GE Transformer Protection: Multilin 850 vs. 845, and When a Basic Relay Is Enough

If you search for 'GE transformer protection,' you'll get sales pages, datasheets, and forum arguments about whether the Multilin 850 or 845 is the better relay. I'm not going to give you one answer, because there isn't one. I've been commissioning transformer and industrial control panels since 2016, and I've personally documented about $14,000 in rework from relay mistakes. That's why I keep a checklist instead of trusting my memory.

I see three situations that look similar but need different answers:

• You're repairing or upgrading an air compressor control panel or similar machine, and you need a relay for basic control logic.
• You're protecting a two-winding distribution or plant transformer and you want a dedicated protection relay.
• You're protecting a larger, three-winding or more critical transformer where a missed fault could be catastrophic.

Those are not the same problem. Don't let anyone sell you a transformer protection relay for an air compressor panel, and don't use a control relay to protect a transformer.

According to GE Vernova's product documentation, both the 850 and 845 are transformer protection systems. The real choice is about winding configuration, monitoring needs, and the people who will commission the relay.

What is an electrical relay, and why the relay symbol matters

An electrical relay is a switch controlled by a separate electrical signal. Energize the coil, and the contacts change state. That's true whether you're looking at a small ice-cube relay inside an air compressor control panel or a multifunction protection relay in a substation.

The difference is what the relay is designed to do. A control relay in an air compressor control panel is doing logic: start, stop, alarm. A transformer protection relay is measuring current and voltage, comparing them, and deciding in milliseconds whether a fault exists. If you use the wrong one, you can burn a transformer that costs ten times more than the relay you saved money on.

If you search for 'relay symbol electrical,' you'll find two common drawing systems. On IEC-style control schematics, a relay coil is shown as a rectangle, and contacts are drawn in their de-energized state. On North American protection drawings, functions are often identified by ANSI device numbers such as 50 (instantaneous overcurrent), 51 (time overcurrent), 87T (transformer differential), and 63 (sudden pressure). Source: IEEE C37.2. The exact graphic style matters less than the device number and the coil voltage.

Get the coil voltage wrong, and you're gonna spend more on labor than the relay saved. I learned that the hard way in 2019. A packaging line's air compressor control panel had a relay symbol that called for a 24 VDC coil, but someone had installed a 120 VAC relay because it physically fit. The panel ran intermittently for a month before we found it. That little mismatch cost $2,300 in downtime. The replacement relay cost $42.

GE Transformer Protection: Three scenarios, three sensible answers

Situation A: You just need a control relay in an air compressor control panel

If all you need is to replace a burnt control relay, do not jump to a Multilin 850. Get an industrial relay with the right coil voltage, contact configuration, and switching capacity. Verify the relay symbol on the drawing. Check whether the coil is 120 VAC or 24 VDC. And check the contact ratings before you install.

That part is not glamorous. It also prevents most field failures. When I was building a small control panel business on the side in 2018, the vendors who took my low-value orders seriously were the ones I still call first for larger projects now. Small orders have a way of becoming bigger ones if you respect them.

Situation B: Two-winding distribution transformer: GE Multilin 850

If you have a GE transformer or another two-winding transformer in the 1–15 MVA range, the GE Multilin 850 transformer protection relay is the sweet spot. It's a dedicated transformer protection and control relay with percentage differential, harmonic restraint, overcurrent backup, and event records in one box. I've set these up on padmount transformers, plant transformers, and small utility applications.

One caution: the 850 is only as good as the CTs feeding it. I almost uploaded settings based on the transformer nameplate instead of checking the actual CT ratio on a 2022 upgrade. So glad I did the pre-check—it was one click away from a differential relay that would have tripped on normal load.

The spreadsheet once told me to buy a surplus 850 and save 40%. My gut said no because the seller could not provide a serial number. I bought new through an authorized distributor. Two weeks later, that same surplus relay was relisted with a dead display. Dodged a bullet.

Configuration tip: use EnerVista, same as the 845. And write down the serial number and catalog number before you call for support. I want to say the 850 has a certain number of digital inputs, but don't quote me on that—the I/O count changes with the order code. That's exactly why the serial number matters.

Situation C: Three-winding or critical transformer: GE Multilin 845

If the transformer has three windings, if you need more flexible CT/VT configuration, or if the cost of an undetected fault is catastrophic, step up to the GE Multilin 845 transformer protection relay. The 845 is built for more complex transformer protection and monitoring. It is not a status choice—or rather, it should not be. On a 5 MVA plant transformer with an on-load tap changer, I'd pick the 845 without hesitating. The price difference is small compared to the cost of a transformer failure.

Does every small customer need the 845? Absolutely not. I'm not going to push a fully loaded 845 onto a facility with one 1 MVA dry-type transformer and a tight budget. Small doesn't mean unimportant—it means you need a proportional answer. The 850 can protect that transformer very well at a lower price.

How to tell which scenario you're in

Ask four questions:

1. Are we controlling a machine or protecting an electrical asset? Control logic gets a control relay. Protection gets a protection relay.
2. How many windings does the transformer have? Two windings usually fits the 850. Three windings or a complicated zone usually points to the 845.
3. Can your team commission a microprocessor relay? If the answer is no, factor in training or a startup service from the manufacturer. That is not a waste; it is insurance.
4. What does your company's risk standard require? Some insurance policies and utility interconnection specs require differential protection. Design to the required scheme, not to the minimum budget.

If you are still not sure, I do not mean that as an excuse to flip a coin. Ask the relay vendor for the application manual and compare the wiring examples to your transformer nameplate. If the transformer is a GE unit, find the serial number before you call. The difference between the 850 and the 845 is not a marketing puzzle. It's a protection decision.

In my first year, I learned that a relay symbol is not a suggestion. If the drawing says 24 VDC, don't let anyone tell you that 120 VAC will work just because the relay fits.