Every April and November — when Kenya's long and short rains arrive — our emergency callouts increase by 30 to 40 percent. The same generators that ran without complaint through the dry months suddenly fail. The cause is almost always moisture. Here is why it happens and, more importantly, how to stop it.
How Water Gets Into a Diesel Generator
A diesel generator is not waterproof. Even units with canopies and weather enclosures have intake and exhaust vents designed to pass large volumes of air for cooling. During heavy rainfall, wind-driven rain enters through these vents and settles on every exposed surface — the alternator windings, the battery terminals, the control panel PCB, and the exhaust manifold.
Condensation is an equally serious problem, particularly for generators in enclosed rooms or basement plantrooms. When a cold night follows a wet afternoon, warm humid air condenses on every metal surface. A generator that runs at 90°C during operation and then cools overnight in a humid space will accumulate moisture in places you cannot see: inside the alternator casing, inside the control panel housing, and between battery plates.
The Wet-Start Problem
Starting a generator with moisture in the alternator windings is one of the fastest ways to destroy it. The resistance of damp windings drops, drawing far more current than the system is designed to handle. We have replaced alternators that failed on their first start of the rainy season — a fault that could have been prevented with a single pre-season inspection.
The Five Most Common Rain-Related Failures
1. Alternator Winding Insulation Failure
Moisture lowers the insulation resistance (IR) of the alternator stator and rotor windings. When insulation resistance drops below 1 MΩ (measured with a Megger at 500V DC), the alternator is at immediate risk of inter-turn short circuits. These cause excessive heat, burning, and total alternator failure — an expensive repair.
2. Control Panel and AVR Corrosion
Printed circuit boards inside the Automatic Voltage Regulator (AVR) and the DSE or ComAp controller are sensitive to moisture. Condensation causes track corrosion, short circuits between components, and random fault codes. We frequently see generators reporting false high-temperature or low-oil-pressure shutdowns that are actually caused by corroded sensor connections.
3. Battery Discharge and Terminal Corrosion
Wet weather accelerates self-discharge in lead-acid batteries and causes rapid sulfate buildup on terminals. A battery that tested at 12.6V before the rains may read 11.8V after two weeks of damp weather — below the threshold needed to crank a large diesel engine. Green or white crystal deposits on terminals indicate active corrosion.
4. Fuel Tank Condensation
Day-tank and bulk-storage tanks that are not fully topped off have an air gap above the fuel. In rainy weather, this air is humid. When the temperature drops at night, water condenses inside the tank and sinks to the bottom of the fuel. Water in diesel causes microbial growth ('diesel bug') within weeks, blocking injectors and lift pumps.
5. Exhaust System Water Ingress
Rain can enter through exhaust outlets, especially when the unit is stationary. Water sitting in the exhaust manifold or turbocharger causes corrosion and, in severe cases, hydrostatic lock when the engine is started — a condition that can bend connecting rods and destroy the engine.
How to Protect Your Generator Before the Rains Arrive
The single most effective thing you can do is schedule a pre-rainy-season inspection before April and before October. A trained technician can complete a full moisture audit in two to three hours. Here is what a proper inspection covers:
Megger test on alternator windings
IR reading above 10 MΩ is healthy; below 1 MΩ requires drying before operation.
Control panel inspection and sealing
Check all cable gland entries, replace dried-out foam seals, apply conformal coating to exposed PCBs.
Battery load test and terminal clean
A battery under 75% state of health should be replaced before the wet season.
Fuel tank water check
Draw a sample from the bottom of the tank using a water-finder paste. Any blue or green colour indicates water ingress.
Exhaust rain cap inspection
Verify the exhaust rain cap closes fully when the engine stops. Replace if the flap is corroded or stuck open.
Canopy seal and drain inspection
Check all rubber seals on access doors, ensure canopy drainage holes are clear and unobstructed.
If Your Generator Is Already Wet
Do not attempt to start a generator you suspect has moisture inside the alternator. The damage from a single wet-start can cost KES 80,000 to KES 250,000 to repair. Instead:
- 1
Open all access panels and use a clean cloth to wipe away any visible water or condensation.
- 2
If you have a portable space heater or a heat gun, apply gentle heat to the alternator end for 30–60 minutes. Do not use an open flame.
- 3
Leave the panels open for several hours in a dry, ventilated environment if possible.
- 4
Call a certified engineer to perform a Megger test before attempting to start.
PowerProKe Rainy Season Check-Up
We offer a dedicated pre-rainy-season inspection package covering all the checks listed above. It takes approximately 3 hours on site and includes a written report with IR readings, battery state of health, and any recommended corrective actions.
Book Inspection — 0762 816 825Key Takeaways
- Kenya's rainy seasons (April–May and October–November) cause a measurable spike in generator failures.
- Moisture damages alternator windings, control panels, batteries, fuel systems, and exhaust components.
- A wet-start with moisture in the alternator can cause catastrophic — and expensive — damage.
- Pre-season inspections are the most cost-effective protection available.
- Never start a generator you suspect has been wetted internally without a Megger test first.
Eng. Okumu
Lead Technical Reviewer & Auditor
Specialization
Alternator Rewinding & Precision Servicing
Experience
18+ Years Industrial Power Systems
All information in this guide has been field-verified by our engineering team to meet current Kenyan Energy Regulatory Authority (EPRA) standards and manufacturer-specific diagnostic protocols for Perkins, Cummins, and Caterpillar systems.