What is SPD?

What Exactly is an SPD?

An SPD (Surge Protective Device), often called a surge protector or lightning arrester, is a critical component installed in your electrical distribution system. Its sole purpose is to protect electrical and electronic equipment from transient overvoltages—sudden, brief spikes in voltage.

These damaging surges can originate from two main sources:

1. External Sources: Such as lightning strikes (direct or nearby) on power lines, which is the most severe cause.
2. Internal Sources: More common but still harmful, these include surges from the switching of heavy machinery, HVAC systems, or other large loads within your facility.

Without an SPD, these surges travel unimpeded through your wiring, degrading or destroying sensitive circuitry in computers, industrial controls, appliances, and HVAC systems.

How Does a Surge Protective Device Work?

The operation of an SPD is elegantly simple. It is connected in parallel to the power supply circuit it protects. Under normal voltage conditions, the SPD presents a very high resistance and is essentially “invisible” to the system—it does not interfere with regular operation.

When a voltage surge exceeding a predefined threshold occurs, the SPD activates within nanoseconds. Its internal components (like metal oxide varistors or gas discharge tubes) instantaneously switch to a state of very low resistance. This creates a safe, low-impedance path to ground, diverting the massive surge current away from the protected equipment and safely into the earth. Once the surge passes, the SPD automatically resets to its high-impedance state, ready for the next event.

Key Features

When selecting an SPD for electrical systems, consider these crucial specifications that define its capability:

• Nominal Discharge Current (In): This indicates the SPD’s standard protective level and durability. Common values are 20kA, 40kA, etc.
• Maximum Discharge Current (Imax): The maximum single surge current the SPD can handle without failure. A higher rating, such as 60kA, offers greater robustness, especially against severe lightning surges.
• Voltage Protection Level (Up): The maximum voltage that will be let through to your equipment. A lower Up means better protection.
• Type/Class: Based on IEC standards, Type 2 (Class C) SPDs are the standard for main distribution board protection against the remainder of lightning currents and switching surges.
• Status Indication & Disconnector: A vital safety feature. A good SPD includes a thermal disconnector that automatically removes a failed unit from the circuit to prevent a fire hazard, along with a clear visual indicator (often green/red) to show its operational status.

About us

For robust, reliable protection of low-voltage AC power systems (like TT, TN-S), a Type 2 SPD is typically the core of your surge protection strategy. Companies like Zhejiang Mulang Electric Technology Co., Ltd. specialize in manufacturing a wide range of these essential devices under their MILELE® brand.

A solid example from their portfolio is the MLY1-C40/385 AC SPD. This device is engineered as a Class II / Type 2 surge protector, making it ideal for installation in main distribution panels. With a nominal discharge current (In) of 40kA and a maximum discharge current (Imax) of 60kA, it provides substantial capacity to clamp common and differential mode surges. Its built-in thermal disconnector enhances safety by isolating the module upon failure, and the clear color-coded window (green for OK, red for fault) allows for instant, easy maintenance checks. Complying with international standards like IEC 61643-1, this 4-pole SPD offers a dependable solution for safeguarding three-phase electrical systems against transient overvoltages.

In summary, an SPD is not just an accessory; it’s a necessary investment in the longevity and reliability of your electrical infrastructure. Understanding “what is an SPD” is the first step toward preventing costly downtime and equipment damage caused by unpredictable power surges.