Does your UPS protect you?

By Natec Systems

Nathan Rector

nater@northcoast.com

http://www.northcoast.com/~nater

 

Does your UPS protect you? The question stems from what a UPS does and doesn't do. A UPS is used to protect against power outages, power surges, and any other "nasties" that decides to come through the power lines.

A UPS works great for what it's designed to do, but there is large blind spot that many people are unaware of. A UPS will not protect you from all the electrical problems that have a tendency to leak into your system.

The blind spot that I'm referring to is in your printers, terminals, and the physical wiring connecting these devices to your computer system. Most people buy one UPS to place on their main file server or mainframe, but never realize, or bother with, how the other equipment connected can affect these machines.

The problem this type of equipment causes is called "Inter-system Ground Noise". To understand what Inter-system Ground Noise is, you'll need to understand how computers use the ground wire.

All computer equipment uses the third prong on the power cord as a safety ground, but they also use it for one other purpose; to provide a point of reference when communicating with another device through data lines.

Since each device has its own power supply, and its own ground wire, they both have a separate point of reference that determines what the communicating voltage should be over the data lines. The two devices use these references to establish a common voltage to use when communicating. This is where the Inter-system Ground Noise comes into play. If any large variation in the voltage from one device occurs, it can alter the common voltage used between the two devices creating spikes and sags in the data line, sometimes to the extent of causing hardware damage.

Inter-system Ground Noise can only be found in the data cables spanning more than one device, such as RS-232 cables or printer cables. There is a misconception that Inter-system Ground Noise is the same as "Common Mode Noise", which UPS's do protect against. This is not the case. Common Mode Noise may be a leading cause most Inter-system Ground Noise, but it not the soul cause.

Common Mode Noise is found in the actual power cables, not in the data cables connected devices. UPS's filter out and protect you from Common Mode Noise, but they can not protect you against Inter-system Ground Noise. It is possible to have Inter-system Ground Noise and no Common Mode Noise. It is just as possible to have Common Mode Noise, but not Inter-system Ground Noise.

These two types of ground noise can cause the same effect, but are two separate problems, even though Common Mode Noise is many times the primary cause of Inter-system Ground Noise.

There are several different ways that Inter-system Ground Noise can occur. One occurrence is caused by the two computers being connected to different electrical circuits that use different grounds. This is common when spanning more than one building.

Another occurrence happens when other equipment, such as copiers, air-conditioners, saws, or anything with a large motor in it, use the same electrical circuit as one of the computers. These kinds of devices have a tendency to create "noise" in the ground wire, also known as Common Mode Noise, affecting the computer's reference point.

Another occurrence is caused by Radio Frequency, or RF, interference. RF interference is caused by any kind of motor, such as saws, generators, and other such equipment. The data cable acts like an antenna, causing data corruption and at times large voltage changes in the cable themselves.

The most damaging cause is from Lightening Strikes or other electrical storms. Some people don't realize that most storms have some kind of electrical energy in them, even if there is no thunder and lightning. The stronger the storm, the more energy the storm creates.

In these cases, data lines sometime become lightening rods. The only difference is that electrical energy isn't sent to a grounding rod, but into your computer system.

There are several ways to deal with Inter-system Ground Noise. One is make sure that ALL of your computer equipment is protected by a UPS. This includes terminals, printers, and anything else that may be connected to your mainframe or file server.

If you are using computer equipment in an industrial environment, then make sure that all the computer equipment is using a dedicated circuit that ONLY the computer equipment is on. Also, use data lines that are relatively immune to Inter-system Ground Noise.

Ethernet cable is one form of wiring that is relatively immune. It was designed to assume Inter-system Ground Noise will occur, and engineered so the noise would not be passed on to the equipment the cable it was connected to. Fiber optic is completely immune to Inter-system Ground Noise. On the other hand, RS-232 cables, long printer cables, and AUI Network wiring are very vulnerable to Inter-system Ground Noise. In the next issue of International Spectrum, I'll cover ways to protect RS-232 cables.

UPS's work well for what they are designed to do, but there are other causes of power problems that they can't protect against. Make sure you are protected against them, or during this storm season you may have hardware problems.