Mistakes BITE
Nov 1, 2006 12:00 PM, BY ROLIN LINTAG, CSTE
Basic maintenance protects your station from equipment failures.
“A little neglect may breed mischief: for want of a nail the shoe was lost; for want of a shoe the horse was lost; and for want of a horse the rider was lost.” This maxim about nails and horseshoes from Benjamin Franklin contains an important message for broadcasters. In much the same way Franklin's rider was horseless for want of a nail, it's the little, overlooked elements of basic maintenance that can unexpectedly cause a broadcaster to lose his station for want of a properly grounded system or because of a loose bolt on a power transformer.
This article focuses on four maintenance topics — proper grounding, mechanical connections, AC power and staff training. These are simple concepts, but they are often taken for granted until problems occur.
Proper grounding
Figure 1. Ground systems prevent lethal currents from flowing through a person.
Click image to enlarge.
Electric current seeks the path of least resistance. The key use of a facility grounding system is to route any unwanted electrical current to the ground or serve as a return path to the source.
Proper AC grounding provides protection by serving as a default current path instead of allowing current to flow through a person's body. (See Figure 1.) This protection is usually provided by the third, round pin on equipment plugs. It's important that the third prong of every AC plug be properly connected to a ground system so any unsafe current is diverted to ground.
Figure 2. Audio system with separate grounding points
A common ground system also helps prevent ground loops from developing in low-frequency circuits like audio. Figure 2 illustrates two circuits connected to ground at unrelated points. Figure 3 shows that a noise voltage then develops because of the difference in the voltage potential between the two grounding points, A and B. The noise voltage source (V
Hearing a hum on an audio circuit can indicate that a ground loop has developed. Check interconnecting cables and plugs, and you'll likely find the culprit.
Instead of grounding each circuit at its nearest ground point, use a single point ground and tie the ground point of both circuits to the same ground point. This way, no ground loop develops.
Lightning protection
Figure 3. Equivalent circuit of Figure 2, where noise voltage VN is added to the audio circuit and a condition known as group loop occurs.
Click image to enlarge.
A good understanding of a grounding system is critical to preventing damage to your gear. In Figure 4, note the simplified equivalent circuit of a lightning rod system connected to an earth ground. The lumped parameters jX
At low voltage DC, most of the impedance to any current going to the earth ground is due to the resistance R. However, for a fast transient high voltage and high current such as in a lightning strike, jX
Copper wires react to RF and lightning much the same. (See Figure 5.) In other words, high current and high frequencies can get shunted to the ground.
Figure 4. Simplified equivalent circuit of a lightning rod
It's important to remember that for the lightning protection to be effective, jX
Use large cables or copper straps to lower both the resistance and impedance of the ground path. Try to avoid folds and kinks.
Unnecessary turns of a copper strap increase jX
This is especially the case with outside connections and on towers. Such maintenance may prevent mysterious equipment failures that seem to happen only during thunderstorms.
Install surge suppressors on individual equipment that you want to protect, and do not rely on a single surge suppressor at the bulkhead panel. Distributing surge suppression throughout the station is a good investment to ensure that voltage spikes do not get through to the equipment.
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