Circuit Breakers
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Circuit Breakers were developed to protect circuits from experiencing out of control power conditions. Circuit Breakers are designed to sense the amount of current and sometimes voltage and other parameters to determin if the circuit is operating within it's designed range. If the circuit starts to operate outside of its designed range an inverse time rule is applied to the severity of the loss of contol wihtin the circuit.
Testing circuit breakers can be broken into four major areas.
1. Insulation Resistance
2. Conductor Resistance
3. Protective function operation
4. Mechanical Stored Energy Mechanism
Insulation is imperitive if the circuit breaker is going to allow energy to flow through it to feed the circuit it is monitoring. Conductor resistance is improtant as to not affect downstream loads. Protective function operation is important to trip the Mechanical stored energy mechanism opening the breaker conductors and isolating the circuit it feeds from the energy source thus stopping the uncontrolled flow of energy that could cause equipment damage, fires, personal injury or even death.
Careful Consideration must be given to the test plan for a circuit breaker. Each component must function properly for safe, reliable and efficient operation of the circuit it feeds.
Incoming Inspection
Visual - Are their any broken or defective parts?
Mechanical - Does the stored energy mechanism open and close the circuit breaker, does the spring charging motor if installed charge the springs properly and shut off when springs are charged.
Insulation - Does the circuit breaker resist phase to phase or phase to ground electrical flow?
Conductor - Does the circuit breaker adequately conduct electricity on all three phases in a balanced way efficiently?
Once each of these have been checked, now the protective features can be tested and operated to determine if it is properly sensing the different conditions that the circuit it feeds was designed for.
Circuti Breakers are designed to monitor circuits from no flow of energy to a maximum designed capacity. This maximum designed capacity is determined by the the conductors ability to conduct fixed amounts of energy over time. For example a 600A circuit is desinged to provide 600A worth of power at the voltage it is rated for. The National Fire Protection Agency provides enforcable designe rules that requires that engineers design circuits to operate at 80% of maximum capacity, so a 600A circuit will have a maximum load capacity of 480A.
Because their are intermitten spikes in loads due to motors or other equipment energizing, the circuit breaker although protecting a circuit designed with 480A of load will still be set to start timing out when it reaches its maximum capacity of 600A. 100% to 600% of a circuit load is considered an overload condition on the circuit and is disconnected before the conductor feeding the circuit breaks down. If the circuit experiences a 110% overload it may take minutes before it disconnects the load. If the circuit sees an overload condition of 500%, it will take just a few seconds to disconnect the load.
Some circuits are desined with a maximum threshold when the conductor simulates a fuse more than a conductor. This is when the conductor vaporizes because it cannot handle the load and ends up exploding. This current threshould is usually accomodated with an Instantaneous trip point on the circuit breaker with no intentional time delay. The circuit breaker will open the circuit before it reaches this breaking point of the system that it feeds. Typically, mechanical motion is the longest time of all the events required to cause this to happen and should be less than on tenth of a second.
Some circuit breakers feed a bus that can withstand more current and has multiple circuits being fed from this bus. For reliability purposes these breakers must stay on through a fault and the breaker dedicated to the circuit should trip before the main breaker. This device coordination is accomplished by the use of a short time setting which allows the breakers being fed by a main breaker an opportunity to trip first. Many times the main breaker will also act as a secondary instantaneous and if the current magnatude reaches a point well above the feeder breaker and it does not trip, then the main breaker will take over and shut down the entire system.
Each of these protection settings are dependant upon phase current over time which means both current and time must be tested to ensure the breaker is operating correctly.
One more common set point on modern circuit breakers is ground fault. This is to limit the amount of current to pass back through the grounding system before tripping on solidly grounded wye systems. Ground fault settings are commonly significatnly less than phase current settings because no system is designed to carry current through the ground conductors. If a phase imbalance is senced in a three p;hase circuit, an imbalance occures in the common ground of the current sensors installed on the circuti breaker and the trip Unit opens the circuit quickly minimizing energy release into grounded systems.
How do you test the protective features of a circuit breaker?
First you start with identifying what protective features are enabled on the trip unit. They are usually as follows:
Long Time
Short Time
Instantaneous
Ground Fault
Extneded Features may include:
Current Imbalance
Voltage Imbalance
Undervoltage
Overvoltage
Underfrequency
Overfrequency
Reverse Power
Zone Interlocking
Then you can develop a test plan. An Instantaneous only circuit breaker only has one pickup with no intentinal time delay and only requires the pickup current and timing point to be tested. A Complex trip Unit has all of these parameters meaning all need to be checked to see if the circuit breaker is sensing and operating properly.
A Tyipcal test plan for an LSIG trip Unit is as follows:
Long time Pickup each phase
Connect the circuit breaker to a current source with a calibrated meter. Close the circuit breaker then energize the current source. Raise the current until the long time mechanism or indicator changes state. Many circuit breakers that have thermal overload units will not provide this indication. For these breakers, pickup cannot be measured.
Results recorded are as found and as left for each phase. The only time this should change is in the case of a replacement breaker or settings change.
Each Trip Unit is designed with an acceptable range for this measurement to fall in. The best source of information for this is to refer to the manufacturers instruction manual to determine what the range is for the trip unit that you are testing. I have seen X +/- 10%, X-X+20%, X-10%+25% as acceptable ranges for different kinds of trip units.
If any of these measurements are found out of range. Confirm the result by replicating the test and then reject the circuit breaker until repaired.
Once the pickup points have been identified and found acceptable, the next concern is how quick will it trip. If it trips too soon, it may not coordinate with a motor starting. If it trips to slow, it might not adequately protect the equipment or circuit that the breaker feeds. Typically a test plan would include 1 or 2 test points. Most typically you will find 300% and 600% being recommended for testing purposes. Many trip units uses the 600% setting for the basis of trip time. Ultimately the test plan can be modified to any other test points, but most commonly 300% is used to simulate a motor overload on a circuit.
When looking at the trip curve you can establish a time range for 3 times the rated current that can fall anywhere from a few seconds to over a minute. Test each phase for by energizing the circuit breaker with 3 times the rated current and when the circuit breaker trips out, record the amount of time it took. If it wihtin the acceptable range then move on to the next phase. If not, reject the breaker because it will not adequately protect the circuit.
Cutler Hammer
Digitrip 1150+