A brief introduction to grounding systems is provided in IEEE Std 142-2007, the definition of “ground” is “a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some other body that serves in place of the earth.” The same standard also gives the following definition of a grounded system as “A system in which at least one conductor or point (usually the middle wire or neutral point of transformer or generator windings) is intentionally grounded, either solidly or through an impedance.” The 2014 NEC definition of Bonding is “Connected to establish electrical continuity and conductivity“. Bonding and Grounding are the framework for providing a safe electrical system where a person is protected from electrical shock and safety hazards.
In relation to the operation and installation of SPDs, the specifier and installer should consider the following information related to grounding and bonding within the power distribution or electrical system.
From IEEE 62.72:
“The importance of ensuring that the grounding system provides a low earth impedance, and not simply a low earth resistance, must be understood. A spectral study of the typical waveforms associated with transient impulses, such as those characteristic of lightning and switching surges, reveals both high-frequency and low-frequency components. The high-frequency component is associated with the extremely fast rising “front” of the transient (typically less than 10 μs to peak current), whereas the low-frequency component resides in the long “tail” or follow-on current of the decaying impulse. High-frequency components are significant for inductive effects (induced voltages in the circuits), whereas low-frequency components are significant for energy effects (deposited energy in resistive elements).”
If an SPD is connected to ground and is expecting ground to be part of its dissipative path, the ground system must be taken into account. It also means the grounding system must be verified as a low resistive and low impedance path.
The SPD grounding lead should be as short as practical and installed in a straight and direct manner as feasible. The self-inductance path of this lead and the grounding system integrity will be the dominant impedance factor during a surge current event such as lightning – due to the relatively short rise time of the lightning current waveform.
Relative to the installation of SPDs, the condition of the grounding system for the common-bonding network must be inspected to ensure compliance to applicable electrical codes and lightning protection system requirements.
Reduction of the resistance or impedance of the grounding system at the service entrance area can reduce the amount of to-ground surge current that enters the facility. However, it is difficult to reduce this parameter after the facility is built. Consequently, the surge impedance of the service entrance and PDS wiring is relied upon to limit the surge currents reaching the SPDs within the facility.
Therefore, just as important as the absolute value of the grounding resistance or impedance is to ensure that all the equipment in the facility is referenced to the common-bonding network. At a minimum, local/regional areas within the network raise/lower to the same relative potential during an impinging surge current event independent of the grounding system.
For more detailed information about grounding, see the references below.
IEEE Std 142IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems
IEEE Std 1100IEEE Recommended Practice for Powering and Grounding Electronic Equipment
IEEE Std C62.41.1IEEE Guide on the Surge Environment in Low-Voltage (1000 V and Less) AC Power Circuits
IEEE Std C62.72IEEE Guide for the Application of Surge-Protective Devices for Low-Voltage (1000 V or Less) AC Power Circuits
IEEE Std PC62.72 IEEE Guide for the Application of Surge-Protective Devices for Use on the Load Side of the Service Equipment in Low-Voltage (1000 V or Less, 50 or 60 Hz) AC Power Circuits
NFPA 70 – The National electrical Code (NEC)
NFPA 780 – Standard for the Installation of Lightning Protection Systems
White paper: Ground Resistance or Impedance?