According to 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.”
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 PC62.72 Draft D1:
Generally, the SPD grounding lead/terminal connects to a local ground/bonding point at the SPD’s mounting location area. This local ground/bonding point then utilizes the facility’s bonding and grounding infrastructure to access the grounding system of grounding electrodes.
Accordingly, the SPD specifier/installer should depend upon the proper application of relevant codes and standards and recommended practices to ensure SPD performance during lightning events. Such documents can vary per global location. Generally, for North America the following documents will apply:
- NFPA 70 – The National electrical Code (NEC)
- NFPA 780 – Standard for the Installation of Lightning Protection Systems
- IEEE Std 142-2007 – IEEE Recommended Practice for Grounding of Industrial and Commercial Power System
- IEEE Std 1100TM – IEEE Recommended Practice for Powering and Grounding Electronic Equipment
The SPD grounding lead should be as short as practical and installed in a straight and direct manner as
feasible. Partial self-inductance of this lead will be the dominant impedance factor for the lead 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 should be inspected to ensure compliance to applicable electrical codes and lightning protection system requirements. Attempts to reduce the net impedance value of the connection of the common-bonding network to the grounding system are unnecessary. Such a reduction does not improve the protective performance of the SPDs inside the facility.
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 not practical or easy 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, more important than 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 detail about grounding, see the references below.
References:
IEEE Std 142-2007 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems
IEEE Std 1100-2005 IEEE Recommended Practice for Powering and Grounding Electronic Equipment
IEEE Std C62.41.1-2002 IEEE Guide on the Surge Environment in Low-Voltage (1000 V and Less) AC Power Circuits
IEEE Std C62.72-2007 IEEE Guide for the Application of Surge-Protective Devices for Low-Voltage (1000 V or Less) AC Power Circuits
IEEE Draft Std PC62.72 Draft D1 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?