Who determines trip curves?

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electrofelon

electrofelon

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Another thread got me thinking about this. So clearly we have fast acting vs time delay fuses, thermal magnetic breakers, hi mag breakers, adjustable trip settings........So who or what determines what settings are acceptable to avoid conductor damage? Article 240 just seems to say "conductors shall be protected against overcurrent" with no further details. Also, OCPd's are not required to be listed, so theoretically they dont have to meet any standard. So is there anything preventing me from making a circuit breaker that has an excessively long time delay? What am I missing?
 
220110-1518 EST

electrofelon:

A circuit breaker is a protective device. Thus, it has to be selected to protect whatever it is supposed to protect.

Now you have to define what is the protection curve to look like. Further that protection curve has to be designed for whatever part of the load that requires the greatest protection. This might mean that you simply need a protection curve that protects the wiring.

When it comes to transformers with short 1/2 cycle very high inrush currents, then you may need to design for this inrush. But that design may result in a breaker much larger than is required for the steady state load. In this case you may want to add protection on the load side as well to reduce wire size and better protect the actual load.

.
 
gar said:
220110-1518 EST

electrofelon:

A circuit breaker is a protective device. Thus, it has to be selected to protect whatever it is supposed to protect.

Now you have to define what is the protection curve to look like. Further that protection curve has to be designed for whatever part of the load that requires the greatest protection. This might mean that you simply need a protection curve that protects the wiring.

When it comes to transformers with short 1/2 cycle very high inrush currents, then you may need to design for this inrush. But that design may result in a breaker much larger than is required for the steady state load. In this case you may want to add protection on the load side as well to reduce wire size and better protect the actual load.

.
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Gar, I understand you would want different characteristics for different types of loads, but who gets to decide the trip characteristics? What is preventing me from making a circuit breaker that takes 6 seconds to trip at 100X rating?
 
Do different brand breakers have different trip curves? Or is there a standard they all have to meet?
 
US molded case breaker performance was originally addressed by product manufacturers in publications like NEMA AB-1. Performance is now governed by UL 489.

Molded case circuit breakers and fused switches are tested using a length of insulated wire, chosen per the NEC. For a test to be successful this wire cannot be damaged nor pull loose from it terminations. Therefore a 600V breaker listed to UL489 will always protect a conductor applied per the NEC.

The US standards do not require universal or common trip curves for molded case breakers, but instead provide maximum time current delay values.
 
jim dungar said:
US molded case breaker performance was originally addressed by product manufacturers in publications like NEMA AB-1. Performance is now governed by UL 489.

Molded case circuit breakers and fused switches are tested using a length of insulated wire, chosen per the NEC. For a test to be successful this wire cannot be damaged nor pull loose from it terminations. Therefore a 600V breaker listed to UL489 will always protect a conductor applied per the NEC.

The US standards do not require universal or common trip curves for molded case breakers, but instead provide maximum time current delay values.
Click to expand...
Good info Jim, thanks. But.......it seems odd there is no specific requirement for listing of circuit breakers. So there is theoretically nothing (other than 110.2 and 110.3) preventing some Chinese fly by night breaker manufacturers to sell breakers with wacky inadequate trip curves?
 
electrofelon said:
Good info Jim, thanks. But.......it seems odd there is no specific requirement for listing of circuit breakers. So there is theoretically nothing (other than 110.2 and 110.3) preventing some Chinese fly by night breaker manufacturers to sell breakers with wacky inadequate trip curves?
Click to expand...
Which may be why the NEC ampacity values are so conservative.
Dont forget, fuse performance is also not addressed in the NEC.
 
There are known "thermal damage curves" for conductors, which is the basis for the NEC ampacity charts. Those same curves are used in system coordination studies and are now built in to engineering software like SKM, ETAP or Easy Power, used by PEs to determine the trip coordination by evaluating them against the trip curves of the breakers. But on smaller stuff, the trip curves for MCCBs or fuses are so far away (conservative) from the conductor thermal damage curves that we generally don't need to worry about it.
 
jim dungar said:
US molded case breaker performance was originally addressed by product manufacturers in publications like NEMA AB-1. Performance is now governed by UL 489.

Molded case circuit breakers and fused switches are tested using a length of insulated wire, chosen per the NEC. For a test to be successful this wire cannot be damaged nor pull loose from it terminations. Therefore a 600V breaker listed to UL489 will always protect a conductor applied per the NEC.

The US standards do not require universal or common trip curves for molded case breakers, but instead provide maximum time current delay values.
Click to expand...
I'm curious of details, especially after finding out how afci's are 'tested' and 'passed' here Jim

~RJ~
 
electrofelon said:
Another thread got me thinking about this. So clearly we have fast acting vs time delay fuses, thermal magnetic breakers, hi mag breakers, adjustable trip settings........So who or what determines what settings are acceptable to avoid conductor damage? Article 240 just seems to say "conductors shall be protected against overcurrent" with no further details. Also, OCPd's are not required to be listed, so theoretically they dont have to meet any standard. So is there anything preventing me from making a circuit breaker that has an excessively long time delay? What am I missing?
Click to expand...
I realize your asking more about MCB protection,
but I also know you do some MV.
You may find this interesting.

We are going through our system now with slower curves on devices, different curves at the subs, and higher pickups. Trying to eliminate so many blinks on every customer like a fuse saving scheme creates.
 
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