In short. A DG set log book has three parts: a daily pre-start check that takes five minutes, a running record taken every time the set runs, and weekly and monthly entries covering test runs, fuel reconciliation and condition checks. Kept honestly, it shows trouble weeks before a breakdown — coolant temperature creeping up, battery voltage sagging, fuel use per kWh rising — and gives your service engineer the facts to act on. The formats below are ready to print, with a filled example week showing how to read them.
Who this page is for: operators, electricians and facility supervisors who run DG sets day to day, and managers who want a log that is actually useful, not a register filled in from memory at month end.
Key facts
- Log by the hour meter, not just the date. Every service interval, warranty term and overhaul decision is counted in running hours.
- Take running readings after 15 minutes on load, when temperatures have stabilised, and again before shutdown on long runs.
- Load percentage = measured kW ÷ rated kW × 100. Rated kW = kVA × 0.8 for a standard set.
- Current imbalance = (largest deviation from the average phase current ÷ average) × 100. Keep it within about 10%, and investigate above 20%.
- A healthy diesel generator at 50–100% load typically uses about 0.26–0.33 litres per kWh, at the higher end of the band at lighter load; confirm against the maker’s data. A steady rise in this figure is an early warning of a fuel, air or injection fault.
- A weekly test run should load the set to at least 30–40% for 30 minutes or more. Repeated no-load runs do more harm than good; see light loading and wet stacking.
Why most log books fail, and how to make yours work
Most DG log books we see on service visits are one of two kinds. Some are blank, because nobody was asked to fill them. Others are full of identical numbers in the same handwriting, written at month end. Neither tells anyone anything. A useful log has four habits:
- Real readings at the time, by the person who took them, with initials.
- Units and normal ranges printed on the sheet, so an abnormal value stands out the moment it is written.
- A remarks column that is actually used: leaks seen, alarms, sounds, smells, smoke colour.
- A weekly look by a supervisor, who signs the page and acts on trends.
Daily pre-start check (set at rest)
Done once a day for standby sets, or before every start for sets that run daily. It takes about five minutes.
- Engine oil level on the dipstick, between the marks. Note any top-up and the quantity.
- Coolant level in the expansion tank, checked cold. Never open a hot radiator cap.
- Fuel level in the day tank, in litres or as a fraction.
- Battery voltage at rest. Check the charger is on and showing a float charge.
- Leaks of oil, fuel or coolant under the set and around hoses and joints.
- Air filter restriction indicator. Note if it is in the red.
- Belts and guards in place; no fraying.
- Louvres and radiator face clear of leaves, plastic and stored material.
- Controller in AUTO (for AMF sets), with no standing alarms, and the breaker in the correct position.
- Enclosure doors closed and latched.
Running record (every run)
Take the readings after 15 minutes on load. On runs longer than four hours, take them again every two hours.
| Reading | Typical healthy range* | What a change means |
|---|---|---|
| Oil pressure | Steady within the maker’s running band; typically about 2.5–5 bar hot at rated speed (confirm the maker’s band) | Falling: oil dilution, low level, worn bearings or a sensor fault |
| Coolant temperature | About 75–95 °C on load | Rising: dirty radiator, low coolant, a belt or thermostat fault, or air recirculation |
| Voltage, line to line (three readings) | 415 V ± 2.5% (about 405–425 V), all phases close | Low or unequal: AVR, sensing or loading problem |
| Frequency | 50 Hz, steady between about 49.5 and 50.5 Hz | Low on load: overload or fuel restriction. Swinging: governor or fuel fault |
| Phase currents (three readings) | Within the rated current; imbalance under about 10% | High imbalance: single-phase loads badly distributed |
| kW and power factor | kW 30–80% of rated for most of the time; PF 0.8–0.95 lagging | Low PF: motor loads without correction. Leading PF: capacitor bank left on |
| Battery charging voltage | About 13.5–14.4 V (12 V system) or 27–28.8 V (24 V system) | Low: alternator or belt fault. High: regulator fault |
| Exhaust colour | Clear to light grey on load | Black: overload or air restriction. White: unburnt fuel or coolant. Blue: oil |
*Always use the manufacturer’s figures for your engine where they differ. Write the maker’s bands on the printed sheet.
Weekly entries
- On-load test run: at least 30–40% load for at least 30 minutes, with the full running record. For AMF sets, simulate a mains failure where it is safe to do so, and note the transfer time.
- Drain the water separator on the fuel filter. Note whether water came out.
- Battery terminals: clean and tight. Check electrolyte level on flooded batteries.
- Charger indication and any charger alarms.
