Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at

March 3, 1994

Mr. Clay Detlefsen, Assistant Director
Environmental & Workers Safety Issues
International Dairy Foods Association
888 Sixteenth Street, N.W.
Washington, D.C. 20006

Dear Mr. Detlefsen:

This is in further response to your October 6, 1993 letter, requesting an interpretation of the Control of Hazardous Energy (Lockout/Tagout) standard, 29 CFR 1910.147, with respect to permit space isolation required by 29 CFR 1910.146.

In your letter you referred to a dairy plant's confined space program and cleaning of processing vats and storage tanks. Also, you indicated that the only hazard within these spaces is a low RPM agitator. Apparently, the agitator is unguarded and thereby poses potential employee exposure to hazardous mechanical energy. The permit-required confined space (PRCS or permit space) standard, 29 CFR 1910.146, applies to processing vats and storage tanks which are confined spaces as defined in paragraph (b) and where there is a hazard, including the agitator as described above. Protection of employees against unexpected energization of the agitator is also covered by 1910.147. Under the PRCS program, the permit space must be isolated as required by [1910.146(d)(3)(iii)]. Isolation means the process by which a permit space is removed from service and completely protected against the release of energy... by such means as... lockout or tagout of all sources of energy... As clarified in the 3rd paragraph of the left hand column on page 4497 of the preamble to the Final Rule (a copy of which is enclosed for your use), mechanical equipment, such as an unguarded agitator, posing a hazard within a permit space must be locked out or tagged in accordance with 1910.147.

With respect to 1910.147, the cleaning of dairy plant processing vats and storage tanks is considered to be "servicing and/or maintenance" and not "normal production operations". The control of hazardous energy (lockout/tagout) requirements at 1910.147(c) through (f) apply. Please note that the aforementioned cleaning activities are not considered minor servicing because they do not take place during normal production operations.

As noted previously, the permit space must be isolated by such means as lockout or tagout of all sources of energy. Use of a control switch locking device which locks the agitator starter switch in the off position to isolate control energy may not reliably prevent unexpected start-up and would not isolate transmission (load) energy to the agitator. Not only does the control energy have to be deenergized to prevent unexpected start-up, but also, the transmission (load) energy must be opened and the energy isolation device locked out to prevent unexpected energization. Additionally, although apparently not applicable to deenergizing the agitator, stored (residual) control energy or transmission energy must be relieved, disconnected, restrained and otherwise rendered safe as required by 1910.147(d)(5).

The lockout/tagout standard (1910.147) is intended to protect employees not only from accidents due to equipment malfunctions but also inadvertent or intentional employee acts. For example, an employee may be cleaning a processing vat or storage tank at the same time that an employee is performing servicing and maintenance on the transmission energy part of the agitator's powering system, for example, at the motor control center as discussed in your letter. Lockout of the energy isolating device controlling transmission energy to the agitator would prevent the other employee performing servicing and maintenance from causing unexpected energization of the agitator.

Your letter does not include sufficient information to evaluate whether the control switch locking device, which locks the machine's starter switch in the off position, deenergizes the control energy and prevents start-up by an intentional or unintentional act or by other activation, for example, because of a failure in the control circuitry. Generally speaking, a control switch locking device alone would not meet OSHA's standards as an energy isolating device because unexpected start- up may be the effect of any of the aforementioned causes.

We appreciate your interest in employee safety and health. If we can be of further assistance, please do not hesitate to contact us.


H. Berrien Zettler, Deputy Director
Directorate of Compliance Programs