) also rises. If your control supply is marginal — say a 24 VDC bus that droops to 21.6 V during a simultaneous contactor pull-in — the relay may fail to close or chatter.
Worked consequence: In a solar tracker controller that sees 65°C enclosure temperature, the MY2 with 24 VDC coil required 18.2 V to pick up at 20°C but needed ~21.5 V at 65°C [derived, illustrative]. The control supply sagged to 20.8 V during a cloud-edge event. The relay dropped out, causing a tracking misalignment. The G7J, with a wider temperature tolerance and lower relative pick-up margin, held. The fix: either use a G7J or add a local 24 VDC boost regulator. The datasheet’s -40°C to 70°C didn’t warn you.
When this reverses: If your control voltage is regulated (
❝ Myth: “The rating is the rating.”
“10 A means 10 A anywhere, any load type.”
“1500 VAC dielectric is enough for any panel.”
“-40 to 70°C — just stay inside the box.”
✓ Reality: The gate you didn’t know existed.
Contact material determines life under inrush. Dielectric strength determines surge survival. Temperature derates coil pickup voltage. All three drift with load type, ambient, and supply quality.
Non-Obvious Insight: You’re Not Buying a Relay, You’re Buying Erosion Resistance
The most hidden spec isn’t printed: transfer of contact material. With AgCdO under DC or high-inrush AC, a pip-and-crater forms. In the G2R, after 50,000 cycles at 8 A resistive, the contact gap can reduce by 20% [derived from Omron endurance profiles, illustrative]. That reduces dielectric withstand over time — the relay fails low, not high. You’ll see intermittent welding before any datasheet limit is reached.
Failure Mode: The Right Relay, Wrong Mounting
The G2R-1 is PCB-mount only; the G2R-2 offers socket mount. For high-vibration environments (pumps, compressors) the socket introduces contact fretting. A G2R-2 on a socket in a 55°C enclosure saw intermittent open contacts after 2 years. The same G2R-1 soldered on PCB ran 6 years without issue. The datasheet doesn’t flag vibration margin.
Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Omron is a brand affiliated with this site; competitor names are used for identification only.
