Here are the OSHA references we found on this topic…
1910.146(c)(2) If the workplace contains permit spaces, the employer shall inform exposed employees, by posting danger signs or by any other equally effective means, of the existence and location of and the danger posed by the permit spaces.
NOTE: A sign reading “DANGER – PERMIT-REQUIRED CONFINED SPACE, DO NOT ENTER” or using other similar language would satisfy the requirement for a sign.
Extract from OSHA Instruction CPL 2.100 Appendix E, Section (c) General Requirements, Q4.
4. How will OSHA interpret the language in paragraph 1910.146(c)(2) requiring employers to inform employees of permit spaces by posting signs or “by any other equally effective means?”
Ordinarily, information about permit spaces is most effectively and economically communicated through the use of signs. Consequently, signs would be the principal method of warning under the standard. Alternative methods, such as additional training, may be used where they are truly effective in warning all employees who could reasonably be expected to enter the space. It is the employer’s obligation to assure that an alternative method is at least as effective as a sign. In some cases, employers may have to provide training in addition to signs, to protect employees who do not speak English or who would have difficulty understanding or interpreting signs. (One method by which OSHA can gauge an employer’s effectiveness is through random interviews of affected employees.)