How to Minimize EMI Problems



 

 


There are some common steps that can be taken to reduce the impact of various types of EMI and there are some steps which are specific to a particular type of EMI.

Good Grounding

Good electrical signal grounding, also known as multi-point grounding, is essential in any effort to ameliorate all types of EMI issues. Once it is determined or suspected that an EMI problem exists, the first step in resolving it should be the installation or renovation of the signal reference grounding system in the affected building.

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.

Cables and Covers

Reducing the impact of TV, radio, radar and microwave EMI can be difficult in the particular situation in some of the situations in which we found ourselves. Short of building Faraday cages around each computer or office, there were a couple things that can be done.

Make sure that all covers are in place and correctly fastened. This includes the card slot covers which are very commonly not replaced after an expansion card is removed and not replaced. The case itself must have all covers on and screwed or fastened in place.

Cables connected at one end to the computer but left dangling at the other end while a peripheral is disconnected can act as antennae to capture signals and draw them into the computer. Disconnect all cables at both ends when peripherals are disconnected for any reason. Cables should be seated properly and if there are fasteners such as screws or wire retaining clips with which to secure the cable to the device, they should be engaged. If a ground wire is present in the cable and it has a separate external connector, connect the ground wire to a grounding terminal on only one end. Connecting the ground wire on both ends can cause groud loops which can be the source of as many problems as the one you are trying to fix.