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*  Produced by:  Ontario Ministry of Labour                         *
*  Provided by:  Canadian Centre for Occupational Health and Safety *
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INFO ALERT:  M1/0893

DATE:  08/93


*** HAZARD SUMMARY:  INTERLOCKED EQUIPMENT ***  

Recently, an incident occurred in which a locked and tagged conveyor
started up while a maintenance crew was about to begin work on it.  No
workers suffered injuries, but the incident could have had serious
results.

The investigation revealed that, although the handle on the door of
the conveyor started was properly locked and tagged in the OPEN
position, the screws securing the door in place were loose.  This
allowed the door to distort when the handle was turned, causing the
tongs on the inside to slide off the handle of the molded case circuit
breaker.  As a result, the breaker remained in the ON position when
the handle was moved to the OPEN position.

After checking that this handle was at OPEN and putting their locks
and tags on it, the maintenance crew went to the conveyor and used its
START button to test whether it was indeed safely off.  The conveyor
did not move.  Then, as the crew was getting ready to do their work,
the conveyor's start-up alarm sounded and the conveyor began moving. 
Its pullcord was used to stop it.

This conveyor is interlocked with other equipment in the area through
PLC ("programmable logic controller") logic in the control room.  The
workers did not know that this logic was active when they pushed the
START button and was what prevented the conveyor from starting then. 
There was no selector switch at the control station by the conveyor to
allow them to switch from remote operation to a manual mode before
using the START button.


*** LOCATION(S) AND SECTOR(S): ***

Mining, industrial and construction; wherever interlocked equipment is
used.




*** SUGGESTED PRECAUTIONS: ***

Before workers attempt to start equipment as part of their lockout
procedures, they must know that the start button they are using is
active and has not been blocked out of the circuit by interlock
circuits.  One method of allowing workers to know that the interlocks
have been removed from the circuit is to have a selector switch
located in the same box as the local START/STOP buttons.

Sections 160 of the Mining Regulations, section 42 of the Industrial
Regulations, and section 188 of the Construction Regulations all
require that all switches controlling electrical equipment be in the
open position and locked out before work on the equipment can begin.



Produced by the Mining Health and Safety Program, Occupational Health
and Safety Branch, Ministry of Labour.


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