The Short Version
If you're choosing between the GE transformer protection relay model 850 and the GE Multilin 845 transformer protection system, here's the honest version: the 850 is not a transformer-specific relay, and the 845 is. But that's not the whole story. I've seen the wrong choice cost more than the relay itself.
I've been specifying and ordering protection relays for eleven years. I've personally made and documented seven significant mistakes, totaling roughly $38,000 in wasted budget. This article is the checklist I wish someone had given me in 2017.
Comparison Framework
Datasheets list I/O, communication protocols, and dimensions. They don't tell you which relay will cause a site delay during commissioning. So I compare these two relays on four dimensions: design intent, protection scope, commissioning reality, and total cost of ownership.
As of early January 2025, this is the framework I use with my own team. It's changed over time, but the core lesson stays the same: protection relay selection is a system design decision, not a parts order.
Design Intent: 850 vs 845
The first mistake people make is assuming that a higher model number means a newer or better version. It doesn't. The GE Multilin 850 is a multi-function feeder protection relay. The GE Multilin 845 is specifically a transformer protection system. Both can be installed in a transformer control panel, but they were designed for different jobs.
The 850 gives you overcurrent, directional overcurrent, breaker failure, auto-reclose, and frequency protection. It's a workhorse for feeders and backup protection.
The 845 is built around transformer differential protection. It compares current flowing into the transformer with current flowing out, applies vector group compensation, and includes restricted earth fault and overexcitation protection. That's a different level of sensitivity.
If an 850 is used where the protection study calls for a 845, the transformer is only protected for the faults the relay can see from one side. That's a hard lesson, not a nice-to-have.
Protection Scope: The Part That Actually Matters
Here's where I'll say something that surprises people. For a large or critical transformer, the 845 is clearly the right answer. For a smaller distribution transformer, the 850 can be a better fit, even though it's not labeled as transformer protection.
Why? Differential protection only works when the CTs are right. The 845 needs matched CT ratios, correct vector group settings, and trustworthy transformer nameplate data. On a small transformer where the client doesn't have a CT test report or someone forgot to order the second set of CTs, the 845 becomes a custom engineering project instead of a drop-in relay.
In 2020, I ordered 850s for a two-winding transformer feeder because I thought the feeder relay would be enough. The client's engineer caught it during drawing review, we swapped to 845s, and the project absorbed a two-week delay plus change-order costs. I still kick myself for not reading the protection scheme one more time.
What did that mistake teach me? The 845's differential protection can catch internal turn-to-turn faults that an 850 simply can't see. For a critical transformer, that matters. But if the CT infrastructure can't support differential protection, the fancy relay is not saving anyone.
Commissioning Reality: Where I've Made Mistakes
Both relays use GE's EnerVista software. Same cable, same logic-editor quirks. The 845 has more setup screens because it needs transformer data, vector group compensation, and CT balancing. More screens means more room for error.
Extension Cord vs Surge Protector: This Isn't a Real Fight
On a bench test, I always plug the relay under test into a 2 outlet surge protector, not an extension cord. "Extension cord vs surge protector" isn't a real comparison. An extension cord extends distance. A surge protector clamps voltage spikes. They serve different purposes.
When I test a relay near a contactor that switches a 10 HP motor, the voltage transients on the bench are real. I once saw a 12 ms power dip that looked exactly like a relay misoperation. It wasn't the relay. The load box latched down because it was powered through an un-protected extension cord. The relay was a scapegoat for bad test setup.
The relay itself might be designed to meet IEEE C37.90.1 surge withstand requirements. That doesn't mean the bench should be powered by a questionable extension cord. A $20 surge protector is cheap insurance, and it's easier than explaining a misoperation that isn't one.
Type D Circuit Breaker on Control Power
On the AC control side, I now specify a Type D circuit breaker for relay power supplies. A Type D trip curve tolerates 10–20 times the rated current, which handles the inrush when a relay's power supply charges its capacitors.
I've seen a Type C breaker nuisance-trip during panel testing. The team spent an afternoon chasing a "relay fault" that was just a breaker with the wrong trip curve. The fix was a Type D circuit breaker and a clearer label on the panel. That's the kind of detail that makes a relay installation look competent.
Cost and Availability
I can't give you a fixed price list because quotes vary by distributor, region, and configuration. In my experience, the 845 is quoted at a premium over the 850. But the larger cost is engineering time. Setting up the 845 correctly takes more design work. Using the 850 for a transformer that really needs differential protection can cost more later.
As of early January 2025, both product lines are still supported, but GE's power equipment business has moved under GE Vernova. The documentation pages have moved around. Verify current model availability, firmware versions, and order-code options with an authorized distributor before you finalize your spec.
Selection Advice
Choose the GE transformer protection relay model 850 if:
- You're protecting a distribution transformer where overcurrent and backup protection are enough.
- You can't verify CTs on both windings.
- You also need feeder protection, and transformer protection is a secondary function.
- Your team needs a relay that can be commissioned quickly without a long differential setup process.
Choose the GE Multilin 845 transformer protection system if:
- The transformer is large, expensive, or part of a critical process.
- You have confirmed CT documents, vector group details, and nameplate data.
- You want restricted earth fault, overexcitation, and differential protection in one package.
- Your protection study specifically calls for transformer differential protection.
The counterintuitive conclusion? For small transformers, the 850 is often the more reliable choice. For large transformers, the 845 is the one. The mistake is treating relay selection as a brand loyalty decision instead of a protection calculation.
Quality Perception: The Last Detail
Clients can't see a trip curve when they open a panel. They see how the system is put together. A neat panel with a proper Type D breaker, a surge protector, and labeled wiring sends a signal: the protection is in good hands. That's quality perception. It's not cosmetic. It's trust.
This was accurate as of early January 2025. The market changes fast, so verify current pricing, standards, and product availability before you budget.
Leave a Reply