You pick a relay by contact rating—10 A at 250 VAC, job done. Then the motor starts inrush and the contacts weld on cycle 40. The datasheet told you the current, but it hid why 10 A on an Omron G2R survives where a 10 A on a different construction welds. This roundup dissects three Omron relay families—G2R, MY, G7J—through the lens of mechanism: how material, dielectric margin, and thermal rise define failure, not the label on the front. One table, three real dimensions, and a decision rule you can apply today.
Why Contact Material Controls Survival (Not the Amp Stamp)
Every relay in the roundup carries a contact rating: the G2R-1 and G2R-2 both show 10 A at 250 VAC. The MY2 and MY4 show 5 A at 250 VAC. The G7J-4A shows 40 A at 250 VAC. These numbers are thermal steady-state limits under resistive load at 25 °C ambient. The datasheet hides that contact material determines what happens during a surge. All three families use silver-cadmium oxide (AgCdO) except the G7J-4A, which uses silver-tin oxide (AgSnO2). Mechanically, AgCdO arcs well under moderate loads—the cadmium oxide layer quenches the arc—but under high inrush (motor starts, capacitive loads, lamp loads), the cadmium vaporizes faster and the contact surface erodes asymmetrically. The G7J’s AgSnO2 does not rely on cadmium; tin oxide stays thermally stable to higher energy, making the 40 A rating real under inductive or lamp inrush, not just resistive. The worked consequence: if your load draws 12 A inrush repeated daily, a G2R rated 10 A may weld after ~2,000 cycles (illustrative, based on typical AgCdO erosion curves), while the G7J at 40 A would handle that inrush as ~30% of its margin and last past 100,000 cycles. The reversal: for purely resistive loads with no inrush—heater elements, incandescent lamps on soft-start—AgCdO is entirely adequate and costs less; the G7J’s AgSnO2 becomes overkill. The rule: match contact material to load surge duration, not just steady current.
Dielectric Strength: The Hidden Safety Margin Under Transients
Dielectric strength is the voltage the relay can withstand between coil and contacts for one minute without breakdown. The G2R Series lists 1500 VAC. The MY Series also lists 1500 VAC. The G7J Series lists 2500 VAC. At first glance all adequate for a 250 VAC system. The mechanism: dielectric breakdown is not about nominal voltage—it’s about transient overvoltage from switching inductive loads (motor contactors, solenoid valves) or lightning-induced surges on long lines. IEEE C62.41 recommends a 2.5 kV transient withstand for industrial control circuits. At 1500 VAC, a 2 kV transient can punch through the coil-to-contact insulation, causing a short-circuit that welds the relay closed. The G7J’s 2500 VAC margin means it survives a 3 kV spike (roughly 120% of rating before statistical failure) without breakdown. The worked outcome: a MY2 relay on a 24 VDC coil driving a 5 A contact in a factory near a VFD drive saw a 1.8 kV transient on the AC line—the MY2 failed shorted on the third event. A G7J in the same panel location survived the same transient profile for 18 months without issue (field observation, illustrative). The reversal: if your environment is a clean lab bench with no long inductive cabling, the 1500 VAC rating is enough and the G7J’s extra margin costs you panel volume and price. The rule: choose dielectric margin based on your worst-case transient environment, not the AC line voltage.
Non-obvious insight: The MY Series and G2R Series share the same dielectric strength (1500 VAC) but the G2R-1 uses a 5 VDC coil while the MY2 uses a 5 VDC coil—same dielectric rating, but the G2R’s PCB mounting means the creepage distance is shorter between PCB traces to the coil pins. In a humid cabinet (condensation), the G2R can track before the MY2 does, even though the dielectric strength number is identical.
Operating Temperature Range vs. Thermal Rise Under Load
The datasheet shows a generous operating temperature range: G2R and MY Series both span –40 °C to 70 °C; the G7J Series pushes to 85 °C. What the datasheet hides: the upper limit applies only at the rated contact current with the coil at nominal voltage. Derate curves are rarely printed. Mechanically, every amp through the contact creates I²R heating—at 10 A, the contact junction temperature can be 30–40 °C above ambient inside a sealed enclosure (illustrative, based on typical thermal resistance for a 10 A contact). If the panel ambient is 60 °C, the internal junction hits ~100 °C, which exceeds the 70 °C ambient limit of the G2R and MY, accelerating contact oxidation and coil insulation breakdown. The G7J’s 85 °C limit plus its larger contact mass and AgSnO2 material means the junction temperature stays below 110 °C at full 40 A load (illustrative, based on datasheet thermal data). The worked consequence: a G2R-2 switching 8 A in a 65 °C cabinet (typical inside an outdoor PLC panel in summer) will see contact resistance double within 6 months, increasing I²R heat further—a thermal runaway. A G7J in the same cabinet at 20 A (underloaded) would stay within its 85 °C ambient and run for years. The reversal: in a temperature-controlled indoor panel at 25 °C, the G2R and MY have ample margin; the G7J’s extra temperature range is wasted. The rule: subtract the contact self-heating from the ambient before comparing to the relay’s temperature range.
Roundup Table: Omron Relay Families by Hidden Specs
| Relay Family | Contact Rating (resistive) | Contact Material | Dielectric Strength | Operating Temp Range | Mounting Type | Best for |
|---|---|---|---|---|---|---|
| G2R-1 / G2R-2 | 10 A @ 250 VAC | AgCdO | 1500 VAC | –40 to 70 °C | PCB / Socket | Clean resistive loads, low surge, moderate ambient |
| MY2 / MY4 | 5 A @ 250 VAC | AgCdO | 1500 VAC | –40 to 70 °C | PCB / Socket | Low-current control, pilot duty, reliable socket change |
| G7J-4A | 40 A @ 250 VAC | AgSnO2 | 2500 VAC | –40 to 85 °C | Panel | High inrush, motor, lamp, hot panel, transient-heavy |
Ratings per manufacturer datasheets; illustrative derating and thermal rise values as noted.
Failure Mode You Won’t See in the Datasheet: The G2R and MY both use AgCdO contacts. In a high-humidity environment (>85% RH) with frequent switching, cadmium oxide can react with moisture to form cadmium hydroxide, which is conductive and causes contact bridging—the relay fails closed even when the coil is de-energized. This is not a spec failure; it’s a chemical failure mode hidden by the contact material. The G7J’s AgSnO2 is immune to this because tin hydroxide is non-conductive. If your relay is in a coastal or washdown area, choose AgSnO2 regardless of current rating.
Decision Rule: One Threshold to Size By
Stop matching by contact amps. Instead, compute effective thermal stress factor = (peak inrush current / rated current) × (worst-case ambient + self-heating margin). If your factor is > 1.2, choose the G7J. If factor
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.
