One of you clowns that went to college.....

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220/221

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Explain it to me again please.

How does a fault on a 20 amp circuit blow a 100A fuse without tripping a breaker? I've seen it happen more than once.


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Part two. How do all 3 100A fuses blow without tripping any downstream breakers?

Situation:

277/480V 3 phase source.

100A switch in 1000A section feeds panel via ATS. ATS is not operational. Installed but for some reason, no control wiring hooked up, never has been.

Lighting load = 14, 20A, 277V citcuits for office lights and 10, 20A, 277V circuits for alll the HID high bay fixtures in warehouse (controled by contactors)

Additional loads:

One, 60A, 480V 3 phase 4wire feed to guard shack appx 1000' away with 120/208, 3 phase transformer and panel.

One, 20A 480V single phase 3 wire feed to gate operator 1000' away. Small 120/208 trannsformer for gate equipment.


SKOP (standard knucklehead operating procedure) is to replace fuses and turn things back on one at a time. Everything comes back on, A phase = about 60A, B and C are about 50 something. No signs of trouble at breakers, contactors, ATS connections or discos.

Old fuses were Bussman LPS-RK
New fuses are different MFG, FSLP. Chart at the supply house says they match. I am clueless what the fuse rating numbers/letters mean.

Realization. Did a breaker trip and they reset it without telling me? Possible I guess.
 
The available fault current was well beyond either devices instantaneous trip setting.

The larger device just happened to trip out faster the the other.
 
Sounds like you have a coordination problem. The same current rating does not necessarily mean they have the same time vs. current charastics. The letters designate the different times at different currents above the fuses rating that it will blow, this creates a time/current curve and can have different shapes for different applications. This curve needs to be properly coordinated with the breakers time current curve to achive the proper coordination, many breakers trip units allow the time current curves to be shifted with pick ip, delays, and curve shapes (I2I on or off).

Either:
1. The system was not properly coodinated in the first place.
2. Someone replaced the wrong fuse at some point.
3. (My guess) someone changed the settings on the breaker.
 
Can you explain available fault current in a way I might understand?

Pretend I'm a plumber or something and spell it out.

I know what available,fault, and current mean but I've never been able to grasp the concept. I just put the stickers on the series rated panels as specified.:wink:
 
If you had a bolted fault (a hard fault, no arcing etc) and you somehow measured the current that would be the available fault current and often that will be many thousands of amps until the fault is cleared or the circuit opened by the breaker or fuse.

The closer you are to the source the higher the fault current will be.
 
220/221 said:
Can you explain available fault current in a way I might understand?
Click to expand...
I'll try, but a plumber probably wouldn't understand, because I am going to start with Ohm's Law. The current you get, in any circuit and in any circumstances, is the voltage supplied by the source divided by the resistance that that source must push current through. When you have a short circuit, it means that the resistance went down to nearly zero, due to a wire, a tool, or some other low-resistance item coming into contact with two hot wires (or a hot wire and the neutral, or some other combination).

But the "total resistance" seen by the source is not, quite, exactly zero. That is because there is resistance within the source itself. So when we talk about "available fault current," we pretend that the thing causing the short circuit has absolutely zero resistance, so that the only resistance in the overall circuit, the only resistance seen by the source, is the resistance of the source itself. Then when we divide the source voltage by that value of resistance, and when we get a high value of current, we will be very confident that the actual value of current that we would ever see during a real fault can never be higher than that value. It is the most that the source can give, because of the limitations of the source itself.

Does that work for you?
 
I think the definition he is looking for is The Avalible Fault Current is the absolute highest level of current or amperage that can or will travel through a fault before it is cleared. the way we come to that value is explained above.
 
perhaps, Joe :grin: , in plumber terms, fault current might be somewhat like the pressure on the water line. Your pipe or valve must be able to withstand that pressure. It might just eb a 3/4" line but if the supply prsure is 500 lbs, the pipe valve must withstand that. The smaller the pipe tapped off the main and the longer the line, the less pressure there is downstream (in water terms only if there is flow) a longer, smaller line equals less pressure. (the effect of a #1 wire tapped off a 500kcmil)
and you can reduce the pressure by a pressure reducer (curent limniting device).
 
oh! plumber's definitions! I think i can pull this one off...


available fault current = worst case scenario

example: Joe the plumber drops his wrench into a live panel that somehow touches two of the bus bars. Chaos ensues - hopefully Joe isn't still holding the wrench.

analogy: Joe the plumber drops his wrench on his pet alligator's (from the sewer) head. Chaos ensues - hopefully Joe isn't still holding the wrench
 
220/221 said:
Can you explain available fault current in a way I might understand?

Pretend I'm a plumber or something and spell it out.

I know what available,fault, and current mean but I've never been able to grasp the concept. I just put the stickers on the series rated panels as specified.:wink:
Click to expand...

Think of hoover dam, there is always water passing through it, rainfall levels (Load) change how much flows, but the bypass has a flow limit (continous current rating of the conductor and your OCPD setpoint).

Available fault current = The dam breaker and all of lake mead comes rushing out really fast, but the volume of lake mead limits how much will flow.
 
infinity said:
Given the title of this thread I'm surprised that anyone even responded. :roll:
Click to expand...
Oh, I don't know. "Clown" might be one of the nicest things I've been called lately. :D
 
I've alerted the mafia..

I've alerted the mafia..

zog said:
Clown is he nicest thing I have been called on this forum lately.
Click to expand...

K, so who's been callin you names? Now they have to deal with me... :grin:
 
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