roy@phri.UUCP (Roy Smith) (11/17/85)
Let me preface this all by urging anyone with a more than passing interest in power distribution to get a copy of Power System Analysis, by Charles A. Gross (John Wiley & Sons, 1979). > Typical reaction at the substation to overload is to shutdown the circuit > and recycle it back up a few moments later. This is retried until the > fault is cleared. The little gizmos that do this are called reclosers. According to Gross they are used on distribution circuits in the 2.4 to 46 kV range (which is usually the high side of the pole transformers). To quote: "Since most faults are self-clearing, reclosers operate on the principle that if the circuit is de-energized a short time there is a good chance that whatever caused the fault was vaporized and the ionized arc path has dissipated." By 'whatever', they usually mean small animal, but copper rods will vaporize and dissipate just as well (assuming your house doesn't first). Typical operation involves opening the circuit when there is a fault and then reclosing it after 20 cycles (1/3 second). If the fault is still there, is opens and tries reclosing after another 15 seconds. If this fails, it makes a final attempt to reclose the circuit after another 45 seconds. If the fault still hasn't been cleared, it locks out and the guys in the yellow trucks have to go out and fix it manually. The problems of fault protection on power distribution lines fills several chapters in Gross (an introductory book). Several things make the subject more complicated than just "big circuit breakers". We're talking 3-phase balanced circuits here. No longer is "short circuit" a useful description. Is it a phase-to-phase, phase-to-neutral, or balanced fault? It makes a big difference which it is. Shorting the two big wires that come into your house will probably be phase-to-neutral. The breakers you are used to seeing are self-contained over-current devices only. Power line breakers often have a remote sensing relay; the decision to open the line is made at point X, but the actual circuit interruption is made at point Y. The relays are usually programmable; one might be set to trip in 6 seconds on a 175% overload and in 1 second on a 2000% overload. Most breakers with current ratings over about 250 A will probably have some sort of setable time-delay; ask the engineer where you work for a tour of the power room to see what I'm talking about. Relays are often directional; a given relay might trip if the fault is on one side of it but not on the other. Relays can also be set up to detect voltage, impedance, or phase problems as well as current overloads. The situation is complex, but suffice it to say that you probably do not want to replace your house's main cartridge fuses with copper rods. As a side note, most circuit breakers are backed up with fuses because breakers are mechanical and may fail when you need them most (especially under extreme overload when they may lock up before tripping). There isn't too much that can go wrong with a fuse. -- Roy Smith <allegra!phri!roy> System Administrator, Public Health Research Institute 455 First Avenue, New York, NY 10016