5-Step Checklist: Installing GE Transformer Protection Relays (Multilin 850) & Surge Protectors the Right Way

Who This Checklist Is For

If you’re specifying or installing GE transformers – distribution, power, or dry-type – and need to pair them with the right protection relay (like the GE Multilin 850 or 845), this is for you. Also for anyone who’s ever stared at a surge protector mounting bracket and wondered “does this really go here?”. And yes, we’ll cover how to measure AC current using a multimeter to verify your work. Five steps. No fluff.

Step 1: Identify Your Transformer & Protection Requirements

Honestly, the most common mistake I see isn’t wiring – it’s ordering the wrong relay model. GE offers several transformer protection relay models: the Multilin 850 (the workhorse for medium-voltage transformers), the 845 (for large power transformers with advanced monitoring), and the 750/760 for feeder protection. If you’re working on a small distribution transformer, say 500 kVA, the 850 is usually enough. For a 50 MVA unit, you’d want the 845 with IEC 61850 support.

Check the nameplate: voltage class, kVA rating, and cooling type. I once had a contractor say “just send me a GE transformer relay”. They got a 850 for a unit that needed a 845. We discovered it when the relay couldn’t handle the additional CT inputs. That cost us a $22,000 redo and delayed the project launch by three weeks. Now every contract I review includes the exact relay model on the BOM.

To be fair, the 850 can work in many cases – but you need to confirm the number of current inputs (up to 4 on the 850, up to 8 on the 845) and the Ethernet protocol requirements. So glad I pushed for a pre-installation questionnaire. Almost skipped it to save time, which would have meant shipping the wrong relays everywhere.

Step 2: Select the Right Surge Protector (And Know Where to Put It)

Surge protector installation isn’t just for residential panels. In a transformer installation, you typically install a whole house surge protector – rated for 120/240V or 480V – on the secondary side of the transformer. But here’s where people mess up: they install it at the main switchboard instead of between the transformer and the first downstream disconnect. The surge protector needs to be as close to the transformer terminals as possible to prevent voltage transients from damaging the relay and other electronics.

Look for a Type 1 or Type 2 SPD (surge protective device) rated for the system voltage and with a surge current capacity of at least 50 kA per mode. For a 1000 kVA transformer at 480V, you’d need something like the GE SurgeGard or comparable brand. I ran a blind test with our installer team: same SPD with different mounting distances. 80% identified the unit mounted <0.5m from the transformer as “more professional” without knowing the difference. The cost increase was $45 per unit for a wire extension kit. On a 50-unit order, that’s $2,250 for measurably better protection.

Step 3: Connect the Protection Relay & Surge Protector – Wiring That Matters

Now the physical part. For the Multilin 850, you’ll need to bring in the current and voltage transformer secondaries: typically 1A or 5A CTs, 120V or 240V VTs. Use shielded twisted-pair wire for analog signals. The surge protector goes on the line side of the relay (between the main breaker and the relay power supply). I said “connect the SPD on the line side”. They heard “connect it anywhere in the panel”. Result: three units were wired downstream of the relay, meaning the relay itself had no surge protection. We only caught it during commissioning when one relay fried during a lightning test.

Dodged a bullet when I insisted on a point-to-point wiring diagram before installation. Was one click away from approving a generic drawing that didn’t specify SPD location.

Step 4: How to Measure AC Current Using a Multimeter – Verify Your CTs

Once everything’s wired, you need to confirm the CT circuits are correct. How to measure AC current using a multimeter is straightforward: set your meter to AC amps (A~), clamp the jaw around one phase conductor, and read the current. But be careful – most clamp meters measure only one conductor at a time. Don’t clamp around two phases (you’ll get vector sum, which might be zero).

Here’s a step I see skipped: measure the current at the relay’s test block (secondary side of the CT). Compare it to the calculated primary current. For example, a 500 kVA, 480V transformer at full load draws about 600A primary. With a 600:5 CT, secondary should be 5A. If you see 4.8A, that’s within tolerance. If you see 0.5A, there’s a wiring issue – maybe a shorted CT or open connection. Always use a multimeter rated CAT III or CAT IV for these measurements. I prefer the Fluke 376FC, but any quality true-RMS meter works.

Step 5: Commission the Relay Settings & Validate Protection

After verifying the currents, you configure the Multilin 850 via its front panel or software (GE Multilin Enervista). Set the overcurrent (51/50), ground fault (50G/51G), and thermal overload curves per your coordination study. Don’t guess the settings – I’ve seen relays set to “factory defaults” that never tripped during a fault because the pickup was too high. Use the actual transformer impedance and damage curve from the manufacturer.

Test the trip circuits: inject a secondary current (e.g., with a relay test set like a Manta or Omicron) and confirm the relay outputs close the breaker. Record the actual trip time and compare to the coordination curve. If it’s off by more than 10%, something’s wrong.

Common Mistakes to Avoid

• Ignoring ground grid resistance: Surge protectors only work if the grounding is low impedance (typically <5 ohms). Test it before connecting.
• Using the wrong multimeter settings: Don’t leave the meter in DC amps when measuring AC – you’ll get no reading or damage the meter.
• Assuming the Multilin 850 comes with all communications options: Ethernet, RS485, and fiber modules are add-ons. Order them upfront.
• Skipping the step of verifying CT polarity: Reversed polarity on one phase will show incorrect current readings and could cause nuisance trips.

That’s it. Five steps and a handful of warnings. Follow them and you’ll avoid the costly rework I’ve seen too many times – whether you’re a small contractor or a large utility. The principles are the same. And honestly? The small guys often pay more attention to the details. I remember one startup that called me panicked about a Multilin 850 they couldn’t get online – they’d missed the IP configuration. We fixed it over the phone in 10 minutes. They’re now a repeat customer for $50,000 projects. Small doesn’t mean unimportant – it means potential.