From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Sinemaster Inverter Generator: Anyone have one? Date: Thu, 17 Mar 2005 14:27:25 -0500 Message-ID: <kflj311t7kbklpbg4jl1g9migb0nnk5hkn@4ax.com> On Thu, 17 Mar 2005 06:46:06 -0800, Dapper Dave <expurgated@gmail.com> wrote: >I know you are talking about a fused circuit here, but I have a related >circuit breaker question. > >I thought circuit breakers would carry only 80% of their rated capacity >indefinitely. E.g., a 30 amp breaker will trip if it carries more than >24 amps for an extended period. > >It that not true? Do fuses work the same way? That's one of those simple questions with a really, really complicated answer. There are probably thousands of let-thru curves for circuit breakers and fuses. For example, a semiconductor fuse will blow instantly when the rated current is exceeded by a certain percentage while a motor starting time delay fuse will pass many times its rating for a considerable period. The response curve is specified according to what is being protected. A semiconductor fuse is protecting the semiconductors in the controller and not the load. An individual motor fuse is protecting the windings from thermal overload and not the branch. An ordinary household breaker is a dual trip device designed to protect the branch wiring but not anything connected to it. There is a magnetic trip that opens the breaker instantly on gross overload like a short. I think the magnetic trip is usually set at 6x the rated current. There is also a thermal trip that responds to the time and magnitude of the overload. The higher the overload the quicker the trip. Most all breakers and fuses must carry the rated current indefinitely. A fuse is somewhat dependent on its mounting to conduct away heat so it isn't all that precise. Most breakers will actually carry a bit more than their rating more or less indefinitely, depending on the ambient temperature. The normal way of sizing breakers is according to the wire size in the branch. If one can get the response curves from the MFR, however, one can do better than just protecting the wiring. Example: When a compressor in a refrigeration unit locks up the thermal overload will cycle until power is interrupted. If it isn't interrupted fairly quickly the thermal overload overheats and the contacts weld shut. At that point the system is entering a "burn out" condition where the windings burn up, the heat breaks down the refrigerant into chlorine and fluorine compounds and the whole mess diffuses throughout the system. A burn out is frightfully expensive to repair, as the system has to be flushed with solvent and a burnout cleaner/dryer installed and later removed. Usually it's cheaper to just replace the system. Unless it's something like a walk-in freezer. If one chooses a breaker with the right thermal response curve then the breaker will trip after a few seconds of locked rotor current before the windings have time enough to start decomposing. The breaker has to be slow enough to ride through even the worst starting inrush and yet trip on locked rotor current. Fairly easy to engineer with the breaker curves in hand. I took that approach with the restaurant. It has saved me a bucketfull of money. I've had two AC locked rotor incidents and one with the walk-in freezer. In all three cases the breaker tripped before the compressors could burn up. All I had to do was recover the freon, acid test it for breakdown products, braze in a new compressor, recharge the system and go. John |