Can a 480V RK1 fuse, and power circuit breaker just not be coordinated on GROUND FAULT? 3

[bdn2004]

Take a look at this Single phase to ground TCC curve...It's a 4000A main breaker, a 1200A feeder breaker and an 80A RK1 branch circuit fuse. It's coordinated. The Main breaker and the feeder breaker are both set on MAX - highest current, highest time delay.

Think of the TCC curves though for any fuse greater than an 80A, the fuse curve is shifting to the right and the feeder breaker is the first device that will trip every time - uncoordinated. Is there anything you can do about this scenario ?

 

[davidbeach]

That’s why there are breaker options that include more segments in the curve.

 

[bdn2004]

There are different options available for ground fault protection? All the ground TCC curves on the low voltage switchgear breakers that I've seen look like the one I pasted.

The phase TCC curves coordinate fine. And we do have more options: Long, Short, Instantaneous.
Why aren't these same curves available for ground faults or are they ?

 

[che12345]

Dear Mr. bdn2004

Q. " The phase TCC curves coordinate fine. And we do have ....Why aren't these same curves available for ground faults or are they ?
A. Most 4kA ACB and 1.2 kA ACB/MCCB are available with [G function]. With I4 selectable from off, 0.2x... 1xIn and with (usually definite) time delay t4 selectable from 0.1s ... 0.8s. In usual cases, it is [NOT] desirable to set the [earth-fault] trip current higher than 0.2x In. If [lower] than 0.2 xIn is required, an Z-CT and an earth-fault protection relay is added, externally.

The 80A fuse is [NOT] expected to coordinate with the up-stream ACB/MCCB on [earth-fault].
Example: An (undesirable) [earth-fault] of 80A occurred at the down-stream of the 80A fuse, neither the fuse nor the up-stream ACB/MCCB is going to trip. This is certainly not desirable. It is also undesirable to set the up-stream ACB/MCCB G function setting to much higher than necessary, in order to coordinate with the 80A fuse; to ensure that the fuse would (blow faster) under [any level of earth-fault].

A possible solution would be to [replace the fuse with a MCCB with G function] or with an external earth-fault protection relay. To achieve [earth-fault coordination], the up-stream ACB G function I1 t1 shall be > ACB/MCCB I2 t2 > replacement MCCB I3 t3. The I3 may be set at say 0.1x of 80A and the time delay say 0.2s.

Che Kuan Yau (Singapore)

 

[bdn2004]

Che,

Changing out fuses to circuit breakers is not an option.
Attached is a technical article from Bussman that does somewhat address this situation.

Basically they don't and can't coordinate without going to a lot of extra expense. Why is still a question to me.
In reading through this stuff there seems to be distinguishing between a high current ground fault and a low level ground fault.
The low level ground fault you tolerate. The high level ground fault you trip the feeder breaker and you BLACKOUT.

I've read before that most faults are ground faults. That's what doesn't register as to why this is acceptable.

 

[dpc]

The NEC requires ground fault protection for larger services and substations for FIRE protection. Coordination is not generally a requirement, except for hospital emergency power systems. What is "acceptable" depends on the facility. The choice was made to use fuses downstream instead of breakers, so a lot of options for improving coordination were given up to reduce costs, most likely. But even with molded case breakers downstream, there would still be coordination issues.

The maximum settings and time delays for ground fault are set by standard. Ground fault functions with optional I2t function - as shown for the main breaker in your curve will greatly improve coordination with downstream fuses. But above some maximum fuse size, there will be coordination issues in the system you have.

Cheers,

Dave

 

[mc5w]

However, full coordination was theoretically outlawed by the 2007 Federal court ruling Manoma Realty Management versus Federal Pacific Electric wherein federal justice Julie Swain ruled that a circuit breaker that is too slow under short circuit conditions is a defective and illegal product. She then limited the jury question to how much money FPE owed to Manoma which meant that FPE was noit allowed to argue that the statute of limitations had expired, they were not allowed to argue that the Stab-lok breakers are legal, they were not allowed to argue that the UL listing was valid, and they were not allowed to argue that the UL labels were valid.

If you are worried about power continuity you need to have dual emergency systems, dual legally required standby systems, and 2 optional standby systems for each area served. Every hospital and data center that is worth its salt has that. There are lots of other failures that will take out power such as a transfer switch failure. Also, a short circuit will collapse the voltage of a local generator to zero because the iron will be saturated. Source: IEEE Orange Book.

When the United Kingdom still had deep pit coal mines the UK National Coal Board required the exact opposite of coordination on ground faults even though the systems in a underground mine that are over 120 volts are resistance grounded. They felt that saving a life or avoiding a fire was more important than avoiding a blackout. Under their scheme, a branch circuit ground fault is supposed to to trip the branch, feeder, and main breakers to reduce the chances of the fault continuing because a circuit breaker jammed. Their methods of preventing fires and electrocution deaths were so effective that in 1971 Congress mandated that the US mines had to switch to the British method of wiring a mine. Eventually, their methods were mandated by US Bureau of Shipping for commercial shore-to-ship power cords.

 

[mc5w]

I forgot to mention that federal justice Julie Swain extended the statute of limitations in a lawsuit from 6 years to at least 41 years. She did not elaborate on why so we have to presume that it was because of FPE's fraud and concealment.
The ruling also theoretically applies to the multibreakers that were manufactured by Federal Electrical Equipment and Colt Patent Fire Arms which were 2 of the 3 predecessor companies.

The ruling also theoretically applies to ITE Pushmatic circuit breakers because those have either a thermal only trip or a thermal allegedly magnetic trip where a magnetic piston pushed on the bimetallic element in an attempt to accelerate tripping curing short circuits.

Also, FPE admitted in 2 US Patents that what they needed to do with the bolted-in and plug-in versions of the Stab-loks was to plagiarize the Westinghouse Quicklag overcurrent sensing element.

I am just trying to tell you that in actual practice full coordination is a moot point,m is illegal, and also conflicts with fully deenergized maintenance. The closest that you will get to full coordination is a Mission Critical breaker that has an instantaneous trip of 2,000% of full load current. For one brand these are only available in the 200 to 400 ampere range.

 

[bdn2004]

Interesting posts. Thanks.

In the case I’m showing in my example the fuse is part of a bus plug located 25’ above the floor. The busway the fuse is connected to can run in some cases hundreds of feet. As it feeds as many as 50 or 60 other busplugs on the same busway.

Somebody 20 years ago determined that a ground fault is unlikely on the feed down from the busplug is all I can figure. All of feeds land in another overcurrent device whether it be a control panel or power distribution panel.

 

[dpc]

bdn2004,

I doubt the original designer was overly concerned about coordination. The system was probably designed to meet the applicable codes when installed. Ground fault protection is often installed ONLY when required by code.

Few 480 V and 600 V systems are selectively coordinated all the way down to the panelboard level. When using standard molded case breakers, it's very difficult to achieve and perhaps not even desirable when arc flash incident energy is taken into account.

Cheers,

Dave

 

[bdn2004]

Another thing I was thinking about with the logic of all this ... as shown the ground fault current on the feeder breaker isn’t guaranteed to trip until 18 cycles of ground current is sustained.

They are always telling you that a ground fault quickly becomes a phase to phase fault in an arc flash explosion. I would think it’s the same with most faults with the wires in such close proximity to each other. The instantaneous elements in the fuses at the high short circuit values will trip faster than 18 cycles. Perhaps that’s the reason this is ok also.