electrical question (wood shop)

On Sat, 01 Sep 2012 15:55:42 +1000, John G <[email protected]>
wrote:

>It happens that Mike M formulated :
>> On Thu, 30 Aug 2012 21:04:39 -0700 (PDT), Ivan Vegvary
>> <[email protected]> wrote:
>>
>>> Finished wiring the wood-shop area. All in conduit, 12 quadplex wall
>>> outlets, and four fluorescent lights on three way switches. Also a 220V
>>> feed to the air compressor. New sub-panel in wood shop is fed by an other
>>> sub-panel 3 feet away on the other side of a wall. Feeder panel has a 50amp
>>> breaker feeding the wood shop sub panel. All wiring within conduit is #12
>>> except the 220 which is #10.
>>>
>>> Question: As I installed each of the quad (gang of four) outlets, I would
>>> go to the breaker, turn it on and check the outlet with a clamp lamp and
>>> then turn off the breaker for further work. When I was installing the
>>> outlet group # 11, the wires shifted withing the conduit and the hot wire in
>>> # 12 touched the metal box. (Yes, I didn't cap the wires while I was
>>> working, I simply bent them out of each other's way. Stupid) Anyway, the
>>> question is "why did the 50 amp breaker feeding my sup panel break instead
>>> of the 20 amp breaker withing the sub panel"?
>>>
>>> Later, after wrestling my large air compressor into it's closet the same
>>> thing happened. I dislodged the safety shield on the belt, the fan blade
>>> could not turn and ergo the motor drew too many amps on start-up. Again,
>>> instead of the 30 amp circuit feeding the compressor breaking, it's the 50
>>> amp feeding the panel that broke.
>>>
>>> Note: 50 amp breaker is used. 30 amp breaker is used. 20 amp breaker is
>>> new.
>>>
>>> Is the 50 amp breaker simply too sensitive?
>>>
>>> I could live with the above, except when the 50 amp breaker trips I lose
>>> everything and am left in the dark. Have to get a flashlight, go outside
>>> one building and into another to access the other side of the wall.
>>>
>>> All comments and advise appreciated.
>>>
>> You don't say whether all the breakers are the same manufacturer.
>> Actually had that happen in a computer center where a 125 amp breaker
>> would trip before the 30 amp parking area breaker. Of course it
>> always led to a 2AM call out so we found that problem quickly. Anyway
>> you have two variables which is the short circuit trip time and the
>> overload trip time. Possible this was the result of the dead short so
>> you might not have to do anything. Possible a defective breaker, most
>> likely the one tripping to fast. A possible problem of a poor ground
>> path to the first breaker.
>>
>> Mike M
>
>Can you explain what part a ground plays in a simple 2 terminal
>breaker?

Simple you can have a line to line short, or a short to ground. If it
is a short to ground and you have a poor ground path the breaker may
sense it more as an overload then a short and not trip or take longer
to trip.

J. Clarke wrote:
> In article <[email protected]>, [email protected] says...
>>
>> On 9/1/2012 7:47 PM, Mike Marlow wrote:
>> ...
>>
>>>
>>> ... but there is a 50/50 chance on either one of them
>>> being faster just based on manufacturing limitations ...
>>
>> Well, only if the two responses are nominally identical...which
>> wouldn't be necessarily so at all.
>
> How much load on the upstream breaker?
>
> Let's say you have a load center with a 50 amp main breaker and four
> 20 amp circuits.
>
> Now, let's also say that three circuits have 15 amp loads on them
> already, and you put a 10 amp load on the fourth. What happens? You
> trip the 50 amp breaker without tripping any of the 20s.
>
> Not saying that's what happened in this case--not enough information
> was given. But it's something to think about.

Correct, but that's a different set of conditions from what the OP
presented.

--

-Mike-
[email protected]

dpb wrote:
> On 8/31/2012 1:16 PM, Scott Lurndal wrote:
> ...
>
>> As for the OP, if the 50 is upstream of the 20, and the 50 blows
>> first on a dead short, the 20 should be replaced ASAP.
> ...
>
> Excepting (unless he mistyped) the 20A is supposedly a brand new
> breaker while the 50A is used. I'm suspecting the 50A (or something
> else isn't kosher that's totally impossible to diagnose from here).

Not to mention that if I recall correctly, there were more than on sub panel
breakers that exhibited this same symptom - so, both of them passed the
current to the sub-panel feed. That would seem to point to the sub-panel
feed.

