Workshop Supply: Load, Earthing and the Tingling Saw Bench
A +2 vocational institute is converting a 6 m × 9 m room into a carpentry-cum-welding practice workshop on a single-phase 230 V supply with a sanctioned load of 5 kW. Connected load: 10 LED fittings at 20 W, six ceiling fans at 75 W, two exhaust fans at 60 W, an arc welding set with 3.0 kW input and a table saw of 1.5 kW, plus eight twin 6 A socket outlets. The existing installation has a 20 A rewirable fuse as the main switch, a single 2.5 mm² copper PVC circuit looping through all sockets, an earth conductor clamped to a 20 mm GI water pipe, and no RCCB. Measured earth resistance is 14 ohms. The fuse blows whenever the welder strikes an arc, the socket outlets feel warm in use, and an instructor reports a tingling sensation when touching the saw bench while standing on a wet floor.
1. Calculate the design current and specify the main switch, sub-circuits and cable sizes, stating your assumptions.
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Connected load: lighting 10 × 20 = 200 W; fans 6 × 75 = 450 W; exhausts 2 × 60 = 120 W; welder 3,000 W; saw 1,500 W. Total = 5,270 W.
Design current: 5,270 ÷ 230 = 22.9 A. For a workshop I would not apply a heavy diversity factor to the welder and saw, because they can run together and the welder has a high inrush; design for at least 25 A, and choose a 32 A main MCB with a C or D characteristic so that welder inrush does not trip it. The rewirable 20 A fuse is both undersized and far too slow - that is why it blows on arc strike.
Sub-circuits (separate MCB per circuit, no daisy-chaining):
- Lighting and fans: 770 W ≈ 3.4 A → 6 A MCB, 1.5 mm² copper.
- Socket outlets: 16 A MCB, 2.5 mm² copper, at most two twin sockets per circuit, protected by a 30 mA RCCB.
- Welder: dedicated 16 A MCB with D characteristic, 4 mm² copper.
- Table saw: dedicated 16 A MCB, 4 mm² copper, with local isolator adjacent to the machine.
- Main/service cable: 6 mm² copper PVC, giving ample margin at 32 A.
Assumptions stated: single-phase 230 V, PVC-insulated copper conductors in conduit, ambient around 35 °C, no derating for grouping beyond normal, and welder and saw not run simultaneously for long periods. If the sanctioned load of 5 kW cannot be raised, the welder or the saw must be interlocked so both cannot run together.
2. Explain the two fault symptoms using electrical principles and state what you would test first.
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Fuse blowing on arc strike: a welding set draws a large transient magnetising/inrush current at strike, several times its 3 kW steady input. A rewirable fuse has a very inverse and imprecise time–current characteristic and a 20 A rating is already close to the 22.9 A design current, so it ruptures on the transient. The 2.5 mm² loop also causes voltage drop and long welds suffer. The answer is a correctly rated MCB with a D curve and a properly sized dedicated circuit, not a bigger fuse.
Tingling on the saw bench: this is the classic signature of a fault-to-earth path through a person. Two contributors: the earth electrode is a water pipe with 14 ohms resistance, well above the 5 ohm limit usually required for a single electrode, so fault current is limited and the metalwork sits at a dangerous potential; and the water pipe is not a reliable electrode - it may be plastic-jointed, may be removed, and may itself become live. Warm sockets point to loose terminations and overload on the 2.5 mm² loop, both of which also generate heat and can lead to fire.
Tests first: earth electrode resistance by the three-pole fall-of-potential method (not a clamp meter on a single rod); earth continuity between the saw frame, the welder case and the main earthing terminal; insulation resistance of the socket circuit at 500 V DC; and polarity. Isolate the saw immediately - a tingling machine must not be used again until the fault is cleared.
3. List the isolation and re-commissioning steps you would enforce, including what must be verified before the supply is switched back on.
Read the worked answer
Isolation (permit-to-work discipline):
- Notify all users and agree a shutdown window.
- Open the main switch and remove the fuse/isolate upstream; lock the isolation point with a personal padlock and attach a 'Do Not Switch On' tag.
- Prove the tester on a known live source, test the conductors dead at the point of work, then re-prove the tester. Never rely on a neon screwdriver.
- Discharge and prove dead at every circuit, not just the main.
Work: install the new main MCB and sub-circuits, a proper earth electrode (GI plate or copper-bonded rod) with the connection in an inspection pit, an equipotential bonding conductor to all metal services, and a 30 mA RCCB. Replace the water-pipe earth clamp with a bonded, not earthing, connection.
Verification before energising:
- Earth electrode resistance ≤ 5 ohms, measured and recorded.
- Insulation resistance of each circuit ≥ 1 MΩ at 500 V.
- Earth continuity from every metal enclosure to the main earthing terminal.
- Polarity correct at every socket, with the switch in the phase conductor.
- RCCB trip test at its test button and with a ramp tester (trip within 300 ms at 30 mA).
- Circuit chart and labelling completed, then remove padlocks in a formal hand-back and re-energise circuit by circuit, testing as each is restored.