Potting compound for high-voltage components
At high voltage, the potting system determines breakdown, partial discharge, and service life. The biggest enemy is air inclusions – even the smallest cavities can be the starting point for gradual failure. This article shows what really matters in terms of material and processing.
To the point
Choose an elastic, partial discharge-resistant silicone with sufficient dielectric strength and encapsulate it completely bubble-free (vacuum). Ensure adequate creepage distances and pay attention to the field conditions at electrode edges.
Why high voltage has different rules
Partial discharge in cavities
Partial discharges begin in cavities and microbubbles as soon as the local electric field strength exceeds the inception voltage. Because the dielectric constant of air is lower than that of the surrounding material, the field becomes intensified precisely within the cavity – this is where erosion begins, degrading the material over hours of operation until breakdown occurs. These failures often only appear after hundreds or thousands of hours in the field. Assessment is carried out via partial discharge measurement according to IEC 60270 ; therefore, the absence of partial discharge is a prerequisite, and a cavity-free encapsulation is mandatory.
Dielectric strength and dielectric
The dielectric strength of solid insulating materials is determined according to IEC 60243. Materials with high dielectric strength and sufficient layer thickness withstand the field load – the required wall thickness results from the voltage and safety factor.
Creepage distances and degree of contamination
Air and creepage distances, as well as the pollution degree, are governed by IEC 60664-1. A dense, full encapsulation achieves pollution degree 1, thus allowing for significantly more compact high-voltage installations. The tracking resistance of the surface is described by the reference number CTI according to IEC 60112.
Field heightening at edges
Sharp electrode edges locally increase the field strength and are frequent starting points for partial discharge. A completely filled, well-wetting potting compound fills critical gaps and mitigates field peaks; if necessary, additional field-controlling measures are implemented.
Heat and high voltage together
High-voltage transformers are subject to both heat and field stress – the choice of materials must accommodate both. The article on inductive components this selection process.
Material selection and products
Elastic, low-stress, and partial-discharge resistant silicones form the basis – pourable enough to completely fill gaps. Suitable examples include SILISIL RTV MF-Flex 20 , PC-Flex 20 , or PRO-Cast 45 ; for particularly sensitive, tightly packed assemblies, a soft MD-Soft 10 is recommended. From the Elkem range, the flowable BLUESIL RTV 3132 and the high-strength, clear BLUESIL ESA 7250 are suitable .
Processing: everything is free of voids
At high voltage, vacuum degassing is not optional, but the crucial step – every remaining air pocket is a potential source of partial discharge. Pour slowly and in one spot, adhere to the pot life, cure completely according to the data sheet, and – where possible – verify the absence of partial discharge on the finished part (step-by-step instructions, avoid errors). Calculate the fill quantity using the potting and mold making calculator.
Frequently Asked Questions
Why does a high-voltage assembly only fail after months? This is typical of partial discharge: An initially invisible cavity slowly erodes the material until the remaining thickness breaks through. Therefore, being free of air bubbles from the start is crucial.
Hard or soft for high voltage? Elastic silicones are usually superior because they remain low-stress and reliably fill gaps; what matters less is the hardness than the absence of voids and the dielectric strength.
Is potting without a vacuum sufficient? At high voltage, no – without degassing, microbubbles remain, which act as partial discharge sources.
Consultation and samples
Tell us about the voltage level, system configuration, and environmental conditions – we'll recommend the appropriate, low-partial-discharge type and provide a sample. Contact us or write to info@silitech.ch.
