Size of static electricity grounding conductors.
- Thread starter TVH
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There is no real current on these conductors. The only consideration is based on physical damage to the conductor. A path of one million ohms or less will prevent static build-up. I am not sure if NFPA 77 specifies a minimum conductor size, but it might.
NFPA 77 does not specify a minimum conductor size for normal static electricity dissipation and control, only a maximum resistance of 1 megohm (1,000,000 ohms). NFPA 33 (spray painting) also specifies a maximum of 1 megohm for bonding and grounding. Virtually any metal wire you can buy will be satisfactory, but in practice most people select the conductor size based on resisting physical damage. A 1/16" SS stranded cable is typical, especially where flexing may be involved as in the examples you cited. The method of attachment requires consideration, especially when it will be repeatedly removed and replaced. Static electricity typically generates 10,000's of volts but only microamperes of current. A 500 volt megohm meter is the preferred instrument for measuring resistance for these types of applications.
Actually, in most instances the 1 megohm resistance is pretty conservative and 100 megohms is pretty much a dead short circuit for static. Some commonly available "conductive" paint/air hoses are more than 1 megohm total resistance, depending on length and resistivity, but may be entirely satisfactory.
Actually, in most instances the 1 megohm resistance is pretty conservative and 100 megohms is pretty much a dead short circuit for static. Some commonly available "conductive" paint/air hoses are more than 1 megohm total resistance, depending on length and resistivity, but may be entirely satisfactory.
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iceworm
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puigrp
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Potential NFPA 77 References
Potential NFPA 77 References
In response to the questions....NFPA 77 ed2007 Section 7.4.1.4 "where wire conductors are used, the minimum size of the bonding and grounding wire is dictated by mechanical strenght not by it current-carrying capacity. Standed or braided wires should not be used for bonding wires that will be connected and disconnected frequently. "
In 7.4.1.3 it does state "that the total resistance to of the ground path to earth should be sufficient to dissipate charges that are otherwire likely to be present. A resistance of 1 megaohm or less is generally considered adequate." That is the total system to ground including the grounds resistance.
Section 7.4.1.3.1 goes on to state "Where the bonding / grounding system is all metal, resistance in continous ground paths typically is less than 10 ohms. ....greater resistance ususally indicates that the metal parth is not continous, usually because of loose connections or corrossion."
So your system that is bonding to earth (metal path to earth) should not have a resistance greater than 10 ohms. There are systems out there the monitor to a less than 10 ohm resistance and even offer dry contacts to control the process if desired. We have a great Handbook that talks of static electricity, applications and solutions for static electrical control in Hazardous areas and different standards that might apply. Be glad to send you electronic copy if you like. It is a great tool.
Potential NFPA 77 References
In response to the questions....NFPA 77 ed2007 Section 7.4.1.4 "where wire conductors are used, the minimum size of the bonding and grounding wire is dictated by mechanical strenght not by it current-carrying capacity. Standed or braided wires should not be used for bonding wires that will be connected and disconnected frequently. "
In 7.4.1.3 it does state "that the total resistance to of the ground path to earth should be sufficient to dissipate charges that are otherwire likely to be present. A resistance of 1 megaohm or less is generally considered adequate." That is the total system to ground including the grounds resistance.
Section 7.4.1.3.1 goes on to state "Where the bonding / grounding system is all metal, resistance in continous ground paths typically is less than 10 ohms. ....greater resistance ususally indicates that the metal parth is not continous, usually because of loose connections or corrossion."
So your system that is bonding to earth (metal path to earth) should not have a resistance greater than 10 ohms. There are systems out there the monitor to a less than 10 ohm resistance and even offer dry contacts to control the process if desired. We have a great Handbook that talks of static electricity, applications and solutions for static electrical control in Hazardous areas and different standards that might apply. Be glad to send you electronic copy if you like. It is a great tool.
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