SPD SELECTION & INSTALLATION - MYTHS & FACTS

Low voltage is defined as any AC RMS voltage up to 1000 V or DC peak voltage up to 1500 V. In general, SPDs need to be installed at main DB, sub DB and at equipment level.

Figure 1-Example of installation of Class I, class II and class III tested SPDs
In the above figure instead of Class I and Class II, Class I + II can be used if Iimp value is taken care It is because SPDs provide an “effective” protection distance of a maximum of 10 meters. It means, if the distance between the SPD and the equipment to be protected is greater than 10 meters, additional SPDs will be required. The crucial point to note in this case is that the “actual” wire length needs to be calculated rather than the physical distance between the SPD and equipment.

Figure 2- Difference between physical distance and actual distance.
Physical location of equipment is important to select the quantity of SPDs. In the below Figure 3- 2 examples are given. In Example1, the number of PCs are 5 & in Example 2, the number of PCs are 4.. But in Example 1, one SPD is enough to protect all the 5 PCs as the “actual ‘ distance from SPD to the farthest PC- PC 3 is only 5 meters, because all the PCs are kept in one room, next to each other. Whereas. In Example 2, 4 number of SPDs are needed to protect 4 PCs , because 1 PC is located in each floor and the ‘actual” distance exceeds 10 m in Floors 1, 2 & 3.Only PC 1 does not require any additional SPD because the “actual’ distance from the SPD which is in Sub DB is only 5 meters away from PC1.

Figure 2- Difference between physical distance and actual distance.
The below selection principle shall be strictly followed: a. Protection between Live conductor and PE is COMPULSORY. b. Protection between Live conductor and neutral is RECOMMENDED to ensure equipment

SPDs provide an "effective" protection distance of a maximum of 10 meters. It means, if the distance between the SPD and the equipment to be protected is greater than 10 meters, additional SPDs will be required. The crucial point to note is that the "actual" wire length needs to be calculated rather than the physical distance between the SPD and equipment.

The below selection principle shall be strictly followed:

• Protection between Live conductor and PE is COMPULSORY.
• Protection between Live conductor and neutral is RECOMMENDED to ensure equipment protection.
• Protection between Live Conductors (3 phase) is OPTIONAL.

Figure- 4 -Connection type CT1 (4+0 configuration) for a 3 phase system with separate neutral and earth terminals (Common mode protection)

Figure- 5 - Connection type CT1 (3+0 configuration) for a 3 phase system with separate neutral and earth terminals (Combination of common mode and differential mode protection)

Figure- 6 - Connection type CT1 (3+1 configuration) for a 3 phase system with separate neutral and earth terminals (Combination of common mode and differential mode protection)

Installation - FACT:

The total length of the cable — from live wire to OCPD, from OCPD to SPD, and from SPD to earth terminal — has to be below 0.5 meter to ensure protection of equipment.

Figure 7- Class I or Class I+II SPD installation to decrease the lead length of SPD supply conductors.

Figure 8- Class I or Class II SPD installation to decrease the lead length of SPD supply conductors.

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