Conducted Immunity Testing - Requirements & Equipment

Conducted Immunity Testing

Conducted immunity focuses on the impact that noise or interference has when transmitted via physical contact from the culprit to the victim circuit. The goal of the immunity testing is to evaluate the potential impact that the associated interference has on the function of the associated equipment under test (EUT). Testing is done for product reliability and certification purposes which can be required before the product can be brought to market.

These series of tests can vary by application as well as regulatory and manufacturer requirements, however will typically involve:

  • Transient Testing
  • Conducted RF Testing 
  • Voltage Dips Testing
  • ESD Testing

Transient Testing

Electrical transients are caused by a wide range of interference events and have the potential to cause impacts to the operation of electrical systems. These rapid changes in energy can be transmitted device-to-device or at the component level.

There are a variety of different causes of transients, some being internal and some external. It is estimated that between 60-80%(1) of surges, a common transient, are created within a facility as opposed to externally.

Transients are defined by their waveform and aim to replicate the underlying event causing the interference. These pulses can be compromised of voltage and current components and can vary by risetime, duration, oscillation as well as associated level. The can also be broken down into impulse transients and oscillatory transients, with impulse being the most commonly tested.

Electrical Fast Transients (EFT)

Electrical fast transient testing, also referred to as burst testing, is one of the most common immunity test that is conducted. These fast rise time, short-duration events, occur in quick succession and are commonly tested on both power and data lines. The most commonly standard for both certification and product reliability is IEC 61000-4-4 which is commonly referenced by more generic standards.

These pulses have a rise time and duration of 5us/50us and are not commonly synchronized when testing AC power. EFT waveform and Pulse Burst

Combination Wave Surges

Electrical surges in accordance with IEC 61000-4-5 have both a current and voltage waveform, being commonly referred to as combination wave surges. While both voltage and current surges are also tested individually, it is far less common. For the our discussion we will focus on the most common waveform within the standard, 1.2/50us voltage open-circuit and 8/20us short-circuit.

When discussing surges it is common to hear them described in a rise time and duration format voltage format and associated test level. This is useful when describing IEC 61000-4-5 as this standard includes various voltage waveform including 10us/700us etc.

Combination Wave voltage and current waveforms

Conducting Testing

Haefely Axos5 transient generator EUT setup

Testing for transients applies the waveforms to the cabling going towards the associated port which testing is being conducted on. This methodology is applied for both power on and power off tests as well as those for power, data, and communication ports. While devices like capacitive coupling clamps are used for testing data lines, coupling decoupling networks or CDNs are commonly used for power line testing.

A variety of criteria can be selected through the associated test generator or associated software including test level, coupling method, etc. It is important to note that all power-on testing must be done through the CDN otherwise damage to the test system will occur! This may require the use of an external power source going into the CDN depending on the EUT/DUT.

Conducted RF Immunity

Conducted RF immunity testing replicates exposure of equipment via the associated cabling to electric or magnetic fields where interference travels down power and data lines towards equipment. These levels of interference which travel down the lines vary in relation to the associated E-Field or H-Field to which the equipment is exposed.

When testing immunity to conducted RF noise or interference there are several injection methods as well as test methods that can be used. The most common, as well as the image shown below is using the substitution method with a coupling decoupling network (CDN) injection device.

Conducted RF Immunity test using CDN

Injection Devices

There are three main injection devices used in conducted RF testing, coupling decoupling networks, electromagnetic (EM) clamps, bulk current injection (BCI) probe. Each of the three offers unique benefits, however testing requirements will typically provide guidance on which is appropriate.

IEC 61000-4-6, one of the most common commercial standards, explicitly states a preference for CDNs over other injection devices. CDNs are not only the most effective injection method, but given their design limit the variability of level injection the associated cabling.

It important to keep in mind with the associated device that the corresponding calibration fixture will be needed. In the case of IEC 61000-4-6 50Ohm to 150Ohm adapters will also be  required during the calibration. How they're utilized during the calibration process can be seen in the associated video. 

RF Immunity Testing

Conducted RF immunity testing using the substitution method as per IEC 61000-4-6 includes first calibrating the associated test system allowing for the system and software to set associated factors for a given test level. When this process has been completed, the calibration factors are removed and associated cables placed through the CDN for testing to begin.

Conducted RF test setups for calibration and testing using CDN

Voltage Dips

Voltage dips are a result of faults in the local network, largely caused by short circuits. These events can be somewhat common and ensuring that products are not impacted by them is commonly done during immunity testing according to IEC 61000-4-11.

This standard places requirements on the number of cycles with the associated percentage reduction in voltage as well as rise/fall times. These levels are commonly 40%, 70%, and 80% of the nominal voltage. The images below demonstrate potential voltage dips on an AC sine wave.

Voltage dips on AC sine wave

Dips Testing

Dips test setup using variac

Dips testing typically involves external equipment separate from transient generators, usually variacs or a switch to meet the requirements of the standard. This switch is typically controlled via the transient generator and typically must be designed for that particular model of generator.   

Electrostatic Discharge Testing

Air & Contact Discharge Tips (NSG 435)

Electrostatic Discharge, commonly called ESD, we included with this grouping of immunity tests. Electrostatic discharge can be thought of as the rapid release of energy between two differently charged objects caused by the buildup of static electricity. This type of testing is done at both component level and system level, with varying different requirements for each.

ESD testing is defined by the voltage test level and current waveform as well as the resistance capacitance requirements. This is commonly 150pF/300Ohms as required by IEC 61000-4-2 one of the most commercial system level standards.

A series of pulses are applied to the EUT or DUT at regular intervals using either the air discharge or contact discharge method. These is achieved through different tips that can be attached to the front of the ESD simulator, which provides the discharge.

Immunity Testing FAQ

What is conducted immunity testing?
Conducted immunity testing is a subset of EMC testing where an equipment is subjected to a variety of interference transmitted via physical contact. 
What is conducted susceptibility testing?
Conducted susceptibility testing is another term used interchangeably with conducted immunity, often in military and avionic applications. 

References/Additional Information

1)What Are Surges | NEMA Surge protection Institute. (n.d.). Retrieved September 7, 2022, from https://www.nemasurge.org/history/

Immunity Test Equipment

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