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In our last blog, we had touched upon the dangers and damage from Surges. We hope that blog answered some questions about Surge Protection –but we do realize that the questions aren’t done yet. Most of the questions that remain are probably about what can help provide the necessary protection – about Surge Protector Devices, that is. So, that’s what this post is about!

What is SPD?

A Surge Protector Device(SPD) is specifically designed to offer protection to electrical devices against voltage spikes. This device is connected in parallel to the power supply circuit of the loads that require protection. An SPD may be used at all levels of the power supply network. An SPD tries to restrict the voltage supplied to the electric device by blocking or shorting to ground any unnecessary voltages that exceed the threshold limit.

What is Surge Protection Device?

Surge Protection Device is a device which offers protection to the equipment against high voltage events occurring in short durations such as lightning strikes (Protection from lightning pulse 10/350 micro sec), switching of large loads (Protection from over voltage 8/20micro sec pulse) etc. An SPD is used on AC power systems which are rated less than 1000 VAC.

What types of components make up a surge protection device?

Among the most commonly used devices for AC voltage surge suppression are MOVs(Metal Oxide Variastor type). This is a solid-state device made up of zinc oxide material. MOV’s are voltage sensitive semiconductors, which can change from high impedance to low impedance when faced with an over voltage situation. MOVs are designed for handling specific voltages and current handling capacities. In some application spark gaps & gas discharge tubes are used.

What are SPD Type Categories?

Type 1 SPDs

Type 1 SPDs are usually recommended for the service sector as well as industrial buildings which are protected by lightning protection systems. They may be installed anywhere between the load side and the service entrance or the main switchboard. Type 1 SPD’s offer protection to electrical installations from direct lightning strikes. They help in discharging the back current from the lightning spreading from the earth conductor to the network conductor.

Type 2 SPDs

Type 2 SPDs are installed across all main power distribution switchboards. They are the main protection systems used for all low-voltage electrical installations. They prevent overvoltages in the electrical installations and protect the loads when installed in electrical switchboards.

Type 3 SPDs

Type 3 SPDs must be installed when the distance between the electrical panel and equipment is more than 10 meters or 30 ft. These devices are cord-connected, direct plug-in, receptacle type SPD’s, installed at the load equipment that requires protection. They have low discharge capacities and hence they need to be installed to act as a supplement to Type 2 SPD’s located nearer to the sensitive loads. Type 3 SPD also protects equipment from over voltage.

What is a Coordinated SPD?

Coordinated protection provides surge protection at every location within an electrical distribution system. An SPD may be placed at the service entrance and the main and secondary distribution along with the branch panels. At every point, the surge is reduced to a level which helps to minimize the surge impacting the sensitive equipment.

What is Let Through Voltage?

The amount of transient voltage passed through a power conditioning device to the load is called the Let Through Voltage. A transient may be defined as a high-amplitude, spike occurring in short durations or a surge which is superimposed on normal waveforms. In other words, Let Through or suppressed voltage is the amount of voltage permitted by the surge suppressor to pass through it during a transient event.

What is nominal discharge current rating?

The Nominal discharge current rating can be defined as the highest value of an 8/20 microsecond surge current conducted through the SPD. The requirement is that the SPD must function even after 15 surges are applied. The highest nominal discharge current rating is 20 kA

What factors are important when specifying an SPD?

When specifying an SPD, it’s important to submit a clear and concise specification, providing all details of requirements, conditions, and design features. A minimum specification would include the following:
Let Through Voltage
Peak search current per mode (L-N, L-G, and N-G)
Voltage service

Do SPD’s protect against direct lightning strikes?

A direct lightning strike is extremely powerful and too big to gain for a surge protection to provide complete protection from. However, proper grounding and bonding of the electrical system (as we have spoken about in a previous blog), as well as using the appropriate surge protection system can prove effective in safeguarding sensitive equipment. An SPD with a higher single surge current rating may deliver the best performance in the even such natural events occur, provided the unit is properly installed and the ground system is sufficient.

What are international performance standards for SPD?

IEC 61643-11:2011 is applicable to devices for surge protection against indirect and direct effects of lightning or other transient overvoltage

UL 1449: Describes the materials and installation requirements for SPDs used for the protection of AC electrical circuits





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