They have a max wattage specified to them, such as 3500 W.

Control

Surge protection and filtering

Surge suppression is usually provided by one or more metal-oxide varistors (MOVs), which are inexpensive two-terminal semiconductors. They act as very high speed switches, disconnecting at the designed voltage. The most commonly used are built to trigger at a voltage somewhat above the local mains supply. In the US, this is (nominally) 115 VAC. It should be borne in mind that this voltage is RMS, not peak, and also that it is only a nominal value.

In most of the developed world, mains electrical circuits are (supposed to be) grounded (earthed), so there will be a live wire, a neutral wire, and a ground wire. 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, mounted 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.

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 less prone to silent degradation than MOVs.

Within the EU power strips with surge suppression circuits are required to meet the requirements of EN 61643-11:2002+A1 (see Official Journal of the European Union at http://ec.europa.eu/enterprise/newapproach/standardization/harmstds/reflist/lvd.html). The standard covers both the performance of the surge suppression circuit and the safety. Likewise power strip with telecoms surge suppression circuits must meet the requirements of EN 61643-21:2001 to demonstrate compliance with the requirements of the Low Voltage Directive 2006/95/EC.

Overload protection is different from surge protection. Some boards only have overload protection, which does not protect from electricity spikes (surges); it only means that the device will trip itself when too many devices are plugged into it. The standard rating for overload protected powerboards is 2400W in Australia, and exceeding that power will make the board trip. An overload protected board cannot cause damage to trip distribution boards that are plugged into them, that damage can only be done by surge protected board by the way the MVO's inside them are manufactured. Both boards have a reset switch. Overload protection boards generally do not have a power light.