Choosing Power Line Filters: Current, Leakage and Safety
How the RF filters on every power line entering a shielded room are chosen, leakage current and safety issues, and common mistakes.
3 min readNovember 6, 2026
A Shielded Cable Alone Isn't Enough
A shielded cable keeps outside fields out and internal signals in thanks to the braid or foil around the conductors. But this protection depends on the shield being correctly bonded at both ends.
The most common mistake is connecting the shield with a single wire, a short lead called a pigtail. At high frequency this thin wire behaves like an inductor and keeps shield current from reaching the enclosure. The shielded cable ends up behaving almost like an unshielded one.
360-Degree Termination
The correct method is to bond the shield around its full circumference to the metal enclosure through the connector shell or a cable gland. This is called 360-degree termination and provides a low-impedance path at high frequency.
The same principle applies in shielded rooms: shielded cables passing through the wall must be bonded circumferentially with a feedthrough on the panel. Otherwise the cable punches a hole in the room's shielding.
When Do Ferrites Help?
Ferrite rings suppress common-mode current on a cable. This current, which makes the cable act like an antenna, is the main cause of emission problems especially between 30 MHz and a few hundred MHz.
A ferrite can quickly get a failing product through a test, but the permanent fix usually lies on the board or connector side. When choosing a ferrite, check that its impedance curve covers the problem frequency and consider the effect of passing the cable through the ring more than once.
Cables Entering a Shielded Room
Shielded rooms prefer fiber optic links wherever possible, since fiber isn't conductive and carries no signal. Where copper is needed, power line filters or shielded feedthrough panels are used.
To spot cable problems on the product side early, conducted immunity testing and radiated emission pre-tests are good indicators.
Frequently Asked Questions
Should the shield be bonded at one end or both?
For high-frequency protection it's usually bonded at both ends. Special cases with low-frequency ground loop problems use different solutions.
Is foil or braid better?
Foil gives full coverage at high frequency, braid offers low impedance and mechanical strength; the best result comes from using both.
Where on the cable should a ferrite go?
Usually as close to the device entry as possible, so noise is suppressed before it spreads along the cable.
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