Q:

Can basic grammar rules be skipped when writing text for machine safety labels?

First of all I'm completely against this idea but a few people who contribute to the technical documentation project constantly suggest that to attain a short, quick, economic, comprehensive message some basic rules can be broken. Things like completely omitting the articles, using simpler words to explain things, using the imperative mode to shorten verbs, even tossing out basic grammar rules etc.
Are there some rules of thumb or even legal directives about the language of machine safety labels?
I'm citing a few examples of text written by those outsiders and my suggested corrections to make my case more clear:

Do not open this cover while powered
  Do not open this cover while the machine is powered
Ensure power is disconnected before servicing
  Ensure that the power is disconnected before servicing the machine
Do not clean the machine using water or materials that can generate an
  explosive atmosphere
  Do not clean the machine using chemicals that can generate an
  explosive atmosphere or using water that can create an electric shock

EDIT
Thanks for your answers. I mostly conclude with every answer but I have to apologize to all of you because I think I did mislead you by not presenting my case in detail. I'll give more details even if it's a little bit late.
We manufacture big machines which have too much surface area and therefore our labels have too much space for details, too. We have 4 kinds of labels:

There are informational labels which are used to designate the inlets (The type of the heating power such as steam or gas and the water inlets), the power switch, and the grounding connection quickly and precisely. Those inlets are also designated on the installation layouts and projects, so these labels are not the only markers of these inlets. They are there mostly to quickly remind the authorized personnel of their functions. They just include the 1 or 2 words describing the inlet type, a pictogram for it and if it's necessary the pressure limits of the heating supply.

I don't have any problems with these labels.

There are two purely graphical labels which don't have any words on them. They only show the rotation direction of the blades of a fan and the bolt locations of the safety belts which have to be unbolted after the installation phase.

I don't have any problems with these labels, too.

There two legends for the explanation of the status light patterns and the explanation of the abbreviations used on the simpler model of the control panels that we are using.

Again, I don't have any problems with these labels.

There are the safety and information labels. We've designed the labels to include as much information as possible but at the same time to display this information in a practical and useful manner. Most of these labels include an internationally valid pictogram, a word describing the message type of the label in colored background and big fonts (Information, Warning, Caution, and Danger), a title of the label in very big fonts (which corresponds to the content that most of you were answering) and the details of the title message in either just one font size or sometimes if the message details are too long in two different font sizes.

I mostly don't have any problems with the title of our labels. They are the type of labels which most of you did mention in your answers.
My problem is with these detailed text parts of our labels. I'm including a screenshot which contains four types of our safety and information labels so you can clearly see how they are constructed.
I'm aware that most of your answers can still be valid for them, too. I'll try to be more open minded when dealing with them in the future but I'll wait for your reactions to the detailed edit to my question before accepting one of your answers
Note: These label are printed on a transparent material which is the reason for different font colors. The ones with black fonts are meant to be fixed on light colored panels whereas the ones with white fonts are meant to be fixed on dark colored panels to enhance the readibility.
Thanks again.

A:

I really like this question.  I cringe to say it, but I somewhat agree about breaking grammar rules in the interest of safety.  I frequently write technical emails/IT system announcements that are sent out to a large group of non-technical people and I find that if I write the emails using the same language that I'd write in my short stories, or even my technical diagrams, then nobody will read them.
Machine safety labels - any safety labels - need to get to the point... Right Now!  The fewer words the better, and the simpler the better because you're working to do two things:

Keep people from getting hurt.
Keep the company from getting sued because someone got hurt.

I'd love it if safety labels were grammatically correct, but I think they just wouldn't be as safe then.

Dear Sir/Madam,
Please do not touch the violently spinning wires in the wire rope
  machine in front of you as they may cause certain parts of your body
  to become separated from the rest of your body, or in the event that
  your different body parts do not separate, your whole body may be
  pulled into the machine.  Either way, a gruesome, bloody, painful
  death will occur and everybody will be sad.
Thank you, and good luck.
Management.

Edit: If you're looking for specific legal advice about this... I'm just a writer and a computer geek... You should ask your company's lawyer and then write the labels whatever they say is the correct way. Your lawyer is going to have specific knowledge about your industry and about the risks involved and will probably be quite happy to put in their two cents. 

