Posted for chuck_me...@electro-test.com:
:-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-) Ed Price ed.pr...@cubic.com Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 (Voice) 619-505-1502 (Fax) Military & Avionics EMC Services Is Our Specialty Shake-Bake-Shock - Metrology - Reliability Analysis :-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-) > -----Original Message----- > From: Chuck Mello [SMTP:chuck_me...@electro-test.com] > Sent: Wednesday, September 01, 1999 7:41 AM > To: s...@world.std.com; 'emc-p...@majordomo.ieee.org'; 'n...@nfpa.org' > Subject: Re: NFPA and overcurrent protection requirements. > > Lauren > The NEC and NFPA 79 are pretty much in alignment with each other for > overcurrent protection of conductors. The basic rule as you properly > stated is that overcurrent protection is to be provided at the source or > the start of the conductors being protected. For the situation posed by > the exceptions to paragraph 8.4 and also by NEC Section 240-21 for tapped > conductors, you have to consider the elements of overcurrent protection > separately to understand the application. > > Overcurrent protection includes short circuit, ground fault and overload > as > the three elements. What the exceptions and the NEC tap rules do is allow > you to have only the short circuit and ground fault protection ahead of > the > conductors as provided by the main or feeder device. That is where the > requirements of the tap conductor having an ampacity of not less than 10 > times the feeder conductor for the 10 foot rule or the tap has to be at > least 1/3 the ampacity of the main conductors in the 25 foot limitations. > The overload protection is then provided by a limited load and within the > enclosure (10 foot rule) or a device (circuit breaker or single set of > fuses) at the load end of the conductors (10 and 25 foot rules). The > position of the overload device in the circuit is not as critical in that > it provides protection ahead of and below the device for the conductors. > Another example of this that is similar is motor protection where the > short > circuit device is at the source of the circuit, and the controller with > the > overload device is possibly at some distance away toward the motor. All > the branch circuit conductors from the short circuit device to the motor > are protected by the overload device no matter where it is located in the > circuit. > > Joe Sheehan at NFPA is the staff liaison for NFPA 79 and is an excellent > practical resource. NFPA policy is that staff can provide an opinion but > formal interpretations have to go through the technical committees which > can take some time. > > ---------- > > From: Crane, Lauren <lcr...@bev.etn.com> > > To: 's...@world.std.com'; 'emc-p...@majordomo.ieee.org'; 'n...@nfpa.org' > > Subject: NFPA and overcurrent protection requirements. > > Date: Monday, August 30, 1999 9:19 AM > > > > Dear Colleagues, > > > > NFPA 79 (1997) provides the following requirement regarding overcurrent > > protective devices... > > > > "8.4 Overcurrent protective devices shall be located at the point where > the > > conductor to be protected receives its supply." > > > > Then two exceptions are provided. However, it is not explicit if the > > exception is regarding the placement of the protective device at the > supply > > point to the conductor, or is the exception regarding the provision of > the > > protective device. In other words, does the exception allow the > protective > > device to be absent under the given conditions? > > > > A similar section in EN 60204 (1997) is 7.2.8 > > > > "An overcurrent protective device shall be located at the point where > the > > conductor to be protected is connected to its supply. Where that is not > > possible, no overcurrent protection is required for those conductors > with > > current-carrying capacity less than that of the supply conductors, > provided > > that the possibility of a short circuit is reduced by all of the > following > > measures:..." > > > > This section clearly states that the exception allows not having any > > protection at all. > > > > It seems reasonable that the intent of NFPA 79 is the same as EN 60204 > .. > > but I want to be a bit more certain. > > > > So...the basic question....IF all the conditions are met for either the > > first or second exception of section 8.4 in NFPA 79, may an overcurrent > > protective device be left out? > > > > And two follow-ups > > > > ...with regard to exception 1, the last condition "the conductor > terminates > > in a single circuit breaker or set of fuses" means, for example, a 16 > awg > > wire may get its supply from a terminal block fed by a 2 awg wire, but > it > > must terminate at the other end in a circuit breaker or fuse sized > correctly > > for the 16 awg wire. Is this correct? > > > > ...with regard to exception 2, the last condition "the conductor > terminates > > in a splitter block, circuit breaker or set of fuses." This means that > the > > 16 awg wire of the above example may also terminate to another terminal > > block. Is this correct? (i.e. splitter block = terminal block). > > > > Thanks in advance for any comments. > > > > P.S. does anyone know how to get interpretations out of NFPA? I have a > > question queued up at n...@nfpa.org that has been answered after a couple > > weeks of waiting. --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
