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My .02 cents

All of this goes back to available fault current, protection hiarchy "?spelling", and impedance. It is not as simple as breaker ratings. I am not even going to begin to try and explain all of this. "I can't" but I have been through a lot of training at various times and this can and will happen in the right conditions. Breakers, OL, and fuses all have different ratings. Fault current inturpting ratings and OL ratings and response times are not all the same.

For what it's worth. DaleP

Re: I only slightly understand

Originally posted by answer
[B
There are two panels in the house side by side 100A each
Breaker was tested and is working fine
[/B]

There is a big difference between 100A and 200A main breaker
Which breaker was tested and how?

I only slightly understand

I only slightly understand this vlotage back up, circuit trip up line theory.
But I thought I'd give some clarifiing info.
New house 4 years ago.
There are two panels in the house side by side 100A each
Breaker was tested and is working fine
wire is 3-8 w G
10 Kw in AH
40 ft panel to disconnect.
?????????????????????

I can chime in a little regarding why 20amp Circuit Breaker would not trip at the end of the circuit, but the 200amp main might.

They key is the "Inductance" of the wire.

You have the main feed coming to the house which terminates at the main disconnect (200amp). From here, there could be a loooooong wire to the 20amp CB as disconnect and to the equipment.

You short out the wire at the equipment. It will cause an abrupt spike in current which travels though the wire. Longer the wire, larger the inductance. When inductance is involved, and when abrupt change in current ocurrs, the inductance generates a current in opposit direction (ie. dead short) This is called "Faraday's Law" in electrical theory text books. Farther from the change, larger the inductance, larger the opposit current.

Thus, the main disconnect might see a larger current than the near by disconnect.

HOWEVER, in relative short distance of the wire, it should really not make that much difference.

Another possiblity is the reaction time of the circuit breaker. There are minimum spec for the reaction time, but there is nothing regulating how fast they *could* react.

I think it is more of a possibility.

Durango
I was reading your post few times to make sure that I would not miss any part of it. I still don’t see how your theory can be correct.
Do you have a link to article or document talking about this issue?
I, specially don’t get part about” fault” traveling from source to transformer and back.
Circuit breaker reacts to over current flowing through it’s bi-metal element. It does not care if this current travels through neutral or grounding conductor.
Breakers are electro-mechanical devices. They are not fail proof.
I have seen 20A breaker been tripped by 15A current.
I also have seen 20A breaker holding 60A for quite few seconds before going off.
In this example where 200A main tripped before 50A, it is possible that 50A breaker was faulty. We have to remember that 200A panel was carrying other loads from the house so it needed much less then 200A to trip.

Just can't figger it...

The HO dodges a HUGE bullet and she won't spring to replace the other two disconnects???

Amazing.

Here is the un-burnt disconnect.

A sguare D if you would belive it. She has two more at her house I told her to replace them, they may be a bad batch from the factory. But no response yet. You bet ya I got photos and her signature on a denial of the disconects.

Originally posted by durango
Hello Wolfdog.......
I still find in the electrical trade a lot of misunderstanding of the terms grounding, grounded, bonding, and grounding electrode systems and how the conductors are used.....

I'm not sure if this was meant in the manner it was delivered.

However, we can agree to disagree and let it go at that.

That is truely hard to believe, I would have not believed it without seeing the pictures. Thanks for sharing the info.

POSSIBLE CONSUMER RECALL

Hello answer:

I personally think you should send these pictures along with a picture of a non burnt disconnect with model, serial, manufacture etc. to the proper authorities for a possible consumer recall if necessary being you stated all the lugs had been checked for tightness in your previous P/M.

There is no telling how many lives across this country might possibly be in danger with these. I'd also be willing to bet they don't have MADE IN USA on them either.

Hello Wolfdog

I agree with some of your statements OHM's Law will dictate the available voltage on systems which have lost their grounded conductor based on available volt-ampere per leg. Most lighting and appliance boards have 240 volt devices which will allow 120 volt loads to continue to work (althought not for long if the voltage exceeds the maximum voltage tested on the equiipment, usually 135 volts max.)
I still find in the electrical trade a lot of misunderstanding of the terms grounding, grounded, bonding, and grounding electrode systems and how the conductors are used.
I did find some lovely electrical engineering ladies at http://www.ferrazshawmut.com/contact/app_ass_na.html that offered some great insight as to how over current protective devices operate and also some good verbal information at Schneider Electric (Square D) on some the statements I have made and now feel quite comfortable in standing behide. Hope you have a good one.

[Edited by durango on 03-19-2004 at 01:13 PM]

BBQ

Post a pic of a pre-BBQ disconnect, same type...


I will try to find one. She has another one of the same on her Condenser.


