Which Test First: Load Bank vs Megger vs Fuel Pressure on a Derate Decision Tree

Why this matters

A generator that makes less power than it should ("deratey," bogs under load, throws an underfrequency or low-power fault) can be losing output to the engine, the alternator, or the fuel system, and you have three big-gun tests to choose from: a load bank, an insulation-resistance (megger) test, and a fuel-pressure check. Run them in the wrong order and you waste an afternoon. Megger a unit that is actually fuel-starved and you learn nothing; load-bank a unit with a wet, low-insulation alternator and you risk a flashover. This tree decides which test to run first based on what the symptom is already telling you, so the first test you run is the one most likely to find the fault and least likely to do harm.

Why this matters in sequencing

The three tests answer different questions:

• Fuel pressure check answers: is the engine getting enough fuel to make rated power?
• Load bank answers: can the set actually deliver rated kW and hold frequency and voltage under controlled load?
• Megger (insulation resistance) answers: is the alternator's winding insulation healthy enough to push rated current without leaking to ground?

The trap is running the load bank first by reflex. If the alternator insulation is compromised, load-banking it can finish the failure. So the sequence is symptom-driven, not habitual.

Symptom presentation

• Engine bogs and frequency sags under load, but recovers at no load: engine power or fuel suspicion.
• Voltage low or unstable under load, frequency fine: alternator/AVR/insulation suspicion.
• Unit makes voltage and frequency at no load but trips on overload or low-power the moment real load hits: could be any of the three; that is what the tree is for.
• Recent flood, condensation, or long idle storage: insulation suspicion rises to the top regardless of other signs.

Quick checks (run before any big test)

• Read no-load voltage and frequency. Correct 60 Hz and rated voltage at no load means the engine governor and the AVR's basic function are okay; the derate appears under load.
• Note environment and history: a unit that sat in a flooded vault, in high humidity, or idle for a long period gets an insulation check FIRST regardless of other symptoms.
• Smell and look at the exhaust under the load that triggers the derate: black smoke points engine/fuel; clean exhaust with collapsing voltage points alternator.
• Confirm the load that triggers the fault is real and balanced; an imbalanced or oversized connected load can look like a derate.

Isolation tree (which test first)

Step 1: Has the unit been wet, flooded, or stored idle long-term, or is the symptom LOW or UNSTABLE VOLTAGE under load with clean exhaust?

If YES: MEGGER FIRST.

• Insulation resistance must be verified before you ever load a possibly-wet or compromised alternator. A low reading (well below the manufacturer's minimum, often referenced to about 1 megohm per kV plus a margin, confirm the unit's spec) means moisture or insulation breakdown. Dry the windings and re-megger before any load test.
• Megger PASS here means the alternator insulation is healthy; you can move on to load testing safely.
• Doing the load bank before this step on a wet machine risks a flashover that destroys the alternator.

Step 2: Does the engine BOG, lose RPM/frequency, and/or smoke black under load, with voltage following the frequency down?

If YES: FUEL PRESSURE FIRST (and engine-side checks).

• Measure fuel supply pressure at the engine under the load that causes the derate, not at idle. Gaseous units: confirm the regulator holds inlet pressure under full draw and does not lock up; a sagging gas pressure starves the engine the instant load is applied. Diesel: check supply pressure, filter restriction, and lift-pump delivery under load.
• A fuel pressure that holds at idle but collapses under load is the fault; no further big test is needed until fuel is corrected.
• If fuel pressure is solid under load and the engine still bogs, the engine is down on power (compression, timing, air restriction); address that before load-banking for acceptance.

Step 3: Insulation is good and fuel is good, but you still need to prove rated output.

Now: LOAD BANK.

• With insulation verified safe and fuel delivery confirmed, apply controlled load in steps to rated kW and verify the set holds voltage, frequency, and temperature, and that it does not trip. This is the test that proves capacity and exposes a marginal engine, a soft governor, or an AVR that cannot regulate under full load.
• Load-banking is also the cure for a wet-stacked diesel, but only after insulation is cleared.

Confirming diagnosis

• Insulation fault: megger reads below spec; drying and re-megger either recovers it (moisture) or confirms a winding fault (stays low). Confirmed.
• Fuel fault: supply pressure collapses under load while holding at idle; correcting it restores power on a load test. Confirmed.
• Capacity/engine/AVR fault: insulation and fuel are good, but the load bank shows the set cannot hold rated kW, frequency, or voltage. Confirmed; diagnosis moves to the specific subsystem the load bank exposed.

Remediation

• Dry and re-test windings with low insulation; replace or rewind the alternator if insulation stays below spec.
• Correct fuel delivery: replace restricted filters, repair lift pump, or resize/repair the gas regulator so pressure holds under full draw.
• For a unit that fails the load bank with good fuel and insulation, follow the specific finding: governor tune, AVR service, or engine power restoration.
• Document the load-bank results (kW, Hz, V, temperatures, duration) as the acceptance record.

References

• NFPA 110, Standard for Emergency and Standby Power Systems, acceptance and periodic load-test requirements.
• NEC Article 445, Generators, and Article 700/701/702 for standby-system testing context.
• IEEE 43, Recommended Practice for Testing Insulation Resistance of Electric Machinery, for insulation-resistance interpretation.
• Generator and alternator manufacturer service manual, insulation-resistance minimums, fuel-pressure specs, and load-test procedure.