To answer this question we need to understand first what is a surge. A surge is a voltage spike (transient) in your building’s electrical circuit that usually lasts less than a second, typically 1 to 30 microseconds. Although a surge is brief, it can cause damage to both your electronics and your appliances. In extreme cases, a power surge can even cause a fire.
These voltage spikes in the electrical circuit can occur for a few reasons.
Tripped circuit breakers
Power transitions in other large equipment on the same power line
Malfunctions caused by the power company
Back to the definition. A power strip is a device that allows multiple electrical devices to be powered from a single electrical socket. A surge protector is a device designed to protect electrical devices from voltage spikes. Although in the market you may find some power strips with a built-in surge protector, sometimes even with multiple USB ports, strictly speaking these “power strips” are already surge protectors, or to be more specific, “power strips with a built-in surge protector”
Other types of surge protectors including surge protection device (SPD) and transient voltage surge suppressor (TVSS) that are installed in power distribution panels, process control systems, communications systems, and other heavy-duty industrial or residential systems, for the purpose of protecting against electrical surges and spikes, including those caused by lightning.
How to Tell if a Power Strip has Built-in Surge Protector?
Although power strips with or without built-in surge protector look similar, you may be able to see the difference when you come across a joules rating on the packaging or description page. Only surge protectors will have this rating. Joules are a unit of measurement for energy (like watts or calories). They measure how long your appliances will be protected. Sometimes, it may only take one massive surge in power (lightning strike) to exceed the number of joules that your equipment is protected against, but sometimes, it could take tens of little surges to do this.
Power strip with a built-in surge protector and an indicator to tell if the surge protector is still functional
Keep in mind that the number of joules your power strip protects against works somewhat like a reservoir. When you initially purchase your surge protector, you may see a label of 2,000 joules, but this will deplete over time. As your surge protector takes hits from the power spikes, the level of protection will diminish, either in one massive strike or over time.
How do Built-in Surge Protectors Work?
The surge protectors built in the power strips are primarily MOVs (metal oxide varistors) in parallel with the sockets’ line and neutral wires after a circuit breaker or short-circuit protector. The MOVs are designed with a clamping voltage, or let-through voltage, which is often 3-5 times higher than household voltage. When the spike voltage exceeds the clamping voltage, MOVs will short the circuit and trigger the circuit breaker to cut off power. Each time the MOVs short, their clamping voltage decrease a little until down to household voltage and finally fuse. LED indicators are often used to tell if the MOVs are fused in some built-in surge protectors.
Specifications on the package indicating clamping voltage and joules rating
When the MOVs are fused, the power strips can sometimes still supply power but the built-in surge protectors can no longer protect the attached devices from the damage of voltage spikes.