As for the tripped Main the distance to the pair of 200A mains is about 60 feet, as the wire feeds. But I think the main only tripped on reset of the 50A air handler breaker. I hit it once before I went up to the attic and it took the whole panel down. I believe the wire was
8-3wG.
I think she did too, she was scared and said she had seen sparks from the breaker. I don't think she, still to this day, comprehends how lucky she is.

Durango

A breaker or fuse is in place to protect the conductors, it is a stopping place for overcurrent and/or ground fault. The purpose is to clear the overload or fault at that point and not let it move further up the system. The purpose of the ground conductor, be it conduit, a seperate conductor, or the bare wire in NM cable; to provide a low impedance path back to the panel and hense the breaker. In the event of a ground fault the current is carried back to the panel, thru the ground, to the breaker/fuse and the fault is cleared at the first overcurrent device before the problem travels back up the system further.

If the event of an "open neutral" affecting a single pole breaker, the conductor for the unbalanced current is lost. Now the single phase breaker is going to "see" somewhere between 0 -230 volts depending on the internal resistance of devices on the other buss. This means that a 120 volt rated breaker may be seeing 50 volts or 230 volts. This accounts for the unstable trip charateristics in an "open neutral" situation. If you have ever encountered this problem, you will notice that the voltage on L1 and L2 will shift as the loads change.

I appreciate the effort fellas, but I still don't get it how a 200 amp main or a fuse like Payson will fault before a 50amp CB...

Obviously, it's never happened to me, and I realize strange things happen with electricity sometimes, but a 50 is still a 50 and should (in my mind) trip before a 200...?

Thanks anyway.

How you doing Wolfman, no I think I will stand by my statement. A grounded conductor is a neutral conductor that is bonded or intentionally connected at the main grounding point of the service equipment or separately derived system (transformer, ups, etc.)where a grounding connection is made to a grounding electrode system(foundation steel, made electrode, metal water pipe or other effectively grounded system). The grounding conductor is used solely to connect equipment or the grounded (neutral) circuit of a wiring sytem to a grounding electrode system usually by a grounding electrode conductor. This conductor connects the grounding electrode to the equipment grounding conductor , or the grounded (neutral) conductor or both at the circuit at the service equipment or at the source of a separately derived system at least according to Article 100 and 250 of the NEC 1999 Edition, sorry didn't have my 2002 available.

A molded case circuit breaker will only function if the fault travels along a grounding path back to the transformer and returns to the device. At least this is what I have been taught in continuing ed. courses. A better source for information on this might be a manufacturer than me.

Sorry about being so long winded on this.

Originally posted by durango
A circuit breaker will only fault/trip if the fault travels back to the utility transformer from the fault location and then back to the breaker. That is why molded case circuit breakers will not trip if the circuit is in an "open neutral" condition.

Maybe you should reconsider these statements.

Consider the express purpose of a ground conductor and the ground electrode conductor.

Why does the Main CB trip (minimum 100A, probably 200) but not the 50 for the A/H?? Was the Question.

I was trying to answer why the 50 amp over current protective device didn't trip or fault but the 200 amp main in the house panel or main disconnect did. You have to look at available fault current, length and size of the branch circuit and feeder, and the transformer suppling the house. When you get a large spike on the system as occurred in the pictures, the circuit breaker (CB) will sometimes not trip at the first overload but may fault the first over current device on the circuit (House Main Over-current Protective Device). A circuit breaker will only fault/trip if the fault travels back to the utility transformer from the fault location and then back to the breaker. That is why molded case circuit breakers will not trip if the circuit is in an "open neutral" condition.

I went to a call with a shorted comp one day. I must have been in a mood because I actually reset the breaker a few times to watch the amp draw as it snapped the breaker silly.

On the 3rd try it tripped. So I then took apart the unit to do a meg ohm test, etc. Comp shorted directly to ground. Then I noticed no more power to anything when I saw the contactor not closed anymore. The whole house was dead.

Went to the main panel expecting the main 200 AMP to be tripped (which has happened before) and nope, it wasn't.

Shortly comes a APS service truck out to the neighborhood. He opened the transformer box and I blew a 15 amp fuse on the primary side of the transformer (12,000 volts)!

I killed the power to 4 houses!

Originally posted by durango
The reason the 200 trips is probably do to the location down stream from the fault site. The further away from the fault the main is, the greater the fault will register on the CB. I intentionally faulted a 20 Federal in the vain hope of tripping it while doing a job years ago, too lazy to crawel two hundred feet back out of an attic, and accidently blew an 800 amp main. Did this while I was a journeyman, know better now....LOL There is an electrical formula to show how this occurs.

Say what?

The reason the 200 trips is probably do to the location down stream from the fault site. The further away from the fault the main is, the greater the fault will register on the CB. I intentionally faulted a 20 Federal in the vain hope of tripping it while doing a job years ago, too lazy to crawel two hundred feet back out of an attic, and accidently blew an 800 amp main. Did this while I was a journeyman, know better now....LOL There is an electrical formula to show how this occurs.