A recent conversation with an engineer highlighted there was a confused relationship between modes of protection and modes of surge. Modes of protection is mainly talked about for power SPDs. As one might expect the definitions found for power SPDs are hardly generic, nor in this case, very precise.
In UL we have:
modes of protection: electrical paths where the SPD offers defense against transient overvoltages. Examples include, Line to Neutral (L-N), line to Ground (L-G), Line to Line (L-L) and Neutral to Ground (N-G). [UL 1449]
This only defines a voltage threat, what means "defense" and only two terminal/node examples are given
For the IEEE we have the related:
modes of protection: electrical paths where the SPD offers defense against transient overvoltages.
—For a single phase ac power SPD connecting to line, neutral, and ground conductors, the modes of protection can be line-to-neutral (L-N), line-to-ground (L-G) and neutral-to-ground (N-G).
—For poly-phase ac power SPDs connecting to line, neutral, and ground conductors, the modes of protection can be line-to-neutral (L-N), line-to-ground (L-G), line-to-line (L-L), and neutral-to-ground (N-G).
In the IEC we have:
mode of protection of an SPD: an intended current path, between terminals that contains protective components, e.g. line-to-line, line-to-earth, line-to-neutral, neutral-to-earth [IEC 61643-11, ed. 1.0 (2011-03)]
This definition doesn't define the threat and only two terminal/node examples are given.
These definitions refer to an SPD, its terminals and AC power distribution conductors and do not represent a generalized protective function definition. In circuit terms terminals become nodes. The current path occurs due to the voltage limiting protective function branch during a surge. In addition, and this causes a lot of debate, there is no indication if the current path is directly between the terminals or via an intermediate junction which connects to other terminals.
These three definitions do not represent a good basis of a generic modes of protection definition. I found one definition proposal that is not specific to power SPDs and is a better starting point:
modes of protection (of a voltage limiting SPD or equipment port): list of terminal-pairs where the diverted surge current is directly between that terminal pair without flowing via other terminals.
Now this definition only lists terminal-pairs where the protective function is directly between the pairs and doesn't involve other terminals. The diagram below clarifies this concept:
1) The (green wire) protective function connects nodes A and B. The arrangement has one direct mode of protection A-B.
2) Three protective functions are used each connecting to a node pair. The arrangement has three direct modes of protection A-B, B-C and A-C.
3) Two protective functions are used each connecting to a node pair. The arrangement has two direct modes of protection B-C and A-C. There is one indirect mode of protection A-C-B. In AC applications the protective function components are likely to be symmetrical and the same voltage, making the A-B voltage twice the A-C or B-C voltage. In signal applications the protective function components may be highly asymmetrical, making the A-B voltage nearly the same as the A-C or B-C voltages.
4) Three protective functions are used each connecting to a node and a common node D. This is a Y configuration. The arrangement has three indirect modes of protection A-D-B, B-D-C and A-D-C.
Example 3) causes some debate as some people say it has two modes of (direct) protection others say it has three modes (two direct and one indirect). The power SPD definitions don't provide any definitive interpretation for these claims.
Moving into the signal protection area there are even more problems to ponder. In example 5) how many modes of protection does this circuit provide?