I am looking for all the meanings of "shall not". Is it closer to "must not" or "might not"?
In this example:
The circuit-breaker shall not trip.
does this mean must not happen or might not happen?
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
"Shall" derives from the Old English "sceal" meaning "must". "Should" is the past simple and conditional form of "shall", just like "would" is the past simple and conditional form of "will". Should is used with a sense of quasi-obligation, synonymous with ought to.
So archetypal uses are statements like, "This shall not stand." The meaning is very much "must."
In the context of instructions relating to a circuit breaker, 'shall not' implies there is a very important set of rules you, the reader, are responsible for.
If, in fact, the breaker -did- trip, you would be guilty of violating those rules.
It is possible that the use of 'shall' is intended to instill almost religious importance to these rules.
You do not give the entire context, however, or possibly the instructions are not thorough. Because, of course, the entire point of a circuit breaker is that it -can- and -must- trip, under fault conditions.
Likely, the statement is part of the instructions for testing the circuit breaker, and the implication is 'if the breaker trips, something is very wrong (perhaps the breaker is defective), and must be fixed right away.
The original question asked for all meanings of "shall not", but then went on to include an example which looks like it came from an electrical requirements document of some sort. Since providing all possible meanings of "shall not" would be both a reference question and vague, I'm focusing on the specific example provided.
The use of the word "shall" to mean an obligation or requirement is very common in standards and regulatory documents.
For example, the IETF, which defines many internet standards, has RFC 2119, which includes the following statement:
1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.
The ASQ section on use of "shall" states:
"This word implies obligation and is traditionally used by laws and regulations."
The IEEE Standards association recommends specifically using "shall" and avoiding "must" in requirements, because "must" could be interpreted as not an obligation, but an inevitability, whereas "shall" specifically connotes an obligation.
There is some dispute over the use of "shall" in legal documents, but with regard to technical specifications and standards, the consensus is clear, that "shall" indicates a mandatory obligation.