1. Contact Metallurgy: AgCdO vs. AgSnO₂ — when 10 A is not 10 A
The G2R-1 is rated 10 A at 250 VAC with AgCdO contacts. The G7J-4A uses AgSnO₂ and is rated 40 A at 250 VAC. Both numbers sit inside the same IEC/UL 61810-1 framework, but the difference isn't about ampacity alone — it's about what happens when the contacts open under load.
Mechanism: AgCdO (cadmium oxide) provides good arc-quenching at moderate currents because cadmium vaporises and cools the arc. But above roughly 15 A (or under high inrush), the arc energy can vaporise so much cadmium that the contact surface erodes unevenly, creating a pitted zone that increases resistance and, in turn, temperature. AgSnO₂ (tin oxide) has a higher melting point and resists material transfer; it's preferred for loads with repetitive high inrush (motors, capacitive banks) or where contact welding must be avoided.
Worked consequence: In a 230 V motor starter cycling 6 A locked-rotor (about 40 A inrush for 80 ms), an AgCdO contact on a G2R-1 will typically show measurable contact resistance rise after 15,000–20,000 operations (illustrative, based on published test data). An AgSnO₂ contact on the G7J-4A under the same cycle would still be below 10 mΩ change at 50,000 operations. The practical choice: if your load exceeds 12 A steady or includes any motor/capacitor inrush above 30 A, the G7J-4A's AgSnO₂ contacts are the threshold that buys real lifecycle.
When this flips: If your load is purely resistive (heater, incandescent lamps) and never exceeds 8 A, AgCdO contacts are actually more forgiving on light arcs — the cadmium suppression works better at low energy. For a resistive 5 A load, the G2R-1 is not just adequate but more durable than a higher-rated AgSnO₂ relay because the lower coil power produces less mechanical wear per cycle.
2. Temperature derating: the 70 °C boundary and what it does to coil power
The G2R and MY series are specified for -40 °C to 70 °C ambient. The G7J series extends to 85 °C. That 15 °C gap seems small — until you derate.
Mechanism: Coil temperature rise is the sum of ambient + self-heating (I²R in the copper winding). At 70 °C ambient, a 24 VDC coil on a G2R-1 (holding current ~35 mA) will reach about 95 °C internal. That's below the insulation class B (130 °C) typical of these relays, but the real constraint isn't insulation — it's the pick-up voltage. As the coil heats, its resistance increases (copper has a +0.39 %/°C tempco). At 95 °C internal, the coil resistance is about 14 % higher than at 20 °C, which means the minimum voltage required to close the armature also rises by roughly the same proportion. If your control rail is 24 VDC ±10 % (21.6 V minimum), the relay may fail to pick up reliably when hot.
Worked consequence: In a panel running at 65 °C ambient (common near a poorly ventilated drive cabinet), a G2R-1 with a 24 VDC coil needs about 21.6 V to pick up at 20 °C, but at 65 °C ambient + 25 °C self-heat, the required voltage climbs to roughly 24.9 V (derived). If your supply drops to 22 V during a brownout, the relay will chatter or remain open — and your load never turns on. The G7J-4A, with its 85 °C rating and heavier coil (about 0.8 W vs. 0.4 W), maintains a wider pick-up margin; its minimum pick-up voltage at 85 °C ambient is still below 21 V (derived from winding data). Threshold: if your panel ambient exceeds 55 °C, choose the G7J series or an MY with a 12 VDC coil fed from a 24 V bus via a small DC-DC converter.
When this flips: In a climate-controlled cabinet (
3. Dielectric strength: the "1,500 V vs. 2,500 V" gap and when it bites
The G2R and MY series are rated 1,500 VAC dielectric between coil and contacts; the G7J series is rated 2,500 VAC. That's a 67 % higher margin.
Mechanism: Dielectric strength testing (per IEC 61810-1) applies a voltage for 60 seconds; the relay must not break down or exceed 1 mA leakage. The test voltage is not the same as the working voltage — it's a safety margin. For basic insulation in a 250 V system, a 1,500 VAC rating is adequate (it's 6× the peak working voltage). But impulse voltages from switching transients (e.g., a motor contactor opening near the relay) can reach 2–3 kV. A 1,500 V-rated relay's internal creepage distance (typically 3–4 mm) may flash over under a 2.5 kV spike if the air has any humidity or dust. The G7J's 2,500 V rating gives roughly 6–8 mm creepage, enough to survive Category II transients in industrial environments.
Worked consequence: In a panel that shares a 24 VDC bus with contactors switching 230 VAC motors, the relay coil sees back-EMF spikes that can exceed 1,000 V. If that spike couples to the contacts (through coil-contact capacitance ~5 pF), a G2R-1 may experience a one-time dielectric breakdown that carbonises the insulation — not a failure today, but a latent tracking path that fails months later. The G7J-4A's higher dielectric margin absorbs that same impulse without tracking. Threshold: if your relay is in the same enclosure as inductive loads (contactors, valves, solenoids) on the same control transformer, choose a >2,000 V-rated relay or add a suppression network.
When this flips: In a pure DC control panel with no motor drives (e.g., a battery-banked PLC), transients rarely exceed 500 V. The G2R-1's 1,500 V rating is overkill, and the smaller package saves 40 % on board area.
Quick-look threshold table
| Threshold | Cross it if… | Then choose |
|---|---|---|
| Contact metallurgy | Load inrush > 30 A or steady > 12 A | G7J-4A (AgSnO₂) |
| Temperature derating | Panel ambient > 55 °C | G7J-4A or MY with lower coil voltage |
| Dielectric margin | Shared bus with motor contactors | G7J-4A (2,500 V) |
| Dropout reliability | Sealed cabinet > 60 °C + 10 A load | G7J-4A |
| Low-heat / low-ambient | Panel | G2R-1 or MY2 (lower coil loss) |
Rule of thumb you can take to the panel:
If your worst-case load (including inrush) is ≤ 1.5× the relay's continuous rating AND your ambient is ≤ 55 °C AND you have no motor contactors sharing a control transformer, any Omron relay from the G2R or MY family will run cool for its rated life. If any of those three thresholds is breached, step up to the G7J-4A or add external suppression. That's the difference between a relay that lasts a decade and one that fails in the only hour you can't afford downtime.
Non-obvious insight: The MY2’s 5 A rating at 250 VAC is not a "weak" relay — it’s actually the most efficient in this group for low-power signal loads because its coil draws only about 0.2 W (derived), generating negligible self-heat. For a 24 VDC PLC output driving 20 relays, using MY2s instead of G2Rs saves 4 W of heat on the card — enough to keep the PLC fan off.
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.
