This is going to be a very short post touching on one aspect of Safe Working Practices.
Often you will come across installations that have a Manual/Auto Selection. What you need to be very aware of is when you have isolated the circuit at a remote location on the control circuit. With the unit in the Auto position, the Start Instruction may still be active when you turn power back on and that means the machine will start without warning.
This is a very easy mistake to make and that is also part why I always state that you must notify the relevant Manager that you have finished work and will be restoring power. Before you restore power, make sure there is no one else working on the machine. You must ensure that it is safe to restore power.
So how could this happen? We will just take a scenario and look at what could cause this. You have a machine that is controlled via a PLC. The PLC is situated in a cabinet and once all conditions are met, it will allow the motor to start. Further along the machine is a control cubicle through which some limit switches and/or sensors are connected. Because of the distance between this and the main cabinet, there is the functionality to isolate these devices in this cubicle without isolating the main control circuit.
Now assume you had a fault on one of the sensors at this position and you have isolated the supply in the control cabinet, fixed the fault and you are now about to restore power. What you are not aware of is that by restoring power, all the start conditions are met and the unit will start. This is where the danger lies. Another person was aware that you are working on the machine and decided to also fix a small item on the machine while you are working on it. This is where it all goes wrong when you restore power.
Agreed, this person should have followed the correct LOTO procedure but did not. That is not a risk you need to take. By informing the relevant Manager and checking the machine before restoring power, this person would have been seen and as such, the incident would have been avoided.
Till next time, Always Think of Safety before you work on any installation!