Don't Skip These 6 Checks Before You Replace a GE 850 Transformer Protection Relay

Let me start with a confession. I've been specifying and buying GE transformer protection relays for a regional utility for about nine years. In that time, I've made six significant ordering mistakes. The total cost? Around $52,000 in reorders, expedite fees, rewiring, and one very tense weekend. I now keep a checklist that I hand to every new engineer who touches a relay order. This is the practical version of that checklist.

If you are about to replace a GE 850 transformer protection relay—or any GE Multilin relay on a transformer—read this first. It has six steps. Do them in order. Don't skip the boring ones.

1. Confirm the exact order code, not just the relay family

Most buyers focus on the big label that says 'GE 850.' That label is not enough. The order code includes optional suffixes that change the auxiliary power range, communication ports, and I/O configuration. A GE 850 with the wrong suffix is a brick with LEDs.

Before you order anything, take a photo of the relay nameplate. Get the catalog number, full order code, serial number, and firmware revision if you can read it. On a live panel, use your phone and zoom in. The nameplate is often dirty or faded. Clean it carefully only if it is safe to do so—otherwise, read it from the drawing package.

I know part-number lookup sounds boring. The same discipline that makes a Super Tech air filter lookup and a Mobil1 oil filter lookup accurate also applies to a GE transformer relay. You don't want the part that 'looks like' the old one. You want the exact cross-reference. A filter mistake costs you an engine. A relay mistake can cost you a transformer.

• Full catalog number and option suffix
• Serial number and hardware revision
• Firmware version
• Auxiliary power: 125 Vdc, 48 Vdc, 110 Vac, etc.
• CT input rating: 1 A or 5 A
• VT input connection and rated voltage
• Required output relays and communication options

2. Back up the current settings before you remove anything

This is the step most people ignore. It's tempting to treat the 850 as a simple swap. But the hard part isn't the relay hardware. It's the settings file, logic, and event history.

Why does this matter? Because if the old relay is still powered, you can retrieve the complete settings and oscillography from the front port or Ethernet port. That file is your safest reference. Without it, you'll reprogram from memory or from drawings that may be outdated.

Looking back, I should have made settings backup a condition of every outage plan. At the time, I thought 'we'll pull the old settings after we take the panel off.' We didn't. We learned that lesson when the relay's file was corrupted. Don't repeat it.

• Connect to the relay via the front USB/RS232 or Ethernet management port
• Save the settings file and active logic
• Export event records and oscillography
• Save a copy in the station file, not just on your laptop

3. Verify CT and VT inputs before you match the catalog number

Here is where a lot of pre-buy errors hide. A relay might be the correct model, correct suffix, but its current inputs are for 1 A CTs while your transformer bushing CTs are 5 A. That mismatch can take a relay out of service instantly when load is applied.

Check the CT secondary rating, ratio, and burden. Check whether the VT inputs are set for wye or broken-delta and whether the relay's rated secondary voltage matches your panel. A 120 V rated input connected to a 67 V phase-to-neutral VT secondary may still work if configured correctly, but you need to confirm it before ordering.

The question everyone asks is 'what's the price?' The question they should ask is 'what's the full input configuration?'

4. Check protection curves, logic, and device numbers

This step is not just about the relay order; it's about the settings transfer. If the 850 is protecting a transformer, the overcurrent and neutral overcurrent elements must match your coordination study. Device numbers used in the logic should follow IEEE C37.2: 50 for instantaneous, 51 for time overcurrent, 50N/51N for neutral overcurrent.

Don't assume that a settings file from one transformer can be copied to another transformer 'because they're similar.' Transformer magnetizing inrush, downstream fault levels, and tap configuration are all different. A setting that worked on transformer A can nuisance-trip on transformer B.

At a minimum, confirm:

• Phase and ground overcurrent curve types and time dials
• Inrush blocking or harmonic restraint settings if the relay supports them
• Breaker fail and external trip logic
• Target and alarm latch behavior

5. Map the communication points before commissioning

One of my biggest regrets: not sending the SCADA point list to the control center until after the relay was installed. The relay was fine. The communication was fine. But we lost two days because nobody had mapped the DNP3 points.

Whether you use DNP3, Modbus, or IEC 61850, verify the mapping for status outputs, alarms, measurements, and controls. The GE 850 has many configurable points. The default map won't match your existing HMI screens unless you update them together.

Do this before the outage, not during it. During an outage, a comm map mismatch is a fire drill.

6. Commission with a proper test set and confirm reset behavior

A relay replacement isn't done until a secondary injection test proves it works. That means testing current and voltage inputs, pickup values, timing, trip outputs, and communication alarms.

If you can't test it, don't energize it. Period.

There's something satisfying about a relay that powers up and passes every test on the first try. After the stress of an outage, that's the payoff.

And one note that almost never makes the project plan: reset behavior. If you have ever searched 'how to reset fuel pump relay,' you got a simple answer: pull it, wait, put it back. That mental model does not work on a GE 850 transformer protection relay.

To reset the 850, you need to clear the fault condition, then use the front-panel reset key or a configured logic input to reset the latched elements. If the fault is still present, the relay will re-trip immediately. Power cycling may clear some elements, but not all.

Test the reset method too. Show the technician which key does what and which SCADA command resets the latch.

Bottom line: pay for certainty when it matters

Before I finish, let me say the thing I wish someone had told me in 2017: if you're in a forced outage, pay for certainty. In March 2024, we paid about $400 extra for a relay to arrive in four days instead of seven. I hesitated because the standard delivery seemed okay. The freight company lost that shipment. We then paid $1,100 to air-freight another relay and worked two nights to get it installed. The $400 was never the issue. The uncertainty was.

Spending money is not the goal. But 'probably on time' and 'that part is probably compatible' are how outages turn into multi-day events. The six checks above don't cost much. The first time they save you from a wrong GE 850 order, they'll pay for themselves.