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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?

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3 Answers 3

"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."

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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 '11 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' –  Bobbi Bennett Jul 31 '11 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 '11 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 '11 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 '11 at 18:05
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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."

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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.

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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 '11 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. –  Bobbi Bennett Aug 9 '11 at 2:35
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