Coordinating Fuses with Circuit Breakers
Coordinating Fuses with Circuit Breakers
(OP)
Hi all, anyone know how to cordinate a 500A HRC fuse protecting a cable for short circuit with a 400A MCCB downstream protecting the cable from overload?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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Coordinating Fuses with Circuit Breakers
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RE: Coordinating Fuses with Circuit Breakers
Now a days its easy to compare the TCC curves using software such as SKM.
Also if the device rating or settings are not asjustable, there is little you can do to affect the coordination or lack of it.
RE: Coordinating Fuses with Circuit Breakers
RE: Coordinating Fuses with Circuit Breakers
RE: Coordinating Fuses with Circuit Breakers
I agree that it will probably be impossible to coordinate unless the fault current at the breaker end is lower than it's instantaneous trip level. Even that probably won't allow for coordination for devices this close.
SquareD has a new molded case breaker with a very high instantaneous trip that can sometimes be used to advantage.
RE: Coordinating Fuses with Circuit Breakers
However, if a forklift truck shears off a #6 copper branch circuit supplied by a SquareD EH34045 branch breaker and JTD200 fuses the fuses will blow - something like 2 phases fully blown and I assumed that the 3rd fuse was half blown. If a forklift shears off a power feed that comes up out of the floor you have a few problems besides that the feeder fuses blew.
Fortunately, the conduit out to the machines were installed in a "trench drain" in the floor that was used exclusively for electrical conduits and compressed air. When a forklift truck would hit a power riser the usual problem was bending of some 50 cent conduit strut clamps. The shearing off of the power line was with a forlift truck that had a rear end that was just the wrong height for shearing just above the transition from galvanized rigid conduit to liquidtight flexible metal conduit.
Breaking off a compressed air outlet was also easy to fix.
RE: Coordinating Fuses with Circuit Breakers
For this reason you can regard it as a high set current item. The MCB is probably the one that will protect for long term currents. I do not think that you will grade for some types of faults however. You need to put the curves on a grah sheet or calculator and have a look at the fault levels that can be impressed on the cable
RE: Coordinating Fuses with Circuit Breakers
raghunath_n00@rediffmail.com
RE: Coordinating Fuses with Circuit Breakers
Most short circuits and ground faults occur in utilization equipment that is far from the source. The high rated feeder fuses or main circuit breaker only has to take care of the minority of faults close to a panelboard. When faults are that close to a panelboard you probably want to have the main shut off anyways to keep a short from spreading to or inside of the panelboard. A large enough fault can turn wires into fireworks fuses so shutting off the whole panelboard very quickly on large faults can prevent a burndown.
In the case of a SquareD residential panelboard with a 200 amp main breaker with 22,000 amps interrupting and 1/2 to 1 cycle clearing on shorts at or above 2,000 amps, you still get coordination because on branch circuits the wire resistance keeps a short from reaching peak value until 1.5 to 3 cycles after a short occurs. That is, the maximum peak of a short circuit occurs during the first 1 to 6 half cycles depending on certain parameters such as distance from the power source.
RE: Coordinating Fuses with Circuit Breakers
I agree that conductor impedance can often provide enough fault current limitation to prevent upstream circuit breakers from tripping on the instantaneous portion of their protective curve, thus providing some degree of coordination.
But for any fault, the peak short circuit current always occurs during the first half cycle. If you separate the total fault current into a symmetrical steady-state ac fault current and a decaying dc current you will see that this must be so.
RE: Coordinating Fuses with Circuit Breakers
Bung
Life is non-linear...
RE: Coordinating Fuses with Circuit Breakers
One of the exercises in circuit analysis class showed that under certain initial conditions the peak current for a first order AC circuit can occur during the second half cycle. The behavior of a second order circuit can be more complex because of stored energy in the capacitors.
Remember, that inductances limit the rate of rise of fault current, what is known as an L/R time constant.
RE: Coordinating Fuses with Circuit Breakers
RE: Coordinating Fuses with Circuit Breakers
If you allow a variable fault impedance, such as an arcing fault, then anything is possible. But for a bolted fault, I believe the peak current always occurs in the first half cycle. If you have a circuit that doesn't follow this rule, I'd be interested in seeing it.