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Selected Letters of Interpretation

OSHA Standards Interpretation and Compliance Letters
Motor starter circuits and energy isolation devices

  • Record Type: Interpretation
  • Standard Number: 1910.147
  • Subject: Motor starter circuits and energy isolation devices
  • Information Date:1991

August 5, 1991



SUBJECT:            Interpretation of "Energy Isolation Device" Application
                   of 1910.147 to Conveyors
This is in response to your memorandum of February 12, requesting answers to three questions concerning the relationship of motor starter circuits and energy isolation devices as defined in the Lockout/Tagout Standard, 1910.147. Please accept our apologies for the delay in responding. Your questions and our answers are listed below.

1. Question: It is recognized that a motor starter is a control circuit device. Did the writers of these standards intend that assured control of the motor starter in the "off" condition be accepted as an energy isolation device?

Answer: The intent of the standard was not to include motor starter circuits within the scope of the definition of energy isolation devices.

For further clarification, the definitions of certain terms that have application to the 1910.147 (Lockout/Tagout) standard can be found in the complementary electrical OSHA standard, Subpart S of 1910. Three appropriate definitions are as follows:

1910.399(a)(31) Controller. A device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.

1910.399(a)(40) Disconnecting Means. A device, or group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply.

1910.399(a)(124) Isolating Switch. A switch intended for isolating an electric circuit from the source of power. It has no interrupting rating, and it is intended to be operated only after the circuit has been opened by some other means.

2. Question: It is our understanding that the electric motor, once it is stopped using the motor-controller "stop" button and is spun down to a full stop, cannot restart without being activated by the motor-controlled starter and its control circuit. It is further our belief that deactivation of the motor-starter device, using two independent keyed energy isolation devices properly wired and installed, can be effective in preventing the motor from starting and energization of its control circuit. Does the standard prohibit the use of this type of control circuit device as an energy isolation device for lockout purposes?

Answer: The intent of the standard was not to accept motor-controlled stop buttons or motor-controlled starter circuits as energy isolation devices. Thus during the servicing and/or maintenance of equipment, such mechanisms will not be sufficient to provide the protection envisioned by the standard. On the other hand, for normal production operations, such as during routine, repetitive package clearing operations on conveyor belts, mechanisms that permit work to be performed by using alternative measures which provide effective protection would be acceptable. Further clarification on this issue is provided in OSHA Instruction STD 1-7.3, Appendix C, Paragraphs A.1. through 4. Also, please see the note under Paragraph 1910.147(a)(2)(ii)(B) of the Lockout/Tagout Standard.

90) that "... A diagram from the 1990 NEC Handbook, which tends to support the company's proposed application of control energy isolation, has been provided as Attachment B." We have not seen Attachment B. However, from our knowledge of the 1990 NEC Handbook, we are not familiar where this support is given. It might be that reference is being made to Article 430-111 of the NEC which states the conditions under which a switch or circuit breaker is permitted as both controller and disconnecting means. This requirement, taken from the 1990 NEC, is repeated here for convenience as follows:

430-111. Switch or Circuit Breaker as Both Controller and Disconnecting Means. A switch or circuit breaker complying with Section 430-83 shall be permitted to serve as both controller and disconnecting means if it opens all ungrounded conductors to the motor, if it is protected by an overcurrent device (which shall be permitted to be the branch-circuit fuses) that opens all ungrounded conductors to the switch or circuit breaker, and if it is of one of the types specified in (a), (b), (c) below:

(a) Air-Break Switch. An air-break switch, operable directly by applying the hand to a lever or handle.

(b) Inverse Time Circuit Breaker. An inverse time circuit breaker operable directly by applying the hand to a lever or handle.

(c) Oil Switch. An oil switch used on a circuit whose rating does not exceed 600 volts or 100 amperes, or by special permission on a circuit exceeding this capacity where under expert supervision.

The oil switch or circuit breaker specified above shall be permitted to be both power and manually operable.

The overcurrent device protecting the controller shall be permitted to be part of the controller assembly or shall be permitted to be separate.

An autotransformer-type controller shall be provided with a separate disconnecting means.

If this is the NEC Article referenced by the Cincinnati Area Office as Attachment B, then it obviously can be seen that it is not applicable to the proposed UPS installation since neither the Air-Break Switch, the Inverse Time Circuit Breaker nor the Oil Switch, as specified in 430-111(a), (b) and (c), is used in the UPS design. It should be further noted that both the Air-Break Switch and the Circuit Breaker must be "operable directly by applying the hand to a lever or handle." And again the UPS method would not satisfy this requirement.

3. Question: It has been proposed, since the motor starter includes control circuit devices and is itself a control circuit device, that it would not be acceptable as an "Energy Isolation Device," per the 1910.147 standard. In the event of control circuit or motor starter failure it could cause the actual three- phase wires feeding power to the motor, coils, armature, and the motor starter circuit to become energized. Do you agree?

Answer: We agree.

The OSHA standard, 1910.147 (Lockout/Tagout), clearly stipulates that in order not to be covered by the standard while performing minor servicing activities during normal production operations, the work must be performed using alternative measures which provide effective protection (emphasis provided). To provide effective protection, the isolation from the source of power must be positive. A dependency on automatically controlled circuits to provide this isolation, even where all ungrounded conductors to the motor are opened, is not positive.

