Grounding and shielding is an often misunderstood process. It is common to hear quotes ranging from “it’s just black art!” to “the rules change all the time!” and “there’s no way to understand it!” These statements are often repeated but not true.
There is a process, it hasn’t changed, and the more one knows about coupling mechanisms the more one will see that it’s sound engineering principals. I have written many EMI and grounding issues in my blog, especially in groundloop section.
Let’s start with grounding. Proper grounding is an essential component for safely and reliably operating electrical systems. Improper grounding methodology has the potential to bring disastrous results from both an operational as well as a safety standpoint. Effective bonding, grounding: The backbone of electrical safety article provides a good overview of proper grounding in electrical systems. Providing and maintaining an effective impedance path to ground is critical to maintain reliable, efficient, and safe operating facilities. It is the foundation of any power generation unit and associated power distribution system. Providing and maintaining an effective impedance path to earth that stabilizes the system voltage is a basic and critical component in maintaining reliable, efficient, and safe operating facilities.
Nowadays solidly or resistively grounded power distribution systems are preferred (in both low voltage and medium voltage systems). Ungrounded systems are no longer recommended (they were once common). Nowadays solid or impedance grounded systems of some form are predominantly installed. Three-phase, 4-wire solidly or resistively grounded “WYE” are preferred over the use of 3-phase, 3-wire ungrounded systems specifically because of the possibility for destructive transient overvoltages that can occur throughout the power system during any re-striking phase-to-ground fault due resonant condition established between the inductive reactance of the system and the distributed capacitance to ground (earth). Experience has proven that these overvoltages may very rapidly cause failure of insulation at multiple locations throughout a power distribution system.
Neutral grounding has been in practice in many systems all over the world. Generally, the neutrals of source transformers or generators with star connected windings are grounded. Grounding the neutral reduces the magnitude of transient voltages, improves protection against lightning, protection for line to ground fault becomes reliable, and improves reliability & safety. The typical disadvantages of grounded systems are related to high fault currents. The operational safety is the primary function of grounding. Grounding systems are designed so that they do provide the necessary safety functions. Grounding also have other functions in some applications (for example work as signal ground reference) but the safety should not be compromised in any case.
Nowadays grounded electrical outlets are used in modern outlets for safety reasons. The modern practices nowadays demand the use of en extra safety wire from electrical outlets/loads to the main distribution panel. This wire is known in Britain and most other English-speaking countries as the earth wire, whereas in America it is the ground wire. Inside buildings the TN-S system is nowadays recommended practice, meaning that the neutral and ground wires are kept separate and they are interconnected only on one place, this place being within the main power distribution board. This is the recommended practice for modern buildings.
Most modern buildings are wired with three-phase power using 5-wire system. This 5-wire system for three phase power has three hot phase wires, one neutral wire, and one grounding wire. For wiring single phase outlets, there are three wires: live, neutral and ground are used. The neutral and hot wires are interchangeable and reversible insofar as the operation of equipment is concerned. In Europe, the normal 3-wire receptacle is symmetrical so that the neutral and hot wire connections can be swapped by simply rotating the plug. Most equipment won’t even know which of its input wires will end up connected to the neutral wire and which will be connected to the hot wire. International office product safety regulations (including IEC 950 and UL 1950) prohibit these wires from being treated differently.
In TN-S system there grounding wires form a tree like structure starting from mains distribution panel (similar as does live and neural wires do). The generally grounding wires should go same route as the current carrying wires (live and neutral) for best performance (but also Signal reference grid approach is possible for grounding). There is a requirement in the TIA 568 B.2 cabling standard that voltage difference between the shield and the ground wire of the work area equipment outlet shall not exceed 1 V rms.
In many older building you can still see TN-C-S and TN-C practices where in some place in the installation there is a combined neutral plus ground wire. This is less safe wiring practice than modern TN-S system and causes lots of ground loop noise problems when used with sensitive equipment. An old grounding system that provides needed basic safety level to the user might not provide sufficient electrical environment where today’s electronics equipment can work well.
Grounding and Shielding Existing Equipment – How to effectively minimize EMI issues when best practices are not available is a good paper mentioned by NASA Tech Briefs and Automation Weekly.
One of the most common direct coupled noise sources is when the ground which is being used for reference or return is not referenced to earth as expected. This is especially prevalent in sensitive high-gain circuits.
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Tomi Engdahl says:
Electrical safety from the ground up
Proper grounding and bonding is critical for electrical workplace safety
http://www.controleng.com/single-article/electrical-safety-from-the-ground-up/704ebba991e540452a58bf62a7eb4a4d.html
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.
Tomi Engdahl says:
Electrical grounding techniques
http://www.suparule.com/docs/earth_ground_testing_technique.pdf
Tomi Engdahl says:
Radiator Ground
http://www.edn.com/electronics-blogs/living-analog/4440273/Radiator-Ground-?_mc=NL_EDN_EDT_EDN_analog_20150903&cid=NL_EDN_EDT_EDN_analog_20150903&elq=daa1a897c39143d293ac4e510c30e0c0&elqCampaignId=24629&elqaid=27906&elqat=1&elqTrackId=e5d617ece5dc4e50b2a9616af5779b31
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.
Tomi Engdahl says:
Return path discontinuities and EMI: Understand the relationship
Minoru Ishikawa -June 11, 2015
http://www.edn.com/design/pc-board/4439672/Return-path-discontinuities-and-EMI–Understand-the-relationship?_mc=NL_EDN_EDT_EDN_today_20151221&cid=NL_EDN_EDT_EDN_today_20151221&elq=f16edb6937274a16b479901f7f319cdb&elqCampaignId=26236&elqaid=29978&elqat=1&elqTrackId=287db830eedd4d7d8fab8b58a7ee48fb
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.
Tomi Engdahl says:
DOCSIS 3.1 noise mitigation: Check your grounds
http://www.broadbandtechreport.com/articles/2018/01/docsis-3-1-noise-mitigation-check-your-grounds.html?cmpid=enl_btr_docsis_31_2018-02-22&pwhid=6b9badc08db25d04d04ee00b499089ffc280910702f8ef99951bdbdad3175f54dcae8b7ad9fa2c1f5697ffa19d05535df56b8dc1e6f75b7b6f6f8c7461ce0b24
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.
Tomi Engdahl says:
Never use pigtails on cable shields
https://www.edn.com/design/test-and-measurement/4433416/Never-use-pigtails-on-cable-shields
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.
Tomi Engdahl says:
Case study: Why did an industrial controller fail the radiated immunity #test at numerous frequency bands? #TBT #interference #EMC #CableShield
Case study: radiated interference to industrial controller
https://www.edn.com/case-study-radiated-interference-to-industrial-controller/?utm_content=buffer03cba&utm_medium=social&utm_source=edn_facebook&utm_campaign=buffer
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.