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#129871 - 10/05/05 09:19 PM Using OCPD at low voltages
PEdoubleNIZZLE Offline
Member

Registered: 06/16/04
Posts: 176
Loc: McKeesport, PA, USA
This may sound like a stupid question, but if you use a fuse or breaker that was rated for 250VAC on, say a 12.5VDC power supply, would the OCPD still be effective? For example, let's say you have a fuse with a resistance of 0.1Ω. That means at 250V 20A, there is 20 times as many watts than at 12.5V 20A, and therefore less heat to blow the element. Does it matter, or is it purely based on amperage? I see regular with the 250V rating protecting the secondary side of a class 2 transformer (not the wall wart type, usually the ones for higher end use... kinda like a laptop pwer supply.) is the 250V just a MAX rating, or a WORKING VOLTAGE rating?
thanks,
Josh.

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#129872 - 10/06/05 04:40 AM Re: Using OCPD at low voltages
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Fuses blow based on the current flowing through them.

The heat produced in a resistive element is I^2R...the higher the current, the greater the heating.

Normally the load will limit the current flow, and the heating in the fuse is essentially constant for the same current flow. A 1 A fuse in a 12V circuit with 750mA flowing through it (9W dissipated in the load) will have essentially the same heating as a 1A fuse in a 240V circuit with 750mA flowing through it (180W dissipated in the load). In the event of an overload (say 1.5A flowing), the fuses will experience the same heating.

In the higher voltage system, more power is being delivered to the load, both during normal and overload conditions. The fuse will reach its trip point in the same way at both voltages.

I would expect a fuse to _open_ differently with different supply voltages. As the fuse opens, more and more of the supply voltage is dropping across the fuse, and less is being dropped across the load. As this happens, the heating in the fuse increases, and the supply voltage will determine the ultimate heating that is possible before the current flow actually stops. I expect that the details of the different operating characteristics are quite complex: high voltage would mean that the metal melts faster, but could mean that arcs get sustained for longer periods.

In any case, when a fuse is rated at a given voltage, it may be used at lower voltage, all other things being equal.

Of greater concern is the difference between DC and AC; a fuse that works just fine with 240V AC supply might not be able to function safely with a 60V DC supply. This is because once the metal melts you must also deal with quenching any arc that forms.

-Jon

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#129873 - 10/06/05 07:54 AM Re: Using OCPD at low voltages
PEdoubleNIZZLE Offline
Member

Registered: 06/16/04
Posts: 176
Loc: McKeesport, PA, USA
Thanks Jon, You're always helping me out with my little hypothetical questions.

So does that mean (in reference to heat being I^2R) if you have 2 "toaster element space heaters", 1 rated for 12 Volts, and one rated for 80 Billion volts, and both draw 5 amps at their respective voltages, they produce the same heat? This kinda confuses me as heaters are rated in Watts (consumed). Does the watt rating just make it look better since the number is higher?

Thanks,
Josh

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#129874 - 10/06/05 08:29 AM Re: Using OCPD at low voltages
Dave T Offline
Member

Registered: 01/22/05
Posts: 157
Loc: Waukesha, WI, USA
Since the common thermal magnetic breaker has a bimetallic element in it to respond to overcurrent it doesn't care if it's AC or DC. However, the contacts do so pay attention to the breakers DC rating if it has one. Because DC doesn't pass through a '0' volt point a AC does it's much harder to extinguish a DC arc as it is more dependent upon the distance the contacts open apart and how well the arc chutes deionize the arc.
With electronic trip breakers that's a whole other issue as the commonly have CTs as current sensors which are insensitive to DC current.

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#129875 - 10/06/05 08:42 AM Re: Using OCPD at low voltages
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
If you have two heating elements, one rated for 12V, and the other rated for 120V, _both_ rated for 5A, then the resistance _must_ be different.
power dissipated is P = I^2*R (square of current times resistance)
power dissipated is also P = I*E (voltage times current)
voltage across a resistor is E = I * R (voltage = current times resistance)

notice that you can substitute E = I * R into P = I * E to get P = I * I * R, or the first equation.

