CONNECTORS & POWER CORDS: Protection & Power Cords
Recent focus has been the protection of flexible cords against over current and arcing-type faults. Article 240.5 of the National Electrical Code (NEC), NFPA 70-2005 specifically addresses the protection of power supply cords provided as part of appliances and portable lamps.
NEC proposals for Article 240.5 made a number of years ago would have required the power supply cords of appliances and lamps to be provided with integral over current protection. Proponents felt that the addition of over-current protection in the cord would protect against fires caused by short circuits occurring in the cord itself. Many of these short circuits were believed to occur as a result of damage to the cords in service.
Others were of the opinion that over-current devices (fuses typically in a plug) do not provide protection for all of the possible fault conditions. That is, over-current protection devices may not respond to intermittent faults for example series arcing that can ignite adjacent combustible materials. Testing was conducted and analysis of results yielded consensus and adoption of the language in Article 240.5(B)(1) of the 2005 NEC.
The NEC now additionally states that flexible cord or tinsel cord provided as the power supply cord of a listed appliance or portable lamp is considered to be protected when applied within the appliance or portable lamp listing requirements. This means that the applicable safety standard determines the extent of protection that is required for the power supply cord of each appliance or portable lamp type. The end product standard can include specific protection types when necessary based on the intended use and actual service history for the product.
In some respects, this affirms the direction taken in UL's Standards, making changes to end product standards as warranted. It also affords a degree of flexibility in approach. For example, the minimum cord construction for many products has been upgraded from SPT-1 to SPT-2 and, where applicable, the number of cycles of cord flexing has been increased. For other products such as Seasonal Lighting Strings, over-current protection is required in the attachment plug.
Recent developments in technology have resulted in the availability of special protection devices intended to provide additional protection for power supply cords. These include the Arc Fault Circuit Interrupter and Leakage Current Detection and Interruption. Both can be configured either in the attachment plug of a power supply cord, or within an enclosure near the attachment plug to protect the free length of the cord. These devices are intended to de-energize the power supply cord (and connected appliance) when damage to the cord results in either arcing in the cord for the AFCI, or the flow of small leakage currents internal to the cord in the case of the LCDI. Single phase cord and plug connected room air conditioners are now required by the safety standard (ANSI/UL 484, Room Air Conditioners) and the NEC (Article 440.65) to employ an LCDI or AFCI as protection against arcing faults within the power supply cord.
With the choices at hand, in what direction does the appliance designer proceed?
This is not an easy question to answer. It requires a strong understanding of the field experience of the appliance in question. How is it used? Does it move around or stay plugged in to the same receptacle? Is an extension cord used? How long will the appliance be in service? Will it be discarded if the flexible cord is damaged or will it be repaired? If there have been field incidents, to what extent did the flexible cord contribute to the incident? Does a contemplated end-product improvement address the issue? Does the improvement introduce other issues that mitigate its value?
These and other safety considerations need to be addressed. So too do production quality control, availability of components and cost. Today, designers must weigh the options for change against remaining with their current designs. Some may elect to take a hybrid approach. That is, pursue increased resistance of the power supply cord to mechanical abuse and incorporate protection against arcing faults in the cord. Prevention and protection are not mutually exclusive safety strategies.
On the forefront of this matter are those who are participating in safety standards and NEC development activities. A joint stakeholder working group for electric heaters and fans has been meeting to consider strategies. Presented with field incident data, primarily from the U.S. Consumer Product Safety Commission, it has been closely examining the data, analyzing potential root causes and establishing standards development plans. These may result in changes to the respective safety standards for electric heaters and fans. The work is ongoing.
A concurrent effort has been undertaken by a working group of NEC Code Making Panel 17 (CMP 17). While some similar ground to the standards development working group is being covered, one of the goals is to make recommendations on future Code changes relating to AFCI and LCDI protective devices: Should they be integral parts of household appliance power supply cords? Extension cords used with appliances? Part of the branch circuit? (See NEC Article 210.12(B) for dwelling unit bedrooms.) Others?
These are fundamental questions. The answers will take time. Standards should ensure safety while retaining product efficacy, both at a cost the market will bear.
Appliance designers are encouraged to become familiar with current trends in the safety standards and the National Electrical Code. They should contribute their by, participating in UL's standards and NFPA's Code development processes. So too should the suppliers of critical components for power supply cords and protective devices. The public's safety is best served with the collective and comprehensive experience of all the stakeholders.
Much time and effort will be devoted in the coming months to making the venerable power supply cord less vulnerable.