Bakelite is the plastic to use for electrical insulation: the scratch carrier gives it a volume resistivity above 10^10 ohm-centimetres and a dielectric strength of 10-15 kV per millimetre, adequate for power and signal work up to several kilovolts. Those are figures for dry, flawless material, and real parts fail from moisture, cracks, voids and conducting surface films long before bulk breakdown, so test every insulator and never use celluloid or casein.
Part of Plastics and Polymers
Plastic electrical insulation is the use of non-conducting polymers to separate and support conductors in wire, switches, terminals and coils.
Why Insulation Is The First Job For Plastics
Plastics from Scratch calls electrical insulation the single most important application of early plastics: without an insulating material there is no insulated wire, switch body, terminal block, capacitor dielectric or transformer bobbin.
The Three Early Plastics Compared
| Plastic | Verdict | Reason |
|---|---|---|
| Bakelite | Use | Heat-resistant, insulating, mechanically strong |
| Celluloid | Never | Burns violently; unsuitable near heat or arcing |
| Casein | Never | Absorbs moisture, so it cannot insulate |
The shellac article in the corpus gives roughly 30 kV per millimetre for dry shellac, another useful insulator for coatings: Shellac. Porcelain insulators for line voltage are a different subject: Electrical Insulators covers ceramic.
Worked Example: What Resistivity And Dielectric Strength Mean
Leakage. Resistance of a block R = ρ × L / A. Take ρ = 10¹⁰ Ω·cm (the lower bound), length L = 1 cm and area A = 1 cm², so R = 10¹⁰ × 1 / 1 = 10¹⁰ Ω, 10 GΩ. At 240 V the leakage current is I = V / R = 240 / 10¹⁰ = 2.4 × 10⁻⁸ A = 24 nA. A tiny current, as long as the material is dry and clean.
Breakdown. A wall 2 mm thick at 10-15 kV/mm withstands 2 × 10 = 20 kV up to 2 × 15 = 30 kV, in the ideal case. A real part is far lower, and the corpus gives no derating factor, so decide the margin by testing, not by this multiplication.
How Real Insulators Fail
- Moisture. Water in or on the surface drops the resistance by orders of magnitude. Keep parts dry and sealed with a varnish where needed.
- Surface films. Dirt, salt and condensation make a conducting path across the surface even when the bulk is perfect. Long surface paths between conductors help; thickness does not. For this reason designs add grooves or ribs to lengthen the path (general knowledge).
- Cracks and voids. Gas pockets from a bad cure or trapped air break down first.
- Carbonised tracks. Phenolic plastics can carbonise under surface arcing, leaving a conducting track that gets worse, so keep arcing off them (general knowledge).
- Heat. Resistance falls as the part warms. The plastics manufacturing carrier gives about 300 °C as the limit for Bakelite, a point at which it decomposes, not a service temperature.
Test Before You Trust
Follow Insulation Materials for how to test aged or improvised material. Visual inspection is not enough: cracks and moisture are invisible. Test dry, then after exposure to the damp conditions the part will see. Safe test practice is in Electrical Safety.
Common Mistakes
- Using the dry bulk figure as a design value.
- Making a thin insulating wall and ignoring the surface path.
- Using casein or celluloid because they were easier to make.
- Leaving unfilled or uncured parts in service.
Failed insulation kills
A shock from failed insulation can be lethal, and a faulty insulator near heat can start a fire. Test every insulating part before it carries power, keep it dry, and never use celluloid, which burns where arcing occurs, or casein, which absorbs moisture. Isolate the supply before touching any conductor.
What’s Next
- Learn next: Insulation and Tools — insulation in practice
- Combine with: Lacquers and Coatings — varnish for sealing and wire