What is leakage current to ground and why is it possibly harmful?

Ground leakage current occurs when wire insulation is damaged or becomes frayed with age. Then, small amounts of a current can flow into the grounded metal cabinet or chassis. The maximum allowed ground leakage current is a few milliamps (thousands of an amp). A GFCI (ground fault circuit interrupter) can be used to detect higher-than-normal ground leakage current and turn off the circuit, which is the reason for mandatory GFCI-protection of AC sockets near earth ground, bathtubs or showers. By detecting the current through a person’s body into the ground and then tripping the circuit, GFCI can prevent electrocution when a person accidentally comes in contact with a live AC source.
Under UL 60950-1 (the standard for safety of information technology equipment including electrical business equipment), the maximum allowed ground leakage current per each computer or office equipment is a few milliamps.
However, when there are hundreds computers, IT equipment and electrical business equipment in each floor of a high-rise building, each floor can produce a combined leakage current of one or two amps. When there are 10, 20 or 30 floors, total leakage current for the building can exceed 10, 20 or even 30 amps.
The presence of constant leakage current violates the NEC (National Electrical Code), which allows momentary current through earth ground wire only for short-circuit protection and surge current when TVSS (transient voltage suppression system) is activated due to transients or lightning strikes.
Another consideration is differences in ground potential between floors. For example, the earth ground wire on the 20th floor may read several volts higher than the earth ground wire voltage on the ground floor. This may cause equipment malfunction and/or damage when data communication wires are connected between ground floor equipment and equipment at upper floors.
Therefore, minimizing ground leakage current in a large building is very important for ensuring personnel safety, equipment functionality and equipment protection from stray voltages and ground loop currents.