Had a 200A 3 ph 208Y120V main blow today due to a dead short in a 20A circuit.

The spike must have had sufficent duration to do this

But the 20A breaker never tripped, and i am at a loss as to explain this


any input apprectiated

Sparky,

I've seen the same thing happen. I always thought that maybe the main was more sensitive somehow to the characteristics of a dead short.

Bill

You said the main blew. I think we are to assume that it was a non-GFI circuit breaker that opened with no damage.

Did both have the same interruptor rating?

If the load through the main was already running 200 A before the fault occurred, it is imaginable. The 20 A CB was cold and the 200 A CB was already warmed up.

Sparky,

Did it happen when you/someone turned on the 20A breaker?

Dspark, Bill,
It happened when the 20A was turned on, the panel is new, not loaded up as yet, did not amp it out but should be about 40A usage give or take.

The interupter rating should be more for the main, however, as you pointed out, there was a hand holding the 20A !

So, somehow the main saw enough AIC ?

Does this make sense?

I had an apprentice miswire a switched leg... wired it like a switch loop... dead short. I should have been watching him closer...

New House, very little load on the main.
The SQ D 100A main tripped before the 15A lighting CB. I replaced the SQ D 15A breaker while the appy rewired the switch (correctly this time).

I never had a good explanation either, but I didn't take any chances... Breakers can be bad out of the box... Even SQ D

Sparky,

That's what I was wondering about. Maybe the hand holding it had something to do with the Main going before the 20A.
Just fishin'


Bill

I see what Bill's getting at... in fact, yes, being a dead short, my hand was forcing the CB closed at the time...

It is my understanding that the short circuit trip curves for most breakers 200 amps and less are almost identical and with the normal manufacturing tolerances it is not unusual for a main to trip out before a smaller branch breaker on a short circuit. Holding the handle of a breaker has nothing to do with its trip time. The breaker will open in the same time even if the exterior handle does not move.
Don(resqcapt19)

Don,
thanks, i was wondering about those breaker locks on the fire breakers....

this has me wondering about series fault..

Are both circuit breakers thermal, magnetic or thermal/magnetic. I wonder if the location on the sine wave (peak, zero crossing or some place in between) at the instant of the fault has to do with which breaker will trip 1st?

Steve,

What brand was the branch breaker??

Have seen this a few times before. Happened to me once, and I'll get to that story later, but had to troubleshoot this one about 10 times when done by other persons - not in the Electrical Trade.

In all cases [including mine], the AIC and time/current curves were exceeded on the branch breaker [1 pole 20 amp unit], so the only thing left was the panel's main.

Some of these panels were protected with breakers which had similar makeup [making the system non-selective], so the main breaker in the service equipment was tripped - or one fuse blew.
These were on systems that had only one Transformer - 208Y/120 VAC 3 phase 4 wire Wye. We'll leave the resons that a ground fault on 480Y/277 VAC 3 phase 4 wire Wye systems of 1KA and more brings down the main!! [I'm sure this has been seen by alot more people than I].

Anyhow Steve, It very well might be that the AIC rating of the breaker was exceeded when you threw the breaker on.

Short circuits consist of three [3] separate levels: Low level, Moderate level and High level.

We see mostly low and moderate level faults, and almost never see high level faults.

Low level faults are the ones that blow sparks all over the place. When they weld themselves solid, they become moderate levels - then the OCPD opens quicker.
Low level faults peak out at levels around 100 amps, so they appear as inrushes to the breaker. All the sparks blown around means the connections are less than solid, so the flowing currents will not be steady enough to "work on" the trip mechanisms.
They eventually trip that device as a preceived overload, rather than overcurrent [AIC].

Moderate faults have two [2] separate levels - low end and high end.

Low end moderate faults occur from spark shows that end up welding solid. They were peaking at 100 amps before, now they quickly rise to 1-2KA and hold this value. The breaker responds to these as an overcurrent [AIC], mainly because the current rose from an overload to an overcurrent.

High end moderate faults occur as did in your case [and in the case I had which will be covered later]. These are "Bolted" faults, which are similar to the High level fault's culprite, but they are downstream from high AIC available areas and occur on small conductors.

On these High end Moderate faults, the current rises to high levels instantly, not over successive steps, so it's nearly impossible for frames of 125 amps and smaller to control these overcurrents.
Their Time/Current curves have been exceeded, and the devices AIC rating is in danger of being exceeded.
In this case, the largest frame / fuse will trip.

Almost the same goes for High Level faults, except they have the highest AIC levels flowing at the most quickest time.

