Surge protection is designed to protect electrical devices from voltage spikes in alternating current circuits which is what supplies all homes in the UK.
Although a voltage spike is a fleeting event it can reach over 1,000 volts. In comparison lightning that hits a power line which typically lasts between 1 and 30 microseconds can give a spike of over 100,000 volts and can burn through wiring insulation and cause fires.
Whilst a normal household spike may seem modest in comparison, they can still destroy a range of electronic devices such as computers, chargers, modems and Tv’s that happen to be plugged in at the time. Spikes can also degrade wiring insulation. Spikes can also occur on telephone and data lines when AC main lines accidentally connect to them, or when lightning hits them or the telephone and data lines travel near lines with a spike and the voltage is induced.
Typically, a surge protection device will trigger at a set voltage which is usually 3-4 times the mains voltage and divert the current to earth. There are also some surge protection devices that absorb the spike and release it as heat. This type of surge protection is rated according to the amount of energy in joules they can absorb.
Whilst most surges are momentary, long-term surges can last seconds, minutes or even hours which are caused by power transformer failures such as lost neutral or other power company errors which are not protected by transient protectors. Long term surges can destroy the protectors in an entire building or area. Even tens of milliseconds can be longer than a protector can handle.
The job of a transient surge protector is to limit the voltage supplied to an electric device either by blocking or shorting current to reduce the voltage below a safe threshold. Blocking is done by using inductors which inhibit a sudden change in current. Shorting is done by spark gaps, discharge tubes, zener type semiconductors and metal-oxide varistors (MOVs), all of which begin to conduct current once a certain voltage threshold is reached or by capacitors which inhibit a sudden change in voltage. Some surge protectors use a combination of multiple elements.
The most common and effective surge protection is the shorting method on which the electrical lines are temporarily shorted together (as by a spark gap) or clamped to a target voltage (as by a MOV) resulting in a large current flow. This then reduces the voltage as the shorting current flows through the resistance in the power lines. The spikes energy is dissipated in the power lines (and/or the ground) or in the body of the MOV, converted to heat. As a spike only lasts tens of microseconds the temperature rise is minimal. If the spike however is large enough or long enough, like a nearby hit by lighting there might not be enough power line or ground resistance and the MOV (or other protection element) can be destroyed and the power lines melted.
Domestic surge protection can be in power strips used inside, or a device outside the power panel. They are installed to protect sensitive electronic equipment connected to the installation, such as computers, televisions, washing machines and safety circuits, such as fire detection systems and emergency lighting. Equipment with sensitive electronic circuitry can be vulnerable to damage by transient overvoltage which is why some people choose to install surge protection devices.