Internal Lightning Protection and Surge Devices: What's the Difference?
1.Relationship Between Internal Lightning Protection and Surge Protection
Internal Lightning Protection (Internal LPS):
This is part of the lightning protection system. It includes equipotential bonding for lightning and/or electrical isolation from the external lightning protection system.
Surge Protection Measures (SPM):
These are measures taken to protect internal systems from lightning electromagnetic pulses (LEMP). SPM is also part of the overall lightning protection system.
According to Figure 1 in GB/T 21714.1, complete lightning protection includes both the Lightning Protection System (LPS) and Surge Protection Measures (SPM).Surge protection (SPM) is covered in GB/T 21714.4, while internal lightning protection is part of the LPS (which includes both external and internal parts), and is described in GB/T 21714.3.
Figure 1 – Relationship Between the Parts of GB/T 21714
2.Origin of Internal Lightning Protection and Surge Protection
2.1 IEC 1024-1:1990
In IEC 1024-1:1990, "Protection of structures against lightning – Part 1: General principles", section 1.2.7 defines Internal Lightning Protection as all additional measures beyond the external lightning protection system. These measures help reduce the electromagnetic effects of lightning currents inside the protected space. So, according to IEC 1024-1:1990, internal LPS includes everything other than the external LPS — which also covers protection against electromagnetic pulses (LEMP).
2.2 IEC 62305-1:2006
By the time of IEC 62305-1:2006, "Protection against lightning – Part 1: General principles" (1st edition), the definitions became more specific:
3.42 Internal Lightning Protection System (LPS): A part of the LPS consisting of equipotential bonding and/or electrical isolation from the external LPS. 3.49 LEMP Protection Measures System (LPMS): A full system of measures to protect internal systems against lightning electromagnetic pulses (LEMP).
The IEC 62305-1 to 4:2006 series replaced earlier standards, combining several IEC TC81 documents. It split the old definition of internal LPS from IEC 1024 into two parts: internal LPS and LPMS (for LEMP protection). In IEC 62305-1:2024, LPMS was officially renamed to Surge Protection Measures (SPM).
3 Internal Lightning Protection Measures
Internal lightning protection is essential for a properly functioning lightning protection system. If no internal protection is in place, lightning currents flowing through the external LPS or other conductive parts of the building may cause dangerous sparks. These sparks can lead to fires, personal injury, or infrastructure damage. Apart from differences in separation distances, internal protection is the same across all lightning protection levels. Sparks may occur between the external LPS and the following components: Metal installations, Internal systems of the building, External conductive parts or pipelines connected to the protected building.
According to standards, sparks occurring in potentially explosive areas of the structure are always considered dangerous. To prevent spark formation, one of the following methods should be used: Equipotential bonding, or; Electrical insulation between metal parts.
Equipotential bonding measures (see Figure 2) include the following:
Bonding conductors, used when natural connections do not provide electrical continuity, e.g., by directly connecting to water or heating pipes;
Surge Protective Devices (SPD), used when direct bonding is not possible, e.g., for low-voltage lines; and
Isolated Spark Gaps (ISG), used when bonding conductors are not allowed, e.g., for cathodic protection pipes or gas pipelines.
Figure 2
Equipotential bonding for lightning protection has the following characteristics based on different types of external LPS:
Isolated LPS: Bonding is only done at ground level;
Attached LPS: Bonding should be done at:
（1）Basement or ground level, and;
（2）Any location where required insulation or separation distance is not achieved.
Electrically insulated LPS: In addition to ground-level bonding, bonding can also be done at air-termination systems or down conductors.
4 Surge Protection Measures
Basic SPM includes the following components:
(1) Earthing and bonding network
The earthing system conducts and dissipates lightning current safely into the ground.
The bonding network minimizes potential differences and reduces magnetic field effects.
(2) Electromagnetic shielding and proper wiring
Spatial shielding reduces magnetic fields inside the LPZ caused by direct lightning strikes or strikes near the building, and helps reduce internal surges.
Shielding internal cables with shielded cables or conduits minimizes induced surges.
Proper internal wiring layout helps minimize induction loops and reduce internal surges. Shielding external lines at the building entry point also reduces surges conducted into internal systems.
(3) Coordinated SPD system
A coordinated SPD system limits the impact of both external and internal surges.
(4) Isolation interfaces
Isolation interfaces reduce the effect of conducted surges on lines entering the LPZ.
Earthing and equipotential bonding must always be ensured, especially at building entry points — either by direct connection or by connecting each conductive service to an SPD as part of the equipotential bonding.
(5) Use of Thunderstorm Warning Systems (TWS)
Clause 7.1 of IEC 62305-1:2024 states that activating a TWS in compliance with IEC 62793 can serve as a protection measure by disconnecting external services, helping reduce the frequency of damage to electrical and electronic systems.
5 Conclusion
Internal system measures aim to prevent dangerous sparking, including lightning equipotential bonding with the first-level SPD. Surge protection mainly addresses surges impacting internal systems, and the SPDs used for this purpose must be coordinated with the first-level SPD, but do not include the first-level SPD itself.