12.1 Shock Hazards & Safe Work Practices
Key Takeaways
- An electrically safe work condition — the equipment de-energized, locked out, and verified absent of voltage — is always the first choice under NFPA 70E Article 120; energized work is the exception, not the rule.
- Current magnitude drives injury severity: about 10 mA is the adult let-go threshold, 50 mA can cause ventricular fibrillation, and 100 mA is frequently fatal.
- Body resistance varies widely with moisture and contact area, so the same 120V contact can produce a minor tingle or a lethal current depending on conditions.
- GFCI protection is supplementary, not a substitute for de-energizing — it trips at ~5 mA to protect against the let-go threshold before the heart is endangered.
- Shock severity depends on current magnitude, duration, pathway through the body, voltage, and body resistance, not on voltage alone.
Why This Matters on the Exam
The Virginia Journeyman Electrician exam blueprint allots 4 questions to Safety. While the counts are modest, these items are high-value because NFPA 70E facts are unambiguous — if you know the standard, you get the point. Many candidates lose safety questions because they guess at physiological thresholds or confuse NEC grounding rules with NFPA 70E work-practice rules. This section covers the physiological effects of current, what makes a shock dangerous, and the hierarchy of safe work practices so you can answer the safety block confidently.
How a Shock Occurs
Electrical shock is the passage of electric current through the body. Three things must be present for current to flow through a person: (1) a voltage source, (2) a conductive path through the body, and (3) a return path back to the source. Ohm's law governs the body just like any other conductor: I = E ÷ R, where E is the applied voltage and R is the total resistance of the body plus contact resistance.
Body resistance is not a fixed number. Dry, thick skin in light contact may offer 100,000 Ω or more; wet skin or a firm grip on a grounded conduit can drop the effective resistance below 1,000 Ω. At 120V and 1,000 Ω, the body sees 120 mA — well into the lethal range. At the same 120V and 100,000 Ω, the body sees only 1.2 mA — a barely perceptible tingle. This is why the same outlet that powers a hair dryer can be harmless one moment and deadly the next: contact conditions change R, which changes I.
Factors Affecting Severity
The severity of an electric shock depends on five factors, and you should memorize this list because exam questions restate it in different orders:
- Current magnitude — the single most important factor; higher current = worse outcome.
- Duration — the longer current flows, the greater the tissue and heart damage; fibrillation risk rises with time.
- Pathway — current that crosses the chest (hand-to-hand or hand-to-foot) passes through the heart and is far more dangerous than current that stays in one finger.
- Voltage — higher voltage drives more current through the same resistance and can break down skin resistance.
- Body resistance — wet skin, sweat, cuts, and tight grips lower resistance and raise current.
Physiological Effects by Current Level
The following table reflects AC (60 Hz) values as widely cited from NFPA 70E Annex K and OSHA 3075. These are the values the exam tests:
| Current (mA) | Physiological Effect |
|---|---|
| 0.5–1 | Perception threshold — faint tingle |
| 1–5 | Mild shock, involuntary startle; let-go still possible |
| 10 | Adult let-go threshold — involuntary muscle contraction prevents releasing the conductor |
| 16 | Average let-go current for adult women |
| 20–50 | Painful, severe muscular contraction; breathing difficulty; possible respiratory paralysis |
| 50–100 | Ventricular fibrillation possible; heart rhythm disrupted |
| 100–200 | Ventricular fibrillation likely; frequently fatal if not interrupted quickly |
| >1,000 | Cardiac arrest, severe burns; usually fatal |
The let-go threshold of roughly 10 mA is a favorite exam fact. Below it you can voluntarily release the energized part; above it, your muscles clamp down and you cannot let go, so duration increases automatically — which is why GFCIs are designed to trip at about 5 mA, below the let-go level.
The Role of Grounding and Bonding
Grounding and bonding reduce shock risk by ensuring exposed conductive parts that could become energized are held at the same potential and connected to earth so overcurrent devices operate. An equipment grounding conductor (EGC) carries fault current back to the source to trip the breaker, clearing the fault before a person touching the enclosure receives a sustained shock. Bonding jumpers eliminate voltage differences between metal parts. These are NEC 2020 Article 250 concepts — they prevent the hazard from existing in the first place. But grounding alone does not make working on energized equipment safe; a de-energized, locked-out circuit is the only condition that removes the shock hazard entirely.
Safe Work Practices — De-energize First
NFPA 70E Article 120 establishes the electrically safe work condition (ESWC) as the default. The principle is simple: if you do not have to work it hot, do not work it hot. To put equipment in an ESWC you must disconnect it, isolate it, lock/tag it, verify absence of voltage with a tested instrument, and ground it if stored energy or induction hazards exist. Only after these steps is the equipment considered safe to touch without PPE.
Energized work is permitted only when the employer can demonstrate that de-energizing introduces additional or increased hazards (NFPA 70E Article 130.2) or is infeasible for diagnostic testing that requires power on. When energized work is justified, an energized electrical work permit (EEWP) is required (Article 130.2(B)).
GFCI as Supplementary Protection
A GFCI monitors current balance between hot and neutral and trips when the difference exceeds about 5 mA — below the let-go threshold. On construction sites NEC 590.6 mandates GFCI protection on 120V, 15- and 20-amp receptacles. GFCIs protect against shock but do not protect against line-to-line faults or overcurrent; they are a backstop, never a reason to leave circuits energized for hands-on work.
CPR and First Aid
NFPA 70E requires that employees exposed to shock hazard be trained in emergency response, including CPR and first aid. The standard calls for response procedures and for a person trained in CPR/first aid to be available when work is performed that could expose a person to shock. Defibrillation (AED) within the first few minutes of ventricular fibrillation dramatically improves survival — another reason rapid de-energization and emergency readiness matter.
Exam Trap: Voltage Is Not the Hazard
A common distracter on the exam states that voltage alone determines shock severity. It does not. A 120V circuit with high body resistance may pass a harmless 1 mA; a 24V circuit with wet hands may pass a painful current. Current, not voltage, injures. Voltage only determines current through a given resistance. If an answer choice says the hazard is proportional to voltage alone, eliminate it.
Worked Scenario
A technician with damp hands grips a grounded metal box that is inadvertently energized at 120V. Skin resistance is 2,000 Ω. The current through the body is I = 120 / 2,000 = 60 mA. This crosses the 50 mA ventricular-fibrillation threshold — a life-threatening exposure. Had the same box been de-energized and verified absent of voltage per NFPA 70E Article 120, the exposure would not exist. This single computation shows why de-energizing is the controlling practice.
At approximately what current level does the adult let-go threshold occur?
Which factor has the greatest influence on the severity of an electrical shock?
Under NFPA 70E, what is the first choice before performing work on electrical equipment?