25 multiple-choice questions for the OSHA Electrical Safety (Subpart S) Practice Test. Pick an answer to see the rule behind it, quoted from the official source.
1. A worker needs to inspect the back of a dead-front switchboard assembly. Under which condition is working space NOT required behind this equipment?
Answer: When there are no renewable or adjustable parts on the back and all connections are accessible from other locations. The exemption for back working space applies specifically when there are no renewable/adjustable parts on the back and connections are accessible from elsewhere.
29 CFR § 1926.403(i): “Working space is not required in back of assemblies such as dead-front switchboards or motor control centers where there are no renewable or adjustable parts such as fuses or switches on the back and where all connections are accessible from locations other than the back.”
2. An electrician is installing a new junction box on a concrete wall at a construction jobsite. According to the mounting requirements for electric equipment, which method is prohibited for securing the box?
Answer: Using wooden plugs driven into holes in the masonry. The regulation specifically forbids the use of wooden plugs for mounting electric equipment on hard surfaces like concrete or masonry.
29 CFR § 1926.403(d): “Wooden plugs driven into holes in masonry, concrete, plaster, or similar materials shall not be used.”
3. An electrician is wiring a receptacle and needs to connect the grounding contact. To what must the grounding contact be connected?
Answer: The equipment grounding conductor of the circuit supplying the receptacle. The grounding contact must be connected specifically to the circuit's equipment grounding conductor, not the neutral or other conductors.
29 CFR § 1910.304(b): “The grounding contacts of receptacles and cord connectors shall be grounded by connection to the equipment grounding conductor of the circuit supplying the receptacle or cord connector.”
4. An electrician is working on deenergized equipment that contains stored non-electrical energy capable of reenergizing the circuit parts. What action is required to ensure safety?
Answer: Block or relieve the stored non-electrical energy so parts cannot be accidentally energized. The rule mandates that non-electrical energy sources which could cause reenergization must be blocked or relieved to prevent accidental energization.
29 CFR § 1910.333(b): “Stored non-electrical energy in devices that could reenergize electric circuit parts shall be blocked or relieved to the extent that the circuit parts could not be accidentally energized by the device.”
5. For systems over 600 volts nominal, where are open runs of non-metallic-sheathed cable or bare conductors permitted to be installed?
Answer: In locations which are accessible only to qualified persons. The rule allows these specific open runs only in areas restricted to qualified persons.
29 CFR § 1926.408(a): “However, open runs of non-metallic-sheathed cable or of bare conductors or busbars may be installed in locations which are accessible only to qualified persons.”
6. An electrician is installing a disconnecting means for an arc welder that does not have an integral disconnect. What is the minimum rating requirement for the switch or circuit breaker used as the disconnecting means?
Answer: It must be rated at least equal to that necessary to accommodate overcurrent protection. For arc welders without integral disconnects, the regulation specifically ties the disconnect rating to the requirement for accommodating overcurrent protection, rather than conductor ampacity or input current.
29 CFR § 1910.306(d): “The disconnecting means shall be a switch or circuit breaker, and its rating may not be less than that necessary to accommodate overcurrent protection.”
7. An electrician is installing a sign at the service entrance equipment for an emergency power system. Under which condition is this sign NOT required?
Answer: For individual unit equipment. The rule states that while a sign is generally required at service entrance equipment, it is not required for individual unit equipment.
29 CFR § 1910.308(b): “A sign shall be placed at the service entrance equipment indicating the type and location of on-site emergency power sources. However, a sign is not required for individual unit equipment.”
8. A technician is maintaining a portable high-voltage system where the equipment grounding conductor continuity is continuously monitored. What automatic action must occur if the continuity of this grounding conductor is lost?
Answer: The high voltage feeder to the portable equipment must deenergize automatically. The regulation mandates that the feeder automatically deenergizes when the monitoring system detects a loss of continuity in the equipment grounding conductor.
29 CFR § 1910.304(g): “The continuity of the equipment grounding conductor shall be continuously monitored so as to deenergize automatically the high voltage feeder to the portable equipment upon loss of continuity of the equipment grounding conductor.”
9. An electrician is protecting a feeder circuit over 600 volts using fuses. What is the maximum continuous ampere rating allowed for the fuse relative to the ampacity of the conductors it protects?
Answer: Three times the ampacity of the conductors. The regulation specifies that for feeders over 600 volts, the continuous ampere rating of a fuse cannot exceed three times the ampacity of the protected conductors.
