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Hello,

Great discussion on fuses. Mark was correct in describing how a fuse
works but failed to answer the question "Why are fuses not rated in
units of power".

Fuses have resistance. The resistance of a fuse usually increases as the
nominal ampere rating decreases. You need to be aware that the voltage
drop across the fuse can be appreciable, especially for fuses with a
low-ampere rating.

Ampere rating. Select the ampere rating for the fuse so that the fuse
can carry the normal operating-current conditions of the application.
The recommended operating current for the fuse is 75% of the nominal
rating. The derating of 25% compensates for the differences between the
documented UL-rating test procedure and the typical application.

Voltage rating. A fuse's voltage rating is a safety issue. This rating
must equal or exceed the application circuit's operating voltage.
Operating a fuse above its voltage rating risks a potential explosion of
the fuse or a fire. Fuses are current-sensitive, and you can
successfully operate a fuse at any voltage below its rated voltage.

Temperature derating. Temperature derating is necessary so that the fuse
can carry the normal operating current of the application. Fuses are
thermal-responding devices, and derating is necessary for ambient
temperatures above 25???C. UL-rating tests are performed at 25???C, and this
derating is in addition to the 25% ampere derating.

Can anyone answer my original question - Why are fuses not rated in
units of power?

Thanks,
Phil

Can anyone answer my original question - Why are fuses not rated in
units of power?

As fuses are used as overcurrent protection devices, the relevant
factor here is the CURRENT that the fuse will carry before melting.
Power is somewhat irrelevant here, as it would be an indirect way to
tell you the important parameter, the current capacity.

I cannot recall anyone designing a circuit saying "then, I want the
fuse to blow while it dissipates 10W or above", but " I want the the
fuse to blow while 10A or above go through this path".

Also, power alone is useless, you need also the resistance of the
fuse to calculate the current capacity, or the voltage drop across
the fuse.


Guillermo.

It is correct that fuses are in units of Current.
Power(watts) is in units of (AMPS)Current and volts. POWER=VOLTS X CURRENT
FUSES PROTECT CIRCUITS (IF SIZED CORRECTLY) WHEN THE Amps is sufficient to "blow" the fuses.

If fuses were sized in Power (watts) they would have to be rated for a certain current, AND voltage and there are many combinations that one could come up with for a certain power rateing.

ie. (power =60watts} could be a current of 1A at 60 Volts, or 60amps at 1 Volt, or 20Volts at 3 Amps. etc so it would not make sense to rate them using power (watts)

From: "Guillermo Brajovic" <guillermo.brajovic@...>
Reply-To: Electronics_101@...
To: Electronics_101@...
Subject: [Electronics_101] Re: Fuses
Date: Sat, 27 Oct 2001 22:35:23 -0000

Can anyone answer my original question - Why are fuses not rated in
units of power?

As fuses are used as overcurrent protection devices, the relevant
factor here is the CURRENT that the fuse will carry before melting.
Power is somewhat irrelevant here, as it would be an indirect way to
tell you the important parameter, the current capacity.

I cannot recall anyone designing a circuit saying "then, I want the
fuse to blow while it dissipates 10W or above", but " I want the the
fuse to blow while 10A or above go through this path".

Also, power alone is useless, you need also the resistance of the
fuse to calculate the current capacity, or the voltage drop across
the fuse.


Guillermo.

_________________________________________________________________
Get your FREE download of MSN Explorer at

Remember, power is dissipated ACROSS an element... the power that is
delivered to other circuits... the actual load power... is not really
related (well, it is indirectly) to the power dissipated by the fuse.

________________________________________________
Kevin Vannorsdel IBM Arm Electronics Development
408-256-6492 Tie 276-6492 kv@... KF6YCI

Please respond to Electronics_101@...
To: Electronics_101@...
cc:
Subject: [Electronics_101] Re: Fuses

Can anyone answer my original question - Why are fuses not rated in
units of power?

As fuses are used as overcurrent protection devices, the relevant
factor here is the CURRENT that the fuse will carry before melting.
Power is somewhat irrelevant here, as it would be an indirect way to
tell you the important parameter, the current capacity.

I cannot recall anyone designing a circuit saying "then, I want the
fuse to blow while it dissipates 10W or above", but " I want the the
fuse to blow while 10A or above go through this path".

Also, power alone is useless, you need also the resistance of the
fuse to calculate the current capacity, or the voltage drop across
the fuse.


Guillermo.



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