Failure modes in Electrical Control Circuits for Isolation

OSHA created a demonstration panel to illustrate the ways in which failure can occur in the sequence to lock out electrical equipment.

Energy isolating devices isolate machines and equipment from energy sources. The standard prohibits the use of pushbuttons, selector switches, and other devices in control circuits as energy isolating devices to control hazardous energies.

The electric circuit on the demonstration panel illustrates a motor control system. The circuit consists of a power circuit which distributes electrical energy from the source (main disconnect) to the motor (connected loud) and a control circuit to control the distribution of power. The control circuit contains a motor controller, system interlock device, on/off key switch, and start/stop push buttons.

The first situation of concern involves locking devices in control circuit. The panel demonstrates several situations, where unexpected energization or startup of the motor can occur following use of this methodology.

The second situation results from trusting the limit switch to stop a motor. These devices prevent energization of circuits by push buttons but will not prevent the start - up of a motor when voltage is present in the power circuit. A motor can start regardless of what occurs in the control circuit. Motor start can occur in several ways.

These situations identify some of the risks associated with reliance on control circuits as the primary method for controlling hazardous energy. A switch or other device in a control circuit is not an energy isolating device.

In industries, one should not rely on the Electrical Control Circuits for isolation. They only bring the machine to a standstill, while the machine remains energized as these electrical control devices such as E-stops, SCADA, etc. do not isolate the energy from its source.

To understand the failure modes in Electrical Control Circuits, one needs to first understand the basics -

What is a control circuit?

Control circuits are the means to control the on-off, speed-up, slow-down, forward-reverse, in-out, pressurized-depressurized, open flow-close flow, and other actions of equipment and machines. Control circuits also control the movement of fluids in piping and other circuits. Understanding about the control circuits is essential to understand the management and control of hazardous energy.

What is Energy Isolating Device?

Control circuits are the means to control the on-off, speed-up, slow-down, forward-reverse, in-out, pressurized-depressurized, open flow-close flow, and other actions of equipment and machines. Control circuits also control the movement of fluids in piping and other circuits. Understanding about the control circuits is essential to understand the management and control of hazardous energy.

OSHA’s standard for Lockout Tagout defines energy isolating devices as “A mechanical device that physically prevents the transmission or release of energy.” The OSHA definition explains that energy isolation devices include, but are not limited to, the following:

• Manually operated switches
• Manually Operated Electrical Circuit Breakers
• Disconnect Switches
• Line Valves
• Block / Blind Flanges

These (EID) Energy Control Devices are to be isolated and locked and tagged out as per the HECP - Hazardous Energy Control Procedures or MSP - Machine Specific Procedures for isolating all types of hazardous energies.

Safety demands Certainty:

Looking at the drawbacks and failure modes in electrical control circuits, Lockout Tagout of a power circuit, energy isolating (disconnect) device is required for safe isolation, because merely locking & tagging the control circuit device is NOT as safe. One must always isolate the main energy source for safeguarding the workers against unintentional activation and release of hazardous energy. Individual jurisdictions have some differences in their regulations’ details, but ultimately the requirements come down to - Complete Employee Protection.