AC SPD Coodination

AC SPD Coodination

Many applications require the use of two or more SPDs in order to reduce the electrical stress on the equipment to be protected to an acceptable value. Very frequently, a Type 1 SPD is installed at the structure entrance and Type 2 SPDs are used in distribution boards or even near equipment to be protected.Using a Type 1+2 SPD at the entrance doesn’t solve the problem, because the protected distance of an SPD is generally limited to a few meters (maximum of 10 m) expect it is of the two-port type that can allow up to 50 m or more (but these SPDs are generally not designed for panel boards).It is known since a long type that in such a case, these two or more SPDs should be coordinated. 

For a long time, safety was the only issue, and coordination was based on energy. Energy coordination means that the sharing of the stress between the two SPDs is related to their energy withstand in order both SPDs are not destroyed.But this is not enough nowadays and it is needed to show that the device protected by the second SPD is also effectively protected. This is called voltage coordination or sometimes protection level coordination. Purpose is to ensure that the voltage at the terminal of the second SPD is not exceeding its voltage protective level Up. 

Furthermore, the residual voltage of this SPD can be reduced below its Up by ensuring that current lower than the nominal discharge current flows through the second SPD. The second SPD can also be a built-in SPD or even surge protective component.The fact the voltage at the second SPD can be lower and sometimes much lower than its Up is very important for critical installations where protection needs to be demonstrated (data center, nuclear installations). In some cases, it is the only way to provide protection especially when a protective level below 1.5 kV is needed.Coordination can be demonstrated by tests or simulations.