Transients are short term, but very fast voltage changes on the power line. High energy transients are known as impulses or surges and they can pack quite a punch.

Low level transients can cause degradation of electronic circuits, process errors, lockups and crashes. Surges can cause immediate and irreparable damage to connected equipment.

Transients can be caused by a number of factors, including:

• Load Switching
Inductive Load Switching can cause very damaging transients and surges. Even everyday items such as dimmer switches and hair-dryers can cause disruptive transients
• Lightning
Lightning is reponsible for the largest transients. Usually you will encounter transients as a result of a localised lightning strike, inducing transient currents into your power and telecom cables. Lightning strikes to overhead power cables occur and can cause serious damage to equipment within a building.

Transients are usually short in duration, typically in the tens to hundreds of μseconds (that is 10-100 millionths of a second), so they're not around for long, but it is this speed that causes the problems. As this is high frequency if they enter a electronic circuit, they can jump across tracks on the circuit board through stray capacitance. This is what causes lock ups and crashes.

Many transients are low energy and don't cause immediate damage, but can cause degradation of components, so equipment fails later on down the line.