Shenhai Explosion-Proof
In explosion-proof electrical applications, insulation materials are differentiated into solid and liquid types, tailored specifically for these applications, unlike broader insulation categories.
Solid Insulation Materials
Referred to as “solid-state insulation materials,” these are substances that remain solid under operational conditions. This category includes materials like insulating varnish, which are initially liquid but solidify upon application.
Commonly utilized solid insulation materials in explosion-proof electrical equipment are listed below.
| Material grade | Compared to the Tracking Index (CTI) | Material name |
|---|---|---|
| I | 600≤CTI | Ceramics (glazed), mica, glass |
| II | 400≤CTI＜600 | Melamine asbestos arc plastic, silicone organic asbestos arc resistant plastic, unsaturated polyester aggregate |
| III-a | 175≤CTI＜400 | Polytetrafluoroethylene plastic, melamine glass fiber plastic, epoxy glass cloth board treated with arc resistant paint on the surface |
| III-b | 100≤CTI＜175 | Phenolic plastic |
These materials are graded based on their Comparative Tracking Index (CTI), a measure of superficial electrical performance. However, their mechanical, thermal, and chemical properties can vary significantly, necessitating careful selection based on the specific environmental conditions of use, including considerations for mechanical strength, heat resistance, and chemical durability.
Ceramic (Glazed) Materials
Comprising inorganic non-metallic insulation substances, these are formed by sintering metal oxides and non-oxygen metal compounds. Their attributes include a hardness range of 1000~5000HV, tensile strength from 26~36 MPa, compressive strength from 460~680 MPa, melting points exceeding 2000°C, low thermal expansion, and high chemical stability and resistance to corrosion.
Polytetrafluoroethylene (PTFE)
This fluoroplastic material sustains long-term usage across temperatures from -180°C to 260°C. It’s highly chemically stable, resistant to corrosion, exhibits a low friction coefficient, and has a significant thermal expansion coefficient.
Phenolic Plastic
A thermosetting plastic, known commercially as “bakelite” or “phenolic board,” it can withstand temperatures exceeding 3000°C and offers excellent burn resistance and chemical stability, although it is brittle and not resistant to alkali corrosion.
In addition to the solid insulation materials mentioned, explosion-proof electrical equipment extensively utilizes various solid insulating substances, including plastic materials for insulating components and certain auxiliary materials in explosion-proof motors.
Liquid Insulation Materials
These refer to insulating substances typically found in liquid form, like transformer oil, and materials like insulating varnish used for coil impregnation, which solidify after specific treatments yet are still considered liquid insulators.
1. Transformer Oil
• Essential for explosion-proof electrical equipment such as transformers, this oil must meet specific standards:
• Ignition point not below 300°C.
• Flash point not below 200°C (closed cup).
• Kinematic viscosity not exceeding 1*10?? m²/s at 25°C.
• Dielectric breakdown strength at least 27kV.
• Volume resistivity at least 1*10??? m at 25°C.
• Pour point not above -30°C.
• Acidity (neutralization value) up to 0.03 mg/g (potassium hydroxide).
Transformer oil, primarily a mineral insulating oil comprising alkanes, cycloalkanes, and unsaturated aromatic hydrocarbons, offers excellent insulating qualities and aging stability. However, its use in Class I mining equipment is restricted due to the potential degradation of its insulating properties over prolonged use.
2. Varnish
Utilized for impregnating electrical coils in explosion-proof equipment, insulating varnish improves their electrical insulation capabilities. Available in solvent-based and solvent-free forms, these varnishes are made up of natural or synthetic resins combined with various solvents like benzene and alcohols for the solvent-based type, and synthetic resins, solidifying agents, and active thinners such as styrene for the solvent-free type.
Both types of varnish offer multiple options to suit different application needs, ensuring adaptability to specific operational requirements.