Power limited circuit vs non limited

[mikefamig]

Can I get some help to clear up the difference in a power-limited circuit and a non power-limited circuit? I understand that a transformer is limited by the fact that it will only supply a limited amount of power by it's design. But where do you draw the line at unlimited? I understand that limited and unlimited wires need to be separated but what is what? 120 vac I understand to be unlimited but isn't it limited by a circuit breaker?
 
I have this definition from the National Fire Academy but am not clear on what it means:
https://www.usfa.fema.gov/downloads/pdf/coffee-break/cb-2007-12.pdf
 
Power-Limited Fire Alarm (PLFA) Circuits.
A power-limited fire alarm
circuit is one that is inherently unable to exceed maximum voltages, or
is equipped with a power-limiting source (transformer or battery) and a
circuit breaker. Generally, these operate in the 24-volt direct current range,
although they may employ higher voltages.
 
Nonpower-Limited Fire Alarm (NPLFA) Circuits.
Nonpower-limited fire
alarm circuits cannot operate at more than 600 volts, and there is no other
power or current limitation for these systems. Usually these are used in 120-
volt alternating current (AC) systems which have been rendered generally
obsolete by the advent of low-voltage, direct current (DC) systems.
 
Any help?
 
 

 

[RAL]

You're right that circuit breakers on your 120VAC circuits do limit the power, but that is different from what is meant in the low voltage environment.
 
The NEC breaks things down into Class 1, 2 and 3 circuits, with different rules for each.  For LV, what you are asking about is Class 2 power limited circuits, which, according to the NEC, means voltages of less than 30 Volts and power of less than 100 VA.   Any circuit that is above 30V or 100 VA is considered not to be power limited, even if there is a protection device, like a fuse or circuit breaker. There is one exception to this, which is that a Class 2 circuit can be from 30V to 150V and less than 0.5VA and still be considered power limited.
 
Here is a good overview.
 
I learned some interesting things in talking with the folks at Altronix when I asked them about the specs on different power supply models.
 
If you look at an AL400ULPD8 power supply, which is 12V or 24V at 4 Amps, it is considered power limited because the power supply can only deliver 24V x 4A or 96VA, which is under the limit of 100VA.
 
The same holds true for an AL400ULPD8CB, with PTCs rather than fuses.
 
For the AL600, it is a little different. 
 
The power supply can deliver 24V x 6A, or 144VA.   That would seem to put it over the 100 VA limit.  Yet the AL600ULPD8CB (with PTCs) is classified as power limited, but the AL600ULPD8 with fuses is not.
 
Each fused output on the distribution board is protected by a 3.5A fuse, which would limit the power to 84VA.  Each PTC is rated at 2.5A, limiting the PTC outputs to 60VA.   Both are under the 100VA limit.  So why the difference in classification?  According to Altronix, it is because, in the field, someone could easily replace the 3.5A fuse with a larger fuse, allowing the 100VA limit to be exceeded.  But they couldn't easily replace the PTCs. 
 
For the AL400 models, it doesn't matter if someone substitutes a larger fuse, because the power supply itself can't deliver more than the 100VA limit.
 
If you were to run the power supplies set for 12V outputs, both the AL400 and AL600 would be under 100VA.  But that doesn't change the listed ratings.

 

[mikefamig]

Thanks for clearing that up, I don't feel so bad now about my confusion.
 
Mike.

 

[Del91574]

In the case of the NEC, a lot is a holdover from the old line voltage fire alarm days....they were mandated long before there were IC's, transistors and smart systems. Actually, the biggest uptick in FACP's was during WW2 (same with alarms in general) as the lack of manpower, per se, was what drove the installation of many of these systems.

 

[LarrylLix]

Circuit breakers and fuses do NOT limit current or power.
 
Circuit breakers limit the length of time a circuit will pass more current or than the breaker rating. This limits the energy that can be delivered into an overload or fault by time limitation.
 
Most of your circuit breakers in your panel are rated a 10,000 amperes maximum current interrupting capacity. This tells us that they cannot limit the current passing through them.

 

[mikefamig]

I found this in the library section that is helpful but it does not mention circuit breakers specifically.
 
http://cocoontech.com/forums/files/file/169-power-limited-circuits-faq/
 
Mike.