The Real Difference Between A Contactor And A Relay (And How It Breaks Transformer Protection)

I'm a quality inspections manager at GE Transformer. I review every protection relay manual that ships with our Multilin SR850 and distribution transformers—roughly 1,200 documents a year. In Q1 2024, I rejected 6% of the control wiring packages in our factory acceptance tests because the same confusion kept showing up. Not the relay's firmware, not the transformer. It's the difference between a contactor and a relay.

A "Mysterious" Trip That Isn't Mysterious

Here's the scene. You're commissioning a small industrial installation. There's a GE Multilin SR850 transformer protection relay guarding a 1 MVA step-down transformer. The test crew closes the main breaker. Nothing. Ten minutes later, the breaker trips "for no reason." The SR850 event log says "Overcurrent." You call the GE support line. They ask: "How many ohms is the tripping circuit? Have you checked the output relay contact?"

That's when you open the GE 850 transformer protection relay manual. (Yes, the one with the contact rating table you skipped.) And you see it. The trip output is rated for pilot duty—meaning it's a relay contact, not a contactor. The manual clearly says the output is for small signals. But someone with good intentions wired it directly to a contactor coil in the breaker's spring charging motor. The coil inrush current was triple the relay's rated make/break. The relay contact welded on the third operation. The SR850 couldn't send the trip signal, but before it welded it was trying to trip on a nuisance current spike caused by the charger motor.

Contactor vs. Relay: It's Not Just About Size

People think a relay is a "small contactor" and a contactor is a "big relay." That's the surface problem. The deeper issue is that they're two different categories of switching devices.

A contactor is designed for power switching. It has arc chutes, robust springs, and can handle high inrush currents—like motor loads. A relay is designed for control switching. It has precise contacts, fast operation, and is intended to switch small signals, not power.

In transformer protection panels, the protection relay is the brain. The contactor is the muscle. If you use the brain's signal contacts to directly control the muscle's power coil, you overload the brain. This is exactly where the setup is reversed: you think the GE Multilin SR850 relay is misbehaving, but the relay is being forced to do something it was never designed to do.

As of January 2025, GE's spec sheet for the 850 relay lists output contacts at 10A/120VAC general purpose, but the governing spec is NEMA A600 pilot duty. That's not marketing. It's a necessary design constraint. When you wire a contactor coil directly to this output, you're violating it.

The GE manual states, in effect, "Pilot duty only. Use an interposing relay for load switching."

The assumption is that GE's protection relays are expensive and overcomplicated. The reality is that the relay's logic is usually correct, and the failure is upstream in the switching schema. The causation runs the other way: an incorrectly rated external component makes a high-quality relay look flaky.

Save $47 Now, Spend $18,000 Later

I worked with an OEM that built a cooling fan control panel for a substation transformer. They wanted to save money on the control circuit. Instead of buying a proper interposing relay, they used a sealed relay caddy that fed the contactor's coil directly from the GE Multilin 850 relay output. The cost difference was $47. They even laughed at the suggestion to add an interposing relay.

Three months later, the fan motor shorted. The SR850 protection relay sent the trip command. The output contact welded. The breaker didn't open, and the transformer ran at 120°C until the Buchholz relay on the tank finally saved it. The transformer survived, but the rewind and replacement bushing work cost $18,000. The OEM paid for it, not the utility. And the $47 "savings" became an insurance claim.

I have mixed feelings when I tell this story. Part of me wants to say "I told you so." Another part says we should have put controls around it. Now we do—every contract includes a requirement for interposing relays in control circuits.

The Same Confusion, Different Hardware

This isn't unique to GE systems. It shows up in other equipment sharing the same control cabinet. Take the ASCO manual transfer switch. It's a robust piece of gear. Inside it, the auxiliary contacts are just that—contacts for the control circuit. If you assume these contacts are equally rated as a contactor's main contacts, you'll overload them, too. When you route an ASCO manual transfer switch to a control panel, check the wiring diagram: those "contact" symbols are relay-level, and you need to drive a contactor if you intend to switch loads.

Then there are indoor control panels. I once saw a Pentair EasyTouch indoor control panel used in a building management system to switch a small chilled water pump. The Pentair panel is designed for pool equipment, but people adapt it. Its internal relay outputs are rated for a few amps, not for motor starting. Using it to drive a pump contactor is fine; driving the motor directly would melt the PCB traces. The distinction: the Pentair's output relays are "control relays," and the contactor is the "power switch."

I want to be clear about the difference between a contactor and a relay in plain terms:

• A relay's job is to pass information—a voltage, a signal, a control command.
• A contactor's job is to pass power—current to motors, heaters, transformers.
• Both use coils and contacts, but they're engineered for different worlds.

I don't look down on adaptations like using a Pentair panel. I've seen small startups do brilliant things with off-the-shelf hardware. The problem isn't being small; it's being unaware of contact ratings. The fix is simple: read the manual of the relay before you wire the panel.

A 5-Point Verification Checklist (From a Quality Inspector)

Here's what I have my engineers do before a panel ever reaches a FAT:

1. Check the SR850 manual's contact ratings. The GE Multilin SR850 transformer protection relay manual lists every output contact's rating, including transient overvoltage. Match that to your load's inrush specs.
2. Identify every contactor in the control circuit. If a contactor coil is driven by a relay output, add an interposing relay—or verify the coil's VA rating is below the relay's make/break rating with margin.
3. Verify transfer switch contacts with a signal light. For an ASCO manual transfer switch, use a 120V lamp or PLC input to test the auxiliary contacts. Don't assume they'll carry motor current.
4. Check PCB relays in indoor panels. If you have a Pentair EasyTouch indoor control panel or similar, look for pad-printed relay ratings on the PCB. Treat them as signal-level outputs unless you see a contactor downstream.
5. Perform a point-to-point wiring check. Use a multimeter to confirm that no power source is routed through a protection relay's output that exceeds its spec. It's basic, but most FAT failures I see trace to this.

Respect the Relay, Save Your Transformer

If you're staring at an expensive GE 850 transformer protection relay that keeps reading "trip," don't blame GE. Blame the relay/contactor misunderstanding that's been hiding in your control cabinet since day one.

I don't care how large or small your project is. I review 1,200 manuals a year, and I've seen this mistake at a 10 MW substation and in an obscure pump room. The fix isn't exotic: open the manual. Use the relay for what it's for. Specify a contactor for power switching. Use an interposing relay when in doubt. And don't be afraid to ask the manufacturer for their spec sheet—that's what we're here for.

Small orders don't bother me. The ones that bother me are the ones where someone skips the basics because they assume a relay is a contactor. That's the path to a $22,000 redo and a lot of apologizing down the line.