Equipment Failure Due To Transient Voltages

Equipment Failure Principals

Once we realize that today's microcircuits fail when exposed to transients of even a few volts, (see TVSS Filtering and Protection Introduction) we need to understand how transients enter equipment. This is key to proper selection of transient suppression equipment and its installation.

1.	Paths of Entry

	Transient current flows into equipment via electrical conductors (See Figure 16). These lines or wires may be AC power hot, neutral or ground, telephone lines, data-com lines, measurement or control lines or DC power busses.

Figure 16

	Wires connected to equipment from outside of buildings represent the greatest threat, next are lines feeding building to building and secondary threats exist where wires are contained within the building. (See Figure 17).

Figure 17

	Under methods and recommended practice, we will discuss matching transient voltage surge suppressors (TVSS) to the threat level. However, at this point we should recognize that any electrical wire connected to electronic circuitry is a potential path of entry and should be carefully analyzed prior to deciding whether it should have protection (See Figure 18).

Figure 18

	A practical approach, which seems to reduce the complexity in analyzing placement of TVSS devices is as follows. Remember that voltage potential applied across an electronic circuit is the base cause of failure. When this potential exceeds the withstand voltage of the weakest component, breakdown or punch through occurs. The principal then is to design TVSS devices, which connect to all points of potential voltage threat and clamp or limit both differential and common mode transients to a level below the equipment failure threshold. (See Section VIII) For example, in power entry, voltage transients must be limited between line to neutral, line to ground, and neutral to ground. In the case of two data lines (telephone wire pair, current loop, telco) both line to line and line to ground modes must be protected.

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