Tech Tip 08
Preventing Damage Due to Ground Potential Difference
- Television, phone, computer system damage during storms.
- Improper grounding.
- Differences in ground voltage.
- Bond phone, CATV grounds to power ground electrode
Most people have suffered sensitive equipment damage during lightning storms, or know someone who has. On the surface, power surges appear to be the culprit and usually receive the blame for this damage. The power system can carry high voltage surges (lightning), but there is a more likely cause.
Many "power surges" are actually voltage differences in the earth that reach sensitive equipment because of grounding errors. Lightning and faults on the distribution system can cause a very large ground potential difference. Sensitive equipment that references multiple grounding systems that are not bonded together, can be exposed to very high voltage differences.
VOLTAGE BETWEEN POWER, CABLE GROUNDS
Televisions often reference two or more grounding systems, significantly increasing the possibility of damage during a storm or line fault. In addition to the power ground, the television may also reference cable television (CATV), an antenna system, or satellite dish grounds. If these grounds are not solidly referenced to the power ground at the house service, very large ground potential differences can appear inside the television. Figure 1 shows a simplified schematic of a television tuner and the power and CATV grounds. The power and CATV grounds may be at significantly different voltages during a storm if they are not bonded at the main power service.
Figure 1: Tuner, CATV and Power Grounds
Figure 2 shows three configurations that occur frequently. The first is an example of incorrect grounding. The middle sketch shows correct, but not preferred, grounding. Power, phone and CATV grounds are connected with No. 6 copper. The right sketch is correct and preferred, with the power and communication grounds referenced to the power ground with a very short bonding conductor. Many new homes are constructed to bring all three to one location.
Figure 2: House Grounding Configurations
WHAT THE CODE SAYS
The 1993 National Electric Code sets the requirements for bonding the communication, radio, and television antenna and CATV grounds to the power ground in Articles 800-40d, 810-21j and 820-40d. The code requires a minimum No. 6 copper bonding conductor between these ground electrodes and the power grounding electrode, where separate electrodes are used. Please see these articles and Article 250 of the National Electric Code for further detail on the proper grounding of low voltage electrical systems.
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.