The Surge protection and filtering in the power strip
Many power strips have built-in surge protectors or EMI/RFI filters: these are sometimes described as surge suppressors or electrical line conditioners. Some also provide surge suppression for phone lines, TV cable coax, or network cable. Unprotected power strips are often mistakenly called "surge suppressors" or "surge protectors" even though they may have no ability to suppress surges.
Surge suppression is usually provided by one or more metal-oxide varistors (MOVs), which are inexpensive two-terminal semiconductors. These act as very high speed switches, momentarily limiting the peakvoltage across their terminals. By design, MOV surge limiters are selected to trigger at a voltage somewhat above the local mains supply voltage, so that they do not clip normal voltage peaks, but clip abnormal higher voltages. In the US, this is (nominally) 120 VAC. It should be borne in mind that this voltage specification is RMS, not peak, and also that it is only a nominal (approximate) value.
Mains electrical power circuits are generally grounded (earthed), so there will be a live (hot) wire, a neutral wire, and a ground wire. Low-cost power strips often come with only one MOV mounted between the live and neutral wires. More complete (and desirable) power strips will have three MOVs, connected between each possible pair of wires. Since MOVs degrade somewhat each time they are triggered, power strips using them have a limited, and unpredictable, protective life. Some power strips have "protection status" lights which are designed to turn off if protective MOVs connected to the live wire have failed, but such simple circuits cannot detect all failure modes (such as failure of a MOV connected between neutral and ground).
The surge-induced triggering of MOVs can cause damage to an upstream device, such as an uninterruptible power supply (UPS), which typically sees an overload condition while the surge is being suppressed. Therefore, it is recommended not to connect a surge-protected power strip to a UPS, but instead to rely solely on surge protection provided by the UPS itself.
More-elaborate power strips may use inductor-capacitor networks to achieve a similar effect of protecting equipment from high voltage spikes on the mains circuit. These more-expensive arrangements are much less prone to silent degradation than MOVs, and often have monitoring lights that indicate whether the protective circuitry is still connected.
Within the EU, power strips with surge suppression circuits can demonstrate compliance with the (LVD) Low Voltage Directive 2006/95/EC by complying with the requirements of EN 61643-11:2002+A1. The standard covers both the performance of the surge suppression circuit and their safety. Likewise, power strips with telecoms surge suppression circuits can demonstrate compliance with the LVD by complying with the requirements of EN 61643-21:2001.