Generator problems
Generator Shutting Down or Tripping: Causes and Repairs
The short answer
A generator that stops when it should keep running has done one of two things. Either the controller has shut down the engine to protect it, usually with an alarm such as low oil pressure, high temperature or underspeed, or the engine is still running but the output breaker has tripped, usually because of overload, a motor starting or an earth fault. If it happens as the load comes on, the set is usually struggling with that load: note what was switching on, and write down the alarm text before you reset anything.
- Write down the alarm text before resetting
- Check whether the engine stopped or only the breaker tripped
- Switch large loads off, then bring them back one at a time
- Do not keep resetting a protection alarm or a breaker
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What exactly stopped?
| Symptom | Likely causes | Repair route |
|---|---|---|
| Engine stops with a low oil pressure alarm | Likely causesLow oil level, oil thinned by heat or fuel, a failed sender or wiring, or a real loss of pressure | Repair routeLow oil pressure shutdown |
| Engine stops on high temperature after a while under load | Likely causesRadiator blocked with dust, low coolant, a slipping fan belt, hot air recirculating, overload, a faulty sensor | Repair routeOverheating |
| Engine stops on underspeed or underfrequency as the load comes on | Likely causesA load step too large for the set, fuel starvation, a sluggish governor or actuator; overspeed when a large load drops off | Repair routeFrequency and speed problems |
| Engine stops on a voltage alarm | Likely causesAVR fault, overload, voltage sagging with a speed dip, or a sensing or wiring problem | Repair routeVoltage problems |
| Engine runs on, breaker trips the moment a load starts | Likely causesMotor or compressor starting current, a short circuit in the load, a worn breaker | Repair routeRunning but no power |
| Breaker trips after running loaded for a while | Likely causesSustained overload, single-phase loads bunched on one phase, a loose and overheating connection | Repair routekVA sizing guide |
| Earth leakage trips when the generator takes over | Likely causesA damp or faulty appliance, switch-on leakage from many appliances at once, or a neutral and earth arrangement that does not suit generator supply | Repair routeChangeover and ATS problems |
| Engine slows and dies under load, no alarm | Likely causesFuel starvation: blocked filters, air leaks, a blocked tank vent, low or contaminated fuel | Repair routeFuel system repairs |
| Low fuel shutdown with fuel in the tank | Likely causesFaulty level sender, a day tank that is not refilling, or the pickup uncovered because the set is not level | Repair routeFuel system repairs |
| Small inverter unit stops, overload light on | Likely causesMore load than the unit can supply, or a load with a high starting current | Repair routePortable and inverter generators |
Load-related shutdowns
Why a generator shuts down under load
With nothing connected, an engine needs very little fuel to hold its speed. When a load is switched on, the alternator suddenly demands torque, the engine speed dips, and the governor has to open up the fuel quickly to recover. Voltage dips with the speed, and the AVR raises the excitation at the same moment. A healthy, correctly sized set recovers within a few seconds. A set with a weak spot does not.
That is why so many faults only show under load. A partly blocked fuel filter or an air leak lets through enough fuel at no load but not at full output, so the engine slows and stops. A sluggish governor lets the speed stay below the underspeed or underfrequency limit for longer than the controller allows. A radiator choked with Highveld dust copes at light load and overheats at full load. Oil thinned by fuel or by too long between changes, or worn bearings, can hold pressure at light load and lose it once the engine is working hard and hot. Turbocharged engines also need a moment to build boost, so a large load step hits them harder than several small ones.
Then there is plain overload. A set is rated in kVA, and at the usual power factor of 0.8 a 100 kVA set delivers about 80 kW. Loads that add up to more than that, or a large motor starting on top of a busy set, will trip the breaker or drag the speed down. The diagram shows how one pump start can double the demand for a few seconds.
Breaker trips
When the breaker trips as the load comes on
Instantly: starting current or a short: kVA sizing guide
A breaker trips at once on very high current. Motors, compressors and pumps started direct-on-line draw several times their running current for a moment, and several starting together can reach that level. A short circuit in the wiring or an appliance trips it the moment it is energised, however small the load.
After a while: sustained overload: Single vs three phase
The thermal part of a breaker trips once current has stayed above its rating long enough to heat it. Too much total load, single-phase loads bunched on one phase of a three-phase set, or a loose connection heating the breaker all cause it. A breaker that has tripped many times can also wear and trip early.
Earth leakage trips: ATS and changeover problems
An earth leakage unit trips when current leaks to earth. At changeover every circuit is switched on in the same instant, and the brief leakage from the filters in many electronic appliances can add up enough to trip a sensitive unit; damp in a geyser element or an outdoor socket shows too. If it trips on generator supply but never on mains, suspect the neutral and earth arrangement. That is set at installation and must be checked by a technician, never altered by the owner.
Load shedding: when everything switches on at once
During load shedding a generator does not take its load gently. When the mains fails, the changeover transfers every circuit it feeds in one step, and everything that was running tries to start together: fridge and freezer compressors, air conditioners, pool and borehole pumps, a geyser element, and inverters and UPS units that start recharging their batteries. A set sized for the running load can be overwhelmed by that combined starting load, and at higher stages it happens several times a day.
The return of the mains can cause trouble too. If the supply comes back and fails again within minutes, as can happen while a network is being restored, the set takes the full load again each time, and start and return delays set too short make it worse.
