Standard Interpretations - Table of Contents|
| Standard Number:||1910.147|
August 5, 1991
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.
Question 1: 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:
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 01-05-019 (formerly 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.
It has been claimed in one of the OSHA Region V Area Office letters (to ARA-Technical Support thru Michael G. Connors dated 12/10/90) 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:
Question 3: 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 over-current 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:
Standard Interpretations - Table of Contents|