Ground Potential Rise (GPR) in pad-mounted cabinets causes ongoing card failures and it is a troublesome issue for every cabinet and equipment manufacturer across the United States. When the voltage in the earth rises and every electrical component within the cabinet does not rise to the same equal voltage potential, chaos begins.
The OEM manufacturer uses different standards and practices to bond, ground and protect the equipment inside of the cabinets that they sell. Some manufacturers refer the end user to a grounding standard that the end user may or may not have a copy of. Some manufacturers refer the end user to the National Electrical Code (NEC) for proper bonding, grounding, and equipment protection. When the end user needs to ground a piece of equipment inside of the cabinet the installer does the best possible job that he has been trained to do. If he has had little or no training on bonding, grounding, and equipment protection then there is a very high percentage that the bonding, grounding, and equipment protection will suffer greatly.
Once the OEM original equipment useful life is over, usually the pad mounted cabinet old equipment gets replace with new OEM equipment. The equipment installer installing the new equipment seldom has original drawings of the OEM bonding, grounding, and equipment protection standard used. In most cases the installer makes sure that he grounds the new equipment to some ground bar in the cabinet. However, he does not test for equal voltage potential between connection points. This can and does lead to disaster in many cases.
Depending on the location, weather pattern and the type of equipment in the cabinet card failures will vary. In many cases, only one cabinet in an area seems to suffer the most failures. Card failures and equipment outages typically occur during some type of weather condition. Lightning strikes are always considered the culprit. You know the saying “when lightning strikes anything can happen.” However, lightning does not always have to have a direct strike on a cabinet to cause equipment damage. A lightning strike as far away as nine (9) miles can cause card failure. When this happens it is from a Ground Potential Rise. A Ground Potential Rise (as defined in IEEE Standard 367) is the product of a ground electrode impedance, referenced to remote earth, and the current that flows through that electrode impedance. The average lightning strike takes about 0.2 seconds from start to finish, it also averages several million volts and around 20,000 amperes. This means that from the point of a direct lightning bolt strikes the earth it could take up to nine (9) miles for that voltage go to zero (0) voltage (remote earth).
In the many different brands of cabinets that we have inspected the voltage potential between every connection point in the OEM cabinet were not always equal. The earth ground in the cabinets that we have inspected have ground resistivity readings ranging from five Ohms (5.0) to a completely open reading. These are just the conditions that a GPR needs to cause card failures and system outages.
Each pad-mounted cabinet must be evaluated for proper bonding, grounding, and equipment protection. If your staff does not have subject matter experts on bonding, grounding and equipment protection you may be in for many card failures that could have been prevented.