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July 6, 2009
Time Delay

I hope that everyone had a safe and sane (and very relaxing) Independence Day weekend.
Saturday night, I found myself entertaining several small children by lighting fireworks on the beach for them.  After lighting the rocket or "fountain of color", I quickly ran back the crowd to "ooh and aah".  Upon sprinting back in the sand, one of the small kids told me that the fireworks needed more time-delay in the fuse so I didn’t have to run so hard.
This got me thinking about the topic of the day...time delay in fuses.
As I’ve mentioned several times before, there are generally two types of overcurrents - Short Circuits and Overloads.  Short circuits consist of very high currents that can cause harm to both people and equipment.  Overloads typically occur when too many devices are added and the normal rating of the circuit is exceeded.
There are times when temporary "overloads" simply cannot be avoided.  This happens when a circuit is first energized or when a motor or fan first starts up.  These "overload" conditions generally only last 1 - 5 seconds with the overload subsiding once the motor has reached it’s operating speed or the circuit has been fully energized.
These temporary "overloads" are sometimes called transient currents and are normally 2-6 times the normal current of the circuit.  These overcurrents are very normal, but can cause problems if the overcurrent protection isn’t properly selected.
In circuits where transient currents exist, a time-delay fuse should be used (sometimes a time-delay fuse is also called a Slo-Blo® fuse, which is a registered trademark of Littelfuse).  The time-delay fuse allows the overload to exist for a few extra seconds before opening and thus avoiding a nuisance opening from the transient current condition.
By the way, the terms "time-delay" and "fast acting" are for overloads and/or transient currents only.  BOTH types fuses are extremely fast acting when it comes to opening short circuits   
I hope this clears up a bit of the time-delay mystery...
Thank you so much for your continued support of OptiFuse where we hope to be fast-acting to solving your circuit protection problems.

Jim Kalb

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