Overcurrent and Overcurrent Protective Devices - …

1 Overcurrent and Overcurrent Protective DevicesFigure 3I =VOLTSTOTAL OHMSI = + + = amperesThe Figure 3 value of overload current may be up to about10 times the normal current value of example:Imagine working at a drill press. Suddenly, the drill becomeswedged in a steel of continuing to draw normal motor load current, thelocked drill motor shaft causes the motor to draw Locked RotorCurrent which is about six times the normal load may be serious unless the Overcurrent protectiondevice opens in time to prevent Multi-Purpose, dual element fuses, properly sized,provide excellent protection for similar term Overcurrent refers to abnormal current flow higher than the normal value of current flow in an electricalcircuit. Uncorrected Overcurrent can cause serious safetyhazards and costly damage to electrical equipment and most circuit breakers are installed in electrical circuits to open and stop the flow of Overcurrent .

2 Fuses and typical circuit breakers respond and open for bothlow and high values of Overcurrent flow. Limiters and mag-netic only circuit breakers respond only to high values ofovercurrent are three basic types of current flow in an electrical circuit:1. Normal intended current flow to operate electrical Abnormal Overcurrent flow with a value of up to 10 timesnormal current flow. This is known as an overload .3. Abnormal Overcurrent flow with a value more than 10 timesthe normal current flow is known as short-circuit or fault current otherwise noted, current (amperes) and voltage(volts) are alternating current/voltage RMS values as read onan ammeter or voltmeter. The alternating frequency is 60Hz(cycles per second). Impedance is a combination of resist-ance and inductive reactance and is expressed in units Protection Device s Law is used to find the value of normal current flow inthe simple circuit illustration.

3 (See Figure 3)*Short-circuit protection onlyFuseFigure 4 Figure 4 is identical to Figure 3 except a short-circuit faulthas been shown that by-passes the impedance of the load of10 ohms. The only impedance remaining to oppose the flowof current is ohms which is the total impedance of theconductors. Short-circuit current now flows around the loadas shown by the arrow in the heavy value of short-circuit current flow for this simple illus-tration is determined by using Ohm s Law:ISC=VOLTSTOTAL OHMSISC=480= 24,000 are many specific ways that short-circuits (faults) mayoccur. Some of the more common are accidents, careless-ness in the misuse of fish tape , tools, etc., crossed phases,contamination, rodents, damaged insulation and unfusedsmall Protection circuit breakersFuses contain an element that acts like a conductor for thenormal current flow. The element s ability to sense abnormalcurrent is the basis for the amp rating of the fuse.

4 When anovercurrent develops that exceeds the fuse rating, heatbuilds up inside the fuse and melts or opens the the element is melted and the arc extinguished, currentflow the overload or fault condition that caused the fuse toopen is corrected, a proper replacement fuse restores theoriginal dependable Breakers contain a resettable latch which acts likean ON-OFF switch for the normal current flow. When abnor-mal current flows through the circuit breaker, the latch trips,and the indicator on the outside of the circuit breaker movesinto the OFF position. Once the Overcurrent problem hasbeen corrected, the indicator lever can be reset into the ON position, which simultaneously resets the latch inside the cir-cuit showing elementduring normaloperationElement intactFuse showing elementmelted by anovercurrentElement meltedBreaker "ON" Overcurrent and Overcurrent Protective DevicesSummaryIt is obviously necessary to rate fuses and circuit breakersaccording to their ability to safely interrupt a given MAXlMUMvalue of short-circuit current UL Fuse Interrupting Ratings*Class H10,000 Class K50,000, 100,000 or 200,000 Classes J, L, R, T, and CC200,000 Class G100,000 Standard UL Circuit BreakerInterrupting Ratings*5,00042,0007,50050,00010,00065,0 0014,00085,00018,000100,00022,000125,000 25,000150,00030,000200,00035,000*Contact manufacturer for availability of 5A specific fuse or breaker may be submitted for testing at oneof the standard levels shown in Figure 5.

5 The successfulproduct testing results in listing of the product as having an interrupting rating ( ) at the level of test current. In otherwords, when a fuse or circuit breaker will safely interruptdeveloped energy at a specific value of short-circuit current,say 200,000 Irms, the product manufacturer may place a ULlogo and words similar to Interrupting Rating 200,000 AmpsA-C on the product label. The product will interrupt any valueof fault current up to 200,000 "OFF" Overcurrent and Overcurrent Protective DevicesWhen fuses and circuit breakers are properly applied withintheir interrupting rating, as required by , deviceopening during short-circuit current flow generates high lev-els of energy inside the current limiting fuses are designed to retain theirhigh interrupting rating in normal use until caused to openand are then replaced with a new fuse.