LISNs - Three Phase

Three-Phase Line Impedance Stabilization Networks (LISNs)

Com-Power’s three-phase LISN portfolio provides a complete range of four-conductor Line Impedance Stabilization Networks for conducted emissions testing of three-phase equipment. With twelve models spanning three series—LI-3P-1X, LI-3P-2X, and LI-3P-4X—these LISNs cover commercial (CISPR 16-1-2, ANSI C63.4) and military (MIL-STD-461) standards at current ratings from 16 A to 100 A per phase. All models support both Delta and Wye power configurations, feature remote fiber optic line switching via the RLI V2.0 interface, built-in transient limiters, and air-core inductors that eliminate magnetic saturation.

How a Three-Phase LISN Works

A Line Impedance Stabilization Network (LISN), also called an Artificial Mains Network (AMN), is a measurement device placed in series between the AC power source and the Equipment Under Test (EUT) during conducted emissions testing. In a three-phase setup, the LISN contains four independent stabilization networks—one for each of the three phase conductors (L1, L2, L3) and one for the neutral (N). Each network performs three critical functions simultaneously:

1. Provides a defined, stable impedance to the EUT. The real-world impedance of a power grid varies unpredictably based on wiring length, connected loads, and time of day. A LISN replaces this unknown impedance with a precisely controlled 50Ω impedance across the measurement frequency range. This ensures that conducted emissions measurements are repeatable and comparable regardless of the power source or facility. The 50 μH inductor in the network simulates the typical inductance of approximately 50 meters of building supply wiring.

2. Isolates the EUT from external noise. The power grid carries RF noise from other equipment connected to the same supply. The LISN’s inductor presents high impedance at RF frequencies, blocking this external noise from reaching the EUT measurement point. This ensures that only the emissions generated by the EUT itself are measured, not interference from the power grid.

3. Couples disturbance voltages to the measurement port. The LISN provides a 50Ω RF output port (N-type connector) that extracts the conducted emissions from each power line and delivers them to the EMI receiver or spectrum analyzer. A coupling capacitor blocks the low-frequency AC power voltage while passing the higher-frequency RF emissions to the measurement instrument.

In a three-phase LISN, one line is measured at a time. The selected line is routed to the 50Ω measurement port, while the remaining lines are internally terminated into 50Ω loads. This maintains the correct impedance on all lines simultaneously, which is essential because conducted emissions on one phase can be affected by the termination on the other phases.

Three-Phase LISN Configuration – The LISN is placed between the power source and EUT, with each phase conductor (L1, L2, L3) and neutral (N) passing through an independent 50Ω/50 μH stabilization network. The selected line is routed to the RF measurement port while unselected lines are terminated into 50Ω.

Three Series for Every Standard

Choosing the correct series depends on the applicable test standard. Each series uses a different LISN network topology and frequency range as required by the relevant standard. The three series are not interchangeable.

Series Comparison

Typical Three-Phase Equipment Tested

Three-phase LISNs are required whenever the EUT draws power from a three-phase supply. Common categories of three-phase equipment that require conducted emissions testing include:

Common Features Across All Series

Regardless of the series selected, every Com-Power three-phase LISN includes: