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Answered by Roger Harrison, director of training, Rockford Systems Inc., Rockford, IL.
For best safety practices, machines must be prevented from automatically or unintentionally restarting when a power interruption occurs. To determine if your machine has drop-out (anti-restart) protection, perform the following steps:
- With the motor running, turn off the power to the machine at the motor starter disconnect, or unplug it from the outlet. Do not do this as a normal means of motor stopping unless the electrical system is designed for it.
- Let the motor ramp down to a complete stop.
- Turn the power back on.
- If the machine starts, you do not have drop-out protection.
When a machine is running and then unexpectedly loses power, the main motor stops after a certain rundown time. The safety concern is what happens when the power returns. Perhaps the greatest hazard is that an unexpected motor startup directly initiates hazardous motion. Several methods can be used to prevent unintended machine motor startup, whether for initial start or restart. One commonsense requirement found in machine safety regulations is to protect the main motor start button from accidental operation, such as accidentally bumping the start button. This can be accomplished by using:
- A ring guard around the start button
- A start button that is recessed or flush to the surface
- A fabricated shield over the start button
Safety regulations that reference preventing accidental operation can be found in OSHA 1910.217 (b)(8)(ii) and ANSI B11.1-2009 (6)(2)(3). OSHA 1910 Subpart O specifies two categories of machines that require a motor starter with an anti-restart feature, also known as drop-out protection. These machines are mechanical power presses (OSHA 1910.217) and woodworking machines (OSHA 1910.213).
In their respective mechanical power press requirements, OSHA 1910.217 (b)(8)(iii) and ANSI B11.1-2009 (6)(2)(4) both state, "All power press controls shall incorporate a type of drive motor starter that will disconnect the drive motor from the power source in the event of control-voltage or power source failure. Operation of the motor-start button shall be required to restart the motor when voltage conditions are restored to normal."
OSHA 1910.213 (b)(3) for woodworking machines states, "On applications where injury to the operator might result if motors were to restart after power failures, provision shall be made to prevent machines from automatically restarting upon restoration of power."
The most common method to prevent unexpected machine restart after a power failure, then power restoration, is a magnetic-type motor starter. Electrical contacts within the starter are magnetically separated upon loss of power. When power is restored, the main motor start button must be pushed to reconnect the electrical contacts and restart the machine.
Another more compact device designed for smaller machines is a drop-out device that is hard-wired to the machine. This Sensing Saf-Start device features a reset button that must be pushed to restart the machine once power has been lost and then restored.
NFPA 79-2007 "Electrical Standard for Industrial Machinery" covers all industrial machines and states in 7.5.3, "Upon restoration of the voltage or upon switching on the incoming supply, automatic or unintentional restarting of the machine shall be prevented when such a restart causes a hazardous condition."
The implication here is for an anti-restart feature – also known as drop-out protection.
For best safety practice, determine if your machine has drop-out/anti-restart protection. Refer to OSHA, ANSI and NFPA 79 for all electrical requirements. It is a safety responsibility to protect against supply interruption or voltage reduction and subsequent restoration.