Tingling grinder body: why the 16 A MCB stayed closed and what to do before production resumes
At a small fabrication unit near Pune, a trainee feels a tingling sensation when touching the metal body of a 1.5 kW, 230 V bench grinder used for buffing. The grinder is fed from a 16 A MCB in a distribution board that has no RCCB. The earth pin of the 3-pin plug is looped into the neutral bar inside the DB, and the single earth electrode at the back wall measures 14 ohm with a 3-pole earth tester. A 500 V megger between winding and body reads 1.2 Mohm with the machine dry. During the monsoon the floor of the shed stays wet, and the operator wears rubber-soled shoes only on some days. The supervisor wants to replace the 16 A MCB with a 32 A one 'so it stops nuisance tripping' and continue production, because an export order is due in two days. He asks the trainee to give a written opinion before anything is changed, and the unit owner is standing next to him waiting for an answer.
1. Explain, with numbers, why the 16 A MCB could not have protected the operator, and why fitting a 32 A MCB would make the installation more dangerous.
Read the worked answer
An MCB responds to current in the circuit, not to the voltage that appears on a metal enclosure.
Fault path. If the winding leaks to the frame, the frame rises towards phase potential. With a 14 ohm electrode the current returning through the electrode is roughly 230 / 14 = 16.4 A, ignoring the winding and body impedance. A 16 A MCB will not trip magnetically until about 5 to 10 times rating (80 to 160 A), and its thermal element at about rated current may never trip in practice. So the fault can persist for hours.
Current through the person. The human body is a parallel path to the electrode. With about 1 kohm body resistance and a wet floor of a few hundred ohm, the touch current is of the order of 230 / 1,500 = 150 mA, far above the 30 mA fibrillation threshold, and none of it is 'seen' by a 16 A MCB.
Why 32 A is worse. The grinder flex and the 2.5 mm2 circuit are protected by the 16 A device. At 32 A the cable is no longer protected against sustained overload, the touch voltage on the frame does not change at all, and the higher let-through energy increases fire risk. The change treats a symptom (tripping) and removes the only protection the cable had.
2. The electrode reads 14 ohm. State the value this installation should meet, and redesign the earthing and protective arrangement so that a person is protected even if the insulation fails again.
Read the worked answer
Target value. IS 3043 guidance for small LT installations is 8 ohm or less (1 ohm is for large substations with high fault currents). 14 ohm fails this, and a single rod in dry black cotton soil also drifts badly in summer, so the measured value under monsoon conditions is probably the best case.
Earthing redesign. Drive two 3 m galvanised iron pipe electrodes spaced at least 6 m apart (twice the rod length), interconnect them with 25 x 3 mm GI strip, and backfill with bentonite or salt-and-charcoal to bring the combined value to 5 ohm or below. Re-measure after 24 hours of settling, not immediately.
Earthing conductor. Run a continuous 2.5 mm2 copper conductor from the earth bar to every socket and to the grinder frame. The present practice of looping the earth pin to the neutral bar must be removed at once: an open neutral then puts phase voltage on every 'earthed' body, and an RCCB would trip continuously.
Protection. Fit a 2-pole 40 A, 30 mA RCCB (or RCBO) at the DB. The RCCB does not clear the fault, it protects the person; the insulation fault must still be repaired by rewinding or replacing the grinder. Add a rubber standing mat and an IP44 industrial plug and socket.
3. Give the safe isolation sequence the trainee must follow before touching the grinder, and the written opinion he should give the owner about continuing production today.
Read the worked answer
Isolation sequence.
- Get the owner's authorisation and inform the operator; identify the correct MCB and label it.
- Switch off, withdraw the plug, lock off and tag the DB with a personal padlock and a 'Do Not Switch On' tag.
- Prove the tester on a known live source, then test the terminals for dead on all poles, then re-prove the tester.
- Discharge and prove dead again at the grinder terminals; confirm no back-feed from any other circuit.
- Wear safety shoes and use insulated tools; keep a dry board and rubber mat on the wet floor.
Opinion to the owner. The machine must not be used until the neutral-to-earth link is removed, the RCCB is fitted, the electrode is improved and the insulation defect is repaired. The measured 1.2 Mohm dry will fall sharply when the winding absorbs moisture, and the present arrangement can kill an operator on a wet floor. Offer a compliant interim measure: withdraw this grinder, use a double-insulated or 24 V tool for the urgent jobs, and shift the order schedule by the one day needed for rewinding and earthing work. Record the isolation, the measurements and the decision in the maintenance log so the stand taken is documented.