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#67202 - 06/29/06 01:45 PM What makes a breaker trip?
Trick440 Offline
Registered: 08/15/05
Posts: 247
Loc: Waterford, MI, USA
I was thinking it was heat.

But on breakers I'm reading that the current interrupting rating is max. rms 10k amps.

[This message has been edited by Trick440 (edited 06-29-2006).]
Shake n Bake
Work Gear for Electricians and the Trades
#67203 - 06/29/06 02:21 PM Re: What makes a breaker trip?
Ron Offline
Registered: 03/13/02
Posts: 582
Loc: White Plains, NY
Most molded case breakers have a thermal sensor and a magnetic sensor. Thermal is for an overload and magnetic is for short circuits.
The 10kAIC that you might see on some breakers, is the maximum amount of short circuit current that it can still reliably open. >10kA may cause catastrophic failure and explode.
#67204 - 06/29/06 02:50 PM Re: What makes a breaker trip?
mahlere Offline
Registered: 11/17/05
Posts: 507
Loc: New Jersey
A squirrel across 2 phases will make a circuit breaker trip
#67205 - 06/29/06 05:24 PM Re: What makes a breaker trip?
e57 Offline
Registered: 05/27/03
Posts: 2876
Loc: S.F.,CA USA
I suppose domestic animales like cats dogs and fish would too...
Mark Heller
"Well - I oughta....." -Jackie Gleason
#67206 - 06/29/06 05:46 PM Re: What makes a breaker trip?
electure Offline

Registered: 12/24/00
Posts: 4259
Loc: Fullerton, CA USA
What makes a FPE Stablok trip?

#67207 - 06/29/06 06:08 PM Re: What makes a breaker trip?
LK Offline
Registered: 01/21/03
Posts: 1429
Loc: New Jersey
Quote: "What makes a FPE Stablok trip?"

The intense heat, as the building burns to the ground.
#67208 - 06/29/06 09:26 PM Re: What makes a breaker trip?
Scott35 Offline

Broom Pusher and
Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA


But on breakers I'm reading that the current interrupting rating is max. rms 10k amps.

To add with the other replies, the "10KAIC" rating of the Breaker you are referring to, means that it may be used on Systems, which the maximum Fault Current Level does not exceed 10,000 Amps (RMS, not Peak).

More to the point; it may be used in Equipment rated no more than 10KAIC, and at a point on the system, which has an available SCA (Short Circuit Amperes) up to a maximum of 10KAIC.

There are 3 commonly used calculation methods available, to figure the SCA at various points along a System. These are:
  • Ohmic Method
  • Per-Unit Method
  • Point-To-Point Method

The "Point-To-Point" Method is most commonly used on systems up to 2.5 MVA, 0 - 600 VAC.

Breakers (and Fuses) without any Fault Ratings listed on them, are to be figured as having a maximum of 5KAIC.

Common AIC limits (ratings) for Molded Case Circuit Breakers and Fuses, used on Low Voltage Power Systems (0-600VAC) are:

* 10KAIC
* 18KAIC
* 22KAIC
* 30KAIC
* 42KAIC
* 65KAIC
* 100KAIC
* 200KAIC

As mentioned, the rated AIC of the device + equipment it is installed in, is the maximum the device may withstand before failure.
Failure may be in the form of:

* Inability to open under fault conditions (contacts of Breaker welded closed, or arc sustained between terminals of the element inside a Fuse),

* Explosive rupture of the device,

* Device catches fire,

* All three of the above.

OCPDs are very interesting, and the design characteristics used when planning a Building's Power System are just as interesting.
Proper Electrical Systems Design Engineering uses an approach of several factors for the OCPDs.

Of course, the maximum SCA is one of the most important design issues, and there are 2 ways to design for a System with a high SCA available.

One method is a Fully Rated System - which uses Equipment rated at the highest level possible at various points.
This is a more "forgiving" approach, and makes System Coordination easier;

The other method is a Series Rated System - which is designed to have an OCPD with a "Lower AIC Rating" to be protected by an "Upstream" OCPD with a higher AIC Rating.

This design approach is a lower priced option, but has some draw backs as compared to a Fully Rated System.

First drawback is the possibility of someone changing out or installing OCPDs with an insufficient capacity on the system.

Second drawback is if the Power Transformer gets changed out in the future, and the new one has a higher Capacity, &/or a lower % Impedance.
This changes the available SCA on the system, and now the Series Rated equipment may fail during a fault condition.

Third drawback is the difficulty in designing a "Selective System" (explained later).

Other drawbacks include:
* All equipment must be classified as compliant for Series rating,
* All equipment used on the system in a series fashion must be from the same manufacturer + be listed as compatible together in a series rated setup,
* Series rating may not be done where Motor Circuits are on the Load Side of Series Rated equipment,
* Non-Selective Coordination may affect the System's overall performance and fault trip abilities.

The next important system design approach deals with "System Selectivity".
This refers to when an overcurrent condition trips one, two, or all of the OCPDs in the path of the Current.

A "Non-Selective" System would be where a Panelboard, fed by a 100 Amp Breaker, has a fault on a 20 Amp 1 Pole Branch Circuit; and instead of only the 20/1 tripping, the 100 Amp breaker trips - and the 20/1 stays closed.

This obviously sucks !

So to avoid the annoyance of Non-Selectivity, the "Target Design Ideal" is to coordinate the OCPDs into a "Selective System".

This design approach normally eliminates the problems of Non-Selective coordination, by applying "Time/Current Trip Curves" against each OCPD's type and location.

One last design factor of consideration for OCPDs (Over Current protection Devices), is the use of Instantanious Trip devices, Time-Delay devices, Current-Limiting devices, Inverse Time-Trip devices, and "Fast Acting" devices.

All of these characteristics above are used to evaluate and properly design a safe + trouble-free Power System.

Feel free to pop in with other questions!

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#67209 - 07/08/06 02:35 AM Re: What makes a breaker trip?
Scott35 Offline

Broom Pusher and
Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
*** BUMP ***

Bumping up thread for more discussions.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#67210 - 07/15/06 02:17 AM Re: What makes a breaker trip?
Scott35 Offline

Broom Pusher and
Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
Posted for ECN Member "KENBO"

***Inside a Molded Case Circuit Breaker***

Submitted By "KENBO"

Member's Text Below:


Here is a photo of a molded breaker I have opened up for my students. It is for domestic use in the UK
Over current flows through the coil, lifts the center pin to break the contacts, arc gets drawn in over the finns and stretched to breaking point.

Hope this helps make some of the explanations clearer.

I do not have any picts of oil filled breakers (yet)


Moderator's Note:
Thanks, KENBO, for the contribution!


posted 07.15.2006 @ 01:23:00
local = x:\elect_eng02\ECNusers\upload\MCCB_INSIDE01.jpg
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#67211 - 07/15/06 04:08 AM Re: What makes a breaker trip?
Trumpy Offline

Registered: 07/05/02
Posts: 8211
Loc: SI,New Zealand
CB's used in Domestic Installations outside of the US (ie 230/240V) have the following rupturing capacity:
  • 3kA
  • 6kA
  • 10kA
  • New one 16kA.

All are single pole.
Let's face it, these days if you're not young, you're old - Red Green grin
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