Why Partial Discharge Testing Needs a Shielded Lab
Why partial discharge measurement in cable, transformer and insulator testing is so sensitive to outside noise, and the role of a shielded lab.
3 min readNovember 3, 2026
A Cable Is an Antenna
However good a shielded room's walls are, every cable entering it can carry outside signals inside. A power cable is a conductor too; it brings radio-frequency noise collected outside into the room and radiates it again inside.
A power line filter passes current at mains frequency while stopping radio-frequency signals. The filter is mounted on the room wall and provides an unbroken bond between the point where the cable passes through the wall and the shielding.
Current and Voltage Rating
The first step in choosing a filter is defining the current and voltage the line will carry. Separate lines are planned for lighting, sockets, air conditioning and test equipment, each passing through its own filter. The filter's rated current is chosen above the line's continuous current.
Inrush currents from high-current devices at start-up must also be considered. Otherwise the filter may heat up or protective devices may trip unnecessarily.
Leakage Current and Safety
RF filters contain capacitors connected between the phase line and ground. These capacitors create a small leakage current. When several filters are connected to the same residual current protection, these currents add up and can trip the protection by mistake.
That's why filter selection is planned together with the building's electrical installation and residual current protection scheme. Bonding the filter body correctly to the shielding and to ground is essential for both performance and personal safety.
Common Mistakes
The most common mistake is routing the cable back outside the room after the filter, or adding an unfiltered line later. A single unfiltered cable can undo all of a room's shielding.
That's why a check measurement is recommended after every electrical change to the room. In our maintenance and modification service, we add new lines together with filter selection and measurement. For building-side preparation, see our building preparation article.
Frequently Asked Questions
Is a separate filter needed for every socket?
No. Filters are chosen per line; several sockets inside the room can be fed after one filter.
Do filters fail?
Correctly chosen filters last a long time. Overload and high temperature shorten their life, so they're chosen with a margin on the rating.
Are filters used for data cables too?
It's possible, but fiber optic links are preferred in most cases; fiber isn't conductive, so it carries no signal.
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