An EMC test setup that doesn't isolate the device under test from the building's mains ground reference can produce measurement results that reflect the building's wiring as much as the DUT's actual emissions or immunity performance. This is a subtler failure mode than most people expect from a "just plug it in" test setup.
The Uncontrolled Ground Path Problem
Every building's mains wiring has its own ground impedance characteristics, influenced by wiring length, connections, other equipment sharing the same circuit, and even building structure. When a DUT is connected directly to building mains without isolation, its ground reference — and therefore its conducted emissions and immunity behaviour — is partly determined by this uncontrolled, site-specific ground path rather than purely by the DUT's own design.
What Isolation Actually Fixes
An isolation transformer breaks this direct ground path, giving the test setup a controlled, known ground reference independent of the building's mains wiring characteristics. This matters specifically for:
- Conducted emissions measurement — where an uncontrolled ground path can introduce noise unrelated to the DUT
- Conducted immunity testing — where disturbances need to be injected against a known, controlled reference to get a repeatable result
- Repeatability between test sites — a test result should reflect the DUT, not the specific building it happened to be tested in
Isolation as a Safety Requirement, Not Just a Measurement One
Beyond measurement quality, isolation transformers are also a standard safety requirement when servicing energised equipment on the test bench — breaking the direct mains ground connection reduces shock risk when working on live circuits, independent of any EMC measurement consideration.
Sizing Considerations
Isolation transformer power rating needs to comfortably exceed the DUT's actual power draw, including inrush current on power-up for equipment with switch-mode supplies or large capacitive input stages — undersizing an isolation transformer is a common practical mistake that shows up as voltage sag exactly when the DUT is powered on.