--

-Mike-
[email protected]

Doug Miller wrote:

>>
> I think you owe Puckdropper an apology for that. He's more nearly
> right than you are.

Point taken on the speed of each breaker affecting which will open first. I
will offer that apology to Puckdropper, however I wouldn't go so far as to
say more nearly right. You are - as is he, correct that the speed of each
breaker is the factor, but there is a 50/50 chance on either one of them
being faster just based on manufacturing limitations. Well taken point
Doug.

--

-Mike-
[email protected]

Puckdropper wrote:

> It's possible that the current drawn would exceed the current ratings
> of both circuit breakers, and in that case there would be a race
> condition to see which one trips first. The used 50A breaker might
> move easier, so it would tend to win that race.

No race condition at all if both breakers are working at rated capacities.
At rated capacities, a 20A breaker will trip before a 50A breaker. By your
own statement above... " It's possible that the current drawn would exceed
the current ratings of both circuit breakers", which should result in the
20A op3ning first. Obviously, there is a problem, because this is not
happening. But - to throw black magic at it is no solution to the problem.
"Maybe, kinda sorta" stuff has no place in this kind of conversation.

>
> Replacing the 50A breaker might let the 20A breaker "win the race"
> more often. This doesn't eliminate the race condition, so the main
> might go first on a dead short every now and again.

Do you understand electricity at all?


--

-Mike-
[email protected]

Bill <[email protected]> wrote in
news:[email protected]:

>
>
> Although I don't have Doug's electrical expertise, I was thinking that
> the surge was just that, a surge. It's wasn't like the turning the
> flow
> from a water faucet from low to high. It was more like turning the
> flow from low to "Niagara Falls"!
>
> CB'ers supposedly handle motor start-up better than fuses(don't
> they?), clearly due to a delay. I think it's built into the mechanics
> of the CB'er. Maybe this delay is a measurable feature of the 2
> CB'ers involved?

It's possible that the current drawn would exceed the current ratings of
both circuit breakers, and in that case there would be a race condition
to see which one trips first. The used 50A breaker might move easier, so
it would tend to win that race.

Replacing the 50A breaker might let the 20A breaker "win the race" more
often. This doesn't eliminate the race condition, so the main might go
first on a dead short every now and again.

Puckdropper
--
Make it to fit, don't make it fit.

Somebody wrote:

> The used 50A breaker might move easier, so
> it would tend to win that race.
-----------------------------------
All bets are off.

A thermal-magnetic, molded case, plug-in cbk'r is a single use device.

They are a single use device that provides inrush (magnetic) or
inverse time delay (thermal) protection.

These cbk'rs only have to clear a fault ONCE to meet design spec.

There is no such thing as a USED cbk'r that you can trust.

Lew

On 8/31/2012 10:21 AM, Doug Miller wrote:
> Ivan Vegvary <[email protected]> wrote in
> news:[email protected]:
>
> [...]
>> Anyway, the question is "why did the 50 amp breaker feeding my
>> sup panel break instead of the 20 amp breaker withing the sub
>> panel"?
> [...]
>> Is the 50 amp breaker simply too sensitive?
>
> Well, it's *more* sensitive, anyway. That doesn't mean, necessarily, that it's *too* sensitive (i.e.
> more sensitive than it's supposed to be). It could be that way by design.

Any consideration given to the 20AMP being defective?

On 9/2/2012 6:34 PM, Doug Miller wrote:

>>> Well, it's *more* sensitive, anyway. That doesn't mean, necessarily, that it's *too*
> sensitive (i.e.
>>> more sensitive than it's supposed to be). It could be that way by design.
>>
>> Any consideration given to the 20AMP being defective?
>
> While certainly possible, I'd consider it to be pretty unlikely -- the OP stated that the 20A
> breaker is new, and the 50A is used.

I agree that it is unlikely, but we BOTH agree that it is possible.
Quality Control isn't always controlling<g>

Logically, it's worth looking at and swapping out the breaker to make
certain that is NOT the case, especially when a dead short on that
circuit won't trip the 20A breaker but will flip the 50A.

