Coordinating Fuses with Circuit Breakers 1

[ROMM]

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?

 

[rbulsara]

Normally devices rated close to each other, or less than 1:2 ratio will not coordinate.

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.

 

[dpc]

For high current faults, there probably is no way to coordinate an upstream fuse and a molded case circuit breaker, especially that close in size. You will find the fuse curve cuts right through the instantaneous portion of the MCCB curve. For fault currents above the fuse max clearing time, either or both device may clear the fault. That the big drawback to using current-limiting fuses to increase circuit breaker interrupting ratings - you have to sacrifice coordination. This is true for any series rating, actually.

 

[RonShap]

Sounds like these devices are in series. There is no real benefit (if you could which you can't) to selectively coordinating those two devices.

 

[alehman]

Unless you want to avoid replacing fuses or avoid risk of a single-phase condition if one fuse blows...

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.

 

[mc5w]

What I have found is that Littelfuse JTD200 fuse ahead of SquareD 1 pole 277 volts 20 amp circuit breakers does coordinate even in an instance where I pinched a branch circuit conductor between a panelboard cover and the box. The panelboard was located next to the supply switchboard and the switchboard was located right next to the service switch.

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.

 

[Rodmcm]

I think you are looking at two things. The fuse use is probably only for current limiting and hence would have been selected to 'knock the top off' any major faut currents, probably to reduce the cable size.
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

 

[RRaghunath]

In voltage installations, the fault current magnitude substantially drops at the remote end due to the impedance of the cable. If you consider this while checking the selectivity (superimposing the I2t characteristicof the fuse with the s/c characteristic of the MCCB), it may be possible to find limited coordination (limited to the range of expected fault current for faults at the remote end, I mean).


raghunath_n00@rediffmail.com

 

[mc5w]

On 120 volts a 10 foot length of #10 copper wire will absolutely limit ground fault short circuit current to 10,000 amps even if the source is infinite and the return path is superconducting. On 240 volts single phase using a common trip circuit breaker 10 feet of #10 copper produces the same result.

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.

 

[dpc]

mc5w,

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.

 

[Bung]

IEC61818 (an IEC standard) is quite a good reference, if somewhat general.



Bung
Life is non-linear...

 

[mc5w]

Close to a generator the peak of a short circuit will occur during the first half cycle. After that, iron saturation tends to limit the fault current and also either the voltage regulator takes over OR the short disables excitation if the field runs off of the 60 Hz. Farther away the transient response during a short can me more complex because of the effects of system inductance and capacitance and resistance.

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.

 

[mc5w]

Also, in the case of a pinched wire the initial breakdown of insulation can keep a full short from flowing until after a few cycles, what is known as a nonlinear resistance.

 

[dpc]

mc5w,

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.