In accordance with relevant specifications, galvanized cable trays must be properly grounded.
(1) The contact resistance at each fixing bolt of the galvanized cable tray splice plate
shall be less than 0.005 Ω; under this condition, the cable tray may serve as a grounding main (powder-coated cable trays are not suitable for use as grounding mains). The resistance value of each cable tray section can be calculated using the following formula:
r = ρ·L/S
Where: ρ = 15 × 10⁻⁶ Ω·cm (at 20°C);
L = length (calculated as 100 mm);
S = cross-sectional area (cm²).
(2) See Table 2 for the unit resistance values of ladder-type and tray-type cable supports.
Table 2 Unit resistance values for ladder-type and tray-type cable supports
(3) See Table 3 for the unit resistance values of cable troughs.
(4) Once the cable trays are installed and connected into an integrated assembly, the resistance of each side rail (or each cable trough section) is:
R = L(r + 1/3r')
Where:
R -- Total resistance of the full length of the side rail (or cable trough) (mΩ);
r -- Resistance per unit length of the side rail (mΩ/m)
