Created by: Glen Zhu | Updated Date: Dec 2, 2022
In the IEC system, SPDs are tested to various Test Classes, intended to assess and assure their suitability for use in different locations and circumstances. Strictly speaking, the Class refers to the type of test, not to the SPD. However, in common usage, SPDs are referred to by their Class, for example, a Class I SPD is an SPD that has been tested to Class I requirements (of a specified severity), and so on.
Class I / Type 1 – Tested with simulated partial conducted lightning current impulses. These SPDs would be used at points of high exposure, such as where the line close to the SPD might be directly struck by lightning, or at the point of entry to a building fitted with a direct strike Lightning Protection System (LPS).
Class II / Type 2 – Tested with shorter duration current impulses. These SPDs would be installed where the surge currents are expected to be less. This could be at the main power entry point of a building in a non-exposed location (surrounded by taller buildings, for example), or at sub-panels within the building.
Class III / Type 3 – Tested with voltage impulses. These SPDs would be installed at equipment to be protected, and are only expected to handle residual voltages surges that “got past” upstream Class I or II SPDs, and the associated small surge currents. Often, for convenience, Class II protectors are used at these locations as well.
In the illustration above, the type of SPDs installed at the Main Distribution Board, Distribution Boards, and the Equipment to be protected would be as follows:
Building Situation | MDB | DB | Equipment |
Highly exposed, of fitted with LPS | Class I / Type 1 | Class II / Type 2 | Class III / Type 3 |
Less exposed, no LPS | Class II / Type 2 | Class II / Type 2 | Class III / Type 3 |
There are a number of IEC/EN standards that work together to provide a system of classifying the power system, the over-voltages that can occur at different points in the system, the performance and application of SPDs, and the relative susceptibility of end use equipment to lightning surges. The most directly relevant are the IEC/EN 62305 series standards dealing with both lightning protection and surge protection, and the IEC/EN 61643 series standards covering testing, selection, and application of SPDs.
Fitting SPDs at all three locations may not be necessary, depending on the building size, and wiring length. Generally, SPDs are always fitted at the point of entry, and in smaller equipment rooms may just be, additionally, at the equipment. In larger buildings, spread over multiple floors or large areas, SPDs would usually be provided at the distribution boards, and additionally at sensitive or critical equipment.
SPDs are primarily rated according to how large a surge current magnitude they can handle, and how well they limit the voltage while conducting that surge current. These parameters are
Test Class | Parameter | Description |
Class I / Type 1 | Impulse Current, Iimp | This current impulse has a 10/350μs waveform |
Class II / Type 2 | Nominal Discharge Current, In | This current impulse has a waveform of 8/20 μs, and is nominal because the SPD has to successfully handle a sequence of 15 of these impulses. |
| Maximum Discharge Current, Imax | This current impulse has a waveform of 8/20 μs, and is the maximum 8/20 μs impulse the SPD can handle. It is an optional parameter. |
Class III / Type 3 | Open circuit voltage of the combination wave generator, Uoc |
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All Classes | Voltage Protection Level, Up |
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It is possible to test one SPD type at more than one Test Class. SPDs are marked and specified with the parameters they have been successfully tested to.
Having determined the Class of SPD required, the correct voltage and configuration needs to be determined. The standard IEC 60364-1 details the following system configurations. In the descriptions that follow, Un is used for the nominal systems voltage, and Uc is used for the maximum continuous operating voltage (this is a parameter of an SPD).
In this system, the neutral and protective earth conductor are combined in a single conductor throughout the system. This conductor is referred to as a PEN, a “Protective Earth & Neutral”. All exposed conductive equipment parts are connected to the PEN.
SPDs installed | Description | Example product |
Phase to PEN (“3+0”) | At least 1.1 x Un | SLP40-275/3S |
For example, on a 230 V Ph-N system, Ph-PEN protection should have a Uc rating of at least 255 V. Generally an SPD with a Uc rating of at least 275 V would be selected for 220 to 240 V systems. Often, to allow for power supply voltage fluctuations, a Uc of at least 1.3 x Un is recommended, such as a Uc of 300 V for a 230 V system, or LSP’s unique trigger release technology would be chosen.
