ECN Forum Forums Technical Discussion Electrical Theory and Applications Dead Shorts

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