With the analog cutoff deadline looming, broadcasters are putting the finishing touches on new digital transmitter and translator installations. What better time to review grounding systems to protect new expensive equipment and ensure years of reliable performance?
Although proper grounding practices may have been followed during initial site construction, problems could have been introduced over the years as either electrical service or equipment was added or modified. Grounding issues account for up to 40 percent of power-related problems and can cause unpleasant off-air or equipment damage situations.
In addition, transient over voltages — high voltage spikes or impulses of short duration — may cause other havoc if not adequately suppressed. The source of these transient surges can be natural, such as lightning. Another source of surges is power grid feeder line or capacitor bank switching. Finally, some surges originate from internal inductive loads. Regardless of the source, these damaging surge voltages may be injected into power and data circuits, causing equipment destruction and safety hazards.
Problems such as these may be avoided by implementing a single-point grounding system, following the National Electrical Code (NEC) when installing the safety ground and grounding electrode systems, and properly selecting the appropriate surge suppressors. (See Figure 1.)
The worst scenario for your equipment, as well as your safety, is to reference each equipment cabinet at a telecommunications site to the earth at different points. Because the earth is a poor conductor, steady state and momentary voltage differences exist in the soil. If equipment cabinets have high-speed communication interfaces between them and each one connects to the earth at different points, representing a multipoint ground, these voltage differences could cause ground loop currents (IG) and equipment downtime, as well as pose a safety threat. (See Figure 2.)
Therefore, a single-point grounding system, where all references to the ground come to a single point in the facility before referencing the earth, is essential to any microprocessor-based installation. In most AC installations, it is normally a question of referencing back to the original neutral-ground bond in the building, or the secondary neutral-ground bond of an associated step-down or isolation transformer.
Whether it is an isolated ground, a neutral conductor or a safety ground connected to an equipment cabinet, all grounds come to a single point at the main distribution panel and then connect to the site's earth grounding system.
Lightning protection system ground
Both the NEC (NFPA 70) and the National Lightning Protection Code (NFPA 780) require a separate, physical lightning protection (air terminal) earth ground electrode bonded to the main entrance grounding electrode system.
The purpose of this is to direct the majority of current from a lightning discharge to a structure into the earth away from the building entrance grounding system, significantly reducing the lightning current into the building electrical grounding system. The required bonding of the two grounded systems maintains a reasonable degree of touch safety between the two grounding electrode systems during the lightning event.
In addition to installing a single-point grounding system, the NEC allows an isolated ground — a term often misunderstood.
The computer industry has recognized for decades that metal conduit, which is allowed as a safety ground system for equipment cabinets by the NEC, often exhibits a great deal of electrical noise due to neutral-ground wire reversals, refrigeration and chilled water pump systems in buildings. Most electronic equipment uses the safety grounding conductor as a logical reference as well as a means to trip circuit breakers during fault conditions.
The isolated ground technique was developed and approved in the code several years ago as a means of establishing a quiet ground connection. Simply speaking, it means a sufficiently sized, insulated conductor, isolated from conduit and subpanel enclosures (that touch conduit). It references the earth only at the building entrance neutral-ground bond or the secondary neutral-ground bond of a step-down or isolation transformer that is part of the associated electrical distribution for the equipment. (See Figure 3.)
The NEC allows a single-point grounding system to be connected to the earth in seven different ways.
Rod and pipe electrodes make up about 90 percent of all grounding electrode system installations. Generally, this is comprised of an 8ft to 10ft stake driven into the earth that extends up and connects to the neutral conductor at the first disconnecting means, which is often the main distribution power panel.
One of the misunderstandings associated with this type of system is that many facilities drive additional ground rods to clear up problems. The NEC requires that all earth ground references be directly bonded to the original neutral-ground bond at the building entrance. This is because if a person is touching one cabinet connected to an independent earth ground rod and something else connected to the main building ground system, the large voltage differences that exist in the earth may harm that individual (especially during lightning events or fault conditions). Remember, the main reason for connecting an electrical distribution system to the earth is for touch safety.
Driving additional rods (multiple grounding) can cause equipment downtime because they create ground loop currents to circulate throughout the equipment cabinets between the different grounds. Instead, the ground should be cleaned up within the structure to a single point coming from each cabinet back to the main building ground point or the nearest neutral-ground bond at the secondary of an associated transformer.
A common mistake that occurs when electricians install additional ground connections is that the resulting current in equipment may easily reach many amps. Changes in this objectionable current create voltage spikes or transient over voltages in the cabinets — any time there is a changing current in a wire, spiking occurs.
Keep in mind, it is legal to add another ground rod at a specific minimum distance and connect it to the original building entrance ground rod. However, it is illegal to ground equipment cabinets directly to separate earth grounding systems.
A second way to connect a single-point grouding system to the earth is with a gound ring. This system is comprised of a minimum No. 2 AWG, bare wire buried no less than 30in under the soil surrounding the building. The ring gives more of an equal potential ground around the facility. It is typically supplemented with multiple earth ground rods. Proper design dictates that there should be one point of contact between the ground ring and the equipment shelter.
Problems occur when installers unwittingly connect different equipment cabinets to various points around that ring. In this case, the inductive nature of the ground ring wire can result in large lightning voltage differences between the cabinets connected to diverse points on the ring. (See Figure 4.)
Other supplemental grounding electrode systems include:
- a concrete-encased electrode, consisting of metal bars encased in concrete, buried in the earth;
- a grounded metal building frame;
- plate electrodes, which are metal plates buried in the earth for a larger surface area;
- a supplemented metal underground water pipe (as a secondary or third connection to the earth); and
- underground local metal structures, with the exception of gas piping.
Tower lighting and controllers
Any time a power or data cable exits or enters a building, injected surge voltages could cause equipment destruction and safety hazards. Proper grounding and surge suppression limit the damage incurred. Another way that electronic equipment can be disrupted is through multigrounded cable shields.
Shields are typically constructed of aluminum foil or mesh that wraps around twisted insulated wires — typically 22-gauge in size — to block undesirable AC and high-frequency fields. Low-frequency power distribution cables running parallel to data lines add noise or hum into low-amplitude signal circuits. In addition, high-frequency fields from colocated transmitters can cause data alterations. If the shield is grounded at more than one end, differing AC potentials in the soil create equalizing AC currents on the shield. These currents introduce unwanted noise into the signal circuit.
Here are a few steps you can take to protect your station:
- Measure the current on your equipment grounding conductor. Anything over 0.25A should be investigated.
- Measure the earth ground resistance, and do all you can to get it as low as possible.
- Verify there are no multiple neutral-ground bonds or earth ground connections.
- Challenge yourself to ensure a robust electrical environment for the new digital transmitters and studio equipment.
Jay Adams conducts educational seminars and site grounding audits for RO Associates.
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