• Zhejiang Xiegong Electric Power Equipment Co., Ltd

Key Explosion-Proof Design Points for Mine Underground High-Voltage Switchgear

Robust Flameproof Enclosure (Type 'd' - IEC 60079-1):

• Material & Construction: The enclosure housing arcing parts must be constructed from durable materials (typically heavy-gauge steel casting or welded steel) capable of withstanding the maximum internal pressure generated during an internal explosion without permanent deformation or rupture. Light alloys may be restricted or require specific approvals (especially in coal mines).

• Flame Path Design: Precise machining and tolerancing of all flanged joints (doors, covers, cable entries, shaft glands) are critical. The width (length), gap (clearance), and surface finish of these flame paths are meticulously defined by standards (IEC 60079-1, etc.) to cool escaping gases below the ignition temperature of the external atmosphere and prevent transmission of an internal explosion.

1. Pressure Relief & Containment:

• The enclosure must safely contain internal explosions and vent combustion products only through the designed flame paths.

• Pressure relief devices (like burst disks) may be incorporated in specific designs to manage exceptionally high pressures, but they must not compromise the flame path integrity or allow flame transmission. Their use requires careful design validation.

2. Interlocked Earthing & Isolation:

• Mandatory Interlocks: Mechanical and/or electrical interlocks must prevent access to live parts (opening doors/covers) unless the circuit is isolated and earthed (grounded). This is paramount for personnel safety during maintenance.

• Earthing Before Access: The design must ensure reliable earthing of all live conductors within the compartment before access is possible.

3. Internal Arc Fault Protection (IAC - Optional but Highly Recommended):

• Switchgear should ideally be rated for Internal Arc Containment (IAC - IEC 62271-200) in addition to flameproofing. This ensures the enclosure can withstand the thermal and pressure effects of a severe internal arc fault, directing plasma and hot gases away from personnel via controlled ducts or vents (often upwards), even if the event doesn't trigger an explosive atmosphere ignition. This is a critical layer of protection beyond basic flameproofing.

4. Enhanced Sealing:

• All penetrations (cable glands, control wiring entries, shafts) must use certified flameproof cable glands and sealing devices compatible with the enclosure's rating and cable type, maintaining the integrity of the flame path.

5. Component Derating & Thermal Management:

• Electrical components (breakers, busbars, CTs/VTs, contactors) must be significantly derated compared to surface use due to the harsher ambient conditions (higher temperatures, humidity, potential dust accumulation impacting cooling). Adequate ventilation or cooling (using certified Ex components if forced) is essential to prevent overheating.

6. Ingress Protection (IP Rating):

• A high IP rating (e.g., IP54, IP65) is essential to protect against dust and water ingress, which could cause tracking, short circuits, corrosion, or interference with flame paths. This is crucial in the wet and dusty mine environment.

7. Material Restrictions (Especially for Coal Mines - Group I):

• Aluminum alloy content in external enclosures is strictly limited to prevent incendive sparks from impact/friction.

• Non-metallic materials must have limited surface area, high resistance to tracking, and anti-static properties to prevent hazardous static charge accumulation.

• In potentially methane-containing atmospheres (Group I - Mines susceptible to firedamp):

8. Certified Components & Overall Certification:

• All internal components operating in the potentially explosive atmosphere zone (relays, heaters, auxiliary devices) must themselves be certified for the appropriate gas group/zone (e.g., Ex 'e', Ex 'i', Ex 'm') or be housed within the main flameproof enclosure or a separately certified sub-compartment.

• The entire assembly must undergo rigorous testing and receive certification (e.g., IECEx, ATEX, NEC 500/505) for the specific mining group/zone (Group I for methane, Group II for other gases/dusts) and Temperature Class (T-rating).

9. Clear Marking & Documentation:

• Permanent, legible marking must indicate the explosion protection type ('Ex db'), Gas Group (I for methane mines, IIA/B/C), Temperature Class (T1-T6), Ambient temperature range, Certification Body, and Certificate Number.

• Comprehensive documentation including installation, operation, and maintenance manuals specific to the explosion-proof features is mandatory.

Crucial Standards: Design and certification primarily follow IEC 60079 series (especially IEC 60079-0, -1 for Flameproof 'd'), IEC 62271-200 (for HV switchgear including IAC), and regional standards like ATEX Directive (EU), NEC Articles 500/505 (North America), or country-specific mining regulations (often incorporating or referencing IEC standards).

This list covers the fundamental explosion safety principles. Specific mine regulations and the nature of the hazardous atmosphere (coal/gas vs. metal/non-metal, dust presence) will dictate precise requirements. Internal Arc Fault Protection (IAC) is increasingly considered essential for personnel safety in high-voltage mining applications.