“Diversity” means using a different kind of technology, equipment, or methodology to perform the same function. Put another way, it’s taking different approaches to solving the same problem . . . .
The goal is to reduce the odds that some kind of common-mode failure strikes all equipment at the same time when it’s needed most. In recent history, the Y2K bug was an example of a potential common-mode failure. It was inherent in most all computers and was feared to have serious consequences on January 1, 2000 all over the world and in many diverse sectors of the economy. Think about a grocery store clerk who relied on a computer to generate receipts and track his sales on that day. To prevent a Y2K bug from shutting him down, his diverse backup would be to resort to hand-writing sales tickets with pencil, paper and mental math.
To give a nuclear power plant example, the NRC suggests that isolation valves should actuate based on signals that are derived from different parameters. Those different parameters, when detected, can all mean the same thing (disaster), but they are different ways of saying it. Three examples of diverse actuation signals given by the NRC are those that detect high containment pressure, high radiation levels inside containment, or any manual or automatic actuation of an engineered safety feature system or subsystem (see Reg Guide 1.141).
One way electrical diversity is implemented is by using different power sources. The plant can operate on the power it produces through its main generator. It can operate using offsite power from the main grid. It can operate from backup emergency diesel generators. All three sources provide the same functionality (power to the Class 1E equipment), but do so in three different ways.
There are different kinds of diversity. Signal diversity is just one type of many. There is also equipment, software, functional, and human diversity, to name a few. For a fuller discussion of these, see G.G. Preckshot, “Method for Performing Diversity and Defense-in-Depth Analyses of Reactor Protection Systems.”