On Mon, 03 Sep 2012 09:27:50 -0700, Larry Jaques
<[email protected]> wrote:

>On Mon, 03 Sep 2012 08:49:35 -0700, Mike M
><[email protected]> wrote:
>
>>On Sat, 01 Sep 2012 15:55:42 +1000, John G <[email protected]>
>>wrote:
>>
>>>It happens that Mike M formulated :
>>>> On Thu, 30 Aug 2012 21:04:39 -0700 (PDT), Ivan Vegvary
>>>> <[email protected]> wrote:
>>>>
>>>>> Finished wiring the wood-shop area. All in conduit, 12 quadplex wall
>>>>> outlets, and four fluorescent lights on three way switches. Also a 220V
>>>>> feed to the air compressor. New sub-panel in wood shop is fed by an other
>>>>> sub-panel 3 feet away on the other side of a wall. Feeder panel has a 50amp
>>>>> breaker feeding the wood shop sub panel. All wiring within conduit is #12
>>>>> except the 220 which is #10.
>>>>>
>>>>> Question: As I installed each of the quad (gang of four) outlets, I would
>>>>> go to the breaker, turn it on and check the outlet with a clamp lamp and
>>>>> then turn off the breaker for further work. When I was installing the
>>>>> outlet group # 11, the wires shifted withing the conduit and the hot wire in
>>>>> # 12 touched the metal box. (Yes, I didn't cap the wires while I was
>>>>> working, I simply bent them out of each other's way. Stupid) Anyway, the
>>>>> question is "why did the 50 amp breaker feeding my sup panel break instead
>>>>> of the 20 amp breaker withing the sub panel"?
>>>>>
>>>>> Later, after wrestling my large air compressor into it's closet the same
>>>>> thing happened. I dislodged the safety shield on the belt, the fan blade
>>>>> could not turn and ergo the motor drew too many amps on start-up. Again,
>>>>> instead of the 30 amp circuit feeding the compressor breaking, it's the 50
>>>>> amp feeding the panel that broke.
>>>>>
>>>>> Note: 50 amp breaker is used. 30 amp breaker is used. 20 amp breaker is
>>>>> new.
>>>>>
>>>>> Is the 50 amp breaker simply too sensitive?
>>>>>
>>>>> I could live with the above, except when the 50 amp breaker trips I lose
>>>>> everything and am left in the dark. Have to get a flashlight, go outside
>>>>> one building and into another to access the other side of the wall.
>>>>>
>>>>> All comments and advise appreciated.
>>>>>
>>>> You don't say whether all the breakers are the same manufacturer.
>>>> Actually had that happen in a computer center where a 125 amp breaker
>>>> would trip before the 30 amp parking area breaker. Of course it
>>>> always led to a 2AM call out so we found that problem quickly. Anyway
>>>> you have two variables which is the short circuit trip time and the
>>>> overload trip time. Possible this was the result of the dead short so
>>>> you might not have to do anything. Possible a defective breaker, most
>>>> likely the one tripping to fast. A possible problem of a poor ground
>>>> path to the first breaker.
>>>>
>>>> Mike M
>>>
>>>Can you explain what part a ground plays in a simple 2 terminal
>>>breaker?
>>
>>Simple you can have a line to line short, or a short to ground. If it
>>is a short to ground and you have a poor ground path the breaker may
>>sense it more as an overload then a short and not trip or take longer
>>to trip.
>
>This may be true in a GFI (ground fault interrupter) circuit breaker,
>but in 120/240v standard breakers, there IS no ground lead.

I don't know what code book your using but you'll never pass a NEC
inspection if the entire system isn't grounded. Your thinking of the
nuetral or identified conducter.

Mike M

On 8/31/2012 1:16 PM, Scott Lurndal wrote:
> Leon <lcb11211@swbelldotnet> writes:
>
>> We have a new home and had a problem with a breaker, it clearly was
>> rated high enough to perform the task. The electrician that came out to
>> replace the breaker indicated that the typical modern breaker is
>> designed to blow up at up to 20% below the rating. BS or not, this is
>> not the first time I have heard this.
>
> IIRC, the code requires that a circuit be loaded to no more than 80%
> ampacity, derated by temperature and raceway loading.

Well that would support the design of modern breakers to enforce that
code. ;~)

>
> As for the OP, if the 50 is upstream of the 20, and the 50 blows
> first on a dead short, the 20 should be replaced ASAP.

Now that is a logical safe first step to diagnosis.