In your example, the 12V 5A resistive heating element must have a resistance of 2.4 ohms, and must dissipate 60 watts. The 8*10^10V 5A heating element must have a resistance of 1.6*10^10 ohms, and must dissipate 4*10^11W (400 nuclear plants worth of output *grin* )

The confusing bit here is that E the voltage in question is _not_ the total voltage of the system, but instead the voltage dropped across the element being considered. With the fuses mentioned above, the supply voltage is being dropped across the _load_; and very little voltage is being lost in the fuse itself.

If you have a 250V 10A rated fuse, you could connect that fuse in a 120V circuit with a 500W lamp. The resistance of the fuse might be 0.01 ohms, and the voltage drop across the fuse might be 0.04V, with a total power dissipation in the fuse of 0.17W. The voltage drop across the lamp is 119.96V, and nearly 500W is delivered. The resistance of the lamp would be about 29 ohms.

Take this same fuse and put in in a 12V circuit with a 50W lamp. The current flow is still 4.2A, and the voltage drop across the fuse is still 0.04V with a power of 0.17W. But now the voltage drop across the lamp is 11.96V, and the resistance of the lamp is 2.9 ohms.

-Jon

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#129876 - 10/08/05 08:58 PM Re: Using OCPD at low voltages
PEdoubleNIZZLE Offline
Member

Registered: 06/16/04
Posts: 176
Loc: McKeesport, PA, USA
Oops, yea, I kinda forgot about R... that's what I get for posting at an odd hour (i forget when I posted it, but anything after being up for 30+ hours is automatically an odd hour for me.

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#129877 - 10/10/05 08:54 PM Re: Using OCPD at low voltages
Larry Fine Offline
Member

Registered: 02/18/05
Posts: 684
Loc: Richmond, VA
Josh, you need to ignore the voltage caross the fuse, which doesn't exist until the fuse opens. No power is disipated across a fuse or breaker.

This is all speaking theoretically, of course.
_________________________
Larry Fine
Fine Electric Co.
fineelectricco.com

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#129878 - 10/11/05 03:35 PM Re: Using OCPD at low voltages
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Larry,

There _must_ be _some_ power dissipation in the fuse or breaker, at least any that have a thermal trip element. It is much less than the power dissipated in the load, and the voltage across the fuse or breaker will be slight unless there is something broken...but there will be _some_ voltage drop.

-Jon

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#129879 - 10/12/05 10:42 AM Re: Using OCPD at low voltages
Larry Fine Offline
Member

Registered: 02/18/05
Posts: 684
Loc: Richmond, VA
Okay, Jon, as long as we're talking theory, voltage drop across a resistance is independent of circuit voltage; only current matters.

E = I x R

Note that only resistance and current are on the right side of the equation.
_________________________
Larry Fine
Fine Electric Co.
fineelectricco.com

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#129880 - 10/12/05 12:48 PM Re: Using OCPD at low voltages
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
I believe that we are in complete agreement.

The power loss and voltage drop across a fuse is set by the resistance of the fuse and the current flowing through it. It does not directly depend upon the supply voltage.

The current flowing through the circuit depends upon the total circuit resistance/impedance and the supply voltage.

The total circuit resistance will be the resistance of the wiring plus the resistance of the fuse plus the resistance of the load.

In general, the resistance of the fuse will be so small as to be negligible; the current flow will be set by the load and the supply voltage. In general the resistance of the fuse is so slight that we can just ignore it.

Just for fun, a Bussmann 7.2kV 25A fuse has a resistance of 0.039 ohms. Take this fuse and misapply it in a 12V application, and the results would be _safe_, but you couldn't really claim that you could ignore the resistance of the fuse *grin* Though this does go back to the original question of applying higher voltage OCPD to lower voltage circuits.

-Jon

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