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.

 

[mpparent]

Potential cable/wiring withstand issues...

Mike

Mike

 

[Mbrooke]

Good point. Any idea what currents typically begin causing problems? I know even the largest breakers come with In dials, so it can be rasied- but I guess not beyond a certain point.

 

[davidbeach]

UL listening of molded case breakers requires them. Buy a high enough rated breaker and the Inst trip stays above available fault current and can be ignored.

 

[Mbrooke]

Thanks

Any idea which specific part of the UL standard requires it? I always assumed that it was not required, at least with 15-225 amp breakers in that for decades some manufacturers did not include instantaneous elements.

 

[waross]

Many breakers have an adjustment, typically a range of x5 to x10 for the inst trip.
Some do not have an adjustment.
The lack of an adjustment does not imply the lack of an inst trip.

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

 

[FreddyNurk]

I can't comment on any UL aspects, but my understanding was that for moulded case at least, its not possible to disable an instantaneous trip as its an inherent part of the circuit breaker's operation. This trip is generally higher than any of the trip unit instantaneous settings would provide too.
This isn't the case for air circuit breakers and it was quite common for some of the applications that I'd seen to not fit an OEM specific trip unit, rather rely on an external relay, CTs / VTs as required and breaker trip coils, which does give more options for protection settings than what the OEM provided unit may offer. I don't see the latter as being any different from an MV breaker with an external relay and a means to actuate the CB contacts to be honest.

EDMS Australia

 

[che12345]

Dear Mr. Mbrooke

Q. Are these "instantaneous elements" required to be enabled or present in molded case circuit breaker and power breakers?
A1. There are different terminologies, testing methods/Standards. In the States (UL, ANSI) and in Europe (IEC). UL uses the term "Power breaker", while IEC uses the term " Air circuit-breaker" etc....
A2. In Europe, IEC 60946-x prevail. Where a "Circuit-breaker is a mechanical switching device, capable of ( making, carrying and breaking currents under normal circuit conditions)* and also ( making, carrying for a specified time and breaking currents under specified abnormal circuit conditions)** such as those of short-circuit"
A3. All moulded-case circuit breaker fulfill the ( making, carrying .... under normal circuit conditions)* but having very limited capability to fulfill the (making carrying for a specified time and breaking current under specific abnormal circuit conditions )**, such as those of short-circuit".
This can be illustrated by a [current-limiting circuit-breaker] with a " break-time short enough to prevent the short-circuit current reaching its otherwise attainable peak value". A 100kA [current-limiting circuit-breaker] does NOT experience 100kA flowing through the contacts during testing! Therefore, "instantaneous elements" is required, as it has very limited capability to fulfill the (making carrying for a specified time and breaking current under specific abnormal circuit conditions )**. It has very low [rated short-time withstand current (Icw)].
A4. In Europe, an Air-circuit breaker is usually of sheet structure housing the circuit-breaker. Invariably, all ACB are tested to Icw to 1s or 3s with utilisation category B (Per IEC 60947-2).
An ACB rated Icw of 75kA 1s would be say 65kA 3s. The full value of the test current flowings through the contacts for the duration stated, during testing. An ACB may have the "instantaneous elements" switched off and the internal short-circuit protective device removed.
Che Kuan Yau (Singapore)

 

[dpc]

Instantaneous (magnetic) trip is absolutely required by UL 489 which covers molded case circuit breakers sold in the US and has been required for decades. As Bill mentions, many smaller molded case breakers have non-adjustable magnetic trips, but the trip is still there. In addition, in some cases there is an adjustable instantaneous trip function PLUS an instantaneous override trip that cannot be adjusted or defeated. The purpose of the instantaneous trip primarily to protect the circuit breaker itself from damage since it is not required to have a withstand rating under UL 489. By contrast, Low Voltage Power Circuit Breakers, built to a different standard, have 30 cycle fault withstand rating and are not required to have instantaneous trip, although they may have instantaneous trip if desired.

This can be easily confirmed by asking any US manufacturer or looking at any UL 489 MCCB time-current curve.