On 8/31/2012 12:06 PM, Mike Marlow wrote:
> Bill wrote:
>
>>
>> Although I don't have Doug's electrical expertise, I was thinking that
>> the surge was just that, a surge. It's wasn't like the turning the
>> flow from a water faucet from low to high. It was more like turning
>> the flow from low to "Niagara Falls"!
>
> Not so bad an analogy, but it does not address the real question - why did
> the 50A breaker trip and not the 20A. Keep working on it Bill - though
> admitedly, it is a bit of speculation after the fact like this.
>
>>
>> CB'ers supposedly handle motor start-up better than fuses(don't
>> they?), clearly due to a delay. I think it's built into the
>> mechanics of the CB'er. Maybe this delay is a measurable feature of
>> the 2 CB'ers involved?
>
> Actually, slow blow fuses can perform better than breakers when it comes to
> start up current. The operative word is "can". As far as the "measurable
> feature of 2 CB's involved" - not even sure what that means, but the short
> answer is... likely, no.
>

We have a new home and had a problem with a breaker, it clearly was
rated high enough to perform the task. The electrician that came out to
replace the breaker indicated that the typical modern breaker is
designed to blow up at up to 20% below the rating. BS or not, this is
not the first time I have heard this.

In my old shop I was running a compressor and freezer on the same
circuit. If I added my router and fan the 15 amp breaker would some
times trip. In the new house on a 20 amp dedicated circuit the breaker
often tripped with only the compressor and freezer. So now the freezer
runs on a separate 15 amp circuit and the compressor and drum sander run
on the 20 amp. No more problems.

In article <[email protected]>, [email protected] says...
>
> On 9/1/2012 7:47 PM, Mike Marlow wrote:
> ...
>
> >
> > ... but there is a 50/50 chance on either one of them
> > being faster just based on manufacturing limitations ...
>
> Well, only if the two responses are nominally identical...which wouldn't
> be necessarily so at all.

How much load on the upstream breaker?

Let's say you have a load center with a 50 amp main breaker and four 20
amp circuits.

Now, let's also say that three circuits have 15 amp loads on them
already, and you put a 10 amp load on the fourth. What happens? You
trip the 50 amp breaker without tripping any of the 20s.

Not saying that's what happened in this case--not enough information was
given. But it's something to think about.

Ivan Vegvary <[email protected]> wrote in
news:[email protected]:

[...]
> Anyway, the question is "why did the 50 amp breaker feeding my
> sup panel break instead of the 20 amp breaker withing the sub
> panel"?
[...]
> Is the 50 amp breaker simply too sensitive?

Well, it's *more* sensitive, anyway. That doesn't mean, necessarily, that it's *too* sensitive (i.e.
more sensitive than it's supposed to be). It could be that way by design.

[email protected] (Scott Lurndal) wrote in news:[email protected]:

> Leon <lcb11211@swbelldotnet> writes:
>
>>We have a new home and had a problem with a breaker, it clearly was
>>rated high enough to perform the task. The electrician that came out to
>>replace the breaker indicated that the typical modern breaker is
>>designed to blow up at up to 20% below the rating. BS or not, this is
>>not the first time I have heard this.
>
> IIRC, the code requires that a circuit be loaded to no more than 80%
> ampacity, derated by temperature and raceway loading.

Not true.

The 80% rule applies *only* to a "continuous load" which is explicitly defined by the NEC as
maximum current for more than three hours.

> As for the OP, if the 50 is upstream of the 20, and the 50 blows
> first on a dead short, the 20 should be replaced ASAP.

Also incorrect. The 50A breaker is the one that should be replaced first.

"Mike Marlow" <[email protected]> wrote in news:45b12$504180ab
[email protected]:

> Puckdropper wrote:
>
>> It's possible that the current drawn would exceed the current ratings
>> of both circuit breakers, and in that case there would be a race
>> condition to see which one trips first. The used 50A breaker might
>> move easier, so it would tend to win that race.
>
> No race condition at all if both breakers are working at rated capacities.
> At rated capacities, a 20A breaker will trip before a 50A breaker.

Not necessarily, Mike -- it depends on the speed with which the breaker reacts to the
overcurrent. If the overcurrent is, say, 40A, of course the 20A breaker should trip first,
because the 50A breaker should not trip at all. But a dead short may result in a brief current
of well over 50A, in which case the breaker that trips will be the one that reacts fastest.

In this case, with a used 50A breaker and a new 20A breaker, one would expect the 50A
breaker to trip a little faster when subjected to a current of, say, 100A.

Even if they're both brand-new, and even if both have the same *design* trip speed, they
still won't respond at *exactly* the same speed, simply due to normal statistical variability in
the manufacturing process. Suppose the breakers are designed to trip within 10ms +/- 0.5%
when an overcurrent occurs -- if the actual trip delay is 9.95ms for the 50A breaker, and
10.05ms for the 20A, they're both within design specifications, but on a 100A overcurrent the
50A breaker will go first every time.

