SOLAR TODAY - May 2011

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Best;Practices
the Quagmire of electrical system grounding
The subject is complex. NEC article 694 tries to clarify it.

By MICK SAGRILLO with technical review by Roy Butler, Four Winds Renewable Energy

mick sagrillo (msagrillo@ wizunwired.net) of sagrillo Power & light is a small-wind consultant and educator.

As originally envisioned, grounding electrical systems in the United States was quite simple. Over the years, however, grounding has become increasingly complex and more confusing.

The rules for grounding electrical equipment are laid out in Article 250 of the National Electrical Code (NEC). In addition, the new Article 694 on small wind turbines in the 2011 NEC covers a few of the differences unique to wind systems and clarifies a few additional considerations about grounding and lightning protection.

Before delving into these, we need to differentiate between grounding for electrical faults and grounding for lightning protection.

lightning Protection

Imagine this: It’s a cold, snowy Sunday morning, and, with a fire going in the woodstove, you head over to your favorite chair with cup of coffee in hand and the Sunday paper under your arm. But wait! That darned cat has already settled into your chair. Shuffling your feet on the carpet as you approach the chair, you stick your finger out toward the cat’s nose and ZAP! A spark flies and the cat flees. You have the chair all to yourself.

What happened during the ZAP portion of this drama?

We all remember from grade school science the “ experiment” where we rubbed rabbit fur on a piece of plastic and created some static electricity that we could discharge on the ear of an unsuspecting classmate. That same experiment was just conducted with your cat. Stocking feet on the carpet, dissimilar materials, static buildup: It’s got to equalize somehow. All you did was simply provide a discharge path through the cat’s nose.

The same thing happens as the air masses roll across the planet: A static charge builds between ground and atmosphere, creating a giant capacitor (not unlike the capacitor in the flash on your camera). At some point, that static builds to a point where it has to discharge to equalize itself. This is what we call lightning, and it occurs several thousand times a minute at locations scattered around the planet.

Understanding how to dissipate that static buildup between the atmosphere (with winds blowing across your wind turbine blades) and the earth (what your tower is attached to) will go a long way toward understanding how to minimize the attraction between lightning and your tower, and subsequent damage. By connecting wires from the tower structure to ground rods in the earth, you are able to dissipate the static charge that builds up on the

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tower and turbine, thereby making the tower less attractive to lightning strikes.

In addition, should it strike, ground wires and rods give lightning a quick route to get from the tower to the earth. Although your tower is built on a concrete foundation embedded in the earth, the concrete is not necessarily a desirable path for lightning to get to the earth. Imagine the problem that would occur if lightning came down your tower and into the metal anchors embedded in the concrete. Depending on the conductivity and moisture content of the concrete, the lightning could shatter the foundation. However, the damage would remain completely unseen, buried below the ground, until a high wind event toppled the system over. Not good.

ground rods … dissipate the static charge that builds up on the tower and turbine … making the tower less attractive to lightning strikes.

equipment grounding

A second type of grounding, called bonding, includes connecting all metal components together electrically. This is required by the NEC so that no one component is isolated in such a way that it might carry a fault current that could harm someone. More importantly to a small wind system is the fact that bonding assures that all components of the system are at the same electrical potential should lightning strike and set up transient voltage on the wiring from the tower to the controls in the house. Equalizing such voltage potentials by bonding all metal components together will not eliminate lightning damage, but it often minimizes it.

Grounding plays another role with wind systems. Just as with any electrical generating device, the metal components of a wind system need to be grounded to protect service personnel and the public from a shock caused by an electrical fault. Since alternators and wires carry an electrical current, and since they can short to the tower or wind turbine, it is prudent to make sure that this equipment is protected by proper grounding.