NFPA 70E, Maintenance and Switchgear Design Choices

What NFPA 70E asks of the owner

NFPA 70E, the standard for electrical safety in the workplace, requires an electrical safety program, a risk assessment procedure and training for qualified persons. Its default is that work is done in an electrically safe work condition: equipment de-energized, locked out and tagged out, tested to verify absence of voltage, and grounded where required. Energized work is permitted only when de-energizing introduces additional or increased hazards or is infeasible because of equipment design or operational limitations, and it requires documented justification.

For switchgear, anything requiring a person to open a door or remove a cover on energized equipment is therefore a decision, not a routine. Design choices that keep people outside the equipment reduce how often that decision arises.

Arc-flash risk assessment: two methods

The arc-flash risk assessment determines whether a hazard exists and, if so, the arc-flash boundary, the incident energy at the working distance and the protective equipment required. The incident energy analysis method calculates incident energy, usually with the IEEE 1584 model, from available fault current, protective device clearing time, electrode configuration, enclosure size and working distance. The arc-flash boundary is the distance at which incident energy falls to the level associated with the onset of a second-degree burn.

The arc-flash PPE category method uses tables that assign a PPE category to equipment types and tasks, valid only within the table's parameters for maximum fault current, maximum clearing time and minimum working distance. The two methods are not to be mixed on the same equipment. Either way, results must be reviewed periodically (NFPA 70E sets a maximum interval) and whenever a system change could affect them, and they are field-marked on the equipment.

Maintenance condition changes the numbers

The incident energy on the label assumes the upstream protective device clears the fault in the time its time-current curve predicts. A breaker with hardened lubricant, a weak trip coil or a mis-set trip unit clears later or not at all, and incident energy rises roughly in proportion to clearing time. NFPA 70E requires the risk assessment to consider the condition of maintenance of the protective devices and notes that improperly maintained devices may increase opening time and the hazard.

NFPA 70B, the standard for electrical equipment maintenance, provides the intervals, condition-based approaches and documentation that give a facility a defensible basis for the clearing times in its study. Documented breaker testing and lubrication, trip unit verification and relay testing are part of the arc-flash program, not separate from it.

Design choices that reduce exposure

In the NFPA 70E hierarchy, elimination and engineering controls rank ahead of administrative controls and PPE. Switchgear design offers engineering controls that either lower incident energy or keep people outside the arc-flash boundary during routine tasks. Each control should appear in the specification with the operating procedure that goes with it.

• Arc-resistant construction tested to IEEE C37.20.7, directing arc products away from the operator for the stated accessibility type
• Remote racking, which moves the operator outside the arc-flash boundary during the highest-risk breaker operation
• Remote operation of breakers from outside the boundary
• Maintenance-mode settings and the other NEC 240.87 arc energy reduction methods, which cut clearing time while a person is near the equipment
• Infrared windows and continuous thermal or partial discharge monitoring, allowing inspection without removing covers
• Drawout construction, allowing a breaker to be isolated in the test or disconnected position rather than worked on energized

Where the boundaries still apply

Engineering controls have conditions. An arc-resistant rating applies only with all doors closed and latched and the pressure relief path clear; open a door to rack a breaker manually and the assembly is conventional switchgear again. Maintenance-mode settings lower incident energy only while engaged and only if the label and procedure reflect the reduced value.

Verifying an electrically safe work condition still requires testing for absence of voltage on each conductor with an adequately rated tester or, where permitted, a permanently mounted absence-of-voltage tester listed for the purpose. Interlocks are not defeated to save time, lockout/tagout applies to every source, and PPE selected for the assessed hazard is worn until the safe condition is verified.

Bringing it together in a specification

A specification that supports the safety program requires the arc-flash and coordination study as a deliverable and labels per NFPA 70E and NEC 110.16 produced from it. It calls for the 240.87 method on each qualifying breaker, remote racking and operation provisions where the facility intends to use them, infrared windows or continuous monitoring, and arc-resistant construction with the accessibility type stated where the risk justifies it. It should also require commissioning tests that verify trip units and relays operate at the settings the study assumed, and a maintenance plan referencing NFPA 70B so those clearing times remain valid.