> By your
> own statement above... " It's possible that the current drawn would exceed
> the current ratings of both circuit breakers", which should result in the
> 20A op3ning first.

No, it should not. It should result in the fastest-reacting breaker opening first. The reaction
speed of a breaker is *not* dependent on its amperage rating.

>Obviously, there is a problem, because this is not
> happening. But - to throw black magic at it is no solution to the problem.
> "Maybe, kinda sorta" stuff has no place in this kind of conversation.
>
>>
>> Replacing the 50A breaker might let the 20A breaker "win the race"
>> more often. This doesn't eliminate the race condition, so the main
>> might go first on a dead short every now and again.
>
> Do you understand electricity at all?
>
I think you owe Puckdropper an apology for that. He's more nearly right than you are.

Unquestionably Confused <[email protected]> wrote in news:50428439$0$15608
[email protected]:

> On 8/31/2012 10:21 AM, Doug Miller wrote:
>> Ivan Vegvary <[email protected]> wrote in
>> news:[email protected]:
>>
>> [...]
>>> Anyway, the question is "why did the 50 amp breaker feeding my
>>> sup panel break instead of the 20 amp breaker withing the sub
>>> panel"?
>> [...]
>>> Is the 50 amp breaker simply too sensitive?
>>
>> Well, it's *more* sensitive, anyway. That doesn't mean, necessarily, that it's *too*
sensitive (i.e.
>> more sensitive than it's supposed to be). It could be that way by design.
>
> Any consideration given to the 20AMP being defective?

While certainly possible, I'd consider it to be pretty unlikely -- the OP stated that the 20A
breaker is new, and the 50A is used.

Bill wrote:

>
> Although I don't have Doug's electrical expertise, I was thinking that
> the surge was just that, a surge. It's wasn't like the turning the
> flow from a water faucet from low to high. It was more like turning
> the flow from low to "Niagara Falls"!

Not so bad an analogy, but it does not address the real question - why did
the 50A breaker trip and not the 20A. Keep working on it Bill - though
admitedly, it is a bit of speculation after the fact like this.

>
> CB'ers supposedly handle motor start-up better than fuses(don't
> they?), clearly due to a delay. I think it's built into the
> mechanics of the CB'er. Maybe this delay is a measurable feature of
> the 2 CB'ers involved?

Actually, slow blow fuses can perform better than breakers when it comes to
start up current. The operative word is "can". As far as the "measurable
feature of 2 CB's involved" - not even sure what that means, but the short
answer is... likely, no.

--

-Mike-
[email protected]

On Mon, 03 Sep 2012 08:49:35 -0700, Mike M
<[email protected]> wrote:

>On Sat, 01 Sep 2012 15:55:42 +1000, John G <[email protected]>
>wrote:
>
>>It happens that Mike M formulated :
>>> On Thu, 30 Aug 2012 21:04:39 -0700 (PDT), Ivan Vegvary
>>> <[email protected]> wrote:
>>>
>>>> Finished wiring the wood-shop area. All in conduit, 12 quadplex wall
>>>> outlets, and four fluorescent lights on three way switches. Also a 220V
>>>> feed to the air compressor. New sub-panel in wood shop is fed by an other
>>>> sub-panel 3 feet away on the other side of a wall. Feeder panel has a 50amp
>>>> breaker feeding the wood shop sub panel. All wiring within conduit is #12
>>>> except the 220 which is #10.
>>>>
>>>> Question: As I installed each of the quad (gang of four) outlets, I would
>>>> go to the breaker, turn it on and check the outlet with a clamp lamp and
>>>> then turn off the breaker for further work. When I was installing the
>>>> outlet group # 11, the wires shifted withing the conduit and the hot wire in
>>>> # 12 touched the metal box. (Yes, I didn't cap the wires while I was
>>>> working, I simply bent them out of each other's way. Stupid) Anyway, the
>>>> question is "why did the 50 amp breaker feeding my sup panel break instead
>>>> of the 20 amp breaker withing the sub panel"?
>>>>
>>>> Later, after wrestling my large air compressor into it's closet the same
>>>> thing happened. I dislodged the safety shield on the belt, the fan blade
>>>> could not turn and ergo the motor drew too many amps on start-up. Again,
>>>> instead of the 30 amp circuit feeding the compressor breaking, it's the 50
>>>> amp feeding the panel that broke.
>>>>
>>>> Note: 50 amp breaker is used. 30 amp breaker is used. 20 amp breaker is
>>>> new.
>>>>
>>>> Is the 50 amp breaker simply too sensitive?
>>>>
>>>> I could live with the above, except when the 50 amp breaker trips I lose
>>>> everything and am left in the dark. Have to get a flashlight, go outside
>>>> one building and into another to access the other side of the wall.
>>>>
>>>> All comments and advise appreciated.
>>>>
>>> You don't say whether all the breakers are the same manufacturer.
>>> Actually had that happen in a computer center where a 125 amp breaker
>>> would trip before the 30 amp parking area breaker. Of course it
>>> always led to a 2AM call out so we found that problem quickly. Anyway
>>> you have two variables which is the short circuit trip time and the
>>> overload trip time. Possible this was the result of the dead short so
>>> you might not have to do anything. Possible a defective breaker, most
>>> likely the one tripping to fast. A possible problem of a poor ground
>>> path to the first breaker.
>>>
>>> Mike M
>>
>>Can you explain what part a ground plays in a simple 2 terminal
>>breaker?
>
>Simple you can have a line to line short, or a short to ground. If it
>is a short to ground and you have a poor ground path the breaker may
>sense it more as an overload then a short and not trip or take longer
>to trip.

