Every generator shed I’ve stepped into has the same smell — diesel, dust, and the quiet hiss of a contactor that’s almost welding shut. On a site in western Colorado, the backup Gen set kicked in after a summer storm, and the control panel’s relay started chattering within 12 minutes. The noise feed wasn’t DC; it was a 60-cycle waveform with harmonics that made the coil see 95–130 VAC in random bursts. The original relay (a cheap 10 A job with no rated dielectric margin) welded one N.O. pole. The whole PLC loop failed open, and the well pump stayed off for 40 minutes.
The fix wasn’t a bigger contactor — it was the correct Omron relay for the feed’s real voltage swing. This roundup looks at four Omron families — G2R, MY, G7J — and what happens when a dirty generator line hits them. The numbers tell the story, and the ratio of coil pickup margin to peak feed voltage is the non-obvious variable that determines whether the relay stays closed or starts welding.
1. Coil Pickup vs. Generator Feed Swing — the Magnitude Ratio
Take the Omron G2R-1 with a 24 VDC coil — it picks up at roughly 16.8 V (70% of nominal, typical for DC relays). On a generator feed that sags to 19 V during a motor start, that’s still 2.2 V above threshold. But the G2R-1 also comes in 12 VDC and 5 VDC versions. If you spec the 5 V coil on a nominal 5 VDC bus fed from a generator that swings ±20% (4–6 V), the ratio of pickup to worst-case low is 3.5 V / 4 V = 0.875 — meaning the coil is only 12.5% above dropout. Any additional harmonic ripple of 0.5 V pushes it into the dropout zone. The mechanism is that the coil’s magnetic force falls off roughly as the square of voltage below nominal; at 85% pickup, the force is ~72% of nominal, and armature sealing starts to fail. The worked consequence is that a relay that chatters even for 20 line cycles will begin to arc between contacts, and silver cadmium oxide (AgCdO, used in G2R-1) wears under DC arcing faster than under clean AC. The reversal: if your generator feeds a closed transition transfer switch that never lets voltage drop below 90%, a G2R with a 24 VDC coil is overkill — an MY2 with 24 VDC (same pickup voltage ratio, same AgCdO) would work and cost less.
2. Dielectric Withstand — Where the Generator’s Spike Hits
The G2R series is rated 1500 VAC dielectric strength; the MY series also 1500 VAC; the G7J series handles 2500 VAC. On a generator feed, the worst transient is usually a re-strike spike from the AVR or a line-to-ground fault that can reach 2.5–3 kV for a few microseconds. The ratio of withstand to peak line-to-neutral (1.8 kV for a 208 VAC system) is 0.83 for a 1500 VAC rated relay — meaning there’s only 700 V margin. The mechanism is that dielectric breakdown is a cumulative fatigue effect; each transient near the limit stresses the internal insulation, and after several hundred events, partial discharge starts across the bobbin. The worked result: if you use a G2R on a generator that has frequent AVR overshoot (common on older brushless gensets), you may see coil-to-contact leakage after 6–12 months. But the G7J’s 2500 VAC rating gives a ratio of 1.39 (2.5 kV / 1.8 kV) — that’s enough headroom to absorb most spikes without stress. The reversal: if the generator is backed by a well-filtered UPS output, the transient amplitude rarely exceeds 1 kV, and a G2R or MY is fine.
3. Contact Rating — 10 A vs. 40 A on a Generator’s Motor Loads
Omron’s G2R-1 and G2R-2 are rated 10 A at 250 VAC; the G7J-4A is rated 40 A at 250 VAC. On a generator feed that powers a 1 HP pump motor (about 8 A running, 40 A inrush), the 10 A relay is only 25% of the inrush surge. The mechanism is that contacts weld when the arc energy during the first half-cycle exceeds the melting point of the contact material. AgCdO (in G2R and MY) has moderate arc resistance but begins to erode at inrush currents above 20 A; AgSnO2 (in G7J) is significantly more resistant to material transfer under high inrush. The worked consequence: a G2R-1 switching a motor on a generator feed — where the voltage can be distorted and the inrush peak is higher than on a stiff grid — will accumulate pitting and eventually weld after 5,000–10,000 operations. A G7J-4A with AgSnO2 contacts will survive more than 100,000 operations under the same conditions (illustrative cycle life, based on manufacturer typical curves). The reversal: if the load is purely resistive (heater, lighting ballast) with inrush under 15 A, the G2R-1’s 10 A rating is adequate and the G7J is oversized.
4. Temperature — What the Shed’s Radiant Heat Does to Coil Resistance
G2R and MY series are rated -40°C to 70°C; G7J series goes to 85°C. In a generator enclosure, ambient temperature near the engine block can hit 65°C after 2 hours of full load (about 120°F). The coil resistance of a copper winding increases roughly 0.4% per °C. At 65°C (25 K rise above 40°C), the coil resistance is about 10% higher than at 20°C. That means pickup voltage also rises by ~10% — a 24 VDC G2R coil that picks up at 16.8 V at 20°C will need about 18.5 V at 65°C. The ratio of generator’s low sag (say 20 V) to new pickup threshold is 20 / 18.5 = 1.08, just 8% margin. The mechanism is that higher coil temperature reduces pull-in force; if the generator feed also sags (which it does under heavy load), the margin disappears. The worked result: in a hot shed, a G2R-1 with 5 VDC coil (pickup ~3.5 V at 20°C, ~3.85 V at 65°C) on a 5 V bus that sags to 4 V — ratio 4 / 3.85 = 1.04, too thin. The G7J with 24 VDC coil (pickup ~16.8 V at 20°C, ~18.5 V at 85°C) on a 24 V bus that sags to 20 V — ratio 20 / 18.5 = 1.08, still marginal but within spec. The reversal: if the generator shed is actively ventilated and ambient stays below 50°C, the MY or G2R are fine.
| Dimension | G2R Series | MY Series | G7J Series | Decision Signal |
|---|---|---|---|---|
| Coil pickup margin (24 VDC, 65°C) | ~8% | ~8% | ~8% | All similar; margin thin for low-voltage coils |
| Dielectric withstand | 1500 VAC | 1500 VAC | 2500 VAC | G7J for transient-prone feeds |
| Contact rating / inrush tolerance | 10 A, AgCdO | 5 A, AgCdO | 40 A, AgSnO2 | G7J for motor or high-inrush loads |
| Max ambient temperature | 70°C | 70°C | 85°C | G7J for hot enclosures |
Rule-based conclusion: For a generator feed where you expect voltage sags >10%, spikes >2 kV, or motor inrush >20 A, pick the Omron G7J-4A with 24 VDC or 12 VDC coil (depending on your DC bus). For clean, ventilated environments with resistive loads under 5 A, the MY2 is sufficient. The threshold: if the ratio of (minimum feed voltage at 70°C) to (coil pickup voltage at 70°C) is less than 1.10, step up to G7J. That one number will save you a call in the middle of a blackout.
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.
References: Omron G2R Series datasheet — dielectric 1500 VAC, temp -40–70°C, coil pickup ~70% nominal. Omron MY Series datasheet — dielectric 1500 VAC, temp -40–70°C. Omron G7J Series datasheet — dielectric 2500 VAC, temp -40–85°C. Omron G2R-1 contact rating 10 A 250 VAC. Omron G7J-4A contact rating 40 A 250 VAC. AgSnO2 vs. AgCdO arc resistance comparison: general manufacturer data, not specific to Omron. Illustrative cycle life estimate: G7J typical curves at 40 A resistive, ~100k operations under clean grid; dirty feed reduces endurance.
