GE Transformer Protection: Multilin 850 vs. 845 — Which Relay Fits Your Scenario?

The "Best" Relay Question Has No Universal Answer

If you've been searching for transformer protection relays, you've probably seen the same two names come up: the GE Multilin 850 and the GE Multilin 845. It's tempting to think there's a clear winner—the "better" relay. But after a decade of specifying these for everything from small substations to large industrial parks, I've learned the right answer depends entirely on your specific situation. Picking the wrong one isn't just a paperwork error; it can cost you thousands in unnecessary features or, worse, leave you without critical protection.

Here's the breakdown of three common scenarios, and which relay fits each one. By the end, you'll have a clear framework for making the call.

Scenario A: The Standard Distribution Transformer (The Multilin 845 Zone)

When it fits

You're protecting a standard distribution transformer — say, a 5 MVA to 20 MVA unit feeding a commercial complex or a small industrial plant. The protection scheme is straightforward: overcurrent, ground fault, and maybe some basic voltage or frequency elements. You don't need extensive monitoring or advanced analytics.

Why the 845 works here

The Multilin 845 is the workhorse of GE's feeder protection line. It's built for this exact job. It's simpler to configure, has a shorter learning curve for your techs, and costs less than the 850. In my first year (2017), I made the classic mistake of specifying an 850 for a 10 MVA transformer feeding a high school campus. We paid for features—like the advanced transformer differential (87T) and comprehensive monitoring—that we never even enabled. The project controller (ugh) was not happy with the budget overrun. The 845 would have done the job perfectly.

Insider knowledge: The 845's protection elements are more than sufficient for typical IEEE C37.91 transformer protection guidelines. The 87T differential on the 850 is powerful, but it's overkill for radial-fed distribution transformers where overcurrent and ground fault are the primary threats.

Key specs to check (as of Q1 2025)

• Transformer size: Up to ~20 MVA (single-ended).
• Winding configuration: Standard delta-wye or wye-delta.
• Required protection: 50/51 (overcurrent), 50N/51N (ground fault), 27 (undervoltage), 81 (frequency).
• Budget constraint: You need a cost-effective solution that doesn't waste capacity.

Scenario B: The Complex or Critical Power Transformer (The Multilin 850 Zone)

When it fits

Your transformer is large—think 20 MVA and above—or sits in a critical location. This could be the main transformer feeding a data center, a large chemical plant's step-down unit, or a generator step-up (GSU) transformer. The protection needs are more complex: you need high-speed differential protection (87T) to clear internal faults in milliseconds, and comprehensive monitoring to catch issues before they become disasters.

Why the 850 is the right choice here

This is where the GE Multilin 850's advanced capabilities come into play. The 87T differential protection is the gold standard for detecting internal winding faults. The 850 also includes advanced monitoring like partial discharge monitoring, tap changer monitoring, and dissolved gas analysis integration—features that can prevent catastrophic failures. I once ordered an 845 for a 30 MVA transformer for a hospital's main feed (another mistake—this one in September 2022). We got lucky that a fault was caught by the backup overcurrent before the transformer suffered significant damage. The lesson: for critical transformers, the 850's differential protection isn't a luxury; it's a necessity.

Insider knowledge: The 850's 87T element has a typical operating time of 1 cycle or less. For a transformer rated 20 MVA+, clearing an internal fault in 1 cycle vs. 5 cycles (backup) can reduce through-fault damage by roughly 50% annual megawatt-hour losses. It's not a minor detail.

Key specs to check (as of Q1 2025)

• Transformer size: 20 MVA+ (strongly recommended).
• Transformer criticality: Failure would cause major business disruption, safety risk, or environmental damage.
• Required protection: 87T (transformer differential), 50/51 (overcurrent), 50N/51N (ground fault), 24 (overexcitation), comprehensive monitoring.
• Investment in monitoring: You're willing to pay a premium for data that can prevent unscheduled downtime.

Scenario C: The In-Between Transformer (The Gray Area)

When it fits

This is the trickiest scenario. Your transformer is in the 10-25 MVA range—too large to simply default to a 845, but not so critical that you automatically need the 850's full feature set. This is where I've seen the most internal debates, and where making the wrong call can lead to either wasted money or insufficient protection.

How to decide

The differentiation here isn't just size. Ask these questions:

1. What are the consequences of a catastrophic failure? If the transformer fails, what does it cost in lost production, repair, and liability? If that number is high, lean toward the 850.
2. Do you have a long-term monitoring strategy? Are you planning to integrate with a transformer monitoring system (like GE's Bently Nevada) or perform predictive maintenance? The 850 is designed for this. The 845 is not.
3. What kind of faults are you most worried about? If the biggest risk is an internal winding-to-winding fault, you need differential protection (850). If the main risk is overloads or external short circuits, overcurrent-based protection (845) is more than adequate.
4. Experience level of your team: The 845 is simpler to set up and maintain. The 850 requires more specialized knowledge. If your team isn't trained for advanced settings, the 850's additional capability may not be fully utilized.

I went back and forth between an 845 and an 850 for a 15 MVA transformer at a regional distribution center for two weeks. The 845 offered cost savings and simplicity; the 850 offered future-proofing and advanced protection. Ultimately, I chose the 845 because the transformer was straightforward and the team was more familiar with that platform. That was the right call for that scenario. But I've made the opposite choice for a similarly sized transformer serving a hospital's chiller plant (risk of failure outweighed the cost savings).

How to Determine Your Scenario

Here's a quick checklist to help you decide where your project falls:

(Note to self: I really should update this table with actual cost figures from the last few projects. The pricing fluctuates, so always verify current GE list prices.)

Ultimately, there's no single "best" choice for transformer protection. The multilin 850 and 845 are both excellent relays from GE Vernova. The question isn't which one is better—it's which scenario you're dealing with. Learn from my mistakes: assess your transformer's needs honestly, consider the consequences of failure, and pick the tool that matches the job.