Best thing to do for understanding this is to review Time/Current curves for a multitude of devices [such as would be installed in any project]. These charts will unmask the hidden stuff behind unknown trips and makes the entire Selective system idea something to really think about.


Now, the time it happened to me, the fault was a high end moderate level one.

On one of our famous overnight bank branch remodel projects [this one was adding IG circuits to an existing teller line], I had my Brother-in-law working with me, and the rest of the crew was working on other stuff.

It was around 3:30 AM and everyone was tired. There was 6 J-Boxes to make up, which I had my B-in-law doing. He was compentant, and I only blame myself for the disaster [almost disaster].

I told him to make up the splices by "tying the Reds and the Whites together", so they continued through as if nothing happened.

Should have realized there was confusion, but I figured it was 3:30 AM run-down!!

Well, after everything was cleaned up and all equipment was put back, I sent everyone home, then commenced to test everything. Time now is 6:00 AM.

While turning on the branch circuits, which derived from a 225 amp copper subfed older Zinsco panel, everything worked out fine until I threw the last breaker: - the one protecting the "Red" circuit in the J-Boxes.

Turned it on, and only heard it sizzle!! Couldn't throw it back off!! It ran it's self for about 5 seconds before tripping the breaker protecting that panel.

Found in all 6 J-Boxes that the Red and White were spliced together - twisted and wirenuts!

Fortunately, there was a functioning spare 1 pole 20 amp breaker in the panel, which I replaced the welded one with.

After correcting the 6 J-Boxes, resetting the LAN, verifying the ATM operation, verifying all LAN equipment operates, conferming correct operation with the Remote Administration company, I left this branch at 8:45 AM - 15 minutes before normal opening time!!

Talk about pulling a Rabbit out of your hat!!!

Scott SET

Scott,
good post,
the panel is a siemens 208Y120V 3 ph subbed out. the branch breakers are T&B interchangables ( listed for 8 panels, that's why i carry them)
i'll have to get the AIC specs next visit there...

got back there yesterday,
all breakers are the same 10,000 AIC.
so would the short circuit trip curves follow suit?

they are all thermal/magnetic, and i suppose it's a lame observation, but the main being much larger must have a larger mechanism?

am i OCPD challenged?

That would be for sure.

I wonder if you terribly exceeded the 10000 A limit and the main CB tolerated it better than the branch CB.

The branch CB appears undamaged, right?

Dspark;
no damage visible, nothing froze shut or open, no heat marks, all meters out ok...

Scott--I don't want to get off the subject but...tied red and white together? Didn't question you on this? You would consider this competent?

Day one of wiring lessons--the neutral is white, the ground is green, and the hot can be any other color. Tie the hots to the hots and the neutrals to the neutrals, and the grounds to the grounds.

Don't make too much out of this, your input is highly valued and respected. Also, never take the blame for someone elses mistake, especially if the bank wouldn't have opened on time. Just kidding.

Steve T,

Like I said, I only blame myself on that one.
He asked me if that's what I wanted - to tie the whites and reds together, and in my 1/3 asleep state, it sounded OK. It was the way I said to tie them together to make them go through.

If I had been alert as usuall [ ], when he double checked my request, it would have been picked up on it.

Made a great impression on me, as this [hopefully] has made me think a little more about how to word something while being tired.

No offense taken at all!! In fact, I would invite a level of sarcasm by everyone for doing it in the first place!!

See ya all either Sunday night or Monday evening.

Scott SET.

Scott's story reminded me of an experience I had this week with a new apprentice.

He was on the ladder setting anchors with a hammerdrill. When he was finished I told him to, "Drop it by the cord". Well, he took me literally and dropped it. Fortunately, he caught it again about 1" from my nose!
It was my fault, I should have told him to "lower it", not drop it.
When you are supervising inexperienced help, make sure that your instructions are clear and unambiguous.
GJ

This has been a good thread and I hate to jump in late with criticism, but......

On a safe work practices note:

Golf Junkie "Drop it by the cord".

It is a violation of OSHA to use the cord of an electric tool to carry, raise or lower the tool. If a tool has a three prong plug the weakest conductor in the cord is usually the smaller EGC. If it breaks or pulls loose at a connection the tool will appear to work okay. If the ECG is broken and the tool is used on a non-GFCI protected source, the short circuit path for a short to the housing will be through the person holding it. Same as if the ground prong were broken off.

Don't ask me when is the last time I broke this rule. Not trying to be holier than thou; just passing along info.

This is the first time I read this thread. Thanks to resqcapt19 for pointing out that a breaker will trip even if the handle is held. That is called "trip free" and is a requirement of a breaker. The latching mechanism is not held by the closing mechanism.

[This message has been edited by gpowellpec (edited 05-07-2001).]