Transients
A transient is a high voltage spike of less than 10 microseconds (µS) in duration. Transients (1) follow the path of least resistance to ground, (2) create damaging excess heat, and (3) can cause immediate malfunctions and failure or gradual, cumulative damage.

Range of Transients
I.E.E.E. (Institute of Electrical and Electronics Engineers) studies show that many transients on the ordinary 120V power line have voltages up to 5,600 volts. The average industrial or commercial circuit receives many transients in excess of 1,000 volts.

External Transient Sources
Transients result from lightning that strikes directly - or even close by - power transmission lines, utility switching, accidental power line damage, and nearby industrial users. Though the power system's protective devices limit the transient voltage at the protective device, reflections and other interactions may permit high-crest voltages at other points in the system. A power-distribution transformer can couple fast-waveform transients, not via its step-down turns ratio, but by the transformer's often high primary-to-secondary interwinding capacitance. A capacitance transfer ratio of approximately 1:6 is very common.
Thus a 13.8 kV-to-240V transformer hit by a 50,000V primary transient would put out an 8,300V surge on the low voltage side. This is almost 35 times the normal voltage. Most electrical wiring will, however, limit the surge to between 2,500 and 6,000 volts, because of insulation, wire spacing, etc. But 2,500 to 6,000V transients are high enough to cause severe damage.

Internal Transient Sources
Some bad things happen when switch or relay contacts de-energize an inductive load. The problem is that an inductive device - like a relay, motor, or solenoid - doesn't de-energize right away, but rather starts generating voltage of its own momentarily, resulting in a transient as high as 2,500V. Also, the switching on and off of motors and/or compressors down the line can regularly introduce transients of up to 2,500 volts into these circuits.

What Kind of Damage do Transients Cause?
If transient voltage is high enough, and it usually is, arcing across the separating contacts of a motor pits and corrodes them into early retirement. A large inductance using several amperes of operating current could even weld the contacts together as its magnetic field collapses, destroying the switch or relay.
Less spectacular, but equally damaging, are the invisible effects of switching transients. For example, contact arcing radiates electromagnetic interference which disrupts the operation of nearby communication and data processing equipment. More serious, however, are the high-voltage spikes generated by de-energizing inductive devices. These transients surge throughout all interconnected circuitry and may damage
or destroy all semiconductors in their path. And even small transients can falsely trigger semiconductors into conduction, upsetting machine logic, timing, and sequencing.
Last, but certainly not least, is the damaging effect transients have on power consumption. The cumulative effects imposed upon contacts, coils, semiconductors, etc. by transients cause efficiency loss on the motors, lighting, and appliances served by electricity. A resistance build-up of one ohm across a motor contact due to contact arcing oxidation can cause a motor efficiency loss of over 13%. If this motor had a design efficiency of 95% to begin with, a 13% efficiency loss would result in over three times the heating of the motor.
Elimination of contact arcing will not, of course, restore pitted contacts to normal. However, oxidized carbon deposits on these contacts can to some extent actually be cleaned away by the natural scrubbing action of the contacts, once the arcing is eliminated. Increased efficiency is the obvious result. This means the user can stop paying for wasted energy due to inefficiency.
Economic benefits of surge suppression far outweigh the initial investment in this technology. This protection can dramatically reduce maintenance costs and increase the life expectancy of computers, motors, relays, lighting, and other electrical equipment. Among customers who track these expenses, paybacks of between six and 12 months are common.

Why Use Surge Suppressors
Surge suppressors, properly manufactured and installed, provide a path of least resistance and clamp transient voltage spikes and surges at the power panel before they can enter sensitive equipment. Manufacturers have found that from 50% to 70% of television warranty failures are caused by voltage transients. But the failure of a heart-lung machine is much more consequential than that of a TV set. And an industrial-control failure can stop a production line and cost thousands of dollars in repairs, lost production, and downtime.