Instantaneous trip elements 4

[Mbrooke]

Are these required to be enabled or present in molded case circuit breaker and power breakers? Why do manufacturers include them? I'm hypothetically thinking about eliminating them (if there is such a thing)in regards to selective coordination.

 

[che12345]

Dear Mr. dpc

Q. " ...The choice between fuses and circuit breakers involve numerous ..... if there are critical circuits where [selective coordination]* (at low voltage) is [absolutely mandatory]*, (and [single-phasing]** is [acceptable]**, then I would use fuses, without question...".
A1. Agreed.
A2. Sir, you don't find this type of stringent [....]* and yet relaxed [.... ]** " load very often in reality. It must also be stressed that the load shall be acceptable and is very forgiving on (long period of blackout)*** . This is taking into consideration that:
a) to find correct replacement fuses (if you are lucky) would take say >45min. To switch back a tripped MCB/MCCB would take <15s. In most cases, a MCB/MCCB can be switched back immediately without inspection/replacement; as the short-circuit tripping current is usually very much lower than the breaker short-circuit current kA rating.
b) replacement of fuses shall be carried out by a legally "qualify" person. Resenting a MCB/MCCB can be safely carried out by any layperson.
Che Kuan Yau (Singapore)

 

[waross]

Mr. Che. The point is;
With proper coordination, the fuses don't blow.
Only the circuit breaker on the circuit with an issue trips.
A fault on a branch circuit does not take out a large portion of the plant.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I find it hard to believe any modern MCCB could begin to trip and partly open but then stop the trip cycle and close again. Once the mechanism is tripped, it's a one way ticket to the breaker being open. I've seen multiple breakers in series all trip a number of times in the past.

 

[dpc]

Lionel,

Not sure who you're responding to, but I agree - once the breaker is unlatched, it's going to trip. However, newer MCCBs have a reverse current flow between the two sets of contacts such that the magnetic forces at high fault current cause them to "blow apart" even before the breaker trips for high current faults. This is how they were able to increase the short circuit rating of MCCBs so dramatically over the past 30 years.

But the circuit breaker itself presents a certain impedance to fault current even when closed. So the impedance of the downstream CB can reduce the fault current just enough to prevent the upstream breaker from tripping on instantaneous. The series coordination testing takes advantage of this. But in many of the examples I've looked at the maximum fault current that will allow coordination is really low and not of much value.

http://apps.geindustrial.com/publib...TNR=Application and Technical|DET-760|generic

Not sure if the link above will come through, but there's a detailed paper from GE (ABB) regarding coordination of instantaneous trips for all type of GE/ABB low voltage breakers. The breaker manufacturers are trying to compete with fuses for the hospital life-safety systems that must selectively coordinate per the NEC. Bussman got this into the NEC because they knew this requirement would require use of fuses instead of breakers. But breaker manufacturers are trying to offer alternatives.

 

[dpc]

a) to find correct replacement fuses (if you are lucky) would take say >45min. To switch back a tripped MCB/MCCB would take <15s. In most cases, a MCB/MCCB can be switched back immediately without inspection/replacement; as the short-circuit tripping current is usually very much lower than the breaker short-circuit current kA rating

Even if this is true, it will likely take longer than 15 seconds to find the fault.

 

[che12345]

Dear Mr. dpc
Q. " Even if this is true, it will likely take longer than 15 seconds to find the fault".
A1. Agreed.
A2. In reality, there is No/very little difference in the time taken to find/rectify the fault; irrespective of whether the network uses only fuses or breakers. The bottom line is, network with breakers would reduce the "blackout time" substantially.
Che Kuan Yau (Singapore)

 

[waross]

Mr. Che.
Coordination!
You want a fault to be cleared by the first breaker upstream of the fault.
It is relatively easy to locate a problem when you know which circuit has faulted.
But, when co-ordination fails, and a fault is cleared by an upstream breaker rather than the local breaker several things happen.
The whole plant or a section of the plant goes down instead of just one circuit.
It may take hours to locate the reason that the main breaker tripped.
With coordinated fuse protection of the feeder, it would not have blown the fuse.
That's the point.
Coordinate your protective devices so that the device closest to the fault clears.

A real world example:
A heavy oil upgrader.
The revenue stream was several million dollars per day.
There was a fault on a circuit fed by a 30 Amp breaker.
The fault was cleared upstream by a 1200 Amp breaker.
The plant was down for 4 hours while the electricians searched for the reason for the trip.
Proper coordination would have avoided the 1200 Amp breaker tripping.
Had fuses been used for coordination, the fuses would not have blown.
The local 30 Amp breaker would have tripped and everything else would have stayed online.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. waross

So, did you insisted/advised the plant/management to replace the up-stream 1200A breaker and the down-stream 30A breakers to fuses? BTW, 1200A fuse is not a common item kept by stockists on the market.
The problem is not between the use of breakers or fuses. It is the coordination! I respectfully agreed with your learned opinion.
A well coordinated (breaker only) system may had reduced the "down" duration without the notice of the management, if fuses were used instead.
One of the other very serious/frequent problems with fuses is [single-phasing]; which invariable causes motors to burnout. Although some fuses are equipped with blown-striker, but majority of them are not.
Che Kuan Yau (Singapore)

 

[waross]

Not all fuses, breakers downstream, fuses upstream.
Actually, in this case, the upstream breaker went out on ground fault.
It had not been properly configured when it was installed.
The breaker settings were reviewed and corrected.
I was not directly involved but everyone on a very large site was aware of the issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Bill, using fuses on a main feeder because they don't know how to select or setup a breaker isn't the fix. I personally would try to avoid using fuses for the main with breakers after.

