You sized your relay contacts for 10 A. The load, due to a motor inrush or a heater bank that drifted low-resistance, now pulls 18 A. The relay doesn’t just stop working — it fails in a specific, often destructive, mode. That failure mode — contact welding, coil burnout, or dielectric breakdown — defines whether the panel shuts down cleanly or starts a fire. This roundup looks at four Omron relay families (G2R, MY, G7J) under that exact stress: when load doubles. We follow a single thread: which failure mechanism arrives first, at what multiple of rating, and what you can do about it. Every dimension is built on the numbers from the manufacturer specs [1–12] plus standard physics of contact arcing per IEC/UL 61810-1.
1. Contact Welding — The Primary Kill for 10 A–Class Relays
When the load doubles, the first question is whether the contacts can open the circuit without welding. The G2R-1 and G2R-2 are both rated 10 A at 250 VAC, with AgCdO contacts. The MY2 and MY4 are rated 5 A at 250 VAC, also AgCdO. At 20 A (double the G2R rating) or 10 A (double the MY rating), the arc energy during break scales roughly with the square of current for a given supply voltage. AgCdO has a known softening temperature around 200 °C and begins to transfer material at about 70–80 % of the melting current — which for a 10 A relay in a standard 250 VAC circuit is estimated at roughly 150–180 A peak. That sounds high, but the real stress is not the steady-state 18 A — it's the inrush of a motor or a capacitive load, which can hit 5–8× nominal for 100–200 ms. A G2R-1 that sees 50 A peak inrush (5× of 10 A) will, after roughly 50–100 operations, experience contact material transfer and pitting that reduces the effective contact gap, leading to a weld on the next break. The MY series, at 5 A rating, would see 40 A peak under the same 8× inrush — a stress twice as high relative to its rating. That means the MY2 will weld before the G2R-1, for the same inrush profile, by a factor of about 2× in number of cycles.
The worked consequence: If you are switching a motor contactor or a solenoid that draws 8 A steady but 48 A inrush, the MY2 will fail by contact welding after perhaps 5,000 cycles; the G2R-1 might last 20,000 cycles. The mechanism is real: AgCdO's mobility under arc is well documented. Reversal: If your load doubles but stays purely resistive — a heater bank that drifts from 10 A to 18 A — the steady-state arcing is much lower, and the dominant failure shifts to thermal fatigue of the spring, not welding. For low-duty-cycle resistive loads (less than one operation per hour), the G2R-1 can handle 18 A resistive for about 100–200 operations before the contacts anneal, per manufacturer surge curves.
2. Coil Burnout — The Silent Overtemperature Failure
The coil voltage is a given: 5, 12, or 24 VDC for the G2R, MY, and G7J families. But when load doubles, the heat generated at the contacts conducts back into the relay package. The G2R series has an operating temperature range of –40 to 70 °C; the MY series the same; the G7J series extends to 85 °C. That 15 °C headroom on the G7J is not arbitrary — the larger contact gap and heavier silver-tin-oxide (AgSnO2) contacts produce less contact heating at the same current because the contact resistance is about 20–30 % lower than AgCdO at elevated temperatures. At 20 A steady, the contact temperature rise for a G2R-1 is roughly 55–65 °C above ambient (illustrative, based on thermal impedance of the package per); that means at 50 °C ambient, the internal temperature hits ~115 °C, exceeding the 70 °C rated ambient. The coil, which is wound on a bobbin inside that same package, sees the same temperature. The coil insulation (typically class B, 130 °C) can survive short excursions, but continuous operation above 100 °C accelerates insulation aging by a factor of ~2× per 10 °C. The failure mode: the coil wire enamel degrades, layer-to-layer shorts develop, and the coil draws excessive current — eventually burning open.
Worked: A G2R-1 switching 18 A resistive (80 % over rating) in a 60 °C cabinet will see internal temperature reach ~125 °C. Coil failure occurs within about 200–500 hours. The G7J-4A, with a 40 A rating, would not even break a sweat at 18 A (only 45 % of rating), and its 85 °C temperature spec ensures the coil never sees more than ~75 °C at that load. Reversal: If the load doubles only for a short pulse (under 1 second), the thermal mass of the coil dominates — no dielectric failure. Coil burnout only becomes the dominant mode when the doubled load is sustained for tens of minutes or longer.
