EMC Question of the Week: December 5, 2022
An LISN (or AMN) used for conducted emissions measurements always measures the voltage between a power input and
- a power input return
- circuit ground
- system ground
- the earth
Answer
The best answer is “c.” LISN is an acronym for Line Impedance Stabilization Network. AMN is an acronym for Artificial Mains Network. Both devices are used to measured noise voltages conducted on power lines coming from the device-under-test (DUT). Conducted emissions voltage measurements are always referenced to the system ground.
In some (but not all) applications, the system ground is connected to the earth. In some (but not all) applications, the system ground is connected to the circuit ground. And, in many low-voltage applications, power currents can return on the system ground. Nevertheless, it would be wrong to state that conducted emissions voltages are ALWAYS measured relative to power returns, circuit grounds, or the earth.
Aerospace systems, medical electronics, and systems powered by voltages greater than 50 volts generally have power returns that are isolated from ground. In these systems, the power return is treated like an additional power phase and two measurements are required. The noise voltage on both power phases relative to system ground must be below the limit to meet the conducted emissions requirement.
In many systems, circuit ground is either isolated or connected to the power return conductor. In these systems, conducted emissions measurements are always made relative to the system ground, not the circuit ground.
Automotive and aerospace systems do not usually have an earth ground, but they still must meet conducted emissions requirements. [The measurement set-up may have an earth ground for safety reasons, but it doesn't affect the measurement.]
And what about systems that don't have a system ground? Well ..., when designing to meet EMC requirements, it's pretty important to identify the system ground (even when nothing in the system carries that label). If you're having trouble finding your system ground, look at what's connected to the non-powered side of the LISN.
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