Phenolic Paper Tube Pressure Rating for Transformer Coils

What Is a Phenolic Paper Tube? Its Role in Transformers

Phenolic paper tube (also called insulation paper tube or bakelite paper tube) is a rigid laminated insulation tube made by impregnating electrical insulation paper with phenolic resin, then winding and hot pressing. Standard grades typically correspond to 3640 / PFCP22 series.

In transformers, phenolic paper tubes serve the following key functions:

Coil former and support: acts as winding skeleton and withstands radial pressure from winding tension

Interlayer insulation spacing: provides insulation barrier between HV and LV windings

Lead wire protection: prevents mechanical damage at conductor exit points

Mechanical spacing structure: forms oil ducts with insulation board for cooling in oil transformers

Application scope: This article focuses on phenolic paper tubes used in distribution transformers up to 35kV.

Pressure Strength ≠ Dielectric Strength — Two Different Concepts

Many engineers confuse mechanical pressure strength and dielectric withstand voltage, but they are completely different parameters.

Dielectric Strength

Represents the material’s resistance to electrical breakdown

Unit: kV/mm (breakdown field strength)

Test method: power-frequency withstand voltage test

Typical value for phenolic tube: ≥8 kV/mm

Mechanical Compressive Strength

Represents resistance to radial or axial compression

Unit: MPa

Typical value for phenolic tube: ≥80 MPa (axial)

Common mistake: “pressure rating” may refer to mechanical strength or dielectric withstand voltage.

Standard Requirements for Insulation Tube Voltage Withstand

Phenolic tube material properties follow GB/T 1303 and GB 7672, while transformer insulation testing follows GB 1094.3 and DL/T 573.

Transformer Rated Voltage	Induced Withstand Voltage	Applied Withstand Voltage	Recommended Wall Thickness
0.4 kV	1.0 kV	3.0 kV	1.5–3 mm
3 kV	6.5 kV	10 kV	3–5 mm
6 kV	12 kV	20 kV	5–8 mm
10 kV	20 kV	35 kV	8–12 mm
35 kV	70 kV	80 kV	12–20 mm

Main Performance Parameters of Phenolic Paper Tubes

Property	Unit	Standard Value	Notes
Thermal Class	–	Class E 120°C	Up to Class F 155°C
Dielectric Strength	kV/mm	≥8	12~15 after oil impregnation
Axial Compression Strength	MPa	≥80	–
Insulation Resistance	MΩ	≥100	–
Water Absorption	%	≤1.0	Keep dry

Phenolic Tube vs FR4 Tube vs Phenolic Cotton Tube

Comparison	Phenolic Paper Tube	FR4 Tube	Phenolic Cotton Tube
Thermal Class	120°C	155°C	120°C
Dielectric Strength	≥8	≥14	≥5
Mechanical Strength	Medium	High	Medium
Cost	Low	High	Medium

Selection Guide by Voltage Level

Confirm transformer type: dry-type or oil-immersed

Check rated voltage and insulation requirement

Calculate wall thickness based on dielectric strength

Verify thermal class meets temperature rise

Match inner diameter to winding process

Control moisture content and storage conditions

Common Failure Causes and Prevention

Failure	Cause	Solution
Electrical breakdown	Moisture or insufficient thickness	Drying and increase thickness
Cracking	Excess winding tension	Control winding tension
Insulation degradation	Thermal aging	Upgrade thermal class
Deformation	Poor storage	Vertical storage

FAQ

What is the difference between phenolic and FR4 dielectric strength?

Phenolic tube about 8kV/mm, FR4 above 14kV/mm.

Does thicker wall mean higher voltage withstand?

Material strength stays same, but total withstand voltage increases.

Can phenolic tubes be used in dry-type transformers?

Yes, but F-class or FR4 is recommended.

Typical size range?

Inner diameter φ10mm–φ300mm, wall thickness 1.5mm–30mm, customizable.