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Tech Tip 15

Consider Fiber Optic Cable When Running Data Lines Between Buildings


  • Data lines run between buildings
  • Computer system lock-ups
  • Data card damage


  • Ground potential difference between building grounding systems
  • Data signal upset or component damaged


  • Use fiber optic data cable
  • Optical isolation
  • Equalize voltage between grounding systems


Frequent unexplained lock-ups of computer systems, and component damage during lightning activity are two problems that may be caused by ground potential differences. Differences in voltage between two separate building grounds often exist in locations that share data or communication equipment. Those differences can occur in large buildings or in completely separate buildings. These locations are often served by separate electrical services.

Monitoring at one location showed computer lock-ups directly correlated with transient differences in voltage between the office and plant grounding systems. The building ground voltage difference upset the data signal ground reference and interfered with normal data transmission. This voltage difference rose to as much as 10 volts (zero-peak) several times each day. Most computer systems operate on DC voltage levels of 5V or less

Lightning currents in the thousands of amps can cause ground voltage differences many times larger than those caused by normal load switching disturbances. Therefore, it is reasonable to expect significant lightning damage to computer systems, or communication systems if the ground potential difference situation is not corrected.


One way to prevent ground voltage difference problems is to attempt to equalize ground voltages by bonding the building grounding systems together with a suitable copper conductor.

The best way to prevent ground voltage differences from causing computer system problems is by using fiber optics. This will completely isolate the two buildings and will do a good job of preventing damage due to lightning. Fiber optic links would also eliminate any “electrical noise” that may be present on the data lines. The choices may range from converting the entire data link to fiber, or to simply installing optical isolators at both ends of each data cable. If optical isolators are installed, surge protection should also be used for each end use device.


Figure 1: Block Diagram of System

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.