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Intriguing Reality of EMI

Understanding what is behind electrical noise, which overpowers the good signal, is what we need to manage.


May 1, 2007  


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Electro-magnetic Interference or EMI is defined as any electrical or electro-magnetic interference that causes undesirable signals on a device, equipment or system.

If you refer to the BICSI Telecommunications Distribution Methods Manual (TDMM), you will find it recommends that during the building design stage, consideration also needs to be given to electromagnetic compatibility, disturbance, emission, environment, immunity, induction, pulse, spectrum and susceptibility.

The manual also references standards with tables, which reference separation of low voltage cabling from AC power cables and devices.

In the real world and based on my own experience, many people think Ethernet is as tough as nails. Just like all those other 2-wire services that service providers offer, right? So, if it’s not crossed, short, grounded or open, it will probably work.

This is NOT true once you move past the delivery of plain old telephone service (POTS). Delivery of even relatively slow-speed, low-frequency services is affected by what you cannot see or test without specialized equipment.

Today’s transmission rates are dependent on clean channels, which need to be tested for things like near-end/far-end X-talk, delay skew, power sum (PS), PSNEXT, PSFEXT, alien x-talk etc.

Signal to noise (SNR) is about as far as many people need to go, but understanding what is behind electrical noise, which overpowers the good signal, is what we need to manage.

Standards suggest that EMI above 5 Milligauss (mG) can affect the transmission of electrical signals. So, what radiates more than 5 Milligauss?

A number of devices such as Power entrance circuit breaker panel — 50mG; Freezer motor — 45mG; Hot water heater — 45mG; Clothes washer — 65mG. (Readings are based on average household measurements). The point is that within all buildings where there is AC power, there is EMI.

Even if the power conductors are in electrical metallic tubing (EMT), there can be measurable, sometimes significant, magnetic interference being radiated from conductors and conduit. The greater the load at a moment in time, the greater the EMI.

For those having worked on high-speed Internet, as I have, you may have experienced technical issues related to delivery of service within buildings.

Voice and HSI on a section of inside wire operate surprisingly differently. If an inside wire running within the stud cavity passes a refrigerator, the magnetic field from the motor can erase the HSI signal as if you took a set of side cutters and cut the wire off, yet you will have perfect dial tone at the far end.

The service also works really well most of the time, but who would ever think their intermittent problems happen when the refrigerator motor is running? How many of you have small businesses with a lunchroom?

Where is your data equipment located, and where are the runs of inside wire placed? Another interesting EMI experience occurred late last year when the Internet service of a person I know stopped working after daylight savings time ended in October.

It turned out his HSI service was installed during the summer and only used until early evening. The modem was conveniently installed beside the desk lamp next to the computer. He hadn’t needed the light on until time changed. Since it is dark an hour earlier, turning on the light to see at the desk caused enough EMI that the Bit Error Rate (BER) caused the service to drop.

The intriguing reality of EMI is that it is often not present during initial installation and testing of ITS infrastructure, because there are no occupants, and few, if any, power load devices are in use.

Today’s BAS and associated infrastructure needs to be designed by certified designers who are familiar with not only EMI, but all the integral subjects that minimize its effects on the performance of networks.

There are many more aspects to this subject that affect the quality of service on your communications and security systems. If you or someone in your company is interested in giving a presentation about this or any other ITS or BAS-related topic, do not hesitate to contact me or the BICSI office.

If you have responsibility for networks, and this subject interests or concerns you, perhaps you or one or more of your employees would benefit from an upcoming BICSI event, becoming a certified BICSI Registered Communications Distributions Designer (RCDD), or specifying, as many are, that you want RCDDs working for you or on your ITS systems.

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In Canada during the remainder of 2007, we are finalizing details for several BICSI breakfast club and regional meetings. The events and their coordinates will be posted on www.bicsi.org under the Training and Upcoming Events tabs.

Richard Smith is the Canadian Region Director of BICSI and the manager of Aliant Cabling Solutions in Moncton, N.B. He can be reached at rsmith@bicsi.org.