This may be true in a GFI (ground fault interrupter) circuit breaker,
but in 120/240v standard breakers, there IS no ground lead.

--
The most powerful factors in the world are clear
ideas in the minds of energetic men of good will.
-- J. Arthur Thomson

Ivan Vegvary wrote:

>
> Is the 50 amp breaker simply too sensitive?
>

It sounds like that's exactly the case. Since it's used, it may have been
tripped many times in its past life, which does change its sensativity.

--

-Mike-
[email protected]

Problem solved! Thanks for all the help.
Scrounged around for another breaker and found a 40 amp double (240 volt). =
Replaced the suspect 50 with the 40 and all is O.K. If I force a short in=
the 20 amp circuit the 20 amp now breaks and the upstream 40 amp holds. O=
bviously the 50 amp breaker was bad. (proved it by putting it back in and =
forcing a short)
BTW, I force a short by plugging in a shorted power cord. (Doesn't everybo=
dy in this group cut off the power cord on a tool prior to throwing it away=
?) =20

Thanks again, you guys are wonderful!

Ivan Vegvary

On Fri, 31 Aug 2012 23:21:50 -0700 (PDT), Ivan Vegvary
<[email protected]> wrote:

>Problem solved! Thanks for all the help.
>Scrounged around for another breaker and found a 40 amp double (240 volt). Replaced the suspect 50 with the 40 and all is O.K. If I force a short in the 20 amp circuit the 20 amp now breaks and the upstream 40 amp holds. Obviously the 50 amp breaker was bad. (proved it by putting it back in and forcing a short)
>BTW, I force a short by plugging in a shorted power cord. (Doesn't everybody in this group cut off the power cord on a tool prior to throwing it away?)
>
>Thanks again, you guys are wonderful!
>
> Ivan Vegvary

Jeeze, Ivan. We usually remove the plug from the outlet before cutting
the cord. "Forcing a short" indeed! ;)

--
I have the consolation of having added nothing to my private fortune during
my public service, and of retiring with hands clean as they are empty.
-- Thomas Jefferson, letter to Count Diodati, 1807

Too bad -none- of the current CONgresscritters are willing to do that. -LJ

On 8/31/2012 11:21 AM, Doug Miller wrote:
> Ivan Vegvary<[email protected]> wrote in
> news:[email protected]:
>
> [...]
>> Anyway, the question is "why did the 50 amp breaker feeding my
>> sup panel break instead of the 20 amp breaker withing the sub
>> panel"?
> [...]
>> Is the 50 amp breaker simply too sensitive?
>
> Well, it's *more* sensitive, anyway. That doesn't mean, necessarily, that it's *too* sensitive (i.e.
> more sensitive than it's supposed to be). It could be that way by design.


Although I don't have Doug's electrical expertise, I was thinking that
the surge was just that, a surge. It's wasn't like the turning the flow
from a water faucet from low to high. It was more like turning the
flow from low to "Niagara Falls"!

CB'ers supposedly handle motor start-up better than fuses(don't they?),
clearly due to a delay. I think it's built into the mechanics of the
CB'er. Maybe this delay is a measurable feature of the 2 CB'ers involved?

On 8/31/2012 12:06 PM, Mike Marlow wrote:
...

> ... As far as the "measurable
> feature of 2 CB's involved" - not even sure what that means, ...

