Live Chassis on a Wet Floor: Why the Fuse Never Blew
At a small flour mill near Kharagpur, a helper touches the metal body of a 1.5 kW, 230 V single-phase atta chakki and is thrown back by a shock. The mill is fed from a 15 A socket through a three-core flexible cable whose earth pin has been cut off at the plug. The socket earth is a 1 m GI pipe driven into dry sandy soil beside the shed; measured with an earth tester its resistance is 92 ohm. The floor was being washed at the time. Protection consists of a 32 A MCB at the pole and a 16 A fuse in the mill starter; there is no RCCB anywhere. The mill runs normally and the fuse has never blown. With the machine running and wet, a clamp meter around the earth conductor reads 1.9 A. The owner says this has happened twice before in the rains and that the workers normally wear rubber slippers.
1. Using the clamp-meter and earth-resistance readings, calculate the touch voltage on the body and explain why neither the 16 A fuse nor the 32 A MCB operated.
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The fault path is: line terminal - damaged lead or winding - metal body - earthing conductor - 92 ohm electrode - mass of earth - transformer neutral earth. The clamp meter shows 1.9 A flowing in that path.
Touch voltage = I x R(electrode) = 1.9 x 92 = about 175 V. The supply is 230 V, so only about 55 V is dropped inside the fault itself (roughly 30 ohm of contact resistance); almost all the rest appears on the body.
175 V is far above the 50 V AC touch limit. A wet hand-to-feet resistance of about 1 kohm would pass roughly 175 mA through the helper, which is in the ventricular-fibrillation zone.
Why no device tripped: the 16 A fuse needs several times its rating (roughly 40-60 A) for a noticeable time to melt, and the 32 A MCB needs even more. Here the current is only 1.9 A, about 12 percent of the fuse rating. Note that even a dead short from line to the body would give at most 230 / 92 = 2.5 A, because the high electrode resistance itself limits the fault current. A low-level earth fault is therefore invisible to every overcurrent device. That is precisely why a 30 mA RCCB, which would see 1.9 A as about 60 times its rating and trip in milliseconds, is required.
2. The owner proposes simply replacing the 1 m pipe with a 3 m pipe and carrying on. Evaluate this proposal, calculate what earth resistance would actually be needed to clear the fuse, and state the sequence of actions you would insist on before re-energising.
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A longer pipe alone is not enough. Driving deeper may reduce 92 ohm to perhaps 20-40 ohm in dry sand, which still gives 6-11 A of fault current and a touch voltage of roughly 180-200 V. The hazard would remain.
Calculate the target instead. To melt a 16 A fuse with certainty you need roughly 3 to 4 times its rating, say 50-60 A. R(earth) must be below 230 / 60 = about 3.8 ohm, and for the 32 A MCB to trip magnetically the resistance must be well under 1 ohm. Design to 1 ohm or less, using a 2-3 m pipe or a plate electrode with salt and charcoal backfill, a watering funnel, and an earth continuity conductor bonded to the machine body.
Sequence before re-energising:
- Isolate, lock off and tag; tell the owner the mill stays off.
- Take the plug with the cut earth pin and the damaged flexible cable out of service permanently.
- Repair or replace the damaged lead and terminal-box gland; dry the winding if damp; megger at 500 V and require at least 1 Mohm to frame.
- Install a proper electrode, bond the body, measure and record the earth resistance.
- Fit a 30 mA RCCB on the socket circuit (and 100 mA at the incoming), and test it with the test button and an RCD tester.
- Keep the socket and floor dry, provide an insulating mat, and record all test results.
3. The owner argues that rubber slippers are enough protection. Explain why this is not a control measure and identify engineering controls that protect the worker even if he does nothing.
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Rubber slippers are personal protective equipment of the weakest kind. They are not tested, they degrade, they are useless once wet or worn through, they only insulate the feet while the other hand may touch a grounded surface, and they depend entirely on the worker choosing to wear them. Here the floor is deliberately washed, so the insulation on which the whole argument rests is exactly what is destroyed. PPE also cannot be relied on for a 175 V touch voltage, because the current path can shift to a knee, hip or hand resting on the frame.
Engineering controls work without the worker's cooperation:
- A 30 mA RCCB, which trips on the earth-leakage current itself and does not need the fault to be large.
- A low-resistance earth electrode (1 ohm or less) that holds the body potential down and lets overcurrent devices operate.
- Class II double-insulated tools, or a 24 V hand-lamp and isolated transformer for wet areas.
- Correct three-core cable with an intact earth pin and proper glanding, so a chafed lead cannot touch the frame.
- Dry standing area, raised socket, and a written rule that cleaning happens only after isolation and lock-off.
The teaching point: fix the installation, then train the person, never the other way round.