APC surge protectors undergo stringent testing by IEEE and are awarded &quot;let-through ratings&quot; which indicate the maximum voltage level that can possibly reach your equipment after a surge or voltage spike has passed through an electric surge protector. Lower IEEE let-through ratings indicate better protection and performance. The APC Essential SurgeArrest series offers optimal performance with a let-through rating of less than 330V for North American products and a rating of less than 15% for products sold worldwide. All APC products comply with local agency safety requirements including UL in North America and CE/Kema/Gost/C-tick/A-Tick/Telepermit in their respective regions.&lt;br /&gt;&lt;br /&gt;SurgeArrest will warn you if its circuitry has been damaged by heavy strike or power line surge and it is unable to provide 100% protection. If still under warranty APC will then replace your damaged SurgeArrest free of charge.