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How to Ground a Surge Protector

How do you ground a surge protector? Do I need a grounding rod?

Hi Jim,
The short answer is: If your surge protection device is more than 20 feet from the electrical service point of grounding, you are required to have a supplemental grounding electrode (for example, a grounding rod) at that surge protection device location.

Location Requirements

NFPA 780 Standard for the Installation of Lightning Protection Systems covers the installation of surge protection devices (SPDs).  NFPA 780 Section 4.18.6.3.1 requires the SPD to be bonded to electrical service point of grounding, per the National Electrical Code.  Section 4.15.6.3.2 states: “If the point of grounding … 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 14.18.2.3.  Inevitably, one or more of these surge protection devices will be more than 20 feet from your service point of grounding and require their own supplemental grounding electrode (e.g. grounding 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 Ground Impedance (and Resistance) by Installing Your Ground Rods Correctly

For an surge protection device to discharge current to ground, you want to ground impedance (and resistance) to be a low as possible.  A lower ground resistance will minimize the voltage seen on the ground 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 ground 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 grounding rod, which should have a sweeping radius all the way to the rod, see Figure 14.13.2.3(B).  It is not recommended to use a T-shaped connection to a grounding ring or grounding rod.

Generally, a better grounding system will have a lower impedance (and resistance) to ground.  So any improvements to your grounding 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 ground 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.

How to ground a surge protector
photo credit: Joy Mystic

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