The cure is usually in the installation rather than the engine: keep geysers, pool pumps and other heavy, non-essential loads on circuits the generator does not feed; bring heavy loads on in stages with timers, or with controller outputs set up to switch them; and fit soft starters or variable-speed drives to large pumps. If the set still trips with the load arranged sensibly, it may simply be too small for the site. Changes to the changeover or distribution board are technician work; see changeover and ATS problems.
A shutdown is the protection doing its job
The controller watches oil pressure, coolant temperature, speed, voltage, frequency and current, and stops the engine before a fault becomes a failure. A protection shutdown is a symptom, not the fault itself: the cause is whatever pushed the reading out of limits. Controller alarms explained sets out how warnings, electrical trips and shutdowns differ.
Sometimes the reading is wrong. A failed sender, a chafed wire or a corroded connector can report low oil pressure or high temperature from a healthy engine, and limits or timers set too tightly can stop a sound set. Both are common, but they should only be concluded after the engine itself has been checked, and settings should only be changed by someone who knows what each one protects. Sensor and controller faults are repaired under control panel repairs.
What you can safely check
Note what you see before you touch anything: resetting, or switching some controllers to off, clears the alarm from the screen. Once it is written down, put the controller in off before you open the canopy, so the set cannot restart while you are close to it.
- The exact alarm text on the controller, and whether more than one alarm is showing
- Whether the engine stopped, or kept running with the breaker tripped
- What was switching on when it stopped, and whether the mains had just failed or returned
- The fuel level, and that the set is standing level
- The oil level on the dipstick, with the engine stopped and left to cool so the oil drains back
- With the engine stopped, whether the radiator or air openings are blocked by dust, leaves or stored items
- Any leaks, smoke or a burning smell around the generator or the distribution board
If the same alarm returns after one reset, stop: repeated restarts on a low oil pressure or high temperature alarm can wreck an engine. Send us the alarm text, a photo of the controller screen and what was switching on, with the make, model and kVA.
Diagnosis
How we find the cause
- 01
Read the event log
Most controllers record which alarm stopped the engine and when, often with readings at the time.
- 02
Check the reading is real
Oil pressure and temperature are measured with independent gauges and compared with what the controller reports.
- 03
Measure the load
Running current on each phase, and the speed and voltage dip as large loads start, show whether the set is overloaded, unbalanced or slow to recover.
- 04
Test the circuit that trips
Insulation and earth leakage tests on the outgoing wiring find faults in the installation rather than the generator.
- 05
Load it under control
Where the fault only shows at full output, a load bank test can be arranged to load the set in steps while it is monitored.
Typical repairs
| Cause found | Usual repair |
|---|---|
| Low oil level or oil pressure fault | Usual repairFind where the oil went or why pressure fell; see low oil pressure shutdown |
| Overheating under load | Usual repairClean the radiator, correct the coolant and fan belt, and clear the airflow |
| Faulty sender, sensor or wiring | Usual repairReplace the sensor or repair the wiring, then test the alarm |
| Everything starting at once | Usual repairMove heavy loads off the generator circuit, stage them, or fit soft starters |
| Overload or unbalanced phases | Usual repairRearrange or stagger loads, balance the phases, or review the generator size |
| Short circuit or earth leakage | Usual repairFind and repair the faulty circuit or appliance, and check the neutral and earth arrangement |
| Worn or faulty output breaker | Usual repairReplace the breaker with one of the correct rating |
| Fuel starvation | Usual repairReplace filters, fix air leaks, clear the tank vent, treat the fuel |
| Protection settings | Usual repairCorrect the settings to suit the generator and the site |
FAQ
Frequently asked questions
Why does my generator trip when the geyser or air conditioner comes on?
A geyser element is a large steady load, and an air conditioner’s compressor draws a surge as it starts. On top of what the generator is already carrying, either can exceed what it can supply. Keeping the geyser off the generator circuit and staggering other loads usually solves it; if it keeps happening, the generator may be too small for the job.
Why does it run fine with no load, then stop when the load comes on?
This usually points to fuel starvation, a governor problem or overload. At no load the engine needs very little fuel; under load a partly blocked filter or a weak fuel supply cannot keep up, and the speed falls until the underspeed protection stops it.
Why does the earth leakage trip when the generator takes over?
Often because every circuit is switched on in the same instant, and the small switch-on leakage from many appliances adds up, or because one circuit or appliance has a real earth fault. If it trips on generator supply but never on mains, the neutral and earth arrangement is the main suspect: it has to suit the installation, or the earth leakage unit may trip, or fail to protect. Note which circuits were on when it tripped and have a technician check it. Never alter earth or neutral connections yourself.
Why does my generator shut down after running for an hour or so?
A fault that takes time to appear is usually heat related: the cooling system cannot shed heat at that load, or the oil thins as it warms and the pressure drops. A fuel tank running low, or a day tank that is not refilling, is the other common cause. The alarm text tells you which to look at.
Can I just reset the alarm and carry on?
Once, after you have written down the alarm and found and dealt with an obvious cause, such as an empty tank or a heavy load that is now switched off. After a low oil pressure, high temperature or overspeed shutdown, do not restart unless the cause is clear and has been put right. If the same alarm comes back, stop and call us: the protection is warning of a real problem, or a sensor needs attention.
Is a low oil pressure shutdown always serious?
Treat it as serious until proven otherwise. The cause is often a low oil level or a faulty sender, but real low oil pressure damages bearings quickly, so do not restart repeatedly to find out. Our low oil pressure guide explains the checks.
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