The Construction Safety Association of Ontario recently published a safety advisory on manual motor controllers (Construction Safety Magazine, Autumn 2000). A manual motor controller or motor starter circuit is used to stop or start a motor. The advisory said manual motor controllers have been installed as disconnect switches on rooftop-type air-conditioning units and for other uses. The advisory warned that using some manual motor controllers as disconnect switches when you do maintenance or repair may expose you to electric shock.

Even if you leave the switch open, it can fail and leave some parts of the units you're working on turned on. Also, some manual motor controller switches have been found installed backward, which means the power is ON when the switch is labeled OFF.

With manual motor controllers (or motor starter circuits), only one conductor is opened. For lockout/tagout – to prevent anyone turning on equipment while repair or maintenance is done – an approved disconnect switch must open all conductors on the power supply and must be able to be locked open. Turning a motor starter switch to OFF is not an acceptable method for lockout/tagout, because that does not guarantee that a machine is turned off.

OSHA does not accept motor starter circuits as complying with its Lockout/Tagout Standard. (See OSHA letter of interpretation http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=20354) The circuits are not an acceptable way to disconnect equipment under the OSHA Construction Standard.