Monday, July 19, 2010

Grounding and Bonding Part II


Last week we discussed a discovery where two panels, in a service for a newly converted gym, were not properly grounded. Article 250 is normally considered to cover two specific topics, Grounding and Bonding. However, it really covers three distinctly separate issues, and those are Grounding, "Earthing," and Bonding. Many in the industry are beginning to separate and treat the "Earthing" component as a majorly separate area. This helps to distinguish between grounding WITHIN a system FROM the grounding OF the system. Our issue here is more of an Earthing concern, where we must insure that stray currents and our "intentionally" grounded conductors have a safe and unfettered path to earth.

The initial 2008 NEC section to confer is Art. 250.52. This article explains what your accepted and physical grounding electrodes must be. Under "Electrodes Permitted for Grounding," there are 8 classes of Grounding Electrodes (GE) and 2 that are specifically prohibited. In most retro-fit remodel jobs, we would use two of these classes the most. These are our "Water Pipe" rule and "Ground Rods." We find the first of these two in section 250.52(A)(1) and the rods under 250.52(A)(5) "Rod and Pipe Electrodes."

There are a couple of installations rules to follow with these two types. The biggest of these is the so called "5-foot" rule. A water pipe that is FURTHER than 5 feet from the ENTRANCE of the building CANNOT be used as an acceptable GE. (Note, there is an exception to this; however, very FEW remodel jobs would ever meet the qualifications for this exception). We must also ensure that a minimum of 10' of continuously bonded metal pipe and casings exist that make DIRECT contact with earth for the entire minimum length. Water meters and the like must be jumpered over with a bonding conductor. Under Article 250.53 we also find a combining rule that REQUIRES an additional supplemental GE be used when we utilize the water piping system as a GE. Thus we see that we're required to use a ground rod or something similar in conjunction with our water pipe.

Ground rods must be at least 8' in length, and according to 250.53, 8' of that length MUST be in contact with soil. Most ground rods are manufactured in 10' lengths so that the bonding connector would not have to be buried in soil. Clamps that are buried must be listed for direct burial. The rod must be driven vertically, unless rock bottom is encountered. (In the Austin area, that is an extremely common situation. In that case, a 45 degree drive/burial angle is allowed or it may be completely buried laying flat, under a minimum of 30" of soil. DO NOT bend your ground rod into a 90 degree "stub up" configuration, where the top portion is above ground and the remainder is horizontally flat. This is a poor practice that is not only a code violation but is also a functionality liability! A situation that I've see and run across too many times beforehand)

There are two final rules to consider. The first is that you are not required to use a bonding jumper for the supplemental GE rod that is larger than a #6 AWG CU conductor. The second is that your resistance reading between the rod and earth shall be 25 ohms or less. The use of special grounding meters should be utilized to determine this value. Where it is greater than 25 ohms, additional rods should be installed at a distance NOT LESS than 6 feet apart. (See Art. 250.56).

Next week we will talk about how to apply Table 250.66 and review more installations rules and requirements. If you have specific questions about any grounding or bonding issues, we encourage you to send us an email. We will respond promptly and your question may even be featured on our web site!

1 comment:

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