Causes of Transients

Transients can be generated internally, or the can come into a facility from external sources. The least common of the two are externally generated transients. They’ve been described as “electronic rust”.

External Sources

Lightning is the most well-known of the externally generated transients. Most lightning transients are not actually the result of direct lightning strikes….they are most often “induced” onto conductors as lightning strikes near the power line. The large electric fields generated during a discharge can couple into the power system, creating induced transients. A cloud-to-cloud discharge
can generate a 70 Volts per meter electric field. On a 1/2-mile length of transmission line this is equal to a 56,000-volt transient–and it didn’t even touch the line!

Other externally generated transients may also be imposed on power lines through normal utility operations. Switching of facility loads, opening and closing of disconnects on energized lines, switching of capacitor banks, re-closure operations, and tap changing on transformers can all cause transients.

Poor or loose connections in the distribution system can also generate transients. They may be caused by high winds, which can blow one power line into another or blow tree limbs into the lines causing arcing. You’ll probably be able to hear a buzzing sound and see sparks when the arc occurs, or you may even be able to smell the burnt insulation around the arc.

Accidents and human error account for some externally generated transients since most power lines are run above-ground. Animals and weather can also produce conditions which generate transients.

Another common source, not commonly known, is neighboring businesses. If you share a transformer with other users, any transient activity generated on his premises will be seen at your electrical main. Remember, you are both physically connected at the secondary side (some people call it the “south side”) of the transformer.

Internal Sources

The vast majority of transients are produced within your own facility. The main culprits are device switching, static discharge, and arcing.

Each time you turn on, turn off, load, or unload an inductive device, you produce a transient. Inductive devices are those devices that use “magnetic mass” to function. Examples of inductive loads are motors and transformers. The inductive “kick” from a 5-horsepower motor turning on can produce a transient in excess of 1,000 volts. A motor with a faulty winding or other insulation faults can produce a continuous stream of transients exceeding 600 volts! Even transformers can produce a large transient, particularly when energizing. Interestingly enough, this isn’t produced the way many people think (from the sudden load on the system), but is a result of the collapse of the magnetic field upon energizing the transformer.

Static electricity (also called “electrostatic discharge”) can generate up to 40,000 volts. This type of hazard is very dependent upon
environmental conditions and areas with lower humidity have the worst problems.

Arcing can generate transients from a number of sources. Faulty contacts in breakers, switches, and contactors can produce an arc when voltage jumps the gap. When this gap is “jumped” the voltage rises suddenly and the most common effect is an oscillatory-ring-type transient. Faulty connections and grounds can produce arcing.

Common Internal Sources of Transient Activity