What Is Diamond Dotted Paper (DDP)?

Published 27 Aug 2026Updated 27 Aug 20265 min readIntermediateInsulation
Quick answer — Diamond dotted paper is a thin electrical insulating paper coated with a thermosetting resin (usually a cresylic/phenolic or epoxy type) applied in a diamond pattern and partially cured. When the winding is heated, the resins bond the layers together, adding strength and holding shape — it is used mainly for interlayer/interturn insulation and as a bonding sheet in transformer coils.
Contents
What is diamond dotted paper (DDP) insulation?

What DDP is

It is a kraft paper that has been treated with a resin applied as a dotted or diamond pattern and then B-staged (partially cured). Making the dots rather than a full coat leaves the paper permeable to the impregnation oil and lets it conform during winding.

Its function

The resin bonds adjacent conductors and insulation on curing, giving a solid, vibration-resistant winding that holds its geometry and improves short-circuit robustness. It also adds some dielectric margin between turns/layers.

Compared with pressboard

DDP is a thin, self-adhesive sheet used inside the coil; pressboard is a thicker, denser structural board for barriers and supports. They are complementary rather than substitutes.

References & standards

Related

Book seriesAcademy

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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