Mitigating arc flash hazards in medium-voltage switchgear

Overview

Peeran's article explains the IEEE 1584 arc-flash calculation for medium-voltage gear. An important point is why arcing current approaches bolted fault current at 4.16 and 13.8 kV: at these voltages arcing current is about 90 percent of bolted fault current, whereas at 480 V it is much lower because of an arc drop of about 200 V. The article also quantifies the effect of mitigation measures.

The validity range of IEEE 1584 is 208 V-15 kV, 700-106,000 A bolted fault current and 0.5-6 in. gaps. Working distances are 36 in. for 5/15 kV gear, 24 in. for LV switchgear and 18 in. for MCCs and panels. MV arcing current follows lg(Ia) = 0.00402 + 0.983 lg(Ibf), and the calculation factor Cf is 1.0 above 1 kV and 1.5 at or below.

Mitigation methods covered include smaller high-impedance transformers, differential protection, current-limiting fuses, relay maintenance modes, light-sensing arc-flash relays, arc-resistant switchgear, crowbar devices and remote operation. Replacing a 3,000 kVA 6%Z transformer with a 1,500 kVA 8%Z unit cut fault current from 6,940 A to 2,603 A and incident energy by 62 percent.

In relay maintenance mode, lowering the instantaneous pickup from 16,000 A to 6,000 A cut clearing time from 1.303 s to 0.015 s and energy from 12 to 1.2 cal/cm2 (HRC 3 to 1). Current-limiting fuses clear within a half cycle (under 0.0083 s). Crowbar devices are covered by IEC 62271-200, with no US standard yet. The article dates from 2013 and rests on IEEE 1584-2002, so later editions may differ.

Key points

1. IEEE 1584 validity: 208 V-15 kV, 700-106,000 A bolted fault, 0.5-6 in. gaps; working distance 36 in. for 5/15 kV gear, 24 in. LV switchgear, 18 in. MCCs and panels.
2. At 4.16-13.8 kV arcing current is about 90 percent of bolted fault current; at 480 V it is much lower due to a roughly 200 V arc drop.
3. MV arcing current: lg(Ia) = 0.00402 + 0.983 lg(Ibf); calculation factor Cf = 1.0 above 1 kV, 1.5 at or below.
4. Replacing a 3,000 kVA 6%Z transformer with 1,500 kVA 8%Z cut fault current from 6,940 A to 2,603 A and incident energy by 62 percent.
5. Relay maintenance mode: lowering instantaneous pickup from 16,000 A to 6,000 A cut clearing from 1.303 s to 0.015 s and energy from 12 to 1.2 cal/cm2 (HRC 3 to 1).
6. Current-limiting fuses clear within a half cycle (under 0.0083 s); crowbar devices are covered by IEC 62271-200 with no US standard yet.

Using it in design

• Implement IEEE 1584 working distances per equipment class in the MV arc-flash module.
• Allow maintenance-mode relay settings as a scenario.
• Suggest transformer size and impedance alternatives when secondary incident energy is high.
• Show mitigation options such as current-limiting fuses, differential protection and light-sensing relays side by side.

Who should read it

Electrical engineers designing medium-voltage switchgear and protection, and consultants carrying out arc-flash studies.

Related standards

• IEEE 1584-2002
• NFPA 70E
• NFPA 70 Art. 110.16
• IEC 62271-200

Publication details

Authors

Syed M. Peeran

Publisher

Consulting-Specifying Engineer

Year

2013

Document language

English

Source type

Expert / community

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation