As for overcurrent protection, medium-voltage utility-type overcurrent protection is a different breed compared to that of typical low-voltage circuits and equipment. For one, sometimes ground-fault/earth-return currents can be below the settings or ratings of overcurrent devices. This may indeed result in flaming tires on incompetently controlled sign trucks.
This is in opposition to how we usually think of equipment grounding and bonding. Besides low current, it is very likely that the potential differences between the crane and the ground mat at a remote source will be large enough to seriously shock or electrocute those standing on the ground in the vicinity of the fault, though not touching any conductive surface short of your shoes contacting the surrounding earth. The rules are different, but in most cases well understood by folks routinely working in medium-voltage applications in utility settings.
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