The short answer is: If your surge protection device is more than 20 feet from the electrical service point of earthing, you are required to have a supplemental earthing electrode (for example, an earthing rod) at that surge protection device location.
NFPA 780 Standard for the Installation of Lightning Protection Systems covers the installation of surge protection devices (SPDs). NFPA 780 Section 126.96.36.199.1 requires the SPD to be bonded to electrical service point of earthing, per the National Electrical Code. Section 188.8.131.52.2 states: “If the point of earthing … is greater than 6 m (20 ft) away, a supplementary earth electrode or electrode system shall be installed at the SPD location.”
Also note: If you have conductors that exit your structure to supply another structure and the conductor run is more than 100 feet than you need two more surge protection devices. Add a surge protection device where the conductors exit the first structure and also add a surge protection device where they enter the second, per NFPA 780 Section 184.108.40.206. Inevitably, one or more of these surge protection devices will be more than 20 feet from your service point of earthing and require their own supplemental earthing electrode (e.g. earthing rod).
We recommend that you add a Type 2 surge protection device to every panelboard in your installation. Multiple surge protection devices work together to form cascading protection. Furthermore, Type 2 surge protection devices include EMI filtering which will help remove internal noise caused by VFDs and switching power supplies.
Minimize Earth Impedance (and Resistance) by Installing Your Earth Rods Correctly
For a surge protection device to discharge current to earth, you want to earth impedance (and resistance) to be a low as possible. A lower earth resistance will minimize the voltage seen on the earth terminal of the SPD. NFPA 780 Annex A states: “It is essential to minimize impedance in this circuit.”
Because lightning energy contains high-frequency components, the conductors between the surge protection device and earth should avoid sharp bends. Sharp bends will look like high-impedance paths to the high-frequency components of the lightning. The radius of conductor bends should be eight (8) inches or more to form gentle sweeping bends. This should include the connection to the closest earthing rod, which should have a sweeping radius all the way to the rod, see Figure 220.127.116.11(B). It is not recommended to use a T-shaped connection to an earthing ring or earthing rod.
Generally, a better earthing system will have a lower impedance (and resistance) to earth. So any improvements to your earthing system will also improve the performance of your surge protection devices.
Minimize Lead Impedance
It is extremely important to minimize the conductor lead length between the phase bus to the surge protection device and from the surge protection device to the earth bus. Every inch of lead length adds 15 to 25 volts over the surge protection device’s Voltage Protection Rating (VPR) or Let-Through Voltage. Therefore, each foot add 200 to 300 volts to the Let-Through Voltage! The VPR printed on the nameplate of the surge protection device is based on only six (6) inches of lead length. Surge protection devices should be purchased pre-installed with your panelboard, bolted directly on the bus bars.
The Let-Through Voltage can be reduced further by twisting all the lead wires together, instead of leaving them loose. Tests by ABB have shown up to a 23% reduction in additional Let-Through Voltage when the surge protection device leads were twisted. Surprisingly, the same testing showed a minimal reduction in performance as larger lead conductors were used. The #14 AWG (2 mm^2) performed almost the same as #10 AWG (5 mm^2) or even #4 AWG (21 mm^2)! Of course, also be sure to follow all the manufacturer’s instructions.
photo credit: Joy Mystic