Circuit breakers have time response curves, too; there are ones
specifically for motor applications as well just as "slo-blo" fuses.

It's possible one of OPs is but it'll be knowable by looking at the
actual manufacturer's part no's...

As for tripping a 50A breaker ahead of a 20A when the short is on that
20A circuit--that ain't the way it should operate, no.

I'd suspect the 50A is the culprit here--check that it's not warm to
touch when operating normally; they will fail w/ age and that's often a
clue.

--

On 8/31/2012 1:16 PM, Scott Lurndal wrote:
...

> As for the OP, if the 50 is upstream of the 20, and the 50 blows
> first on a dead short, the 20 should be replaced ASAP.
...

Excepting (unless he mistyped) the 20A is supposedly a brand new breaker
while the 50A is used. I'm suspecting the 50A (or something else isn't
kosher that's totally impossible to diagnose from here).

--

On 8/31/2012 1:02 PM, Leon wrote:
...

> We have a new home and had a problem with a breaker, it clearly was
> rated high enough to perform the task. The electrician that came out to
> replace the breaker indicated that the typical modern breaker is
> designed to blow up at up to 20% below the rating. BS or not, this is
> not the first time I have heard this.
...

You can always go to manufacturer datasheets and see...eg,

<http://products.schneider-electric.us/support/technical-library/?event=detail&oid=0900892680117fff&cat=0b008926800fd695>

--

On 9/1/2012 7:47 PM, Mike Marlow wrote:
...

>
> ... but there is a 50/50 chance on either one of them
> being faster just based on manufacturing limitations ...

Well, only if the two responses are nominally identical...which wouldn't
be necessarily so at all.

--

On 9/1/2012 2:21 AM, Ivan Vegvary wrote:
> Problem solved! Thanks for all the help.
> Scrounged around for another breaker and found a 40 amp double (240 volt). Replaced the suspect 50
with the 40 and all is O.K. If I force a short in the 20 amp circuit
the 20 amp now breaks and the
upstream 40 amp holds. Obviously the 50 amp breaker was bad. (proved
it by putting it back in and
forcing a short)

Good job. Before the woodworkers are done here, they will be plotting
things out on a Cray supercomputer, and fighting for government approval
of some sort of new electrical code.

BTW, I force a short by plugging in a shorted power cord.

Works, but I'm sure it violates a gov't code somewhere, and if you kill
yourself, neighbors, family pets, your insurance company will certainly
deny all claims.

(Doesn't everybody in this group cut off the power cord on a tool prior
to throwing it away?)

I thought I was the only one, I have a giant box of them. I tend to
save all sorts of stuff on the way to the trash. Pisses the wife off
when she catches me, until she needs me to fix something she broke.

--
Jack
Add Life to your Days not Days to your Life.
http://jbstein.com

Leon <lcb11211@swbelldotnet> writes:

>We have a new home and had a problem with a breaker, it clearly was
>rated high enough to perform the task. The electrician that came out to
>replace the breaker indicated that the typical modern breaker is
>designed to blow up at up to 20% below the rating. BS or not, this is
>not the first time I have heard this.

IIRC, the code requires that a circuit be loaded to no more than 80%
ampacity, derated by temperature and raceway loading.

As for the OP, if the 50 is upstream of the 20, and the 50 blows
first on a dead short, the 20 should be replaced ASAP.

scott

It happens that Mike M formulated :
> On Thu, 30 Aug 2012 21:04:39 -0700 (PDT), Ivan Vegvary
> <[email protected]> wrote:
>
>> Finished wiring the wood-shop area. All in conduit, 12 quadplex wall
>> outlets, and four fluorescent lights on three way switches. Also a 220V
>> feed to the air compressor. New sub-panel in wood shop is fed by an other
>> sub-panel 3 feet away on the other side of a wall. Feeder panel has a 50amp
>> breaker feeding the wood shop sub panel. All wiring within conduit is #12
>> except the 220 which is #10.
>>
>> Question: As I installed each of the quad (gang of four) outlets, I would
>> go to the breaker, turn it on and check the outlet with a clamp lamp and
>> then turn off the breaker for further work. When I was installing the
>> outlet group # 11, the wires shifted withing the conduit and the hot wire in
>> # 12 touched the metal box. (Yes, I didn't cap the wires while I was
>> working, I simply bent them out of each other's way. Stupid) Anyway, the
>> question is "why did the 50 amp breaker feeding my sup panel break instead
>> of the 20 amp breaker withing the sub panel"?
>>
>> Later, after wrestling my large air compressor into it's closet the same
>> thing happened. I dislodged the safety shield on the belt, the fan blade
>> could not turn and ergo the motor drew too many amps on start-up. Again,
>> instead of the 30 amp circuit feeding the compressor breaking, it's the 50
>> amp feeding the panel that broke.
>>
>> Note: 50 amp breaker is used. 30 amp breaker is used. 20 amp breaker is
>> new.
>>
>> Is the 50 amp breaker simply too sensitive?
>>
>> I could live with the above, except when the 50 amp breaker trips I lose
>> everything and am left in the dark. Have to get a flashlight, go outside
>> one building and into another to access the other side of the wall.
>>
>> All comments and advise appreciated.
>>
> You don't say whether all the breakers are the same manufacturer.
> Actually had that happen in a computer center where a 125 amp breaker
> would trip before the 30 amp parking area breaker. Of course it
> always led to a 2AM call out so we found that problem quickly. Anyway
> you have two variables which is the short circuit trip time and the
> overload trip time. Possible this was the result of the dead short so
> you might not have to do anything. Possible a defective breaker, most
> likely the one tripping to fast. A possible problem of a poor ground
> path to the first breaker.
>
> Mike M

