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. 

Product Safety & Risk Assessments

There is more to a safe product than exceeding safety requirements.  Assessing whether a product is safe, the product needs to:

·         Exceed all industry safety standards
·         Exceed all “best engineer practices”
·         Be designed for the intended environment and application
·         Be designed to protect users against foreseeable misapplications
·         Be provided with effective labeling and instructions

Assessing compliance to a specific industry standard is relatively straight-forward.  For an industrial control panel that is intended to be applied in the US, the following standards are minimum requirements:

·         OSHA 29 CFR 1910 – General Industry
·         UL 508A – Standard for Safety, Industrial Control Panels
·         NFPA 70 – National Electric Code
·         NFPA 79 – Industrial Machines

Depending on the technology deployed in the industrial control cabinet and its intended application, additional standards may be relevant.  This includes those related to electromagnetic compatibility, life safety, emergency power systems, hazardous conditions, etc.

When attempting to evaluate the equipment to intended environments and applications and to protect users from foreseeable misapplications, one needs to look beyond industry standards.  One method to analyze these scenarios is to perform a risk assessment.

A risk assessment is a tool used to identify hazard conditions associated with the design, manufacture, installation, operation or de-commissioning of the product.  The major components of risk assessment are:

·         Identify the hazards
·         Assess the hazards
·         Develop controls & make decisions
·         Implement controls
·         Supervise and evaluate

When a thorough risk assessment is used in conjunction with meeting industry standards, “best engineering practices”, and proper labeling and instructions, the product will be better designed than those that do not undergo a risk assessment, and be less likely to cause injury from normal or abnormal operation.

In addition to the increased safety attributes from conducting a risk assessment, there are also financial incentives for an organization.  This includes those associated with increased profitability as fewer resources are required to be accrued to account for the potential of product damages, recalls, or related injuries.

Product Safety

Product safety is an essential aspect of the design and deployment of any product into the marketplace.  Many manufacturers are aware of the requirements to have their product evaluated to national or international safety or performance standards (e.g. ASTM, ANSI, IEC, IEEE, NFPA, UL, SAE).  However, these standards are only the minimum requirements.

Companies need to go beyond meeting minimum product safety standards for two reasons: legal and financial.

Legal Aspects of Product Safety:

The US courts have ruled that a company that merely complies with the minimum product safety standards has done an insufficient job of evaluating the safety of their product.  The US and international courts have also ruled that companies that have been subjected to product safety recalls of their products must report these cases to other markets where the products are sold.

Financial Aspects of Product Safety:

If a company’s product is subjected to a recall, either forced by a government or voluntary because of a poor design, the company can struggle with market acceptance of current or future products.  This reduces the sales and profit potential of the company. 

Large companies can overcome the negative consequences associated with products that have poor safety records or that face product safety recalls.  But, at what costs too marketshare, earnings potential, or overall corporate image? 

Small and medium size companies typically do not have the financial resources to overcome poor product performance or product safety recalls.  These companies are usually required to liquidate, sell out, or are reduced to much smaller organizations.

To avoid product recalls or reduce product failures in general, the best method is comply with all relevant national and international standards regardless of where the product is being sold, to thoroughly review the design and performance attributes of the product, and to perform risk assessments on how the product is intended to be used and not used by the customer.