Electrical Power Surges range from small harmless occurrences to thunderous events that can cause extensive damage. Here is a review from small to large. Small harmless surges occur in electrical power systems almost continuously. A surge is a temporary sudden increase of voltage in the electrical circuit. In electricity, volts are the force that push electrical current through the circuit. Volts act like pressure pushing water current through a pipe. In electricity, the current is called amperes, amps, or, often just current.
Anytime electrical current flows through a circuit, a magnetic field builds around the flow of current. If one could see the magnetic field, it would look like rings around the wire that is carrying the current. Anytime that the flow of current is switched off, as when the refrigerator switches off, the magnetic field around the wire collapses on itself, inducing a voltage in the circuit. This voltage is a small surge. It happens for a moment, then it dissipates and is gone. These are the small harmless surges.
A large surge can occur when power to a neighborhood or other large part of the community is interrupted. Large currents being suddenly interrupted cause large surges. Power interruptions are caused when circuit breakers at power substations trip then automatically re-close. The breakers trip when shorts are caused by things hitting the power lines. Vehicles can hit poles, trees fall into lines, bird's wings short between lines, wind slaps lines together, etc. These cause the large surges that damage electrical equipment, appliances, and devices.
Still larger and more dangerous surges are caused by lightning. Lightning is a big surge in the sky. When it strikes on or near an electrical circuit it becomes a power surge that can damage any electrical equipment connected to the power system. Lightning, like any flow of electrical current, generates a circular magnetic field around it. Being a very large current, it generates a very large magnetic field, and therefore a large surge. Even if lightning only strikes near a power system, the magnetic field will induce a power surge in any nearby electrical system.
If lightning directly strikes a power line, it causes a direct surge as well as the magnetically induced surge. The only larger surge is the EMP. Any electromagnetic surge can be called an EMP, electromagnetic pulse, but the term is usually reserved for the surge caused by an atomic explosion. Where lightning's electromagnetic field is limited to a few hundred feet, that of a nuclear explosion can extend miles. If you are closer than that, you probably need not worry about the EMP.
So, small surges to a few hundred volts, are harmless. Harmful surges to a few thousand volts can cause damage to electrical equipment, appliances and devices. Larger surges, such as lightning, up to 50,000 volts will cause damage.
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