Application Note CCC 94-002 Revised February 2001

External Sources of Transient Voltage Surges and Electrical Line Noise

Jim Tiesi, Mgr. Product Mgmt & Applications Engineering Control Concepts / Liebert

Binghamton, New York

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Lightning: Lightning strikes are the most visible and the most destructive high voltage transient. Although a direct hit can delivery hundreds of millions of volts and hundreds of thousands of amperes, this is not the only concern. Through mutual induction, any conductive circuit within a few miles radius could experience a several thousand volt transient.

Utility Switching: The path of electrical power from the utility generator to a commercial building is not a dedicated line. It is vast network of generators and substations interconnecting hundreds or thousands of miles. Normally every component is electrically tied together and in sync. Demand changes or service requirements result in frequent switching of network grid tie points. Any time a substation is switched into or out of the network, a voltage transient is produced. The transient can reach hundreds or thousands of volts depending on the current, impedance and distance from the switching point(s).

Capacitor Switching: Power factor correction capacitors area added to the network by utilities as well as large commercial power users. The purpose is to compensate for excessive inductive loads such as motors and to boost lower power factor at distribution extremities. These banks of capacitors are typically switched in at the same time every morning and

switched out at the same time every night. Those with Transient Voltage measuring equipment can see voltage disturbances at the same time every day. The transient can reach hundreds or thousands of volts depending on the impedance, or distance from the utilitys or commercial users capacitor banks.

Faults: A fault is a catastrophic phase to phase or phase to neutral/ground connection ideally causing a fuse or circuit breaker to operate instantly. When a fault occurs, the distribution voltage falls from normal to very near zero and the systems corresponding fault current increases until the protective circuitry operates and the fault is cleared. Though the loads downstream of the fault clearing device experience a blackout, the loads upstream of the fault clearing device are subjected in certain conditions to substantial voltage transients.

Construction: Arc welders in use in construction close to ones building can produce damaging voltage transients and Electrical Line Noise.