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Bus Bar Insulation Materials — Supporting, Spacing, and Isolating Bus Bars

Bus bar insulation serves two functions: mechanical support (resist electromagnetic forces during faults) and electrical isolation (prevent phase-to-phase and phase-to-ground arcing) — thermoset laminates are the dominant material class for both, with GPO-3 and G10 covering most of the market.

TL;DR — Key Takeaways

  • GPO-3 (glass-mat polyester, UL 94 V-0, CTI Group I) is the industry standard for bus bar support blocks and spacers in low-voltage switchboards
  • G10 (glass-epoxy) is preferred for medium-voltage bus bar insulation where high dielectric strength through-thickness is the primary driver
  • FR4 replaces G10 wherever UL 94 V-0 is required
  • Cotton-phenolic (CE) is used for low-cost mechanical bus bar clamps in legacy and budget installations (Class A, 600V dry)
  • Insulation sleeves (heat-shrink tubing or extruded PVC) cover bus bar surfaces; rigid thermoset provides structural support — the two functions are distinct

Bus Bar Insulation Requirements

Mechanical Requirements — Fault-Current Force Resistance

During a three-phase fault, bus bars experience large electromagnetic forces proportional to the square of the fault current. For a 65kA symmetrical fault rating system:

  • Fault forces on adjacent parallel bus bars can reach 2,000–5,000 lbf per meter of unsupported span
  • Bus bar support spacing (cleats/brackets) must be designed so the bar does not deflect enough to contact adjacent phases or ground
  • Support material must not creep or compress under sustained load, allowing gaps to close over time

Material requirement: High compressive strength (flatwise, perpendicular to load), low creep, good stiffness. G10 (compressive strength 40,000–50,000 psi) and GPO-3 (25,000–35,000 psi) both qualify.

Electrical Requirements — Insulation Coordination

Per IEC 60664-1 and ANSI/UL insulation coordination standards, bus bar insulation must provide:

  • Minimum creepage distance between live parts at different potentials (depends on CTI group of material and working voltage)
  • Minimum clearance (air gap) through air — does not depend on material CTI, only on working voltage and altitude
  • Adequate dielectric strength in the solid insulation path (through the support block or spacer)

Bus Bar Support Components and Materials

Support Blocks and Cleats (Molded or Machined)

Support blocks grip the bus bar and attach to the enclosure structure. Two manufacturing approaches:

Molded GPO-3: The most common for standard low-voltage bus bar supports. GPO-3 can be compression-molded into the cleat geometry in one step — no machining required. Low per-piece cost at volume.

Machined G10 or FR4: For medium-voltage applications and custom geometries, support blocks are machined from G10 or FR4 sheet or plate. Higher per-piece cost but superior electrical properties and precision.

Phase Spacers

Phase spacers maintain fixed separation between phases in a three-phase bus arrangement. They are loaded in compression and must:

  • Not creep under continuous compressive load (G10: < 1% creep at 5,000 psi, 100°C over 1,000 hr)
  • Provide electrical insulation across their thickness
  • Be machined to precise thickness to control phase spacing

Material: G10 or FR4 plate cut and bored to final dimensions. Standard thicknesses: 0.250″, 0.375″, 0.500″, 0.750″.

Bus Bar Sleeves and Wrapping (Non-Laminate)

For surface insulation covering bus bar flat bars:

  • Heat-shrink tubing (PVC, PVDF, XLPE): Applied over the bus bar surface; provides additional creepage path length; not a structural material
  • Tape-wrapped insulation (glass-polyimide, Mylar): For high-temperature applications
  • Extruded PVC sleeving: Low-cost; covers bus bar surface for accidental-contact protection (not for primary HV insulation)

These non-thermoset products complement rigid thermoset supports — they cover the bus bar surface, while the thermoset clamps provide the structural mechanical support.

Material Properties Comparison for Bus Bar Insulation

Fault Current Withstand — What the Support Must Resist

During a close-in fault, the instantaneous peak current can be 2.5× the symmetrical fault current. The electromagnetic force between parallel conductors is:

F = (2 × I² × L) / (d × 10⁷) (Newtons)

Where I = peak current (amps), L = conductor length between supports (meters), d = center-to-center conductor spacing (meters).

For a 65kA symmetrical fault (peak ≈ 163kA), 1-meter span, 100mm center-to-center:

  • F ≈ (2 × 163,000² × 1) / (0.1 × 10⁷) ≈ 53,000 N ≈ 12,000 lbf

This force is applied impulsively — the support must have sufficient strength and rigidity to prevent conductor deflection into adjacent phase. G10 (flexural strength 55,000+ psi, modulus 2,200,000+ psi) easily handles these loads in properly designed cleat geometry. GPO-3 (flexural 25,000–35,000 psi) requires more careful structural geometry for fault ratings above 65kA.

Request GPO-3, G10, or FR4 bus bar insulation with UL 94 documentation

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