Before I adopted a modified equipment-ground cord, residual voltage always disrupted the field-training demonstrations I conducted in high-rise or office buildings. Materials that should have drained a charge to near zero in in less than i see might show a residual voltage of more that 100 V after 30 sec or more. Field meters that would normally return to zero would not do so even when equip-potentially bonded to the operator and the conductive work surface being used in the demonstration area.