Why Saving 100 Milliseconds on Fault Clearing Time Is Worth More Than Any PPE Upgrade

The IEEE 1584:2018 empirical model calculates incident energy through a multi-step process, but the relationship with arcing time is approximately linear for the durations relevant to LV switchgear (100–2000 ms). The simplified relationship is:

E = C × t

Where E is incident energy (cal/cm²), t is arcing duration (seconds), and C is a constant determined by arcing current, electrode configuration, enclosure size, and working distance. The full IEEE 1584:2018 model uses intermediate variables (normalised incident energy, distance correction, enclosure correction), but the time dependency remains essentially linear.

This linearity has a profound practical implication: halving the clearing time approximately halves the incident energy. No other single variable in the arc flash equation offers this degree of leverage. Reducing arcing current requires changing the system design (transformer impedance, cable lengths). Increasing working distance requires redesigning the switchboard. But reducing clearing time requires only a protection coordination decision.

Consider a standard scenario: a 415 V main switchboard fed by a 1000 kVA transformer, with 25 kA prospective symmetrical short-circuit current. The switchboard is a box-type enclosure, 508 mm width × 508 mm height × 508 mm depth. Working distance is 450 mm (the IEEE 1584:2018 default for LV switchgear). Electrode configuration is VCB (vertical conductors in a box).

Using the IEEE 1584:2018 empirical model, the arcing current is approximately 12.8 kA. The incident energy varies with clearing time as follows:

The relationship is starkly clear. At 100 ms clearing time, an electrician can work on this board in a long-sleeve arc-rated shirt. At 2 seconds, the board requires a full arc flash suit — or is labelled “DANGER — Do Not Operate” and requires the circuit to be de-energised for any work.

The difference between 100 ms and 2000 ms is typically the difference between a current-limiting MCCB (which clears in half a cycle, approximately 10 ms) and a standard thermal-magnetic breaker operating in its long-time delay region (which may take 1–2 seconds for a fault current well above the overload setting but below the instantaneous trip).

The arc flash clearing time is determined by the protection device upstream of the fault location and its time-current characteristic at the arcing current. Three engineering decisions directly control this:

1. Instantaneous Trip Setting

If the arcing current (12.8 kA in our example) is above the instantaneous trip threshold of the upstream breaker, clearing time is one half-cycle (approximately 10 ms for 50 Hz systems). Incident energy drops to less than 1 cal/cm². The key decision: set the instantaneous trip low enough to catch arc faults, while maintaining discrimination with downstream devices.

2. Zone-Selective Interlocking (ZSI)

ZSI allows upstream breakers to trip in their instantaneous region when no downstream breaker has detected the fault. This preserves discrimination during normal downstream faults while providing fast clearing for faults at the main switchboard. ZSI can reduce clearing time from 300–500 ms (time-delayed for discrimination) to 50–100 ms (instantaneous when ZSI confirms fault is local).

3. Arc Flash Detection Relays

Dedicated arc flash relays (e.g., ABB REA, Schneider Vamp) detect the optical flash and overcurrent simultaneously, tripping the upstream breaker in 5–50 ms. This is the fastest available clearing method and can reduce incident energy to below 1 cal/cm² for any realistic LV scenario. The cost is $2,000–$5,000 per switchboard — less than a single arc flash suit and incomparably less than a burn injury.

The hierarchy of controls in IEEE 1584:2018, Clause 4 and in workplace safety regulations globally places PPE at the bottom — the last resort when engineering controls are not feasible. Yet in practice, many facilities accept high incident energy levels and simply specify higher PPE categories.

This approach has three critical flaws:

• PPE compliance is imperfect. An arc flash suit in a locker does not protect the electrician who opened the switchboard “just to check the voltages” without donning it. Engineering controls protect everyone, every time, automatically.
• PPE has an upper limit. Category 4 PPE is rated to 40 cal/cm². Above this level, the area is labelled DANGER and work is prohibited while energised. A facility with 50 cal/cm² incident energy cannot solve the problem with better PPE — it must reduce the energy through engineering changes.
• PPE degrades work quality. An electrician in a full arc flash suit with a hood and gloves has severely restricted vision, dexterity, and thermal comfort. Troubleshooting, testing, and switching operations take longer and are more error-prone. The PPE that protects against one hazard increases the probability of other errors.

Every calorie per square centimetre reduced through faster clearing time is a calorie that does not need to be stopped by fabric against skin.

For a 10-switchboard industrial facility, the comparative costs are instructive:

The engineering solution (ZSI or arc flash relays) is cheaper over 10 years than the PPE program it replaces. And the engineering solution eliminates the risk entirely, rather than attempting to contain its consequences against human skin.

This is before considering the cost of a single arc flash injury. The average US workers’ compensation claim for an arc flash burn exceeds $1.5 million. Hospital stay, rehabilitation, lost productivity, OSHA fines, legal costs. A single injury exceeds the entire 10-year cost of protecting every switchboard in the facility.

ECalPro’s arc flash calculator implements the full IEEE 1584:2018 empirical model, including all electrode configurations (VCB, VCBB, HCB, VOA, HOA) and enclosure correction factors. The sensitivity analysis feature allows engineers to sweep clearing time from 10 ms to 2000 ms and visualise the incident energy curve, with PPE category boundaries overlaid.

The practical workflow is:

1. Enter system parameters: voltage, prospective fault current, electrode configuration, enclosure dimensions, working distance
2. Enter the current protection clearing time (from the breaker TCC at arcing current)
3. View the incident energy and PPE category
4. Use the sensitivity slider to explore how clearing time changes affect the result
5. Identify the clearing time threshold that achieves the target PPE category
6. Take that threshold back to the protection coordination study and select a device that achieves it

This closes the loop between arc flash assessment and protection coordination — two studies that are traditionally performed by different teams at different project stages, but are physically inseparable.

Standards referenced: IEEE 1584:2018 (Clauses 4, 5, and Annex D), NFPA 70E:2024 (Table 130.5(C)), IEC 60909-0:2016 (fault current inputs).