Transients

Transients should be distinguished from surges. Surges are a special case of high-energy transient which result from lightning strikes. Voltage transients are lower energy events, typically caused by equipment switching.

They are harmful in a number of ways:

• They deteriorate solid state components. Sometimes a single high energy transient will puncture a solid state junction, sometimes repetitive low energy transients will accomplish the same thing. For example, transients which exceed the PIV (peak inverse voltage) rating of diodes are a common cause of diode failure.

• Their high-frequency component (fast rise times) cause them to be capacitively coupled into adjoining conductors. If those conductors are carrying digital logic, that logic will get trashed. Transients also couple across transformer windings unless special shielding is provided. Fortunately this same high frequency component causes transients to be relatively localized, since they are damped (attenuated) by the impedance of the conductors (inductive reactance increases with frequency).

• Utility capacitor switching transients are an example of a commonly-occurring high-energy transient (still by no means in the class of lightning) that can affect loads at all levels of the distribution system. They are a well-known cause of nuisance tripping of ASDs: they have enough energy to drive a transient current into the dc link of the drive and cause an overvoltage trip.

Transients can be categorized by waveform. The first category is "impulsive" transients, commonly called "spikes," because a high-frequency spike protrudes from the waveform. The cap switching transient, on the other hand, is an "oscillatory" transient because a ringing waveform rides on and distorts the normal waveform. It is lower frequency, but higher energy.

Causes

Transients are unavoidable. They are created by the fast switching of relatively high currents. For example, an inductive load like a motor will create a kickback spike when it is turned off. In fact, removing a Wiggy (a solenoid voltage tester) from a high-energy circuit can create a spike of thousands of volts! A capacitor, on the other hand, creates a momentary short circuit when it’s turned on. After this sudden collapse of the applied voltage, the voltage rebounds and an oscillating wave occurs. Not all transients are the same, but as a general statement, load switching causes transients.

In offices, the laser copier/printer is a well-recognized "bad guy" on the office branch circuit. It requires an internal heater to kick in whenever it is used and every 30 seconds or so when it is not used. This constant switching has two effects: the current surge or inrush can cause repetitive voltage sags; the rapid changes in current also generate transients that can affect other loads on the same branch.

Measurement and recording

Transients can be captured by DSOs (Digital Storage Oscilloscopes). The Fluke 43 PQ Analyzer, which includes DSO functions, has the ability to capture, store and subsequently display up to 40 transient waveforms. Events are tagged with time and date stamps (real time stamps). The VR101S Voltage Event Recorder will also capture transients at the receptacle. Peak voltage and real time stamps are provided.

Transient voltage surge suppressors (TVSS)

Fortunately, transient protection is not expensive. Virtually all electronic equipment has (or should have) some level of protection built in. One commonly used protective component is the MOV (metal oxide varistor) which clips the excess voltage.

TVSS are applied to provide additional transient protection. TVSS are low voltage (600V) devices and are tested and certified to UL 1449. UL 1449 rates TVSS devices by Grade, Class and Mode. As an example, the highest rating for a TVSS would be Grade A (6000V, 3000A), Class 1 (let-through voltage of 330V max) and Mode 1 (L-N suppression). The proper rating should be chosen based on the load’s protection needs:

• A lower Grade might result in a TVSS that lasts one year instead of ten years. The solid state components in a TVSS will themselves deteriorate as they keep on taking hits from transients.

• A lower Class might permit too much let-through voltage that could damage the load. Class 1 is recommended for switch mode power supplies.

• A Mode 2 device would pass transients to ground, where they could disrupt electronic circuit operation.