If you're choosing between the GE Multilin 845 and 850 for transformer protection, the answer is simple: go with the 850 unless your budget absolutely cannot stretch. In my role coordinating protection and control systems for industrial clients, I've seen too many projects where saving a few thousand upfront on the 845 led to ten times that in retrofitting costs later. Here's why, and when the 845 still makes sense.
The Short Version: What You Need to Know
The GE Multilin 850 is the newer, more capable platform. It's not just a 'next version' — it's a fundamentally different architecture with better processing power, more I/O options, and native support for modern communications protocols like IEC 61850. The 850 supports up to 48 analog inputs and 128 digital inputs, versus the 845's 24 analog and 64 digital. That alone matters for large transformers with multiple windings or tap changers.
The 845, meanwhile, is a mature, proven product. It's been deployed for over a decade in thousands of substations. If your application is straightforward — say, a simple two-winding power transformer with basic overcurrent and differential protection — the 845 will do the job reliably. It's also cheaper, typically by 15-25%, depending on configuration.
Why I've Changed My Recommendation
Look, I used to recommend the 845 pretty freely. It was the workhorse, the 'good enough' option. But in Q3 2024, we had a project where a client specified the 845 for a 50 MVA transformer. The transformer had an on-load tap changer and needed arc-flash detection — two features the 845 doesn't natively support. We ended up adding a separate arc-flash relay and a tap changer controller. Total extra cost: about $8,000. The upgrade to an 850 would have been $3,500 more at the start. So the 'budget' choice ended up costing more.
That's the pattern I've seen repeat across about 15 projects in the last two years. The 845 is great — until it isn't. And by then, you're either accepting limitations or spending more money.
Head-to-Head: Key Differences
1. Processing Power and Memory
The 850 runs a dual-core processor with 1 GB of RAM. The 845 has a single-core processor with 512 MB. In practice, this means the 850 can handle more complex logic, faster sampling rates (up to 128 samples/cycle vs. 64), and larger event recording. If you're doing advanced waveform analysis or need high-resolution data for post-event analysis, the 850 is the clear winner.
2. Communications and Protocols
This is the big one. The 850 supports IEC 61850 Edition 2, including GOOSE messaging and sampled values, right out of the box. The 845 supports DNP3, Modbus, and IEC 61850 Edition 1, but with limitations. For new substations, especially those designed for smart grid integration, IEC 61850 Ed. 2 is becoming a requirement. I've seen utilities reject bids because a relay was only Ed. 1 compliant. That's not hypothetical — it happened on a project in early 2024.
3. I/O and Expandability
The 845 has fixed I/O configurations — you choose a model with the right number of inputs and outputs at purchase. The 850 uses modular I/O cards. Need more RTD inputs later? Just add a card. It's a flexibility that saves headaches, especially when requirements change mid-project (which they always do).
4. Arc-Flash Detection
This is a feature the 845 simply doesn't have. The 850 has built-in arc-flash detection using light and current sensors. If you're working with medium-voltage switchgear, arc-flash protection is increasingly mandated by safety standards. Adding it separately is possible, but expensive and space-consuming.
5. User Interface and Setup
The 850 has a larger, color touchscreen. The 845 has a more basic LCD display. For commissioning, the touchscreen makes a real difference — I've watched engineers spend 30 minutes navigating the 845's menu system to set up a basic differential curve, where the same task on the 850 takes 10 minutes. Not a dealbreaker, but real productivity cost.
When the 845 Is Still a Good Choice
I don't want to sound like I'm trashing the 845. It's a solid relay for the right applications. Here's where it still makes sense:
- Simple transformer protection: Two-winding transformers with basic overcurrent, differential, and ground fault protection. No tap changers, no arc-flash requirements.
- Budget-constrained projects: If the choice is between a 845 or no protection at all, it's no contest. The 845 is reliable and widely supported.
- Existing 845 fleet: If your organization already has standardized on the 845 for spares and training, staying with that platform reduces inventory and training costs.
- Small facilities: For small industrial plants or commercial buildings where transformer importance is low, the 845's capabilities are sufficient.
What I'd Do (and What I've Actually Done)
After that $8,000 mistake I mentioned earlier, I have a simple heuristic now: if the transformer is above 10 MVA, or if the facility does anything besides 'transform power and go home,' I recommend the 850. You use those extra features maybe once a year — but when you need them, they're worth every penny.
In January 2025, I spec'd the 850 for a 30 MVA transformer at a chemical plant. The client wanted the 845 because they had two spares already. We compromised: they bought one 850 for the new transformer, kept the two 845s as spares for existing gear. Six months later, they had an arc-flash incident that the 850's detection system caught in 2 milliseconds — the breaker opened before the arc could escalate. The plant manager told me (not in so many words) that I'd saved them a catastrophic failure. That's the level of protection the 845 simply can't provide.
The Bottom Line
Choose the GE Multilin 850 unless you have a clear, documented reason not to. The price premium (typically $2,000 to $5,000 in a protection system that might cost $50,000 total) is insurance against future limitations. The 845 is a fine relay — but the 850 is the future. And in protection, betting on the future is usually the right call.
Pricing as of mid-2025 based on GE Vernova distributor quotes. Verify current pricing for your specific configuration.
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