- Visual check of hoses, mounts and the exhaust for cracks, soot at joints and loose clamps.
- Supervisor review of the week’s pages, signed.
Monthly entries
- Fuel reconciliation: opening stock + fuel received − closing stock = fuel used. Compare it with running hours and with kWh if a meter is fitted.
- Specific fuel consumption: litres used ÷ kWh generated. Track it month by month.
- Longer load run, or a load-bank test if the building load stays light. Aim for at least an hour above 50%.
- Running hours since last service, against the service interval. Book the service before it falls due.
- Earth pit and cable inspection: visual check for damage, corrosion and loose connections.
- Housekeeping: the DG room or yard clear, the drain-pan not overflowing, and extinguishers in date.
Worked example (illustrative): reading one week of a 125 kVA apartment set’s log
The set: 125 kVA, so rated kW = 125 × 0.8 = 100 kW, with a 12 V battery. It runs during power cuts plus a weekly test on Saturday mornings.
| Day | Hours at start | Run (h) | Oil (bar) | Coolant (°C) | V L1/L2/L3 | A L1/L2/L3 | kW | Hz | Batt. rest (V) | Fuel added (L) | Remarks |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Mon | 4,812.0 | 2.5 | 4.1 | 84 | 414/415/413 | 88/72/61 | 48 | 50.0 | 12.7 | — | Power cut 6–8:30 pm |
| Tue | 4,814.5 | 0 | — | — | — | — | — | — | 12.6 | — | Pre-start check OK |
| Wed | 4,814.5 | 3.0 | 4.0 | 87 | 413/414/412 | 95/74/63 | 52 | 49.9 | 12.5 | — | Slight oil mist at rocker cover |
| Thu | 4,817.5 | 0 | — | — | — | — | — | — | 12.4 | — | Pre-start check OK |
| Fri | 4,817.5 | 1.5 | 4.0 | 89 | 414/414/413 | 90/73/62 | 50 | 50.0 | 12.3 | 120 | Radiator face dusty |
| Sat | 4,819.0 | 0.5 | 4.1 | 90 | 414/415/414 | 92/74/63 | 51 | 50.0 | 12.3 | — | Weekly test; transfer 8 s |
| Sun | 4,819.5 | 0 | — | — | — | — | — | — | 12.2 | — | Pre-start check OK |
Reading it like a service engineer:
- Load: about 50 kW on 100 kW rated, or 50%. That is a healthy level.
- Current imbalance on Wednesday: the average is (95 + 74 + 63) ÷ 3 = 77.3 A. The largest deviation is 95 − 77.3 = 17.7 A, giving 17.7 ÷ 77.3 × 100 = 22.8%, well above 10%. The L1 phase carries too much of the common-area load. Action: an electrician redistributes single-phase circuits at the next shutdown.
- Coolant temperature: 84, 87, 89, then 90 °C at similar loads. That is a 6 °C creep in one week, and the Friday remark explains it: a dusty radiator face. Action: clean the radiator from the fan side outwards, then watch the next readings.
- Battery at rest: 12.7 V falling to 12.2 V over the week. A fully charged 12 V battery rests at about 12.6–12.7 V; 12.2 V is roughly half charge. With a healthy charger on float, the reading should never slide like this. The charger is not keeping up. Action: check the charger output and the battery condition before the set fails to start during the next power cut.
- Fuel: about 7.5 hours at about 50 kW = 375 kWh. Fuel drawn, from the day-tank readings, was 104 litres, so 104 ÷ 375 = 0.28 L/kWh. That is within the normal band; keep it as this month’s baseline.
- Remark: the oil mist at the rocker cover is minor now, but it is logged with a date, so the next service can deal with the gasket.
Result: three problems found and acted on in one week, none of them yet a breakdown. That is what a log book is for.
