On completion of the equipotential bonding phase, lightning current is distributed at the grounding resistance Re and across the conductors that are part of the equipotential bonding installation, while at the same time the potential increase induced by the lightning strike decreases accordingly. Described in [6.20] is a method for distributing lightning current across the various conductors and leads. It can be assumed that around half of the lightning current will be dissipated by the grounding resistance and the remaining
Telecommunication
cables
Connection to low voltage utility grid
D Water conduit
Connection to foundation grounding
half by the building’s nL in equal shares. In a conductor comprising nA leads, lightning current iL is distributed equally to all leads. In the case of bilaterally shielded conductors, however, it can be assumed that most of their lightning current will flow to their shielding, providing that this current can be conducted there without causing damage. This scenario can be expressed as follows:
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i Lightning current in one conductor: iL ~ 0.5 nL |
(6-9) |
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Lightning current in one lead: iLA — — ~ — nA nA |
(6.10) |
where:
i — lightning current
nL — number of conductors connected to the building nA — number of conductor leads
Using the values in Table 6.3, the maximum anticipated lightning current imax can be inserted into the two equations above according to the desired protection class, whereupon the maximum current iLAmax for the various leads and conductors can be calculated and then specified accordingly.
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When sizing equipotential bonding conductors, it is also useful to determine the maximum current they will exhibit, iPAmax, so as to ensure that the right wire gauge Apa is used. According to [6.2], copper equipotential bonding conductors may carry the following maximum lightning currents (this limit is imposed in order to prevent excessive warming of the insulation):
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where iPAmax — maximum equipotential bonding conductor lightning current, in kA (the sum total of all connected conductor and lightning protection device current) and Apa — equipotential bonding conductor gauge in mm2.
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Hence, if a copper equipotential bonding conductor exhibits a specific current iPAmax (in kA), the following minimum gauge APAmin (in mm2) should be used:
As the two equations above are so-called adjusted quantity equations, they only apply to numerical values if iPAis given in k A and APAis given in mm2.
The equations above can also be used to estimate whether a conductor can handle a specific lightning current.
In the interest of attaining satisfactory mechanical robustness, national lightning protection standards for equipotential bonding conductors often specify a 6 mm2 minimum gauge for copper wires (5 mm2 according to [6.22]). For bus bars that interconnect equipotential bonding conductors and the grounding installation, the minimum gauge for copper wires is 16 mm2 (14 mm2 according to [6.22]).