With the proposed UPS method of conveyor stoppage, one scenario may occur as follows:

An UPS package jams on the conveyor and other packages quickly begin to pile up. The attendant immediately actuates the stop button at one of the keyed lockout (field station) devices. The conveyor comes to a stop and the attendant climbs aboard the conveyor to free the jammed packages. Lacking the direct control of a manually operated switch or breaker to remove power by disconnecting all power conductors, motor stoppage becomes dependent on the proper functioning of the control circuits. In this case, we assume that the automatic control circuitry in the motor control center malfunctions such that only one phase of the three phase source to the motor is opened (not an uncommon occurrence especially where the overcurrent protection device opens the third phase (L3) without disturbing the motor starter circuit). Because of the additional load placed on the belt by the jammed packages, the motor, now only operating on two phases, has insufficient torque and stalls, and the belt stops. The attendant, believing that the conveyor has been safely stopped because the maintained stop button on the keyed lockout device was actuated, climbs upon the belt in order to free the jammed parcels. However, as the jam is removed, the resulting load on the conveyor motor is reduced and the belt starts again with sufficient start-up torque from the two remaining phases which have not been disconnected. The attendant becomes unbalanced by the moving conveyor, slips and falls, and is injured.

Other scenarios can also be postulated as a result of the lack of effective (positive) energy isolation.

Wiring Considerations. In accordance with 29 CFR 1910, Subpart S, Electrical Standards, the UPS conveyor motor control installation must comply with the 1910.305(j)(4) requirements. The following table estimates UPS compliance based on the UPS submitted schematic diagram:

OSHA Standard UPS Compliance


A disconnecting means shall be No located in sight from the controller location (Refer to 1910.305(j)(4)(i) for definition of "In sight from.")


If a motor and the driven machinery are not in sight from the controller location, the installation shall comply with one of the following conditions:

(1) The controller Unknown disconnecting means shall be capable of being locked in the open position.

(2) A manually operable switch that will No disconnect the motor from its source of supply shall be placed in sight from the motor location.


This disconnecting means shall plainly indicate whether it is Unknown in the open (off) or closed (on) position.


The disconnecting means shall be readily accessible. If No more than one disconnect is [Refer to UPS compliance with provided for the same 1910.305(j)(4)(ii)(a)] equipment, only one need be readily accessible.

         Occupational Safety and Health Administration
               230 S. Dearborn Street, Room 3244

DATE:               August 8, 1991

MEMORANDUM FOR:     Assistant regional Administrators
                   Area Directors District Supervisors

FROM:               William Q. Wiehrdt
                   Assistant Regional Administrator
                   Technical Support

SUBJECT:            Interpretation of "Energy Isolation Device" Application
                   Of 1910.147 To Conveyors
Enclosed is a recent interpretation from the National Office on the Lockout/Tagout Standard. The memo discusses several issues related to conveyor usage. Please share this information with your staff.


                            REGION V


DATE:               January 31, 1991

REPLY TO:           William M. Murphy        Facsimile Transmittal
                   Area Director

                   IN ACCORDANCE WITH 1910.147

TO:                 William Wiehrdt
                   ARA-Technical Support

THROUGH:            Michael G. Connors
                   Regional Administrator
In memos previously sent to the Regional Office on December 10, 1990 (copy attached, less original attachments), December 10, 1990 (copy attached), January 7, 1991, and January 24, 1991, the Cincinnati Area Office requested an official interpretation of the term "energy isolation devices" as applied under OSHA's lock out standard (1910.147). This request was in regard to a proposed abatement method submitted by United Parcel Service (UPS) for the deenergizing of conveyor belts to protect workers when they climb onto the conveyors to clear jams and/or locate missing parcels. Included in the information submitted was an electrical schematic of the locking device to be used.

In the memorandum from the regional Office which this office received on January 26, 1991 it appears that the response to our inquiry is that an energy isolating device is "designed to positively disconnect power..." and that the device in question is an acceptable device.

Despite your response, there are still unanswered questions in this matter. It appears from our discussion with the company and review of the information supplied that the locking switch involved is a switch on a control circuit which controls a motor starter. The lock out standard defines an energy isolation device as one which "physically prevents the transmission or release of energy". It also indicates that the term "does not include a push button, selector switch, and other control circuit type device".

We believe that the schematic submitted by UPS reflects that the keyed lockout switch is merely a selector switch which is a control circuit that stops the motor through a motor starter. In our view, this would not meet the "energy isolation device" definition based upon the switch being a control circuit device.

If this device would be considered an unacceptable control type circuit, then we would request some further clarification of the acceptability of devices. Specifically we would need additional information as to why this method does comply and what types of control circuit methods and devices would comply.

Our purpose in raising this issue was not simply based on a concern for this particular facility, but on the possible ramifications at UPS facilities throughout the United States. It is important to ensure that modifications instituted by UPS are consistent with agency policy and in accordance with the standard. Based upon the nationwide implications we recommend having this issue forwarded to the National Office for their review and response.

Finally, time is of the essence in this matter. As I have pointed out in my previous memos on this subject, the formal settlement of two contested cases with UPS rests in part on the agency's interpretation of "energy isolation devices". We would appreciate a thorough but expedient review of this information in order to conclude the case.

If any further information is needed regarding this matter, please contact our office.


U.S. Department of Labor
Occupational Safety and Health Administration
Washington, D.C. 20210

Reply to the attention of:

December 14, 1990

MEMORANDUM FOR:     William M. Murphy
                   Area Director
                   Cincinnati Area Office

FROM:               W. O. Wiehrdt
                   Assistant Regional Administrator
                   Technical Support

SUBJECT:            United Parcel Service: Lock Out Proposal
As discussed between you and James Kontos, the correct interpretation of "Energy Isolation Devices" is: "A mechanism, tool, or other piece of equipment designed to positively disconnect the power capable of doing work. This mechanism will release energy only by the person who locked the energy, no one else."

The electrical schematic supplied to us with your second memorandum depicts this type of device; however it does not show that the lock out control is on the "off" position. As shown in the drawing now it is a close circuit "on position."

We have marked the schematic for your information.


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