When a reading means “stop and call”
| Observation | Action now |
|---|---|
| Oil pressure falling, or a low oil pressure alarm | Shut down. Check the level. Do not restart repeatedly. Call your service engineer. |
| Coolant above about 100 °C, or a high-temperature alarm | Reduce load or shut down. Do not open the radiator cap hot. Check the fan, belt and radiator face. |
| Frequency below 49 Hz on load, or black smoke | Shed load. The set is likely overloaded or short of fuel or air. |
| Fuel or oil leak near the exhaust or turbocharger | Shut down immediately. Fire risk. |
| Battery at rest below about 12.2 V (or 24.4 V) | Check the charger and the battery today, before the next start is needed. |
| New knock, rattle or vibration | Shut down and inspect. Note when it started and at what load. |
Log sheet 1: daily pre-start check and running record
| Site | Set no. / kVA | Rated kW (kVA × 0.8) | Month | Maker’s oil pressure band (bar) | Maker’s coolant band (°C) |
|---|---|---|---|---|---|
| Date | Oil level ✓ | Coolant ✓ | Fuel (L) | Batt. rest (V) | Leaks / belts / louvres ✓ | Controller AUTO, no alarms ✓ | Hours start | Hours end | Oil (bar) | Coolant (°C) | V L1/L2/L3 | A L1/L2/L3 | kW | PF | Hz | Charging V | Exhaust colour | Fuel added (L) | Remarks / alarms | Initials |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Log sheet 2: weekly entries
| Week ending | Test run: kW / minutes | AMF transfer time (s) | Water drained from separator? (Y/N) | Battery terminals and electrolyte ✓ | Charger OK ✓ | Hoses, mounts, exhaust ✓ | Current imbalance (%) | Issues and actions | Supervisor signature |
|---|---|---|---|---|---|---|---|---|---|
Log sheet 3: monthly entries
| Month | Opening fuel (L) | Fuel received (L) | Closing fuel (L) | Fuel used (L) | Running hours | kWh generated | L/kWh | Long load run or load bank: kW / minutes | Hours since last service | Next service due at (hours) | Earth pits and cables ✓ | Housekeeping and extinguishers ✓ | Manager signature |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Exceptions and cautions
- The healthy ranges on this page are typical. Your engine’s maker sets the real bands, and some electronic engines run hotter or at different oil pressures by design. Write the maker’s figures at the top of each sheet.
- Prime-power and continuous-duty sets that run every day need running readings every two hours, and a shift handover line.
- Sets with remote monitoring still need a paper or digital log of the physical checks: leaks, belts, louvres and housekeeping are not visible to a sensor.
- Never bypass a protection to keep a set running because a log entry looks normal. A trip means stop and investigate.
Common mistakes
- Logging by date only, with no hour-meter readings.
- Recording one voltage and one current instead of all three phases, so imbalance is invisible.
- Weekly test runs at no load, for five minutes, which glaze the engine instead of exercising it.
- Filling the log at month end from memory.
- Never calculating fuel per kWh, so slow fuel theft or injection wear goes unnoticed.
- Remarks left blank when something was seen. The remark is often the most valuable entry on the page.
What to send us
Send photographs of the last two or three weeks of log pages, along with the set’s kVA and hour-meter reading. Our service engineers will read the trends, flag anything that needs attention, and set the service plan by hours. Site staff can also send individual readings or photographs of a gauge, an alarm or a leak to the service engineer as they occur. Use the service enquiry, or WhatsApp 98852 01608. We work across Telangana, Andhra Pradesh and every district within about 300 km of Hyderabad, with project support across India.
Questions engineers ask
What should be recorded in a DG set log book?
Daily pre-start checks (oil, coolant, fuel, battery, leaks, louvres, controller status), running readings on every run (hours, oil pressure, coolant temperature, three voltages, three currents, kW, PF, frequency, charging voltage, exhaust colour), plus weekly test runs and monthly fuel reconciliation.
How often should a standby generator be test run?
Weekly, on load: at least 30 to 40 percent for 30 minutes or more. Add a longer run above 50 percent, or a load-bank test, each month if the building load is light.
How do I calculate the load percentage of my generator?
Divide the measured kW by the rated kW and multiply by 100. For a standard set, rated kW is kVA times 0.8, so a 125 kVA set is rated at 100 kW.
How do I work out current imbalance?
Find the average of the three phase currents, take the largest difference between any phase and that average, divide by the average and multiply by 100. Keep it within about 10 percent.
What fuel consumption is normal for a diesel generator?
Between 50 and 100 percent load, a healthy set typically uses about 0.26 to 0.33 litres per kWh, at the higher end of that band at lighter load. Confirm against the maker’s data for your engine. Track your own figure monthly; a steady rise points to a fuel, air or injection problem, or to fuel going missing.
What battery voltage shows a problem?
A fully charged 12 V starting battery rests at about 12.6 to 12.7 V. Around 12.2 V is roughly half charge and needs attention before the next start. On load, charging voltage should be about 13.5 to 14.4 V.
Is a digital log as good as a paper log?
Yes, if readings are entered at the time by the person who took them, and the physical checks are recorded as well as the controller data. Remote monitoring cannot see a leak, a loose belt or a blocked louvre.
Related pages
- The DG set document file
- Generator maintenance planning
- Light loading and wet stacking
- Controller alarms explained for site staff
- Fuel theft and refuelling control
- Battery and starting system service
- Generator full-load current chart
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 5 November 2026.