What is the meaning of spd in electrical?

When an electrical surge occurs, voltage that greatly exceeds accepted peak voltage levels can pass through building circuits to electrical equipment. Without proper protection, this equipment is susceptible to damage or failure from a voltage surge. The type of protection required
to negate these spikes can be provided by a surge protective device (SPD).

Specifying the correct SPD requires that you identify and understand the ratings associated with its application. There are many performance values and ratings associated with an SPD, such as maximum continuous operating voltage (MCOV), voltage protection rating (VPR), nominal discharge current (In), and short circuit current rating (SCCR). The most misunderstood rating is the surge current rating, typically quantified in kilo-Ampere (kA).

What is the Type of Surge Protective Devices (SPD)?

The Type of Surge Protective Devices (SPD) is a classification method used to categorize devices that protect electrical systems from voltage surges, based on their protection functions, installation locations, and the ability to withstand various surge currents. SPDs are classified according to two major standards: IEC (International Electrotechnical Commission) and UL (Underwriters Laboratories). Each standard has its own classification and requirements to ensure the devices protect electrical systems from surge-related incidents.

Type of Surge Protective Devices According to IEC Standard

The IEC 61643-11 standard specifies performance requirements and testing methods for SPDs used in AC power systems. According to this standard, SPDs are classified into three main types with the following characteristics:

Type 1 SPD (Class I):

- Function: Protects the electrical system from direct lightning strikes.
- Installation Location: Installed at the system entrance, near the main distribution panel.
- Surge Waveform: 10/350 µs. This waveform simulates direct lightning strikes, with a rise time to peak in 10 µs and a decay to 50% in 350 µs.
- Surge Current Withstand (Iimp): High, typically from 10 kA upwards.
- Nominal Discharge Current (In): From 10 kA upwards, according to IEC 61643-11, Class I. This is the current the SPD can withstand multiple times without damage.
- Voltage Protection Level (Up): From 1.5 kV to 2 kV. Up is the maximum voltage the SPD allows to pass during discharge.
- Applications: Suitable for high-rise buildings, industrial facilities, and areas with a high risk of lightning strikes.

Type 2 SPD (Class II):

- Function: Protects the electrical system from surge voltages caused by indirect lightning strikes or switching operations.
- Installation Location: Installed at sub-distribution panels or after a Type 1 SPD.
- Surge Waveform: 8/20 µs. This waveform simulates surge voltages propagated within the electrical system, with a rise time to peak in 8 µs and a decay to 50% in 20 µs.
- Nominal Discharge Current (In): Medium, typically from 5 kA to 20 kA. This is the current the SPD can withstand multiple times without damage.
- Voltage Protection Level (Up): From 1.5 kV to 2 kV. Up is the maximum voltage the SPD allows to pass during discharge.
- Applications: Suitable for commercial, residential areas, and regions with moderate lightning risk.

Type 3 SPD (Class III):

- Function: Protects sensitive electronic equipment from residual surge voltages after they have been attenuated by Type 1 and Type 2 SPDs.
- Installation Location: Installed near sensitive electronic equipment such as sockets, small distribution boards, or terminal devices.
- Surge Waveform: 8/20 µs and 1.2/50 µs. These waveforms simulate residual surges, with faster rise times (1.2 µs) and slower decay times (50 µs).
- Nominal Discharge Current (In): Low, typically less than 5 kA.
- Voltage Protection Level (Up): From 1 kV to 1.5 kV. Up is the maximum voltage the SPD allows to pass during discharge.
- Applications: Suitable for sensitive electronic devices such as computers, telecommunication devices, and medical equipment.

Type of Surge Protective Devices According to UL Standard

The UL 1449 standard specifies requirements for SPDs used in electrical systems in North America. According to this standard, SPDs are classified into four Types:

Type 1 SPD:

- Function: Protects against surge voltages caused by direct or nearby lightning strikes from outside the electrical grid.
- Installation Location: Installed before the electricity meter, either before or after the main circuit breaker.
- Surge Current Withstand: Designed to withstand high surge currents.
- Applications: Suitable for large industrial and commercial buildings.

Type 2 SPD:

- Function: Protects against surge voltages propagated within the system or from the electrical grid.
- Installation Location: Installed after the main circuit breaker or at sub-distribution panels.
- Surge Current Withstand: Designed to withstand surge currents from the power grid or internal system faults.
- Applications: Suitable for residential and commercial areas.

