This subject of reliance on branch circuit protection for loads has been discussed for a long time and involves a lot of historical tradition and code and standards activities. The following is my understanding based on some code committee activities, but I have not researched this historically.
You will generally find two kinds of faults which need protection, overloads and short circuits. Overloads are generally faults in an operating product which provide moderate increases in the load. Short circuits may occur between supply conductors or to ground and involve very high currents. The branch circuit protection provides overload and short protection for the branch circuit wiring. The overcurrent is sized to prevent overheating of any branch circuit wiring on either a low current long term or high current short term basis. In some cases you will find breakers which have inductive trip for shorts and thermal trip for overloads. Appliances and the associated cords are expected to be protected by product design. Traditionally we have been able to rely on the branch protection for shorts (which are high current and should trip the branch overcurrent protection). For overloads, internal protection is provided such as additional smaller overcurrent protection, impedance protection, fault testing, etc. In the US the traditional sizing means a cord as small as 18 AWG could be used, since it would usually trip a 15 or 20 amp circuit when shorted. An exception was shaver (tinsel) cords which typically opened under fault and were attended. As has been mentioned in this thread, these basic ideas did not always work as planned. For example, loose connections will often overheat connectors, carbonizing the plastics and resulting in a resistive heat which over time ignites. Short circuits are often high impedance, either because few wire strands are involved or because an arc is involved. Extension cords were overloaded. It turns out circuit breaker curves sometimes permitted an arc to melt the copper and blow the circuit open, sometimes repeatedly, without tripping. Fuses were much more responsive in this case. Gradual improvements have been seen in codes and standards. For example, smaller gauge extension cords and outlet strips now have overcurrent protection. Arc fault breakers have been introduced to detect and open wiring shorts. Breaker curves have been readjusted. Recent code cycles have considered introducing plug located overcurrent as Britain has done. Ground fault interrupters have also played a part in reducing ignition faults. Experience with these improvements will be considered as future code and standard changes are proposed. Bob Johnson ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