Can you explain what part a ground plays in a simple 2 terminal
breaker?

--
John G

"Jack" <[email protected]> wrote
>
> BTW, I force a short by plugging in a shorted power cord.
>

Only thing with that is that it melts the plug contacts a bit. Plug the
cord in then short the ends together, or my favorite is a u shaped piece of
wire against the sides of the plug's hookup screws. Even better, they make
plug testers that you plug in and push a button. Not too expensive, and
tells you if you have all of the wires hooked up correctly all the way back
to the breaker box.

Jim in NC

On Thu, 30 Aug 2012 21:04:39 -0700 (PDT), Ivan Vegvary
<[email protected]> wrote:

>Finished wiring the wood-shop area. All in conduit, 12 quadplex wall outlets, and four fluorescent lights on three way switches. Also a 220V feed to the air compressor. New sub-panel in wood shop is fed by an other sub-panel 3 feet away on the other side of a wall. Feeder panel has a 50amp breaker feeding the wood shop sub panel. All wiring within conduit is #12 except the 220 which is #10.
>
>Question: As I installed each of the quad (gang of four) outlets, I would go to the breaker, turn it on and check the outlet with a clamp lamp and then turn off the breaker for further work. When I was installing the outlet group # 11, the wires shifted withing the conduit and the hot wire in # 12 touched the metal box. (Yes, I didn't cap the wires while I was working, I simply bent them out of each other's way. Stupid) Anyway, the question is "why did the 50 amp breaker feeding my sup panel break instead of the 20 amp breaker withing the sub panel"?
>
>Later, after wrestling my large air compressor into it's closet the same thing happened. I dislodged the safety shield on the belt, the fan blade could not turn and ergo the motor drew too many amps on start-up. Again, instead of the 30 amp circuit feeding the compressor breaking, it's the 50 amp feeding the panel that broke.
>
>Note: 50 amp breaker is used. 30 amp breaker is used. 20 amp breaker is new.
>
>Is the 50 amp breaker simply too sensitive?
>
>I could live with the above, except when the 50 amp breaker trips I lose everything and am left in the dark. Have to get a flashlight, go outside one building and into another to access the other side of the wall.
>
>All comments and advise appreciated.
>
You don't say whether all the breakers are the same manufacturer.
Actually had that happen in a computer center where a 125 amp breaker
would trip before the 30 amp parking area breaker. Of course it
always led to a 2AM call out so we found that problem quickly. Anyway
you have two variables which is the short circuit trip time and the
overload trip time. Possible this was the result of the dead short so
you might not have to do anything. Possible a defective breaker, most
likely the one tripping to fast. A possible problem of a poor ground
path to the first breaker.

Mike M

dpb <[email protected]> writes:
>On 8/31/2012 1:16 PM, Scott Lurndal wrote:
>...
>
>> As for the OP, if the 50 is upstream of the 20, and the 50 blows
>> first on a dead short, the 20 should be replaced ASAP.
>...
>
>Excepting (unless he mistyped) the 20A is supposedly a brand new breaker
>while the 50A is used. I'm suspecting the 50A (or something else isn't
>kosher that's totally impossible to diagnose from here).

Assuming everything was wired correctly, and the 20 isn't a delayed trip device,
perhaps the 50 trips more quickly. But I agree with the "totally impossible to diagnose from here".

scott