Type 3 SPD:

- Function: Protects sensitive electronic devices from residual surge voltages.
- Installation Location: Installed at electrical outlets or near sensitive devices.
- Surge Current Withstand: Designed to withstand residual surge currents after passing through Type 1 and Type 2 SPDs.
- Applications: Suitable for household and office electronic devices.

Type 4 SPD:

- Function: Modular or assembly SPDs integrated into electrical equipment.
- Installation Location: Typically integrated within devices or distribution boards.
- Surge Current Withstand: Designed to meet the requirements of integrated electrical equipment.
- Applications: Suitable for electrical devices with built-in SPDs.

SPD: Working Principle

The operation of an SPD is simple yet effective. When a power surge occurs, the MOVs quickly reduce their resistance, increasing their conductivity. This allows them to divert most of the surge current safely to the ground before it can reach and damage connected devices. By doing so, the surge is neutralized, protecting downstream equipment from high-voltage or current spikes.

What are transient overvoltages?

Transient overvoltages are brief, high-magnitude voltage surges that occur over a short period. These surges arise from the sudden release of stored energy or are induced by external factors. They can be classified as either naturally occurring, such as lightning strikes, or man-made, like switching operations in electrical systems.

How do transient overvoltages occur?

Transient overvoltages caused by human activity often result from the operation of motors, transformers, and certain lighting systems. In the past, these events were uncommon in residential settings. However, the rise of modern technologies like electric vehicle chargers, air and ground source heat pumps, and variable-speed washing machines has significantly increased the likelihood of transients in domestic electrical systems.

Natural transient overvoltages are typically triggered by indirect lightning strikes. For example, a direct lightning strike on nearby overhead power or telephone lines can send a surge along the lines. This can lead to severe damage to electrical installations and connected equipment.

How to Properly Size SPDs

There is very little published data or even recommendations on what level of surge current (kA) rating should be used in the different locations. The Institute of Electrical and Electronics Engineers (IEEE) has provided some input on what surge ratings are and how to interpret them but does not publish recommendations. Unfortunately, there is not a proven equation or calculator available to input system requirements and receive a solution. Any information a manufacturer provides, via calculators or other means, is merely their recommendation.

There is a tendency to assume that the larger the panel, the larger the kA device rating needed for protection. Another misconception is that if 200 kA is good, then 400 kA must be two times better. As you will see in this whitepaper, this is not always the case. As a result of its many years of knowledge, experience and expertise in the electrical industry, Emerson has generated some guidance on how to apply surge current ratings. (See figure 1, next page)

Selecting the correct type of surge protective device and understanding their classifications according to IEC and UL standards is crucial to ensuring that your electrical systems and electronic devices are adequately protected from voltage surges. Each standard provides a different approach to safeguarding electrical systems, depending on the specific requirements of the application and location.

The primary purpose of a surge protective device is to shunt and suppress the transient voltages that are being introduced into an electrical distribution system from either an external or internal source. Selecting the proper surge current (kA) rated SPDs throughout the electrical distribution system provides the best performance life for equipment. When selecting the appropriate SPDs for your facility, keep these key points in mind:

1. Providing proper surge suppression to a facility and the equipment within requires more than a single SPD located at the service entrance. We recommend cascaded SPDs with a proper surge current rating for each location. This will provide superior suppression for a service panel or critical load. A single SPD, no matter how big or expensive, will not provide the same level of system protection.

2. Over-sizing an SPD for its application cannot hurt a system, but under-sizing the SPD can result in premature failure of the SPD, leaving systems exposed to transients and their effects.

3. For direct lightning strikes, SPDs alone are not a replacement for a comprehensive lightning protection system (refer to UL96A Master Lightning Certification).

Cautions When Installing SPD

To ensure surge protective devices (SPDs) function effectively, careful installation is essential. Key precautions include:

- Install SPDs in parallel, positioned directly before circuits or devices, to redirect surge currents away from sensitive equipment.
- Keep the connection wires within the switchboard as short as possible, with a maximum length of 0.5 meters.
- Using only a Type 1 surge protector may not be adequate for managing high-energy surges and reducing overvoltages. It is advisable to complement it with a Type 2 or Type 3 surge protector.
- All installations must be carried out by qualified electricians following local electrical regulations to ensure proper grounding and secure mounting of the device.

Conclusion

In conclusion, surge protective devices are essential for safeguarding electronics in both industrial and commercial settings. Installing a correctly rated and certified SPD provides reliable protection against power surges that exceed the capabilities of standard circuit breakers.