Do you know the special grounding requirements involved in bonding to steel?
When you think of grounding, you may visualize ground grid conductors, electrodes, interconnections -- all buried in the soil. But, many of the currents these components must carry and dissipate enter the below grade ground system from an above grade structural member. Bonding the ground conductor to the building structure seldom gets adequate planning or execution. And this reduces grounding integrity.
Bonding to steel is one of the toughest types of grounding connections. Two applications typically require bonding to steel: (1) grounding indoor and outdoor structures, reinforcing bars (rebar) in concrete construction, fence and gateposts, steel enclosures, etc. (2) power; signal and power bonds to railroad rails and bonds on crane rails.
You can make connections with mechanical lugs, crimp or compression lugs, or exothermically welded lugs. For good connections, follow these rules:
• Use two-hole lugs, rather than one-hole lugs. If jarred, a one-hole lug may twist and loosen the connection to the steel.
• Always use a tinned lug. Tinned copper lessens the corrosion cell between the copper lug and steel.
• Use the proper hardware -- don't try to second-guess the connector design.
• Properly torque bolts for sufficient clamping without warping.
• Clean and dry the surfaces (lug and steel). Corrosion increases resistance.
• Periodically inspect all mechanical bonds throughout the lifetime of the equipment, system, or facility. Test and maintain to assure continued performance.
Mechanical connections. Mechanical lugs have two bimetallic interfaces. One is between the conductor and the lug: The other is between the lug and the steel surface.
No metallic surface is perfectly smooth. When two surfaces contact, they touch only at the tips of the peaks (asperities). Thus, the actual contact area for current flow is much smaller than the apparent area of contact. This creates contact resistance between the mating surfaces of mechanical connections.
Since grounding conductors are usually copper, and grounding clamps are usually copper alloy or steel, you make a bimetallic couple (copper and steel) when using a clamp. In the presence of moisture, a corrosion cell occurs in which the steel corrodes to protect the copper. No matter how tightly you bolt the connection, the bimetallic interface still exists; and so does the corrosion, which increases the connection's resistance.
Mechanical connections come in two styles:
• The device captures the conductor within the connector and holds the surface of the connector against the structure.
• The device holds the conductor against the structure.
Within these styles are many shapes. Clamps to a round surface, such as a fence post, use a U-bolt around the post.
Exothermically welded connections. The exothermically welded connection eliminates the conductor-to-steel bimetallic interface. A welded connection has no mechanical interface in the electrical path between the two dissimilar metals. Therefore, no bimetallic corrosion can occur in the current path.
Connections to rebar present a special situation, due to deformations on the bars. Unless you grind off the deformations, a mechanical connector will contact the rebar in only a couple of "high" spots. This results in a high-resistance connection. An exothermically welded connection doesn't require you to grind off the deformations.
Surface preparation. Regardless of the type of connection, you must remove all insulating material (paint, grease, etc.) on the steel and any rust or mill scale to ensure a good connection. Exothermically welded connections require extra cleaning on galvanized surfaces. You must remove all heavy layers of galvanizing. On an electro-galvanized surface, you need only to brush to a bright finish.
Putting it all together. While neither the exothermic weld nor the bolted connection is superior to the other for all applications, each has its place. To make either connection work properly, you must have clean wire, clean connectors, and clean surfaces when you make the connection. Plan your connections carefully, and ensure you are routing and supporting the wiring for the least amount of mechanical stress.
Switzer is a Senior Staff Engineer, ERICO, Inc., Solon, Ohio.
Sidebar: Bolted or Exothermic
Both bolted and exothermic connections have their place. For example, you need bolted if you want to facilitate removal or replacement of the bond or steel item. Or maybe you just want to ground a steel equipment cage so it does not build up a charge -- resistance is not critical. In other applications, you need an exothermic weld because maximum grounding system performance is critical -- such as for the lightning protection systemof an air traffic control tower or a substation grounding system.
For many installations, the performance of bolted connections is fine. For others, it's not. The big advantage of the bolted connection is low initial cost. Whether this is low lifetime cost compared to exothermic depends on such factors as maintenance costs, quantity of connections, and how much resistance your system can tolerate in each connection or the connections in aggregate.
When you use exothermic, you buy a mold even if you make only one connection. However, it's a reusable mold, and you recover some of the fixed cost of the mold with each connection. So, you must look at the quantity of each kind of permanent connection you are making before you can do an accurate price comparison. In most installations, you'll probably use some combination of these kinds of connections -- just as you will use both screwdrivers and wrenches in your work.
