How I Learned the Hard Way: A 6-Step Checklist for Ordering GE Transformer Protection Relays

If you're ordering GE transformer protection relays — specifically the Multilin 850 or 845 series — you might think it's straightforward. Pick the model, add the options, place the order. Simple, right?

I thought so too. Until I got my first order wrong. Not just a little wrong. $3,200 wrong.

That wasn't the only mistake. The third one, a mix-up on the 87T differential protection settings, cost us a 2-week project delay. That's when I stopped winging it and built a checklist.

Here's the 6-step checklist I use now for every GE Multilin relay order. It's saved us an estimated $8,000 in potential rework over the last 18 months.

Step 1: Confirm the Application (Not Just the Model Number)

This sounds obvious. It wasn't to me.

My first mistake: I ordered a GE Multilin 845 relay for a transformer differential application. The 845 is a great relay. But for a simple two-winding transformer, the Multilin 850 would have been a better fit — and about 30% cheaper.

Here's what I check now:

• What exactly are you protecting? Transformer, generator, motor, feeder? Each application has a preferred relay model.
• How many windings? The 850 handles two-winding transformers with 87T differential protection. The 845 handles up to five-winding configurations. Over-specifying costs money; under-specifying causes failures.
• What's the voltage class? GE Multilin relays have different input voltage ranges. Match it to your PT and CT ratios, not just the nameplate voltage.

I now keep a one-page reference card showing which GE relay model maps to which primary application. It's taped to my monitor.

Step 2: Verify the Multilin 850 87T Differential Settings (This Is Where I Blew $3,200)

Here's the painful part.

In September 2022, I submitted an order for a GE Multilin 850 transformer differential relay with the standard 87T protection settings. Looked fine on the spec sheet. The problem? Our transformer had a unique tap configuration and an unusual impedance mismatch.

The relay arrived. We installed it. The differential protection kept tripping on inrush. False trips. Every time the transformer energized, the relay thought it was a fault.

We spent a week troubleshooting. Finally called GE support. Turns out, the 87T slope settings needed adjustment for our specific transformer parameters. But the relay we ordered didn't have the flexible slope options we needed. We had to order a different variant. $3,200 wasted.

What I do now:

• Get the transformer's impedance, tap range, and winding configuration before specifying the 87T settings.
• Check if the relay order code includes the flexible slope option (usually designated by a specific alphanumeric suffix in the model number).
• If the transformer isn't standard, order the configurable version. It costs a bit more upfront, way less than a redo.

"5 minutes of verification on the transformer specs beats 5 days of troubleshooting." — My rule, learned the expensive way.

Step 3: Cross-Check the Relay Model Against the Protection Functions You Actually Need

GE Multilin relays pack a ton of functions. The 850 alone lists dozens: 87T, 50/51, 67N, 49, 24, 64REF… it goes on.

The mistake I see people make: ordering based on the model name without confirming the function set is enabled.

Example:

Someone needs an under voltage relay (27 function) for a secondary protection scheme. They order a GE Multilin 850. But the 850 base model doesn't include the 27 function in its standard package. You need to select the option code for it.

Worse than wrong? Incomplete.

My checklist item:

• List every protection function you need (49, 50, 51, 87T, 24, 27, 52, etc.).
• Compare against the order code matrix for that specific relay.
• If you're unsure, the GE Multilin ordering guide (available on their site) breaks down which option codes enable which functions.

I've caught two near-misses using this step. Could have been another week-long delay each time.

Step 4: Match the Auxiliary Power and Relay Output Configuration

This step is boring. It's also where things silently go wrong.

I once ordered six GE Multilin 845 relays for a substation upgrade. Everything looked correct. The relays arrived and we started racking them. Then we noticed: the control power input was 48 VDC. Our station battery was 125 VDC.

Six relays. Wrong power supply modules. All had to go back.

Here's what I verify before clicking 'order':

• Control power: 24 VDC, 48 VDC, 110-125 VDC, or 110-240 VAC? Check twice.
• Output relay contacts: Form A (normally open), Form C (changeover), or solid state? And the rating — 10A at 250 VAC is common, but verify.
• Communication ports: Do you need RS485, Ethernet, fiber optic? The standard relay might not include your preferred port.

The power supply module is usually a separate line item or a specific option code. Don't assume it's included.

Step 5: Review the Serial Number Matrix for Firmware and Hardware Revisions

Not a step most people take. I learned it from a field support engineer.

GE Multilin relays have a serial number matrix that encodes the firmware version and hardware revision. If you're ordering a relay to match an existing installation — say, you're adding a second transformer and want the new relay to communicate with the old one — mismatched firmware can cause compatibility issues.

Example:

An older Multilin 850 with firmware version 4.0 might not support the same IEC 61850 data mapping as a new one with firmware version 7.0. The relays will work. They won't talk properly.

Quick check:

• If it's a replacement or expansion, note the serial number of the existing relay.
• Ask your supplier or GE representative the firmware version associated with your order.
• Specify the firmware revision if compatibility matters. This is not an option on every order, but it's often available if you ask.

Not ideal, but necessary. Better than finding out during commissioning.

Step 6: Double-Check the Accessories — Don't Assume They're Included

This step sounds trivial. It isn't.

I ordered a GE Multilin 850 relay last year. The relay came. No mounting bracket. No RS485 terminator plug. No Quick Start Guide. All things I'd assumed were in the box.

They weren't.

Had to order them separately. A $40 bracket caused a 3-day delay because we didn't have it on hand.

My final checklist:

• Does the order include a mounting bracket or panel adapter? (Not always standard.)
• Are communication terminators or cables listed? (RS485 networks need a terminator resistor.)
• Is the power supply module confirmed? (Often a separate item in the order code.)
• Do you need a USB-to-serial adapter for local programming? (Older relays use serial ports.)
• What about the manual or documentation? (Downloads are free, but a printed Quick Start Guide is sometimes included.)

A $50 accessory missing can halt a commission. I know. I've been there.

One More Thing: The "I'll Figure It Out" Trap

The most frustrating part of my early mistakes? I kept telling myself, "I'll figure out the specifics when the relay arrives."

You'd think written specs would prevent problems. But interpretation varies wildly between sales engineers, catalog listings, and actual hardware. I learned: verify before you buy, not after.

Took me three mistakes to accept that. Now I use this checklist for every single GE transformer protection relay order — whether it's a Multilin 850, 845, or even a simpler product.

Prices as of early 2025; verify current rates with your distributor. And if you're dealing with a non-standard transformer, take extra time on Step 2. Trust me on this one.