What is your Design Safety Margin?

When designing components, equipment or machines, engineers use information from a variety of sources.  This could be information from specifications from a component manufacturer, customer requirements, or requirements from a safety standard.  To ensure sufficient tolerance in the design of the component, equipment or machine the design engineer needs to account for component, manufacturing, installation, and environmental variability.  Accounting for this variability is accomplished by incorporating a safety margin or de-rating factor into the design process. 

Example 1

Let’s consider the operating voltage of a general use AC capacitor.  The manufacturer‘s specifications state that the component has a maximum operating voltage of 600 V.  The equipment where this AC capacitor is intended to be installed in is to operate at 480 V with a tolerance of 10 percent.  The engineer establishes a 15 percent safety margin to the operating voltage of the capacitor.  Working through the math, the following is obtained:

1.                  Voltage tolerance of 480 V source is 408 V to 552 V.

2.                  New maximum operating voltage of the AC capacitor is 540 V

In this example, the maximum AC capacitor rated by the manufacturer was reduced from 600 V to 540 V.  Since the equipment is intended to operate on a power source that has voltage range up to 552 V, this capacitor should not be used or steps should be taken to ensure that the capacitor is appropriate for the limitations established.  This could include using an alternate component with a higher rated operating voltage, using capacitors in series, or controlling the voltage tolerance of the source.  It is never a good idea to reduce the safety margin unless a sound engineering analysis has been performed.

Example 2

Let’s consider the spacings between phases of opposite polarity for input terminal lugs of a power distribution unit (PDU).  The UL safety standard states that the terminal lugs shall have a spacing of least 1 inch at 480 V.  The PDU is intended to operate at 480 V with a tolerance of 10 percent.  The engineer establishes a 15 percent safety margin.  Working through the math the voltage ranges from 408 V to 552 V.  At 552 V the spacing between the terminal lugs is 1.15 inches.  Accounting for the defined safety margin, the spacing between the terminal lugs is increased to 1.33 inches.

In this example, the spacings of the field connected terminal lugs need to be increased from the dimension of 1.0 inch to at least 1.33 inches between phases of opposite polarity.  The increase in separation of the terminal lugs in the PDU may not affect the terminal lugs specified, but it may affect the particular location that they are installed.

Conclusion 

Every component, equipment or machine has specifications that can be subjected to the addition of a safety margin by the design engineer.  The addition of the safety margin may or may not change how a component, equipment or machine is designed, installed or used.  When incorporating a safety margin reduces the specifications to a point where the component, equipment, or machine is outside of the tolerances, alternative designs need to be considered.  Regardless of the actions taken, safety margins need to be considered in the design process of all equipment or machines.