Circuit Breakers
Over-current and Ground Fault protection on Irrigation Craft pump stations is provided by circuit breakers. Therefore, Irrigation Craft equipment is fuseless. The advantages of circuit breakers are:
Safety - Circuit breakers reliably disconnect the entire load, whereas fuses often disconnect only one of the circuits.
Motor Protection - Circuit breakers protect three phase motors better than fuses, especially large pump motors. When a fuse disconnects the load, only one of the three phases is opened, allowing the motor to operate under "single phase" conditions, requiring another device to protect the motor from this quickly damaging condition. A circuit breaker however, opens the entire circuit resulting in total disconnection of the entire circuit.
Cost - Circuit breakers save the owner money by eliminating expensive fuses and the time to order and pick up the fuses. When a fuse opens, company policy often requires replacement of the entire fuse set because the remaining fuses in the set may have been weakened or partially damaged during the fault, with the weakest fuse opening first.
Specifiers who study hard to serve their customers well, are attracted to the large gap in safety, cost, and reliability, that circuit breakers offer over fuses. The initial purchase price can be deceiving. The cost of just one service call and a set of fuses may cover the additional cost of circuit breakers.
Type of Circuit Breakers
Irrigation Craft uses Siemens molded case circuit breakers for the main electrical disconnect, serving as the UL 489 Branch Circuit Protector (BCP). All main disconnect breakers are Thermal and Instantaneous Magnetic Trip.
Thermal provides accurate inverse time delay low level fault current protection.
Instantaneous Magnetic Trip protection provides fast reaction to high level fault currents.
Irrigation Craft Main Disconnect circuit breakers have high AIR ratings. AIR ratings indicate the breakers ability to handle large fault conditions safely.
AIR (Amperage Interrupting Rating)
Also called
AIC (Amperage Interrupting Current)
Determined by testing at Underwriter's Laboratories (or other approved agency). The AIR rating is the maximum amount of current that the circuit breaker has demonstrated, can be safely disconnected in a fault condition exhibiting a symmetrical wave form.
AIR for Siemens circuit breakers used by Irrigation Craft:
Note: The type of breaker used is determined by the amount of current being handled by the breaker and by the intended use.
CQD - Individual Motor Protection Only to 100 Amps
120 Volts - 65,000 Amps AIR
240 Volts - 65,000 Amps AIR
480 Volts - 14,000 Amps AIR
ED4 - Main Disconnect and Motor Protection to 125 Amps
120 Volts - 65,000 Amps AIR
240 Volts - 65,000 Amps AIR
480 Volts - 18,000 Amps AIR
FXD6 - Main Disconnect and Motor Protection 125 - 250 Amps
240 Volts - 65,000 Amps AIR
480 Volts - 35,000 Amps AIR
JXD6 - Main Disconnect and Motor Protection 250 - 400 Amps
240 Volts - 65,000 Amps AIR
480 Volts - 35,000 Amps AIR
LXD6 - Main Disconnect and Motor Protection 400 - 600 Amps
240 Volts - 65,000 Amps AIR
480 Volts - 35,000 Amps AIR
Safety
Siemens circuit breakers were chosen based on our confidence in Siemens engineering and support. These breakers have both instantaneous magnetic trip for high fault currents, and inverse time delayed thermal protection for low fault current response. These two features and the general design of the Siemens breakers provide major safety advantages over fuses.
Circuit breakers provide greater safety than fuses in the majority of fault situations on pump stations. This is because circuit breakers ALWAYS open ALL circuits simultaneously when a fault occurs, because all poles of a circuit breaker are mechanically linked. When one pole of a circuit breaker opens, all poles are forced open, completely de-energizing the faulted component. In a fault situation fuses cannot fully disconnect the load unless every fuse on every pole opens, a rare event. When a fused circuit faults, the majority of the time only one fuse opens, leaving one or two fuses feeding power into the faulted component. Each fuse experiencing high current flow will open, however damaged components typically do not cause high current flow into each pole.
Reliability
Circuit Breakers are more reliable than fuses. Pump stations with circuit breakers operate longer periods of time without problems compared to stations with fuses. Fuses cause unnecessary shut downs because fuses wear out and open when there is no fault.
Fuses wear out due to mechanical fuse element fatigue. Each time current changes in a circuit, the fuse element heats up or cools down, causing the fuse element to change shape. Additionally, motor starting current is 3-15 times greater than the normal motor run current, so that each time a fuse element experiences current flow beyond the fuse current limit, (but for a time period lower than time limit point of the element), the fuse element torques or twists, producing mechanical fatigue of the element. The result of this fuse element twisting is that specific areas of the element become stressed and this always leads to one result, the fuse element runs hotter and hotter, and eventually the fuse element burns open when there really is no fault in the circuit. Circuit breakers are not subject to this surge current fatigue problem.
Reliability and Safety
If people become frustrated because a fuse fails too often for no apparent reason, people will, and often do, replace fuses with larger fuses to "correct" the problem, thereby causing a potentially dangerous situation. Also, if the correct fuse is not readily available, people may use a larger fuse and thereby cause a dangerous situation.
Replacing troublesome fuses with larger fuses, or bypassing the fuses entirely, is an old and well known problem. Warning labels will not stop some people from doing something wrong, if they are under pressure to "fix" equipment or "keep" the equipment running reliably, or if they have to pay a service company hundreds of dollars for fuse replacement.
Initial Cost
Circuit breakers have one disadvantage compared to fuses, initial cost. The cost of a circuit breaker ranges from 3 to 19 times the cost of a comparable fusible disconnect switch with its fuses. Moreover, inventory for circuit breakers is much higher than for fuses. Two fused disconnect part numbers cover the 10-100 amp range, with the only other inventory required being the fuses for each amperage. With circuit breakers however, 12 different part numbers were required to cover the 10-100 amp range, and each of those 12 different part numbers cost 3 to 19 times more than comparable fused disconnects with the fuses. These two factors cause equipment with circuit breakers to cost more than equipment with fuses.
Cost of Ownership
Circuit breakers offer savings to the owner compared to fuses. A circuit breaker can be reset immediately and easily, and circuit breakers last a long time compared to fuses. Fuses are one-time components that must be replaced each time the fuse acts. Moreover, each time a fuse acts the fuses must be replaced in sets, so 2 or 3 fuses must be replaced each time, not just one (2 fuses on two phase systems, 3 fuses on three phase systems). Moreover, fuses used on motor starting services wear out and must be replaced periodically.
The labor is the most expensive item in the total cost of fuse replacement. However fuses themselves are also very expensive to purchase. Retail fuse costs range from approximately $10.00 each up to $250.00 for each fuse, and correct procedure requires that the entire set of fuses be replaced, not just the fuse that failed. Therefore, the cost of fuses ranges from $20.00 to $750.00 or more for an entire set of three fuses. Add to this the cost of labor to purchase the fuses and to actually replace the fuses, and the costs quickly climb to one hundred to one thousand dollars or more depending on the size of the equipment, the site location and depending on who is doing the work.
Compounding this cost problem is the reliability issue. Fuses are less reliable than circuit breakers because fuses wear out (read the section on Reliability). So the majority of fuse replacements over the life of a system should really be labeled fuse maintenance, because fuses fail due to aging.
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