Air-Conditioner Point: Cable Drop, Warm Plug and Earth Resistance
An ASSEB trainee is asked to inspect a newly wired 1.5-ton split air-conditioner point in a Guwahati flat. The outdoor unit is about 18 m of cable run from a 16 A MCB. The installer used 2.5 mm² copper conductors. At 230 V the running current is 7.2 A, but the compressor inrush is about six times running current for 0.3 s. The customer complains that ceiling-fan speed drops and the plug top feels warm whenever the compressor starts. The earth wire is 1.5 mm² and the earth-pit resistance measured with a three-pole tester is 12 Ω. The local supply often sits at 205 V in the evening. The cable is clipped to a hot wall and passes through one conduit bend. The trainee has a digital clamp meter, a multimeter and a low-resistance ohmmeter, and must decide whether to certify the point, replace cable, or improve earthing.
1. Calculate the steady-state voltage drop in the 18 m run at 7.2 A using ρ = 0.0172 Ω·mm²/m for copper. Treat live and neutral as a go-and-return pair and state whether the result is within a 3% limit at 230 V. Then explain why the result alone does not explain the fan flicker.
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Calculation
- One conductor: R = ρL/A = 0.0172 × 18 / 2.5 = 0.1238 Ω.
- Go-and-return loop: 2R = 0.2477 Ω.
- Steady-state drop: V_d = 7.2 × 0.2477 = 1.78 V.
- At 230 V: 1.78/230 × 100 = 0.78%, below the 3% limit of 6.9 V.
Reasoning The steady-state drop is small, so the cable is not obviously undersized for 7.2 A. But the complaint occurs at compressor start: inrush is about 6 × 7.2 = 43.2 A, which gives 43.2 × 0.2477 ≈ 10.7 V for 0.3 s, plus extra source and termination impedance. That short dip explains the fan flicker. The warm plug top is not explained by this calculation; it points to a loose termination, poor plug contact, or a shared circuit. The 205 V evening supply makes the same absolute drop a larger percentage, about 0.87%, but the main issue is starting current and connection quality.
2. The plug top is warm and the earth resistance is 12 Ω. List the checks you would make before deciding to replace the 2.5 mm² cable, and state what you would do about the earthing. Give a reasoned recommendation.
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Checks before replacing cable
- Isolate and prove dead; inspect the plug top, socket, MCB terminals and neutral link for loose strands, tinning, corrosion and weak spring tension. A warm plug top is often a termination fault, not a cable-size fault.
- With the AC running, measure voltage at the socket, at the MCB and across the plug/MCB joints. A large difference across one joint locates the bad connection.
- Clamp live and neutral separately to check imbalance, and confirm whether the AC is on a dedicated circuit.
- Confirm the cable is copper, not copper-clad aluminium, and check for damage at the conduit bend. A 2.5 mm² copper cable protected by a 16 A MCB is not overloaded at 7.2 A, so the calculated 0.78% drop does not justify replacement by itself.
Earthing 12 Ω is too high for a domestic installation; common practice is to aim below 5 Ω, and not to accept more than about 8 Ω without review. Improve the earth electrode with an additional rod or backfill, verify the 1.5 mm² earth continuity and connections, and re-measure with the three-pole tester. Also confirm that the earth conductor size meets the local code for the 16 A circuit.
Recommendation Do not certify yet. Repair terminations, improve earthing, and recheck voltage during start-up. Keep the AC on a dedicated circuit if flicker persists; replacing the 2.5 mm² cable alone would not fix a loose plug top or high earth resistance.