This can get confusing for the new breakers in larger frame sizes such as the Square D MasterPact since many of these breakers can be ordered as UL 489 molded case breakers, which will have an instantaneous trip OR as as Low Voltage Power Circuit Breaker, which may or may not have instantaneous trip.

 

[davidbeach]

The UL 489 version explicitly lists in INST trip. The LVPCB version has an a non-adjustable “Instantaneous Override” set at the interrupting rating of the breaker.

 

[Mbrooke]

Can anyone quote that part of UL-489? I know of old UL listed molded case breakers without a magnetic trip. Not simply due to the lack of adjustment, but legitimately only a bi-metal. Think of FPE, Bulldog Pushmatic, some ITE, Challenger, ect.I guess at some point in time they mandated it... but I'm now left wondering why that is... I'm being told to help fulfill the switch gear's withstand rating... which would make sense for the MCB itself.

But what about all this 60s-70s gear that survived without any magnetic or instantaneous function?

 

[davidbeach]

Because of those? Zinsco may have been the worst, but there’s a lot of that old crap still out there that no decent AHJ will let you touch without requiring a full replacement.

 

[Mbrooke]

Zinsco had magnetic trip if I remember right. But yes to them seizing and burning up. From what I've seen magnetic trip was not a big concern in the 60s and 70s. My thinking is that if manufacturers could get away with it decades back- why not today? I can't find any specific literature (besides in relation to 15 and 20 amp single pole breakers) that gives an answer based on electrical theory.

I like your advice on power circuit breakers btw

 

[waross]

Available fault currents have been increasing for generations.
Distribution transformers have tended to get bigger as impedance voltages have dropped.
I can't find it now, a lot of my very old books have gone missing over the years, but I remember reading about the magnetic forces from a fault ripping busbars loose from their mounts.
That was when copper bars were mounted to fibre blocks with wood screws.
The importance of instantaneous trips has become more important over the years as Available Short Circuits Currents have increased.

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

 

[Mbrooke]

I'll take your word for it. Though I do see FPE equipment being generously rated.

With that said, is anyone here ok with taking fuses over breakers?

 

[davidbeach]

What are you trying to achieve? Lowest first cost or a decent system? They’re very different.

 

[Mbrooke]

My primary concern is selective coordination. IMHO fuses appear to be the best at achieving this... Sounds like you have a preference for breakers however. I'm all ears

 

[davidbeach]

Selective coordination doesn’t happen with molded case breakers when the available fault current is above the instantaneous trip point.

That’s the industry official line.

But that’s all based on one breaker and one fault. I have reason to believe that in real life it’s different than that. What I believe happens, without shareable data, is that breaker dynamics enter the picture. What I believe (can’t prove) happens is that multiple breakers in series all begin the interruption process and all begin to open. A bit of a gap, and a bit of a gap, and another bit of a gap and suddenly the fault current is reduced to the point that only the last device actually opens and the others “say oh, never mind” and close again before they’ve passed the point of no return. Can’t prove it, but can’t otherwise explain events I know to have occurred.

 

[Mbrooke]

I have a different theory but could be wrong: Most faults are not bolted faults but rather sputtering or high current arcing faults both drastically increasing RMS impedance. This limits the currently substantially such that it doesn't go over any upstream breaker's magnetic trip thresholds that happen to be close to the system bolted fault current.

I've looked at a lot of data from UL and others who've studied short circuits and the oscilliogram data typically looks nothing like a sine wave. Quarter and half cycling has also been observed.

 

[dpc]

The dynamic impedance of molded case breakers during faults is a thing that has been studied. The bottom line is that the molded case breakers do coordinate somewhat better than it would appear from just looking at the time-current curves. In fact, manufacturers have developed the concept of "series coordination" of MCCB (not to be confused with "series rating".) Certain combination of molded case breakers have been tested and the manufacturers have published table showing that a certain pair of breakers will coordinate up to some maximum value of fault current. This is always between specific breakers from ONE manufacturer.

The choice between fuses and circuit breakers involve numerous tradeoffs. However 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.