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?

  • 1
    In American English usage, shall is used only in legal formulae, archaic language like prayers and hymns, and in the invitation formula Shall we VP? and the offer formula Shall I VP?, both with first person subject. Other than that, it's not used at all. Of course, as a modal -- any modal -- there are many, many possible "meanings" of shall, and even more of shall not, depending on how precise you want the "meanings" to be. Dec 5, 2014 at 21:29

4 Answers 4


"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."

  • Thank you for your reply.You mean that in this case it is an obligation or the possibility ? (there is the option to trip but if not, ok no problem)
    – Walker
    Jul 31, 2011 at 14:44
  • That is an interesting difference in meaning, where 'shall' is an obligation (using OP's words), but 'should' is an obligation that possibly can be broken. @Walker: I do not think anything implies 'there is an option to trip but if not, ok no problem' Jul 31, 2011 at 17:03
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    "shall" specifies obligation. For instance, consider the specification for a 10 amp circuit breaker: 'The circuit breaker shall not trip when there is 10 amps of less current through the circuit.' The circuit breaker is obligated not to trip when there are 9 amps of current.
    – Jay Elston
    Jul 31, 2011 at 17:03
  • I think that Raven's association of "shall" and "should" (and "will" and "would"), though historically accurate, is unhelpful here. In some of its meanings "should" functions as the past of "shall", but in other uses they are quite different.
    – Colin Fine
    Jul 31, 2011 at 23:01
  • Thank you everyone, you are great! You helped a lot. Is it possible to show me any official dictionary with details like these about "shall not"? Thanks again!
    – Walker
    Aug 1, 2011 at 18:05

Shall is used for both expressing a strong assertion or intention, and expressing an instruction or command.

They shall succeed.
You shall not frighten me out of this.
You shall not steal.

In the last sentence, you can replace "shall not" with "must not."


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.

  • Thank you, you are right about this but with your point of view I don't know why "must" or "should" is not used. So the writer is not expressing an obligation but a possibility. And I have to search for how correct is this explanation
    – Walker
    Aug 1, 2011 at 18:17
  • @Walker: Could you give more context for the excerpt? I have -some- background with electrical things and technical manuals. The use of the word 'shall' in that context is unusual. Can you explain what you mean by an obligation, and what you mean by a possibility? I do not see how you conclude that the writer is expressing a possibility, without an obligation. Aug 9, 2011 at 2:35

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.

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