Thank you Richard – this situation vividly illustrates the danger of nuisances tripping – what message does that give to the Home Owner – “FIT A BIGGER FUSE****!” That is why (some) CSA standards require a Turn-On Test – ON-OFF 10 times within a minute – protection devices MUST NOT OPERATE. I have been recommending this test to clients as GOOD ENGINEERING PRACTICE for the last 10 years. Best regards Gregg
From: [email protected] [mailto:[email protected]]On Behalf Of [email protected] Sent: Wednesday, January 29, 2003 7:58 AM To: Rich Nute Cc: [email protected] Subject: Re: Circuit Breaker Tripping Dring Fault Tests Rich and group, Just one interesting point continuing the thought of fuse sizes for branch circuit protection. Awhile back, I was conducting a series of UL tests on a household food mixer. For one of the abnormal tests, UL required me to use a 30 A time-delay branch circuit fuse (lamp base) in the power supply to the unit under test. I thought this was extreme, believing that a 20 A fuse would be the maximum anyone would ever use. When questioned, I was told that such fuses are available and still in use, primarily in old residential areas. I visited my local hardware store and, sure enough, they were readily available, right along with the other more common sizes. You may want to consider this when considering such tests in the future. Good day, Richard Pittenger Agency Approval Engineer Hobart Corporation Rich Nute <[email protected]> Sent by: [email protected] 01/28/2003 07:23 PM Please respond to Rich Nute To: [email protected] cc: [email protected] Subject: Re: Circuit Breaker Tripping Dring Fault Tests Hi Peter: > For safety, it is not clear from the standards whether > the main branch circuit breaker tripping during fault > conditions is an acceptable result. > > I see no reason why this should not be acceptable. What > is your view? Some third party labs find it acceptable > and others do not. Some products are provided with internal overcurrent protection and some are not. Clearly, those that do not have internal overcurrent protection rely on the branch circuit protection. If a product has an internal overcurrent protective device, and the fault is on the load side of that device, then the internal device should provide the protection and not the branch circuit device. (Otherwise, the internal device provides no protection, and might as well be removed.) If the fault is on the supply side of the internal device, then clearly the internal device cannot provide protection, and the branch circuit must provide the protection. The real question is whether or not the product is safe when the fault current is just below the operating point of the branch circuit device. Examining this question requires an understanding of the fault and whether its resistance can be high enough to not trip the branch circuit yet not create a hazardous condition (such as a fire). If the fault resistance always is no more than 120/20 = 6 ohms, then I would say that the branch circuit could be relied upon to provide protection against the fault. Note that in the USA, a 120-volt branch circuit can be provided with either a 15-amp or a 20-amp overcurrent device. Therefore, the product must be safe when the fault current is 20 amps, just below the overcurrent device operating point. That means that the product must be capable of dissipating 2400 watts without catching fire or destroying internal insulation that serves a safety purpose. Best regards, Rich 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: Ron Pickard: [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: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
