What is Surge Protection Device?

Surge protection and the associated surge arresters on the market can be confusing to a homeowner. Understanding the terminology can help.
Surge Protection Devices have several names: surge protectors, surge suppressors, transient voltage surge suppressors (TVSS), or secondary surge arresters. But they essentially have the same function of protecting against power surges. Other common terms you may hear when shopping for surge protection devices are listed below.
Surge Protectors: For the type of surge protectors one would find around the home, this a general term that can refer to TVSS or secondary surge arresters. These surge protection devices are designed to protect equipment "downstream" against power surges by reducing the amount of voltage they let through.

Many electric utility companies also use secondary surge arresters and devices called lightning arresters throughout their electrical grid to protect their equipment from lightning damage. The devices they use are more durable, but can't reduce the power surge down to the lower voltage levels that in-home products can.

However, the utility company's lightning surge protection measures can help the homeowner by reducing the energy level of a power surge before it gets to the home.

Secondary Surge Arrester: These surge protection devices are designed to go on the inside or outside of the house. If tested, they are tested according to the recommendations of the Institute of Electrical and Electronics Engineers (IEEE) standard C62.11, Metal Oxide Surge Arresters for Alternating Current Power Circuits, with a 10,000-volt, 5,000-amp power surge. IEEE C62.11 is not a test and does not assign a clamping voltage for secondary surge arresters. This makes it difficult to compare the capabilities of one product to the next.

These surge protection devices include the meter-mount surge protectors and the plug-on surge protectors that snap into the electrical panel.

Transient Voltage Surge Suppressor: TVSSs are generally designed to go on the inside of the house. If tested, they are tested according to Underwriters Laboratory (UL) standard UL 1449 with a 6,000-volt, 500-amp power surge. UL 1449 assigns a clamping voltage to the TVSS which can be used for comparison from one product to the next.

These surge protection devices include the point-of-use surge protectors and service entrance surge protectors mounted on the electrical panel.

Metal Oxide Varistors (MOVs): MOVs are a common technology (not the only type) and are at the heart of the surge protector's (TVSSs) ability to protect against power surges. Generally, the larger they are and the more there are equates to better lightning surge protection and a more durable, longer-lasting surge protection device.

MOVs redirect the electrical current in the event of a power surge. How an MOV works is easier to understand if you think of it as a water spigot. Under normal conditions, without power surges, the MOV is a "closed valve" allowing current to flow in the electrical circuit and not through the MOV.

If there is a power surge, the MOV clamps the voltage by redirecting the electrical current (opening the valve) from the electrical circuit into the grounding system until the surge voltage drops below the clamping voltage of the protective device. When the power surge is over, the MOV returns to the "closed-valve" position.

During the power surge, all of the excess energy of the surge is diverted by the MOV, causing it to get hot. The temperature of an MOV disc can vary from room temperature to several hundred degrees after a power surge has been redirected.

The higher the voltage of the power surge, and the longer it lasts, the more energy that must be diverted and the hotter the MOV becomes. MOVs are sacrificial, meaning they will divert a finite number of power surges until they are eventually destroyed. They may reach end-of-life after only a single large surge or over several years from several smaller surges.

Thermal Fuse Protection: Because MOVs heat up when handling a power surge, there is a potential for the surge protection device or material surrounding the surge protection device to catch fire. The 2nd Edition of UL 1449 tests the fire safety of the TVSS surge protection devices by requiring severe overvoltage tests, causing the MOVs to fail.

The surge protection devices passes if it does not create a fire or electrocution hazard. This is commonly accomplished by the use of thermal fuse protection. Under the previous version of UL 1449 surge conditions could cause the surge protector to overheat and catch fire. The thermal fuse reduces that risk.
L-N, L-G, & N-G Protection: The electrical system in your home is typically a three-wire system. The wires are the ground, line (hot), and neutral. A power surge can exist across any of these wires. The lightning surge protection should protect against surges coming through any of these wires. When a surge protection device indicates the following, you know all wires are protected: Line to Neutral (L-N), Line to Ground (L-G), and Neutral to Ground (N-G). Secondary surge arresters installed at the service entrance have only Line to Neutral (L-N) protection because there is no ground wire in the locations where they are installed.