In this system, a separate neutral and protective earth conductor are run throughout. The Protective Earth (PE) conductor is normally a separate conductor, but can also be the metallic sheath of the power cable. All exposed conductive equipment parts are connected to the PE conductor.
SPDs installed | Description | Example product |
Phase to PE (“4+0”), or | At least 1.1 x Un | SLP40-275/4S |
Phase-N, and N-PE (“3+1”) | SLP40-275/3S+1 |
For example, on a 230 V Ph-N system, Ph-PE (or Ph-N) protection should have a Uc rating of at least 255 V. Generally an SPD with a Uc rating of at least 275 V would be selected for 220 to 240 V systems. Often, to allow for power supply voltage fluctuations, a Uc of at least 1.3 x Uo is recommended, such as a Uc of 300 V for a 230 V system, or LSP’s unique trigger release technology would be chosen.
In this system, the supply is configured as per TNC, while the downstream installation is configured as per TNS. The combined PEN conductor typically occurs between the substation and the entry point into the building, and earth and neutral are separated in the Main Distribution Board. This system is also known as Protective Multiple Earthing (PME) or Multiple Earthed Neutral (MEN). The supply PEN conductor is earthed at a number of points throughout the network and generally as close to the consumer’s point-of-entry as possible.
SPDs installed | Description | Example product |
MDB: Phase to PEN (“3+0”) | At least 1.1 x Un | FLP12,5-275/3S |
DB: Phase to PEN (“4+0”), or | FLP12,5-275/4S | |
Phase-N, and N-PE (“3+1”) | FLP12,5-275/3S+1 |
For example, on a 230 V Ph-N system, Ph-PE (or Ph-N) protection should have a Uc rating of at least 255 V. Generally an SPD with a Uc rating of at least 275 V would be selected for 220 to 240 V systems. Often, to allow for power supply voltage fluctuations, a Uc of at least 1.3 x Un is recommended, such as a Uc of 300 V for a 230 V system, or LSP’s unique trigger release technology would be chosen.
A system having one point of the source of energy earthed and the exposed conductive parts of the installation connected to independent earthed electrodes. The incoming supply neutral is not earthed at the main distribution board.
SPDs installed | Description | Example product |
Phase to N, N-PE (“3+1”) | At least 1.1 x Un | FLP12,5-275/3S+1, SLP40-275/3S+1 |
For example, on a 230 V Ph-N system, Ph-N protection should have a Uc rating of at least 255 V. Generally an SPD with a Uc rating of at least 275 V would be selected for 220 to 240 V systems. Often, to allow for power supply voltage fluctuations, a Uc of at least 1.3 x Un is recommended, such as a Uc of 300 V for a 230V system, or LSP’s unique trigger release technology would be chosen.
In the TT system, in order for overcurrent protective devices (fuses and circuit breakers) to operate in an intended manner, it is important that SPDs must not connect directly from phase to protective ground, but from phase to neutral and neutral to ground. Therefore, the Neutral-to-PE SPD carries both the PE to neutral impulse current and the PE to phase impulse currents. This SPD is recommended to be a GDT (Gas Discharge Tube) due to its generally superior energy handling characteristics.
A system having no direct connection between live parts and earth, but all exposed conductive parts of the installation being connected to independent earthed electrodes. The source is either floating or earthed through a high impedance (to limit fault currents). This means that during a Phase to Earth fault, the systems continue to operate. This is detected, and maintenance efforts commenced to rectify the fault. However, during this time, the Phase to Earth voltage rises to the usual Line to Line voltage, and installed SPDs must withstand this during this time. Most installed IT systems do not utilize a neutral conductor – equipment is powered from line to line. The IT system is typically used in older installations in countries such as Norway and France. It is also used in special applications, such as intensive care wards of hospitals and special industrial applications.
SPDs installed | Description | Example product |
Phase to PEN (“3+0”) | At least 1.73 x Un | SLP40-275/3S |
SPDs installed | Description | Example product |
Phase to PEN (“4+0”) | At least 1.73 x Un | FLP12,5-275/4S, SLP40-275/4S |
For example, on a 230 V Ph-N system, Ph-PE and N-PE protection should have a Uc rating of 440 V (allowing for the L-L voltage and a 10% tolerance). Often an additional safety margin is applied, to allow for instabilities that can occur in the ungrounded IT system, such as a Uc of 480 V.
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