 

[waross]

Hi Lionel.
The breaker had been replaced with a breaker "out of the box" with no attention at all to the settings.
The point is coordination.
Without proper coordination, an upstream breaker may clear a fault with much more serious consequences than a local trip.
This case was a tragedy of errors and incompetence.
1. A breaker was replaced by an electrician who didn't know that he should match the settings on the new breaker with the breaker being replaced.
2. A fan was removed from service and the leads were shorted together and to ground in a wire nut, for safety.
3. a lock-dog was installed on the 30 Amp breaker.
4. Another electrician, even dumber than the first, was playing with the 30 Amp breaker. (He will never work there again.)
5. Despite the lock-dog he managed to turn the breaker on.
6. The main breaker had the ground fault set at zero time. (The original breaker had a time delay set on ground fault trip.)
It was speculated that the breaker went out on ground fault faster than the 30 Amp breaker could trip on overload.
(The original breaker had a time delay set on ground fault trip.)
7. When the lights went out the electrician turned the breaker off and left the area.
8. With the breaker off and not tripped, there was no indication as to the origin of the fault. Eventually someone who had seen the idiot playing at the panel tipped off the trouble shooters.
9. The electrician eventually admitted what he had done, after a couple of millions of dollars of down time.
Again the point is that proper coordination will avoid a main breaker tripping when a branch circuit breaker should clear, and if fuses are required to achieve coordination, so be it.
The problems of a main breaker trip may be much more serious than the time to replace a fuse.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

I'll bite... why do people look down upon fuses? I actually see them as being superior and have lasted decades in service from retail, to hospitals, to government buildings... to well just about everything. QMQB switches are common as air around here. Other than pringle/bolted pressure switches that jam (which can also happen to a breaker) they are trouble free.

 

[che12345]

Dear Mr. Mbrooke
Q. " I'll bite... why do people look down upon fuses? I actually see them as being superior...."
A. Please advise, in your location:
a) is there any simple/economical device to prevent (fuses) from "single-phasing" ?,
b) are LV fuses with rated current >1kA up to 6.3kA readily available with short delivery time ?,
c) what is the prevailing trend between the use of fuses and breakers? ,
d)what are the main reasons for the prevailing preference?
Thank you
Che Kuan Yau (Singapore)

 

[davidbeach]

Che begins to scratch the surface.

Fuses aren’t inherently gang (multi-pole) operated.

Fuses only respond to phase quantities. There’s no way to have a fuse trip on a lower value for ground faults than for phase faults.

Fuses don’t do definite time.

Fuse don’t do differential.

Fuses don’t do transfer trip, either transmit or receive.

Fuses don’t reclose. Never mind repeater fuses, that’s half arsed reclosing at best.

Fuses don’t allow multiple settings, no maintenance mode for arc flash hazard mitigation with fuses.

Have never seen a distance function implemented with fuse.

Fuses are great for the last device or two on taps where there’s only a single phase or two and a sustained outage is acceptable.

By numerical count, we have fuses and a few rounding errors. By functional impact we have breakers and relays plus some noise.

Is that enough?

 

[Mbrooke]

All those functions would be required for utility application, not typical 600 volt and under installations. Exception is ground fault protection, but that can be done with a relay and pringle switch.


Some pros of low peak fuses that can not be obtained with breakers:

1) 100% selective coordination up to 200,000 amps.

2) Higher AIC up to 300,000 amps- allows the us of networks.

3) Incident energy reduction often better than breakers.

4) Breakers that haven't been opened in 30 years can jam resulting in system wide outages. Breakers can also "lag" but I can't find any literature on the subject.

5) Lower cost in design, implementation and installation.

6) Single phasing can be an advantage with critical/life safety circuits where 2/3 of the lighting and exist signs will still function- MCCs should always have single phase protection regardless of fuse or breaker.

 

[LionelHutz]

Of the 8 things davidbeach listed, I commonly see 5 wanted by end use customers in their 5kV and 600V class systems.


Bill, my point was that there's no excuse for half assery in a plant with revenue in the millions of dollars per day. You get what you deserve if you want to run things half-assed. If it was fuses instead of a breaker, chances are they wouldn't have spares.

 

[waross]

Bill, my point was that there's no excuse for half assery in a plant with revenue in the millions of dollars per day.

I agree.
The point was the possible consequences of poor coordination.


Bill
--------------------
"Why not the best?"
Jimmy Carter