Once we understand that there exists a life shortening or outright destructive threat to our electronic equipment and the ways this equipment is vulnerable (See Figure 22), we need to examine the choices and principals in the selection of protective equipment.

Figure 22

Many questions arise as one considers purchasing transient voltage surge suppressors for his equipment or system. Should I buy protection, how much do I need, where should it be placed, how do I connect it? And so on.

One must consider the cost of equipment replacement whether immediate or through shortened life. The costs of down time or out of service time and cost of repair must also be examined. The threat level in this location should be examined, isokaunaric maps consulted for lightning frequency, power quality examined and industrial environment examined for switching/inductive transient generators such as HVAC systems, copiers, motors, pumps, control equipment, etc.

A study is not required to determine whether we need suppression, of course, if we have already experienced failures or upset.

The key is to match the TVSS equipment to the threat level and then be certain all doors or paths of entry are closed, or if some are left open, the risk has at least been examined. A good general guide is found in Figure 23.

Because of the unpredictable nature of transients the threat level and frequency of occurrence may vary widely for a given location. For this reason, it is good practice to use primary protective suppressors for all outdoor and building entry areas. It is also good practice to put primary suppression on any lines going between buildings (See Figure 24).

Previously, we noted all modes should be suppressed and clamped together during a transient. Since the path to ground is several ohms of impedance at transient frequencies at any protected device, clearly the voltage produced can be reflected back into the equipment on the opposite end of the data or telco line. Therefore it is always good practice to put suppression on both pieces of equipment that are connected together.

Suppression equipment must always be located as close as possible to the equipment it is to protect and the suppressor ground made common to the chassis. A good earth ground should be provided at this common point where possible (See Figure 25).

The recommended practice as noted in ANSI/IEEE C62.41.1991 for AC power is to place primary protection (main power entry) at the building entrance, branch panel protection at each branch panel and equipment level protection right at the equipment. All distributed outlets (duplex receptacles) that power sensitive electronic equipment (telephone systems, computers, fax machines, measurement or control equipment, office equipment, medical equipment) should have protection added. All signal/data, telco phone lines, current loops and control signal lines should have primary suppression (See Figure 26).