Standards About Grounding Practices of Electrical Systems

Powering and grounding an electrical equipment in the United States must, at the least, comply with regulations in the National Electrical Code (NEC). As illustrated in the CNC example in the article below, however, there are many misconceptions on the practices of power system grounding.

In fact, grounding and earthing are often confused, which is the direct cause of  many improper grounding problems that can expose the operating personnel to serious bodily harms and/or cause costly damages to your equipment.

What’s more, improper grounding can degrade the power quality and induce severe noises into sensitive electronics. It can cause your otherwise “flawless” product to misbehave embarrassingly/badly, and cost you big bucks for troubleshooting and support.

Keep in mind, NEC is to ensure safety of people and equipments. We must not violate NEC in order to remedy a performance (noise) issue. Letting a piece of equipment use an isolated earth ground rod, for example, may keep some noise issues away from your installation on a good day. But it is a common example of such violations that can have dangerous consequences.

Therefore, knowledge of proper system grounding is a critical know-how that can help improve your ROI (return on investment) of your projects.

Here are a few resources and guidelines for your quick reference.

NEC Article 250: NEC provides the minimum requirements for a safe electrical installation. Article 250 is Grounding and Bonding.

IEEE Standard 1100-2006 (IEEE Emerald Book): IEEE Recommended Practice for Powering and Grounding Electronic Equipment.

IEEE Standard 142-1991 (IEEE Green book): IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems.

To get started, here is a good article to read:

Understanding the Differences Between Bonding, Grounding, and Earthing

 

Thanks

Gregory

 

Board Layout: Identifying Current Paths

I have seen a lot of PCBs with this issue. Yet many people simply dismiss it. But it is an important EMC issue that has the potential to cause severe troubleshooting headaches when sensitive analog front-end is also in the system, or your products have to pass third-party EMC testing for UL/CENELEC certifications.

Besides, in a big project where many engineers are involved, it is next to impossible for the poor analog guys to “prove beyond reasonable doubt” that certain noise problems are caused by such seemingly very mundane layout “mishaps”, above all other “bigger” possibilities.

I know, I know. I am sounding a little un-team-player here as if I am trying unduly hard to teach an engineer to dodge a blame. But, you know what I mean? You do not want such a thing blowing up at your face at a later stage of the project. It is a potential 20dB improvement that is easy to achieve.

So, what is the “current returning paths” I want to talk about?

Let’s take a look at the drawing in Fig 1.

Ground-plane with cutoutFig 1

The pink dotted line shows the ground current return path via the ground plane of the PCB. So, what’s the problem?

If the signal frequencies between the two amplifiers are DC or low, there is indeed no problem. But what happens at some higher frequencies?current density at 1mhz

Dr. Archambeault at IBM has done some extraordinary experiments over this issue.  The figure on the right shows his results at 1Mhz, where returning current of his 1Mhz signal ignores the shortest path on the ground plane(blue) to return to the source. Instead it follows the same route of the signal path(red trace).

Going back to our drawing in Fig 1, the behavior Dr. Archambeault has shown us means that our return current will follow the edge of the ground copper cut until it gets back under the signal trace. We have hence created a “nice” loop antenna with “edges” to radiate/bounce these signals. In fact, other experiments by Dr. Archambeault further show that over a frequency range of 20-1000Mhz, the near field radiation caused by a grounding split is much greater than that of no split, in excess of 20 dB !!

There are many other EMC implications as well, such as high frequency ground return current that accidentally expose other elements of the circuits to the interferences.

Now, you may like to say that your signals are of low frequencies. So this should not affect you.

Is that really? Not so fast, mister.  When digital pulses are involved, the harmonic content of the signal have more to do with the rising and falling speeds of the pulses than the base frequency of the signal itself.

The base frequency may not be very high, but a very fast rising/falling edge may force its harmonics far into the your view, like it or not.

Well, now that will be the subject of my possible future blog.

Thanks

Gregory

Electromagnetic Compatibility (EMC) – Directives & Standards

Quote: “The aim of electromagnetic compatibility considerations is to avoid or minimize the influence of electromagnetic phenomena on a device, equipment or system and on living or inert matter. To achieve this, methods of measurement and test, as well as limits and levels of electromagnetic emission and of electromagnetic immunity are defined in a large number of EMC publications.

EMC publications and standards, developed by the IEC and other standardization bodies, can be divided into four general categories:

  • Generic EMC Standards,
  • Basic EMC Publications,
  • Product family EMC Standards and
  • Product EMC Standards.

These terms are defined in the corresponding chapters.”

Pretty good read.

You can download the PDF here.  Enjoy!

Gregory