29 CFR § 1910.304(f): “The continuous ampere rating of a fuse may not exceed three times the ampacity of the conductors.”
10. A worker is installing a ventilation unit for electrical equipment that relies on natural air circulation for cooling. To comply with safety requirements, how must the unit be positioned relative to walls and other equipment?
Answer: It must be installed so that room air flow over exposed surfaces is not prevented. Equipment relying on natural cooling must be installed in a way that ensures air can freely circulate over its surfaces without obstruction.
29 CFR § 1926.403(d): “Electrical equipment which depends upon the natural circulation of air and convection principles for cooling of exposed surfaces shall be installed so that room air flow over such surfaces is not prevented by walls or by adjacent installed equipment.”
11. An industrial worker is inspecting a junction box and finds an unused opening. What is the required action to comply with the mechanical execution of work standards?
Answer: Effectively close the unused opening to afford protection substantially equivalent to the wall of the equipment. The regulation requires that unused openings be effectively closed to provide protection that is substantially equivalent to the equipment's wall.
29 CFR § 1910.303(b): “Unused openings in boxes, raceways, auxiliary gutters, cabinets, equipment cases, or housings shall be effectively closed to afford protection substantially equivalent to the wall of the equipment.”
12. When an electrician installs a wireway to house and protect electric wires and cable, what is the required sequence for installing the conductors?
Answer: The conductors are laid in place after the wireway has been installed as a complete system. The definition specifies that conductors are installed after the wireway itself is fully installed as a complete system.
29 CFR § 1910.399: “Wireways. Sheet-metal troughs with hinged or removable covers for housing and protecting electric wires and cable and in which conductors are laid in place after the wireway has been installed as a complete system.”
13. Before an electrician begins work on deenergized equipment, what action must be taken regarding stored electric energy if it might endanger personnel?
Answer: It must be released, with capacitors discharged and high capacitance elements short-circuited and grounded. The rule requires that stored electric energy be released, specifically mandating the discharge of capacitors and the short-circuiting and grounding of high capacitance elements.
29 CFR § 1910.333(b): “Stored electric energy which might endanger personnel shall be released. Capacitors shall be discharged and high capacitance elements shall be short-circuited and grounded, if the stored electric energy might endanger personnel.”
14. When an employer selects electrical equipment for a jobsite, what is the primary basis for determining that the equipment is safe for use?
Answer: The equipment is free from recognized hazards likely to cause death or serious physical harm. Safety is determined by ensuring the equipment lacks recognized hazards that could cause severe injury or death.
29 CFR § 1926.403(b): “The employer shall ensure that electrical equipment is free from recognized hazards that are likely to cause death or serious physical harm to employees.”
15. Low-voltage switches (600 volts, nominal, or less) are installed in a room that also contains exposed live parts operating at over 600 volts. What is required to separate the low-voltage equipment from the high-voltage space?
Answer: A suitable partition, fence, or screen. The regulation mandates physical separation via a partition, fence, or screen when low-voltage equipment shares a space with high-voltage exposed parts.
29 CFR § 1910.303(h): “the high-voltage equipment shall be effectively separated from the space occupied by the low-voltage equipment by a suitable partition, fence, or screen.”
16. An electrician is connecting a grounded conductor to a terminal. Which practice is prohibited to ensure safety and code compliance?
Answer: Attaching the conductor to a terminal so as to reverse designated polarity. The rule strictly forbids attaching a grounded conductor in a way that reverses the designated polarity of the terminal or lead.
29 CFR § 1926.404(a): “No grounded conductor shall be attached to any terminal or lead so as to reverse designated polarity.”
17. An electrician is installing threaded conduit in a Class I, Zone 1 hazardous location. To ensure the explosionproof integrity of the system and prevent sparking during a fault, what specific threading standard and tightness must be used?
Answer: Threaded with an NPT standard conduit cutting die providing 3/4-in. taper per foot and made wrench tight. The regulation mandates the use of NPT dies with a 3/4-inch taper per foot and requires the conduit to be wrench tight to maintain explosionproof integrity.
29 CFR § 1910.307(g): “All threaded conduit shall be threaded with an NPT (National (American) Standard Pipe Taper) standard conduit cutting die that provides 3⁄4-in. taper per foot. The conduit shall be made wrench tight to prevent sparking when fault current flows through the conduit system and to ensure the explosionproof or flameproof integrity of the conduit system where applicable.”
