Resistance to ground

The resistance to ground should be sufficiently small to prevent spark ignition at the maximum anticipated charging current to the system. This can be achieved by ensuring either that the energy stored is less than the MIE or that the minimum ignition voltage cannot be attained (A-4-1.3). The necessary resistance depends not only on the flammable mixture but also on the electrical circuit. 

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The static decay test is a good measure of the static dissipative nature of an object. However, the test is controlled by the object s resistance to ground. Objects that have a high resistance, such as insulators, will not readily accept an induced charge or decay an induced charge. The static decay test is most suited for objects with resistances of 10 or less (52). 

It is essential that persons be grounded in hazardous (classified) locations. For most (memical operations, the resistance to ground from the body should not exceed 100 megohms. A lower allowable resistance may be specified for locations where the presence of pri-maiy explosives, hydi ogen-air mixtures, oxygen-enriched mixtures, or certain solid-state devices requires faster charge dissipation. 

Refer to Figure 22.1. The approximate resistance to ground in a uniform soil can be expressed by... 

The resistance to ground for a 600 mm x 600 mm plate grounding, considering a sandy soil, treated artificially and having attained an average soil resistivity of 10 k2m... 

From the above the resistance to ground of a plate grounding is inversely proportional to the square root of the linear dimension fA) of the plate. The variation in resistance with the size of the plate is shown in Figure 22.2. Considering the resistivity of soil as 10 k2m, since the ground resistance is proportional to the resistivity of soil, there would be different parallel curves for the ground resistance for different values of resistivity of soil. 

Figure 22.2 Variation in resistance to ground with the linear dimensions lor a plate gounding, for the same resistivity of soil...

The resistance to ground of a 19 mm internal diameter pipe, 2.44 m long, with p as 10 fim... 

The resistance to ground would vary with depth of the electrode. A minimum depth of 1.5 in from ground to the top of the electrode is considered mandatory, and even deeper to reach damp soil. 

The safe step voltage, j, should not be more than the total resistance to ground through the body, f 2isb (Section 22.10.1) X safe body current, 4, as a function of time, where... 

Conductive objects may accumulate charge via conduction, induction, ion collection or contact-separation in relation to a nonconductor. In general, charge accumulation on conductive objects can be completely prevented by bonding and grounding. The only decisions to be made are how small the resistance to ground must be (4-1.3) and how small a conductive object needs to be before it can be neglected. The latter depends on the results of... 

These measures prevent hazardous potentials from developing on conductors at the greatest anticipated charging current. This does not require a short circuit since charging currents are typically less than 100 juA. Depending on the system, allowable resistance to ground may be 1 or more. 

The resistance to ground from any point along a flexible metal conductive hose should be 10 2 or less except where insulating flanges are employed to avoid stray current arcs. 

Conductive hoses containing a continuous wire or braid bonding element should have a resistance <1 k 2 per meter of hose length and the resistance to ground from any metal connector should be less than 1 M 2. 

To avoid ignition via these indirect sparking mechanisms, the resistance to ground of conductive objects that could be encountered by discharges on powder beds should be less than 1 k 2 and preferably less than 100 2, so that the maximum potential attained is less than 1 kV. 

Resistances to ground were measured for a small van with four tread 4 polyester, 2 nylon ply tires (sidewall 6 polyester plies) showing the effect of ground surface [107]. The resistance at a source potential of 50 V was mea-... 

The combination of conductive flooring and conductive (ESD) footwear is the preferred method of grounding. Untreated concrete flooring with conductive footwear is usually adequate, but the resistance to ground should be measured. Where this method is impractical, personnel-grounding devices are available. 

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