A:

Of course you can't just ignore all basic grammar rules.  For example, writing:

Not cover the opening machines power be while do.

obviously makes no sense to anyone, even though it's got all the right words (plus or minus a few grammatical suffixes) in there.  It's just broken English.
But you can totally write, say:

Do not open cover while machine is powered.

or, if space is really at a premium (or you just want to make the letters as big as possible to make them stand out), e.g.:

DO NOT OPEN WHILE POWERED!

So what's the difference, then?  The difference is that, in effect, English has a special linguistic register for terse messages like signs and headlines that modifies or relaxes some otherwise (nearly) universal grammar and style rules in the interests of brevity.  But not all rules; only those that do not introduce any significant ambiguity in the message (or for which an alternative method for conveying the same distinction is available).  In effect, it's a different language variety with (somewhat) different grammar rules from general literary English — but it does still have grammar, because a language with no grammar at all is not a language, but just a bunch of random words thrown together.
So what are the differences between general English and this "headlinese"?  I haven't personally read any scholarly studies on the grammar of this special language register (although it has been studied, and the Wikipedia page I linked to even references a couple of articles on the subject), but just off the top of my head, the most notable differences are:

Omission of definite and indefinite articles.  When you really think about it, articles are rarely essential for conveying the meaning of a sentence.  Plenty of languages get by just fine without them — the map on the linked page shows pretty strikingly that the (near) compulsory use of articles in front of nouns is a distinctively Western European areal feature.  In many languages it's a relatively modern one, too; classical Latin didn't have articles, but most of the Romance languages that descended from it developed them (generally from demonstrative pronouns that became a compulsive part of the noun phrase) under the influence of neighboring Germanic languages.
Omission of the copula.  Have you ever seen a sign that said "Door is closed," or a headline that read "Politician is caught lying"?  Again, plenty of languages get by just fine without a copula at all, anyway.
Use of stand-alone noun phrases instead of full sentences.  Related to the above, it's very common to replace sentences that in standard English would use the copula "is" with just a plain noun phrase (with, effectively, an implied "there/this is").  This is very common in signs: "NO SMOKING", "WET FLOOR" or "OUT OF USE" communicate their ideas much more concisely than "Smoking is not allowed here," "The floor is wet" or "This machine is not currently usable."
Omission of implied referents.  The default assumption is that a sign refers to the object or the location it's attached to.  Thus, you don't need to write "This store is closed" or "No trespassing on this property." Of course, such omission also occurs in standard literary English (even if some grammarians frown upon it), but it's extremely common in signs.

There are also some grammar and style features that, while not directly serving the interests of brevity, have nonetheless come to be commonly associated with this particular style of English.  A notable example is title case, which (as the name indicates) is very popular in titles and headlines, but also commonly found in other places that employ a similar terse style.  (Exercise: Open a drop-down menu on your computer and look at the entries; there's something like a 50-50 chance that they'll be In Title Case.)
Anyway, my point is that, as long as you follow the established style and grammar conventions relevant to the context you're writing in, you certainly can (and, arguably, even should) break any "rules" of prescriptivist "standard English grammar" that conflict with actual established usage in your field.  That's not the same as throwing grammar, willy-nilly, out of the window — that's just understanding that grammar, like all things in human communication, is a context-dependent and flexible construction that can and does adapt to fit the needs of the medium.

A:

The label should be as short as possible without creating ambiguity.
In many workplaces, the employer is required (OSHA, ISO, FDA, etc.) to train anyone who would be working in a particular area with the hazards of the environment and the equipment.  The label acts as a reminder (as well as a legal obligation).  Everyone in that lab knows lighting a flame near the hydrogen hood is a bad idea.  Anyone entering that lab should require safety training.
If the labels you used as examples are positions such that there is no ambiguity as to which system has to be turned off or which cover can't be opened, then clarify.  If it's clear, don't add additional wording.  If the label is on the wall, then, yes, it should specify you're talking about the spectrophotometer with the yellow cover.
If you are working in a small business that is not government regulated, then the need for more explicit labels may be required.

