Hello from San Diego:
Fred Waechter asks several questions about fuse selection.
2. For fuse selection in an environment threatened by lightning, is
IxIxT rating or interrupt rating more important? I assume IxIxT for
normal operation and interrupt rating for surge operation.
I would think that the most important parameter would not be either the
IxIxT rating or the interrupt rating but the voltage rating.
For fusing to be successful, the fuse must be able to quench the arc
produced after the link has melted. Upon melting, an arc develops
between the two ends. The heat of the arc continues to melt back the
ends of the link until the distance is too long to support the arc. The
voltage rating of the fuse relates to the maximum spacing between the
melted ends of the link.
If you exceed the fuse voltage rating, you can expect the fuse to
explode.
The interrupt rating relates to the ability of the fuse to dissipate the
power in the arc, ExI, at maximum rated voltage, without exploding.
With lightning, I don't know whether the voltage can be predicted, but I
would expect it to be in the thousands of volts. I would think that you
probably can't effectively use a fuse rated for thousands of volts -- it
would be too big. Therefore, I doubt that it is practical to consider
using a fuse to provide protection against lightning.
Having said this, if lightning strikes, the fuse will open. It likely
will be completely destroyed. However, you cannot rely on it to protect
circuits on the non-lightning side of the fuse.
5. Question - How can I be sure for European fuse application, if I
apply 3000A current surge, that fuse will not disintegrate, as fuse
standard calls out 1500A? I accept different manufacturers will
probably have different ratings which will exceed 1500A spec., by
different amounts.
If you connect the fuse to a circuit which exceeds the interrupt rating,
you can expect the fuse to explode due to the power dissipated in the
arc.
However, if you are using a standard power cord and plug on a standard
outlet (e.g., 16 amp Schuko plug), then the resistance of the plug
contacts and the resistance of the power cord will limit the current
to very much less than 3000 amps or even 1500 amps.
For example, to get 3000 amps from a 230 volt circuit, the circuit
impedance must be:
E 230
R = ------ = -------- = 0.0766 ohms
I 3000
No. 18 AWG is 6.636 ohms per 1000 feet. Assuming an 8-foot power
cord, the resistance of one wire is 0.0640 ohms. The circuit,
through the cord on one wire and back on the other wire is 0.1280
ohms. The maximum current through the wire alone is:
E 230
I = ------ = --------- = 1,797 amps
R 0.1280
This is just the power cord alone. It doesn't include the contact
resistance of the plug, and it doesn't include the wiring from the
source to the outlet.
So, it is unlikely you can get even 1500 amps into a cord-connected
product from the conventional 230-volt supply. So, a 1500-amp fuse
is okay.
Best regards,
Rich
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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