18. An industrial worker is assessing whether a specific electrical panel is 'accessible' as applied to equipment. Which condition correctly describes an accessible piece of equipment?
Answer: It admits close approach and is not guarded by locked doors, elevation, or other effective means. Equipment is considered accessible if workers can approach it closely without being blocked by locked doors, significant elevation, or other effective barriers.
29 CFR § 1926.449: “Accessible. (As applied to equipment.) Admitting close approach; not guarded by locked doors, elevation, or other effective means.”
19. An industrial electrician is installing custom-made electrical equipment designed specifically for a single customer. To ensure the installation is considered 'acceptable' under OSHA Subpart S without a nationally recognized testing laboratory listing, what must the electrician rely on?
Answer: A determination by the manufacturer that the equipment is safe for its intended use, based on test data kept by the employer. For custom-made equipment, acceptability is established by the manufacturer's safety determination based on retained test data, rather than third-party lab certification or external agency inspection.
29 CFR § 1910.399: “With respect to custom-made equipment or related installations that are designed, fabricated for, and intended for use by a particular customer, if it is determined to be safe for its intended use by its manufacturer on the basis of test data which the employer keeps and makes available for inspection to the Assistant Secretary and his authorized representatives.”
20. An industrial worker is evaluating a new piece of electrical equipment for safety compliance. Which service is a 'Qualified testing laboratory' authorized to provide regarding product labels?
Answer: Authority to recall the label if a hazardous product is installed. The rule lists the authority to recall a label for hazardous products as a specific service of a qualified testing laboratory, while the other options are not supported by the rule.
29 CFR § 1926.449: “Service-value determinations through field inspections to monitor the proper use of labels on products and with authority for recall of the label in the event a hazardous product is installed;”
21. An electrician is installing an aerial communications circuit that runs partly outside a block. According to OSHA standards, what specific device must be provided on this circuit?
Answer: A listed primary protector. The regulation mandates a listed primary protector for aerial circuits not confined within a block.
29 CFR § 1910.308(e): “A listed primary protector shall be provided on each circuit run partly or entirely in aerial wire or aerial cable not confined within a block.”
22. An electrician is installing equipment in a Class I, Zone 2 location containing a specific flammable gas. They have a piece of equipment listed for a Zone 1 location of the same gas. According to OSHA rules, what is the correct action?
Answer: Install the Zone 1 listed equipment in the Zone 2 location. The regulation permits using equipment rated for a higher hazard zone (Zone 1) in a lower hazard zone (Zone 2) provided the gas type is the same.
29 CFR § 1910.307(g): “Equipment that is listed for a Zone 1 location may be installed in a Zone 2 location of the same gas or vapor.”
23. An electrician is installing a DC circuit for fixed equipment that requires grounding. Unlike AC circuits, which specific allowance does OSHA provide for the routing of the equipment grounding conductor?
Answer: It may be run separately from the circuit conductors. OSHA permits the equipment grounding conductor to be routed separately from the circuit conductors for DC circuits, whereas AC circuits generally require them to be contained together.
29 CFR § 1910.304(g): “For dc circuits only, the equipment grounding conductor may be run separately from the circuit conductors.”
24. An electrician is selecting a two-pole circuit breaker to protect a 3-phase, corner-grounded delta circuit. Under what condition is this specific breaker type permitted for this application?
Answer: Only if the circuit breaker is marked 1Φ—3Φ to indicate such suitability. Two-pole breakers are prohibited for this specific circuit type unless they carry the specific 1Φ—3Φ marking.
29 CFR § 1910.304(f): “A two-pole circuit breaker may not be used for protecting a 3-phase, corner-grounded delta circuit unless the circuit breaker is marked 1Φ—3Φ to indicate such suitability.”
25. An electrician is selecting a cable type for a factory assembly where one or more conductors are insulated with highly compressed refractory mineral insulation and enclosed in a liquidtight and gastight continuous copper or alloy steel sheath. Which cable type matches this description?
Answer: Mineral-insulated metal-sheathed cable (Type MI). The definition of Type MI cable specifically describes the use of highly compressed refractory mineral insulation and a liquidtight/gastight continuous metal sheath.
29 CFR § 1910.399: “Mineral-insulated metal-sheathed cable (Type MI). Type MI, mineral-insulated metal-sheathed, cable is a factory assembly of one or more conductors insulated with a highly compressed refractory mineral insulation and enclosed in a liquidtight and gastight continuous copper or alloy steel sheath.”