Case study: radiated interference to industrial controller
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As an EMC consultant, I seem to be running into more and more issues with ESD and radiated susceptibility. I believe this is due to the fact noise margins are gradually being reduced as supply voltages move from 5 to 3.3 to 1.8 to 1.2 volts. In addition, IC chips are scaling down in size, and quite frankly, designers still don’t understand basic EMC design principles, as I wrote up recently in an editorial for Interference Technology’s 2014 Test & Design Guide

Generally, the first thing I like to do is to sniff around with a near field probe and current probe to get a feel for any radiated emission issues. Finding nothing major, the project engineer demonstrated how he could affect the controller using just a Family Radio Service (FRS) walkie talkie from about 10 feet away. I recently measured a typical FRS radio at a 1m test distance and it read about 2V/m. Using Equation 1, at 3m (about 10 feet), we’re talking just a 1.3 V/m field strength, where I’m assuming the actual power output from the FRS radio is 0.25W, the antenna gain is 0.7 and the distance is 3m.

We actually performed most of the testing using that FRS radio. Initially, though, the resolution using the radio was too coarse, so a near field probe was connected to an RF generator, tuning it to one of the failing frequency bands (Reference 6). By probing around, we narrowed the issue down to one of several cables running through a mechanical arm on the machine.

A shielded box with several cables running through grommets. Penetrating a shield with a cable without terminating the shield allows RF interference into the enclosure.

we discovered the designer had failed to connect the cable shield! Once the cable shield was bonded to the chassis structure at both ends, the controller was completely immune to RF signals.

It’s my experience that many designers seem unsure how and where to connect cable shields. I’m simplifying somewhat, but connecting the shield at one end provides a good E-field shield. Connecting it at both ends (to the same structure) provides a good H-field shield. Most digital circuitry relies on low impedance, low voltage switched currents. Therefore, it’s more important to shield for the resulting H-fields. On the other hand, things like switch mode power supplies utilize high impedance with switched high voltages and so E-field shielding is a practical solution. Additionally, connecting each end of the shield to two differing potentials – for example, one end to digital return and one end to chassis – can introduce a potential difference which can inject high frequency switching noise into the signal wires.

There’s an additional point to be made regarding cable shields and that is the type and quality of the shielding material. Some less expensive cables use loosely formed shielding, with distributed gaps along the length. These should be avoided, due to poor shielding effectiveness. More expensive cables have a tighter weave on the shield with correspondingly better shielding performance.

In conclusion, it turns out that most of the client projects in which I’ve been involved that fail one, or more, EMI tests are due to basic design issues, such as poor routing of clock traces, penetration of I/O cables through shielded enclosures, and poor termination of cable shields. For more on shielding and bonding, check the references.

Ask any EMC engineer to name the most common problems associated with system-level emissions and all of them will include poor cable shielding and terminations. Often, the poor terminations occur because someone takes a cable’s shield, twists it into a single wire (often called a “pigtail”), and connects it to “ground.” Let the emissions begin.

Since the dawn of time, individuals have known that they need to protect themselves from lightning. In the beginning, humans were only concerned with protecting themselves. As time went on and infrastructures were constructed, it became evident that those things needed to be protected, too. Through trial and error, society figured out how to design and construct lightning rods that could take the energy generated from lightning and harmlessly return it to the earth.

A balancing act

With the advent of DOCSIS
3.1, companies not only have to be concerned that the grounds in hubs and headends are adequate in the sense that they meet the absolute ohm specification of the ground for safety of people and protection of property, but also that the various elements of that ground are balanced. That means that they must make sure that each of the various metallic “runs” that make up the ground have the same resistance.

Elements of the hub and headend ground

There are various elements that make up the hub and headend ground, including shelves that are bonded to racks with screws and wires, racks that are bonded together to make aisles, as well as aisles that are bonded to bus bars.

Why balance is important

Balancing the elements of a ground is always important because of the antennas that imbalances create for RF. But we had enough power difference between the signal and the noise to more-or-less harmlessly “absorb” the noise. What changed?

Because of potential energy coming into the plant, when we go from 64-QAM (quadrature amplitude modulation) to 256-QAM channels, we need to lower our noise floor by 3 dB just to stay even with MER (modulation error rate) and BER (bit error rate). Bonding up to 32 of these channels adds to the potential for interference, for noise.