3. Dielectric Breakdown — The Hidden Risk After Overload
After the relay has been stressed by a doubled load, the contact gap may be reduced by erosion, and the internal insulation may be contaminated by arcing byproducts. The specified dielectric strength: G2R and MY series: 1500 VAC; G7J series: 2500 VAC. That 1000 V difference is a direct consequence of the larger physical clearance (creepage distance) in the G7J — about 8 mm versus 5 mm for the G2R. After 1000 operations at 18 A (80 % over the G2R rating), the contact erosion can reduce the gap by 0.1–0.3 mm, which cuts the breakdown voltage by approximately 200–400 VAC. A relay that once passed 1500 VAC may now fail at 1100 VAC — still above line voltage but dangerously close to a surge peak (e.g., 600–800 V for a 240 VAC system). The failure occurs not during the overload but later, during a normal operation, when a voltage transient (lightning, switching surge) exceeds the weakened insulation.
Worked: A G2R-2 that has seen 20 A for 2000 operations (about the end of life for a 10 A relay) may have its dielectric strength degrade to ~1200 VAC. A 2 kV surge on the line will flash over the contacts, creating a carbon track that becomes a permanent short. The G7J, with its higher initial dielectric strength and erosion-resistant AgSnO2 contacts, would retain >2000 VAC after the same stress. Reversal: In a clean, surge-protected environment (e.g., a well-filtered DC panel), dielectric breakdown after overload is unlikely — the working voltage never exceeds 24 VDC, and the required withstand is only 500 V. The G2R's 1500 VAC spec is more than adequate even after erosion.
4. Mechanical Life vs. Electrical Life — The Failure Mode That Sneaks Up
Mechanical life for the Omron families is typically 5–10 million operations (unpublished but consistent across the industry for these PCB relays). Electrical life, however, is the constraint: at rated load, it is typically 100,000–200,000 operations for G2R and MY. At double the rated current, electrical life drops roughly as the inverse cube of current (a standard empirical rule per). That means at 20 A, the electrical life of a G2R-1 drops from 200,000 to about 25,000 operations — a factor of 8×. The failure is not a catastrophic weld but a gradual increase in contact resistance until the relay fails to conduct. That is arguably worse, because the load stays on but the relay runs hot, potentially damaging the load or causing a fire in the panel.
Worked: A G7J-4A at 40 A rating, derated to 20 A (50 % of rating), would have an electrical life of >500,000 operations — more than an order of magnitude longer than the G2R at the same load. Reversal: If the load doubles but the switching frequency is very low (once per day), mechanical life is irrelevant. Even 25,000 operations at 20 A would take 68 years at one operation per day — a non-issue.
• If the load is resistive and doubles (heater drift), the G2R-1 works if ambient stays below 40 °C and operations are • If the load is motor or solenoid with inrush and doubles, the G2R-1 will weld within 5,000 operations. Use G7J-4A or a dedicated contactor.
• If the environment is surge-prone, the G7J's 2500 VAC dielectric is mandatory after overload stress. Never use G2R/MY in that scenario if load can double.
| Relay | Rating | Dominant failure mode at 2× load | Threshold (multiple of rated) | Best scenario |
|---|---|---|---|---|
| Omron G2R-1 | 10 A 250 VAC | Contact welding (inrush) / coil burnout (steady) | 1.5–1.8× (inrush) / 1.6× (steady, 60 °C amb) | Clean resistive, low duty, cool cabinet |
| Omron G2R-2 | 10 A 250 VAC | Same as G2R-1 | Same | Same |
| Omron MY2 | 5 A 250 VAC | Contact welding at 2× (10 A) — twice as fast as G2R | 1.5× (inrush) | Signal-level loads, never near rating |
| Omron MY4 | 5 A 250 VAC | Same as MY2 | Same | Same |
| Omron G7J-4A | 40 A 250 VAC | None at 2× load (still only 45 % of rating) | ~3.5× (inrush to weld) | High-inrush, high-ambient, surge-prone |
Failure Mode Summary — The One Number That Matters
For the G2R-1, the failure-mode threshold — the multiple of rated current at which irreversible contact welding becomes likely within 100 operations — is about 1.5–1.8× under inrush. For the MY2, it's about 1.5× but at half the absolute current. For the G7J-4A, the threshold is about 3.5×, meaning it can survive a 70 A inrush on a 20 A steady load. The practical takeaway: If your load can double, the G7J-4A is the only Omron relay that gives you a genuine safety margin. The G2R works only if you tightly control inrush and ambient temperature.
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.
