MCB tripping after ten minutes - compressor, capacitor or supply?
A 1.5 TR fixed-speed R-410A split unit in a Bhubaneswar flat trips the 20 A MCB at the distribution board about 10 to 12 minutes after start on a hot afternoon. The technician measures 198 V at the socket with the unit off and 187 V while the compressor is running; running current is 9.2 A against a nameplate 6.4 A. The run capacitor is marked 35 uF, 440 V and reads 24 uF on a digital multimeter with a capacitance range. Compressor winding resistances are C-R 2.1 ohm, C-S 3.9 ohm and R-S 6.0 ohm, and insulation resistance to earth is 200 Mohm at 500 V. The condenser coil has a visible mat of dust and cotton fluff, and the indoor filter is clean. The helper says the compressor is weak and quotes 9500 rupees for a replacement.
1. Assess the evidence. Is the compressor the faulty item?
Read the worked answer
No, the compressor looks electrically sound, and each measurement needs to be read against the others.
Winding resistances: C-R + C-S = 2.1 + 3.9 = 6.0 ohm, which equals the measured R-S. The windings are continuous and balanced, with no short between windings and no open circuit. Insulation resistance of 200 Mohm at 500 V is far above the usual 1 Mohm minimum, so there is no earth fault or moisture ingress.
The real findings are elsewhere. The capacitor reads 24 uF against 35 uF, about 31 per cent low, well outside any normal tolerance; a weak run capacitor reduces starting and running torque, so the compressor draws more current for the same load. Supply voltage is 187 V against a 230 V nominal, about 19 per cent low, and a motor delivering roughly constant power draws more current as voltage falls. The blocked condenser raises condensing pressure, which raises compressor load again. Three separate causes all push current in the same direction, which explains 9.2 A without any compressor fault.
A 'weak' compressor is a mechanical diagnosis, and nothing here supports it. Replacing it would cost the customer money and leave the real faults in place.
2. List your corrective actions in order and state the current you expect afterwards.
Read the worked answer
Order matters, because each step changes the readings the next step depends on.
- Switch off, isolate and lock off at the DB. Clean the condenser coil from the inside out and straighten any bent fins; clean the outdoor fan blade.
- Test the run capacitor again out of circuit and replace it with an identical rating (35 uF, 440 V or higher). Check the fan capacitor at the same time, since the same ageing affects both.
- Check that the unit is on a dedicated circuit, that the MCB is the correct type and rating, and that terminal screws at the socket, MCB and compressor terminal block are tight. Loose terminals add resistance and heat and can cause nuisance tripping on their own.
- Ask the flat owner about other loads on the same circuit and about the supply voltage at other times of day; 187 V under load suggests a weak supply that should be reported to the utility.
- Restart, let it stabilise for 20 to 30 minutes, and re-measure voltage, current, suction and discharge pressures.
Expected current is about 6.4 to 6.8 A at 230 V. At 187 V it may legitimately read around 7.5 A. If current stays near 9 A with a good capacitor, a clean coil and full voltage, then a mechanical or winding fault becomes credible, and the next step is a proper compressor test rather than guesswork.
3. The customer asks whether a bigger MCB would stop the tripping. How do you answer, and what would change your diagnosis?
Read the worked answer
Explain that a bigger MCB is not a repair. The breaker is doing its job: it is protecting the cable and the compressor from a sustained overload that should not exist. Fitting a 32 A breaker in place of 20 A would let 9 A flow through a machine designed for 6.4 A, cooking the windings and creating a fire risk in the wiring, while the actual faults - low capacitor, dirty condenser and low supply voltage - remain. The breaker must match the cable and the manufacturer's requirement, no larger.
What would change the diagnosis:
- A capacitor that reads within a few per cent of 35 uF, a clean coil and 230 V supply, yet current still over 8.5 A: suspect a mechanical fault such as a tight bearing or slugging, or a partially shorted winding.
- Insulation resistance below about 1 Mohm, or a wide imbalance between the three winding readings: suspect winding breakdown or contamination.
- A compressor that hums, draws locked-rotor current and trips in seconds rather than after ten minutes: suspect the start circuit or a seized compressor.
- Tripping that happens even with the compressor disconnected, or with a different appliance on the same circuit: suspect the wiring, the MCB itself or the supply.
Write the findings on the job sheet so the customer can see why the capacitor and coil, not the compressor, were billed.