How To Choose A High-quality Surge Protector?

How to choose a high-quality surge protector? The varistor type surge protective device is subjected to power line voltage for a long time. Due to the (transient) overvoltage of the power grid and the surge shock, its performance will gradually decline, the Uc value will slowly decrease, and the leakage current will gradually increase. When the current flowing through the surge protection device reaches the mA level, the surge protector (SPD) can be heated and catch fire. However, the mA level current is not enough to trigger the backup protection action of the surge protector (SPD), so for this kind of fault, the SPD is generally disconnected by the action of the thermal protection trip device inside the SPD, which is why the voltage-limited power supply SPD requires an internal disconnector.

How to choose a high-quality surge protector? In addition to the discharge capacity, the safety performance of the SPD product itself should also be focused on. Now, taking the power supply SPD of the Class II experiment (8/20us) as an example, let's talk about the safety requirements of SPD.

1. Switching power supply SPD needs to have continuous current interruption capability

When introducing the GDT power frequency continuous current problem, the concepts of switching surge protector (SPD) and voltage-limiting surge protector (SPD) were mentioned. Due to the existence of power frequency continuous current, switching SPD is generally only used between N-PE (under normal circumstances, there is no voltage difference between N-PE). If the switching SPD is to be used between the line and the neutral (ground), it must have continuous current interruption capability. In order to prevent the surge protector (SPD) from being unable to interrupt the continuous current, a backup protection device is also required.

2. Voltage-limiting surge protector (SPD) needs to have thermal protection disconnection function

Due to the continuous current interruption problem of the switching surge protector (SPD), the power surge protector (SPD) is mostly voltage-limiting type, and its commonly used component is varistor. Varistor, as the name implies, is that the size of its resistance is extremely sensitive to voltage. When the power line voltage is lower than Uc, it is in a high-resistance state, which is approximately an open circuit. When a surge occurs, the voltage is much higher than Uc, and it presents a low resistance state, which is close to a short circuit. When the surge disappears, it immediately returns to a high resistance state, and no continuous current will occur.

Although the voltage-limited surge protector (SPD) has no continuous current problem, it will fail due to various reasons. There are usually two types of failure states:

① Short circuit state;

② Variable resistance state (deterioration);

The open circuit state of the surge protector (SPD) is considered to be a safe state. When the surge protector (SPD) has the above two fault states, we need to use the internal and (or) external separation mechanism of the surge protector (SPD) to make the SPD enter the open circuit state, and remove the surge protector (SPD) from the circuit to avoid accidents such as the surge protector (SPD) catching fire and the main circuit power failure.

The varistor type surge protector (SPD) is subjected to power line voltage for a long time. Due to the (transient) overvoltage of the power grid and surge shock, its performance will gradually decline, the Uc value will slowly decrease, and the leakage current will gradually increase. When the current flowing through the surge protector (SPD) reaches the mA level, the surge protector (SPD) can be heated and catch fire. However, the mA level current is not enough to trigger the backup protection action of the surge protector (SPD), so for this kind of fault, the SPD is generally disconnected by the action of the thermal protection trip device inside the SPD, which is why the voltage-limiting power supply SPD requires an internal disconnector.

3. SPD should have sufficient short-circuit tolerance

The varistor type SPD may also directly enter the short-circuit state. Common reasons are:

① Transient overvoltage (TOV) of the power grid. High-voltage TOV may directly break down the varistor and cause the SPD to short-circuit;

② The SPD is hit by a surge that exceeds its tolerance and is broken down and short-circuited;

When the SPD enters the short-circuit state, the current flowing through the SPD is very large, which may reach hundreds or even thousands of amperes. The large current causes the SPD to heat up quickly. In this case, the internal disconnector of the SPD cannot guarantee reliable disconnection, and the SPD has already caught fire. In order to avoid accidents caused by SPD short-circuit failures, it is necessary to configure a backup protection device for the SPD.

Therefore, whether it is the problem of continuous current of the switch-type SPD or the problem of short-circuit failure of the voltage-limiting SPD, we need to configure backup protection when using the SPD.

However, even if a backup protection device is equipped, the following problems may occur:

① The SPD has caused a fire accident before the backup protection device is disconnected;

② The SPD is disconnected somewhere inside under the short-circuit current, but the disconnector does not act (indicate). That is to say, the SPD is damaged, but the display function is normal, which will cause the system to be in a state without SPD protection, and the next lightning strike may cause equipment damage.

Therefore, the SPD is required to have a sufficiently large short-circuit tolerance. The short-circuit tolerance of the SPD is preferably not less than the expected short-circuit current of the power system to avoid the above two problems.