Electrical Safety, Testing and Troubleshooting

The hazards

Electricity kills in three ways: by passing current through the body (shock), by the intense heat and blast of an arc, and by fire from overheated conductors and connections. Each is controlled by a hierarchy: eliminate the hazard (work dead), engineer it out (insulation, guards, protection, see Protection, Earthing and RCDs), apply procedures (lockout, permits) and, last, use personal protective equipment. This page is general information, not a replacement for training and the standards that apply to your work: electrical work must be done by competent people.

Electric shock

Current through the body (50 Hz)	Effect
about 0.5 to 1 mA	threshold of perception
5 to 10 mA	painful; at about 10 mA the muscles may not let go
30 mA	breathing difficult; the limit an RCD protects against
50 to 100 mA or more for a fraction of a second	ventricular fibrillation, usually fatal without a defibrillator
amperes	burns, tissue damage, cardiac arrest

The severity depends on the current and the time, with the fibrillation limit about I = 116 ÷ √t mA (Dalziel): 580 mA for 40 ms (an RCD), 183 mA for 0.4 s (a breaker), 52 mA for 5 s. The body’s resistance varies from about 1 000 Ω (wet skin, a good contact) to 100 000 Ω (dry, a poor contact), so the current from 230 V can be 2 to 230 mA. Treat any voltage above 50 V AC (120 V DC) as dangerous, and any wet place as worse. A person who has been shocked should be seen by a doctor, even if they feel well, because heart rhythm problems can appear later.

Arc flash and arc blast

A short circuit through the air makes an arc at 10 000 to 20 000 K that vaporizes the copper, expands it by a factor of 67 000, and releases an enormous heat and pressure wave. The hazard is the incident energy at the working distance, in cal/cm² (1.2 cal/cm² causes a second-degree burn on bare skin in a fraction of a second). IEEE 1584 gives the full method; the old Ralph Lee equation E = 2.142 × 10⁶ V I_bf t ÷ D² (J/cm², V in kV, current in kA, t in s, D in mm) gives an estimate for a quick view of the factors:

Safe working: isolate, lock out, prove dead

• Plan: identify every source of energy (including a second feed, back-feed from a generator or solar array, capacitors, stored mechanical energy, hydraulic accumulators).
• Isolate: open the isolator and the switch the correct way; check it visually where possible.
• Lock out and tag: each worker puts a personal lock on the isolator; no lock, no work. Keep the key.
• Prove dead: with a tester that you have proved on a known live source, test every conductor to earth and to each other, then re-prove the tester on the known source (the three-point test).
• Discharge and earth: capacitors and drives hold a charge for minutes (read the label); apply earths where a back-feed is possible.
• Live work only when dead work is impossible, with a permit, trained people, insulated tools, the right PPE and a second person who knows how to cut the power and give first aid.

Testing: insulation, earth and continuity

Test	Instrument and voltage	What it checks	Pass guide (typical)
Insulation resistance	insulation tester, 500 V (1 000 V for 690 V circuits)	leakage through insulation between conductors and to earth	≥ 1 MΩ for installations; motors ≥ (rated kV + 1) MΩ, much more when new
Polarization index	insulation tester, 10 minute test	dryness and aging of a motor or cable	ratio of the 10-minute to the 1-minute reading ≥ 2
Continuity of protective conductors	low-resistance ohmmeter, 200 mA	unbroken earth paths	a few milliohms per conductor, agree with the calculation
Earth-fault loop impedance	loop tester	Zs low enough to trip the device quickly	below the tabulated maximum (see Protection, Earthing and RCDs)
RCD trip time and current	RCD tester	trip at 30 mA and within 40 ms at 5 × IΔn	per the standard
Earth electrode resistance	earth tester (fall-of-potential)	quality of the electrode	depends on the system; as low as practical

Worked example: testing and diagnosing the press motor

Troubleshooting

Symptom	Likely causes	Check
Motor does not start, hums	single phase (lost fuse), jammed load, broken rotor bars, low voltage	voltage on all three lines at the motor; shaft turns by hand
Trips on start	breaker too small or wrong curve; starting current too high; overload set too low	peak starting current against the magnetic trip; relay class
Trips after a time running	overload (mechanical), high ambient, blocked ventilation, voltage unbalance, worn bearings	running current against the nameplate; temperatures; unbalance
RCD trips	leakage: damp equipment, drive filters, insulation damage	isolate circuits; measure leakage with a clamp meter; insulation test
Motor overheats	overload, poor cooling, too many starts, low voltage, harmonic heating on a drive	current, airflow, duty cycle, supply quality
Lights flicker or dim when a motor starts	high starting current and a weak supply or long cable	voltage dip at the load; soft start or a larger cable (Cable Sizing and Voltage Drop)
Burning smell, discolored terminal	loose connection, undersized cable, an overloaded circuit	isolate; thermal image; torque the terminals; inspect
Tingling from metalwork	earth fault or a poor protective conductor; stray leakage	isolate at once; test continuity and loop impedance
Neutral hot or overloaded	unbalanced single-phase loads or triplen harmonics	neutral current with a true-RMS clamp (Power Factor, Harmonics and Energy)

Thermography and maintenance

A thermal camera finds loose connections, overloaded phases and failing bearings before they fail: a joint 10 to 15 K hotter than the same joint on the other phases under load is a warning, and over 40 K an urgent job. Scan panels yearly under load. Maintenance includes cleaning and tightening, checking protection settings and breaker operation, testing RCDs and earth continuity, insulation tests on motors and cables, battery and UPS checks, and keeping accurate drawings and labels. Record every reading: the trend tells more than a single value.

Related

• Previous: Solar PV, Batteries and Backup Power. Back to the overview: Electrical Power.
• Workshop Safety for the wider safety guide.
• Protection, Earthing and RCDs for breakers, RCDs and earthing.

FAQ

Is a 12 V supply safe to touch?

Normally yes: below about 50 V the current through the skin is small. But batteries can deliver huge short-circuit currents that melt tools and burn, and arcs at 24 V DC and above can be hazardous.

What should I do if someone is stuck to a live conductor?

Switch off the supply if you can reach the switch safely. Otherwise push them clear with a dry non-conductive object. Call for help, start CPR if there is no breathing, and use an AED.

Can I trust a non-contact voltage tester?

It is a useful first check, not proof of dead: use a proved two-pole tester. Shielded and low-voltage cables can mislead it.

How often should installations be tested?

Fixed installations typically every 5 years (1 to 3 for harsh or commercial sites); portable tools and RCDs more often. Follow the local code and the risk assessment.

References

• NFPA 70E (electrical safety in the workplace), IEEE 1584 (arc-flash hazard calculations), IEC 60479-1 (effects of current on human beings), IEC 61557 (test equipment), EN 50110 (operation of electrical installations).
• IEC 60364-6 (verification) and BS 7671 (inspection and testing); IEEE Std 43 (insulation resistance testing of rotating machinery).
• Cadick, J., Capelli-Schellpfeffer, M., Neitzel, D., Al-Hamdi, A., Electrical Safety Handbook, McGraw-Hill; HSE (UK) guidance note GS38.

Last reviewed: October 2026. This page is general information, not professional safety advice; incident-energy figures use a simplified formula.