EMI – Electrical Noise



 

 


Electrical noise from electric motors and other electrical appliances can be transmitted through power lines and building wiring into your computer. This is particularly true when a motor or the wiring in a building is defective in some way. Although the wiring might be perfectly safe and meet all building codes, it may not be satisfactory for the proper operation of your computers or the safety of your data.

The Coke Machine

One of my favorite illustrations of EMI is that of noise from an electric motor that happened in about 1977 while I was working for IBM as a Customer Engineer in Lima Ohio. Part of my job was to assist new customers in planning the installation of their large computers — this was before PCs — so that problems with EMI would not occur.

There are a number of things which must be done to ensure a high tolerance for EMI in a computer installation, and I outlined these to the customers well in advance of the installation. The advice I gave to this particular customer was precisely carried out; they complied with everything I recommended. Nevertheless, shortly after the installation was completed the customer began having strange problems. The computer would lock up at random times. Strange, inexplicable error messages would appear on the error display and would be recorded in the error logs.

None of the errors made any sense in the context of the customer’s installation. One error message, for example, indicated a problem with a peripheral device of a type which was not even installed on the computer and which was not configured. Another error indicated a hardware problem with the system’s memory. I replaced the memory multiple times but the error continued to occur, still completely at random. Based on other errors, I also replaced other hardware components of the system.

As this scenario continued with no noticeable progress towards a solution, I decided that the answer had to be related to an EMI problem. One afternoon I visited the customer to prove this point.

Grounding for Computers

You should understand at this point in the story is that the best way to minimize all types of problems from EMI is to ensure that the computer system has a proper signal ground. This is not the safety ground that electricians normally associate with the word “ground,” but rather a computer signal reference ground. Guess what? They can and should be the same for computer and electronic equipment. But there is also a difference in how to achieve this.

A safety ground is a third wire that differs from the two insulated copper wires that carry the electrical current required to operate your computer. The third, or ground wire in a normal electrical installation is a bare wire that is connected to the grounding buss in the electrical distribution panel. The wire from that ground buss may lead back to other panels where it is connected to the ground buss in that panel. In many cases the ground busses from all of these panels will connect to the power company ground wire. There may be a very large number of ground wires all connected together throughout the many ground busses in all of these electrical distribution panels. That is fine for safety and may work fine most of the time unless there is a problem. It may not be good enough to ensure the correct operation of computer equipment when problems occur.

A ground wire suitable for computer equipment is covered with green insulation to prevent it contacting other wires, the metal frame of the building, metal water pipes or other conductive objects. When it is to be used for a stable signal ground, a ground stake at the building location is required. This ground stake is ideally a one to two inch diameter solid copper stake that has been driven at least ten feet into moist ground. Lacking this type of ground stake, a copper water pipe that extends at least ten feet into moist earth from a basement floor is acceptable. The wire that runs from the computer, including those from any PC’s electrical outlet should run directly from the outlet to the ground stake and should not be connected to any of the ground busses or to any other wire or connection along its route to the ground stake. The reason that no other connections are allowed is that the wire must not carry any current. Those of you who know Ohm’s Law will remember that a current carrying wire, no matter how small the resistance, will have some voltage potential above that of ground reference level as a result of the current flow. This voltage potential is what causes a problem for computer equipment.

Finding the Problem

In the case of the customer who was experiencing the strange problems, the customer had run an insulated green wire, as specified, from the computer to a copper well casing  that was 90 feet deep with 50 feet of water in it. I knew this to be an excellent ground, but I still suspected a problem with it.

I asked my customer to have his plant electrician show me the route of the ground wire all the way from the computer to the connection to the well casing. Only 20 feet from the computer, in the second power distribution panel from which we removed the cover and inspected, the ground wire had a very large and ugly wire with black insulation spliced into it. AHA! I connected the current probe to my oscilloscope and clamped the probe around the black wire. That way I could see whether any current was flowing which would degrade the usefulness of the wire as a proper signal reference ground.

At that moment the company Vice President of IT walked by and asked how I was progressing on the problem. I told him about the wire I had found and had just begun to explain what I was doing with the oscilloscope when we both heard an electric motor start up and at the same time the oscilloscope screen showed a very large spike of radio frequency noise on the ground wire. At almost exactly the same moment, someone from the computer room yelled that the computer had crashed. This is about as convincing a demonstration as one could ask with a VP standing there!

The Explanation

The large wire connected to the computer ground was also connected to the outlet that supplied the Coke machine with its power. This wire had been added after the computer green wire was installed, so the electrician who  did the work on the computer did not know about it. When the refrigerator compressor was turned on, the normal noise from the motor’s startup traveled down the ground wire from the outlet to the green wire from the computer and thence to the copper well casing. This caused a temporary disruption of the signal ground reference and the computer crashed. A few minutes after we figured this out, someone came into the room and used the postage meter to mail a couple letters and a very similar spike was displayed on the oscilloscope because the postage meter was plugged into the same outlet as the Coke machine.

The Fix

The company electrician cut the intruding wire off the computer’s green wire ground and attached it to the ground buss in the electrical panel. The problem never occurred again.

Entry Paths

The primary entry path for conducted electrical noise is via the power lines into the power supply or possibly through attached cables which may also transmit the noise into your computer.