Workshop radial circuit that trips on saw start-up
A workshop extension has a 230 V single-phase radial circuit 48 m from the distribution board, wired in 2.5 mm squared copper and protected by a 16 A type B miniature circuit breaker. It supplies a bench saw driven by a 2.2 kW motor (power factor 0.80, efficiency 0.82, direct-on-line starting) and 500 W of fluorescent lighting. The electrician measured 226 V at the board under load. The saw starts and runs, but the breaker trips perhaps one start in five, always at the instant of start-up and never while cutting. The workshop owner has already replaced the breaker once with the same type and wants to fit a 32 A breaker so it stops tripping. Resistivity of copper is 0.0175 ohm mm squared per metre.
1. Calculate the running current and the voltage drop on this circuit, and decide whether the 16 A breaker and the 2.5 mm squared cable are adequate.
Read the worked answer
Motor input power and current. Mechanical output 2200 W, efficiency 0.82, so electrical input = 2200 / 0.82 = 2683 W. At pf 0.80, apparent power = 2683 / 0.80 = 3354 VA, and current = 3354 / 230 = 14.6 A.
Lighting: 500 / 230 = 2.2 A (unity power factor assumed).
Total running current: 14.6 + 2.2 = 16.8 A against a 16 A breaker. The circuit is overloaded in normal running, not just at start-up. A breaker carrying 16.8 A will not trip instantly, which is why the saw runs, but it is being thermally stressed and the cable is running above its design current.
Voltage drop. Go-and-return resistance of 2.5 mm squared over 48 m: R = 2 x 0.0175 x 48 / 2.5 = 0.672 ohm. Drop at 16.8 A = 0.672 x 16.8 = 11.3 V, which is 4.9 percent of 230 V - right at the 5 percent limit before the supply is even considered.
With the measured 226 V at the board, the saw terminal voltage is about 226 - 11.3 = 215 V, some 6.6 percent below nominal. That is enough to raise motor current and temperature and shorten its life.
Verdict: neither the breaker nor the cable is adequate for this load. The 2.5 mm squared cable is thermally protected by a 16 A breaker but fails on voltage drop and on continuous rating once the lighting is added.
2. Explain the start-up tripping quantitatively and evaluate the owner's proposal to fit a 32 A breaker.
Read the worked answer
Start current. Direct-on-line starting draws roughly six times full-load current for a fraction of a second: 6 x 14.6 = about 88 A.
Breaker magnetic thresholds (type B: 3 to 5 times In; type C: 5 to 10 times In):
- 16 A type B trips magnetically between 48 and 80 A. An 88 A inrush is above that band, so tripping on perhaps one start in five is exactly what the curve predicts - the current lands near the threshold and random variation decides the outcome.
- 16 A type C would trip between 80 and 160 A: 88 A is inside the band but close to the lower edge, so it is better but not a clean solution.
- 20 A type C trips between 100 and 200 A, comfortably above 88 A, and still carries the 16.8 A running load.
Why the 32 A breaker proposal must be refused. The breaker is there to protect the cable, not to stop nuisance tripping. A 32 A device on 2.5 mm squared cable leaves the cable unprotected against overload; the fault current required to trip it magnetically is also higher, so the disconnection time on a short circuit may not be met. The result is a fire risk in a cable buried in insulation or conduit, and it converts a small nuisance into a serious hazard.
Correct remedy: replace the run with 4 mm squared copper and fit a 20 A type C breaker. New drop: R = 2 x 0.0175 x 48 / 4 = 0.42 ohm, so 0.42 x 16.8 = 7.1 V, about 3.1 percent - compliant with margin.
3. List the verifications and documentation required before the modified circuit is re-energised, and explain why each matters on this job.
Read the worked answer
- Insulation resistance at 500 V DC, minimum 1 megohm to earth and between conductors. New cable, new terminations at both ends - this catches a nicked conductor before it becomes a fault.
- Earth fault loop impedance (Zs) measured, compared with the maximum permitted for a 20 A type C breaker at the required disconnection time (of the order of 1.15 ohm for a 0.4 s disconnection). If Zs is high, the breaker will not trip fast enough on a short circuit.
- RCD test if the circuit supplies socket outlets, 30 mA, with the trip time recorded at half, one and five times rated residual current.
- Polarity and continuity of line, neutral and protective conductors, and confirmation that the saw is bonded and its isolator is lockable.
- Supply capacity check at the board: 16.8 A added to existing demand against the main breaker and supplier fuse. A new circuit can be perfect and still overload the intake.
- Documentation: an updated distribution board schedule showing the new breaker rating, cable size and circuit description; the test certificate and schedule of results signed by the competent person; a note in the maintenance file that the circuit was uprated and why; and a written instruction to the owner that the saw must not be run from the lighting spur.
The records matter because the next person to open the board must not see an undocumented 20 A circuit and assume the cable is 4 mm squared without checking. Mis-documented circuits are a standard cause of unsafe additions.