LDHT Engineering, power system engineering consultancy

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Arc flash

Incident energy at each work location, arc flash boundary and suitable personal protective equipment (PPE).

The incident energy of an arc is proportional to its duration. At equal current, an arc cleared in 70 ms releases five times less energy than an arc cleared in 350 ms. The protection settings therefore also determine the risk for the person carrying out the work.

The calculation follows IEEE 1584 up to 15 kV. Above that, we apply the Lee method, which is more conservative, and we state it in the note. PPE categories follow NFPA 70E, in addition to your site rules and NF C 18-510.

For direct current (PV plants, batteries), the calculation methods are specific and we treat them separately.

01What we do

Calculation

  • Incident energy per switchboard and cubicle
  • Arc flash boundary
  • PPE category per work location
  • Direct current: PV and batteries

Risk reduction

  • Maintenance settings
  • Logic discrimination to reduce clearing times
  • Arc detection
  • Switchboard labelling

02Examples

Incident energy versus fault clearing time, 20 kV switchboard0.00.10.20.30.40.50.6020406080100Arc clearing time (s)Incident energy (cal/cm²)8 cal/cm²25 cal/cm²40 cal/cm²Ibf 12.5 kAIbf 8 kAarc detection: 6.9 cal/cm²51 time-delayed: 34.6 cal/cm²20 kV · working distance 910 mmLee method (conservative)
Fig. 01Arc in a 20 kV switchboard (8 kA). The incident energy is 6.9 cal/cm² when cleared in 70 ms by arc detection, and 34.6 cal/cm² with a protection time-delayed to 350 ms.Worked example
Time-current curves of two cascaded protections, 250 ms margin at maximum short-circuit current1001 00010 00020 0000.010.1110100Current (A, referred to 20 kV)Time (s)Isc max 8 kAP2 · MV feederIs 200 A · TMS 0.10P1 · TR incomerIs 400 A · TMS 0.30Δt = 250 ms
Fig. 02Two cascaded protections (IEC 60255, inverse curve), with a 250 ms margin at the maximum short-circuit current.Worked example

03Our offer

Usual content of an offer for this type of assignment. The exact scope, deliverables and price are set in the quotation.

  1. Package 1Data

    Short-circuit currents, clearing times, switchboard geometry.

  2. Package 2Calculation

    Incident energy and arc flash boundary per work location.

  3. Package 3Risk reduction

    Proposed maintenance settings, logic discrimination, arc detection.

  4. Package 4Labelling

    PPE table and labels in the chosen format.

04How we work

  1. 01Data

    Short-circuit currents and clearing times per switchboard.

  2. 02Calculation

    Incident energy, arc flash boundary, per work location.

  3. 03Reduction

    Maintenance settings, logic discrimination, arc detection.

  4. 04Labelling

    Labels and PPE table.

05References

Integrator, West Africa

9 MWp, 30 kV PV plant

LV, MV and DC arc flash: incident energies and PPE per work location.

Industrial site

Protection coordination and relay verification

MV protection coordination and on-site check of the settings.

All references

06Frequently asked questions

Which standard?

IEEE 1584 for the calculation, NFPA 70E for PPE categories, in addition to your site rules.

Can you reduce the incident energy?

Yes, by reducing clearing times through maintenance settings, logic discrimination, arc detection or differential protection.

07See also

For a project, give us the voltage, power, location and deadline in a few lines.
Reply within 48 working hours.

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