By balancing the ground circuits, noise is reduced. Lab experiments and tests in actual hubs confirmed that if an unbalance of 0.8 ohms in the ground circuits can be reduced to 0.3 ohms, the noise floor in the 5 MHz to 50 MHz spectrum can be reduced by 8 dBmV.

How to determine if grounds are balanced

We cannot measure the resistance value of a ground at a shelf or similar place in a hub or headend. We can, however, easily measure and compare the continuity and balance of the various ground circuits of a hub or headend.

Fixing the balance

Daisy-chained ground circuits can be changed to home runs if the headend and hub grounds are not balanced.

It’s conventional wisdom that a solid, continuous return path provides a better result in electromagnetic compatibility (EMC). This article discusses the relationship between return path discontinuities and EMC.

A quality signal channel has a nice, uniform trace and a continuous return path from driver to receiver. Disruption to the return path introduces noise, and is typically caused by:

Changing the reference plane(s) along the signal path
Discontinuities within the reference plane

There are two modes of high-frequency current flow

Normal mode: This is the simpler mode. Current goes along a closed circuit loop, so the total current along the loop becomes zero. The loop is small/narrow enough, so the radiation from the incident current is canceled by the return current.

Common mode: Noise power goes through both of the traces and, lacking an appropriate, closely spaced plane, something like the enclosure can become the return path. The noise induced by the currents on the signal traces is not canceled by a nearby return current, so strong radiation could occur. This physically larger circuit can act as antenna, so it may cause EMI as well as an EMS (electromagnetic suseptability) issue. The common-mode noise source could be the reference plane discontinuity mentioned in Normal mode.

Most of my life, I have been given to understand that steam radiators can be assumed to have an electrical connection to ground.

I once visited a neighbor’s home and while I was there, he showed me how his heating system had been recently “updated”. Instead of steel pipes, there was PVC piping all over the place. It was my guess that at least some of his radiators were no longer grounded.

If you ever find yourself in an unknown radiator milieu, this is a possibility to be borne in mind.

Risks associated with shock and electrocutions from inadvertent contact with energized parts have long been recognized as a threat to electrical workers. In recent years, OSHA and industry associations recognized the severity and urgency of the situation which resulted in the development of new standards for electrical workplace safety. In essence, they mandate that work on electrical equipment must be performed in a manner that does not expose the worker to undue risk of injury.

While arc flash awareness has been growing (as well it should), the dangers of shock and electrocution should not be overlooked. In fact, electrocution is the second leading cause of construction site fatalities in the US.

Ground faults are unintentional current paths to ground, which turn into arc flash events if not appropriately addressed. One way to address ground faults is through ground fault protection systems applied on circuit breakers.

New innovations in circuit breaker and switchgear manufacturing are constantly advancing the state of the art in arc flash mitigation toward the goal of worker protection.

Protective devices (circuit breakers and fuses) are installed in electrical systems to protect against a short circuit or a major fault current. Unfortunately, a person can be electrocuted below the point at which the protective device would operate. Bonding of the electrical system components and equipment helps reduce this type of hazard.

OSHA requirements

Much the same as the NEC, the OSHA standards (29 CFR 1910) recognize two types of grounds:

System or service ground: One of the current carrying conductors (typically the neutral conductor) is grounded at the service entrance to the building. This is primarily designed to protect machines, tools, and insulation against damage due to surges and high voltages on the utility line.
Equipment ground (bond): This is intended to offer enhanced protection to workers. If a malfunction causes the metal frame of a tool to become energized, the “equipment grounding conductor” provides another path for the current to flow through the tool to the electrical power source. Further, the equipment grounding conductors are connected to the earth at the service point.

Under certain conditions OSHA permits the power system to be ungrounded. In this case, none of the current carrying conductors is connected to the earth. However, equipment grounding conductors must be provided and must be connected to the earth at the service entrance point.

Grounding installation and maintenance

Components of a grounding system are subject to corrosion due to electrochemical, electrolytic, or chemical reactions. In fact, if the system has been in place long enough, a ground grid can be completely consumed. Facilities that have sensitive electronic equipment are particularly vulnerable to disruptions. Qualified field service personnel should inspect a facility’s grounding on a routine basis.