OverCurrent Protection devices trip time!!!

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faresos

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When we say the fuses and circuit breaker are rated at 80% of the actual rating of the device. Lets say we have 4000A fuse or circuit breaker, and let say the connected load is actually is running at 4000A. My question is, How long it will take this overcurrent device to trip? is it 3 hrs if it was continuos load? I have been looking at trip time curves and it doesn't show it. Thanks for your thoughts..
 
It depends completely on the manufacturer, type, model, etc. of the OPD in question. The answer can be anywhere for never to sometime. There are issues of tolerances, electronic vs. thermal trips on breakers, fuse link characteristic if fuses.
 
It would depend on how much heat was allowed to build up during the time that the load was present. This woud be influenced by ambient temperature, the presence of other breakers and the amount of heat and/or insulation that they provide, air flow around the breaker, the amount of conductor material in the termination and its ability to work as a heat sink, etc.
In theory a ciruit breaker should be able to carry its full rated load, at its rated ambient temperature indefinitely. It will also carry current well above its rated ampacity for a period of time. I think the standard is that they must carry a 125% overcurrent for a minimum of 300 seconds (5 minutes) without tripping. That is for inverse time circuit breakers; instantaneous trip breakers are another story. Of course the old FP breakers seem like they will never trip regardless of load!
 
If you really meant to pose your question in terms of a 4000 amp device that is passing a continuous current of 4000 amps, the answer is that it will never trip.
 
Keep in mind that the electronic units do not work base on thermal operation. Therefore, all of the ambient temp factors would not come into play of these.
 
charlie b said:
If you really meant to pose your question in terms of a 4000 amp device that is passing a continuous current of 4000 amps, the answer is that it will never trip.
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Thanks all for your reply..

Charlie, if a continuous current of 4000 amps passing thru 4000A(Fuse or CB rated @ 80%) and that will never trip the device, so what is the benefit of using 100% rated overcurrent devices?

Thanks,
 
As defined by UL Class L fuses have to have case sizes and minimual hold times., usually 500% for 4 seconds, not all Most will hold 100%-110% rated current indefinately at 25 degrees C or 68 degrees F. You rate (125% or higher) Overcurrent protection devices to forgive the inductive spikes or inrush. Unless you have a purely resistive loas, I would never recommend sizing at 100% of FLA, you may encounter nuisence tripping.
 
davidr43229 said:
As defined by UL Class L fuses have to have case sizes and minimual hold times., usually 500% for 4 seconds, not all Most will hold 100%-110% rated current indefinately at 25 degrees C or 68 degrees F. You rate (125% or higher) Overcurrent protection devices to forgive the inductive spikes or inrush. Unless you have a purely resistive loas, I would never recommend sizing at 100% of FLA, you may encounter nuisence tripping.
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I'm planing on using 4000A service and will be using fuses as my main service. The total demand load is about 2800KVA for 340,000SF Courthouse (9-story building). the service size was based on square footage, the demand amp is 3370A. The utlity will provide a 2500/3333KVA transformer with dual temp. rating. I just want to make sure for some reason if the load is increased I want to be covered and thats why I want to make sure the 4000A fuse will handle this load for quite some times.

Thanks again..
 
Faresos,
You are indeed cutting it close to the wire with this. At 3,370 FLA and a 4000 amp Class L fuse , you have sized it at 115%. I understand the need to keep the budget close as well, since you would be jumping up to a larger frame.
Several questions come to mind here. If the ambient stays at 68 degrees maybe you will be alright, however without knowing the largest motor/s.... ie: roof top units, compressors, motors....... I would feel "uneasy" at that range. I would assume the spec calls out for "Spares".
 
My understanding is that when operating between 80% to 110% of rated current, a 80% rated fuse or circuit breaker will carry that current forever. And since the rating is being exceeded, eventually the 80% rated fuse or circuit breaker will fail, due to overheating.

There are available 100% rated circuit breakers, which require slightly oversized wire for heat removeable purposes. However even with those devices, operating at 100-110% of rated current will still cause a problem.
 
It makes me nervous when you say that the load has been calculated on a square foot basis. The only load that may be calculated on a square foot basis is lighting, and that is MINIMUM; you would need to compare the per square foot lighting load to the actual lighting load and use whichever figure is GREATER. Lighting will probably be a very small percentage of the total load on this service. A complete article 220 load calculation must be done, especially if you want to size your service that close to the actual load.
 
For what its worth.
Most of the 4000A fusible bolted pressure switches I have seen are 100% rated devices which must be cabled with 90C conductors sized for 100% based on the 75C tables (actually I can not think of any are only 80%). But the switchgear manufacturer must size the connected busbars for 100% also.
 
The same is for 100% rated breakers also.
This is the way I have always looked at it.
Also, for the common breaker if one sized the cable solely based upon the 125% of the continuous load and then size the breaker to protect the cable one could say the breaker is automatically applied at 80% anyway.
The UL testing for breakers and NEMA AB4 requires that there by a given length of rated cable attached to the breaker being test. The cable does act a heat sink by not adding heating to the breaker but to reduce heating.
If on were to apply a common breaker at more than 80% depending upon the size of the cable it may alter the trip curve because it hasn't been tested as such.
 
4000 A fused switches versus a circuit breaker? I'd go with a breaker, mainly becasue I have seen too many large fused switches that failed. Most common cause was too many ground fault trips causing the arcing parts to pit and jam, which then prevented complete closing of the main contacts. That led to heating and eventual melt down. Root casue was probably GF protection set too tight.

I would be uncomfortable about putting 100% load on a fuse. There are enough things that could reduce the trip point and cause a nuisance trip.

But with proper maintenance and good design, a large fuse would provide good protection and probably provide better coordination.
 
your safety..

your safety..

Several questions come to mind here. If the ambient stays at 68 degrees maybe you will be alright, however without knowing the largest motor/s.... ie: roof top units, compressors, motors....... I would feel "uneasy" at that range. I would assume the spec calls out for "Spares".
Click to expand...
...as davidr43229 said...

Mind also your loading during its peak time and I prefer circuit breaker compare to fuse, with that...the convenience on replacing that fuse at that main, the arcing that it gives, every faults and trips, the cost? Let?s just stick to 80% loading...which comes first the "your safety and reliability or the effect in the future". Which will cause you too much? You might do some additions in the future.
 
4000amp Main

4000amp Main

Faresos, At 3370 amps I would be looking at using a 100% rated MCB with LSIG options. If you do use fuses you would need a HPC switch with phase failure protection to prevent single-phasing if one fuse were to blow. From a fault-current stand point there will not be much cut-off with a 4000 amp fuse and with todays current limiting breakers you would have some adjustability at the main. You would need to specify the settings for the MCB, it has been my experience that some Engineers go through the trouble to specify the MCB with LSIG but never let the contractor know what the settings should be.

Grant
 
RCWILSON:

Circuit breakers are susceptible to pitting as fusible bolted pressure switches, just because you do not see the pitting does not mean it does not exist. Both devices have arcing contacts (movable and stationary. In a bolted pressure switch these are replacecable.


The biggest issue we see with bolted pressure switches and circuit breakers is misuse. The electrician?s fail to read the instructions on how to operate the closing and/or opening mechanism, or continual reclosing the device into a fault.

AFTER any fault the device should be serviced, per manufacture's specifications, meggered, and contact resistance measured (DLRO micro-ohm) and the GFP tested (if installed).


Lastly have you ever opened a circuit breaker after a fault, do it sometimes you will realize circuit breakers have their limitations.
 
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