Re: Fuses

Posted by Texasmark1 on May 07, 2015 at 06:29:42 from (172.242.8.12):

In Reply to: Fuses posted by HenryO on May 06, 2015 at 13:33:11:

Don't know why I'm pitching my hat in this ring buttttttt.

Protecting the wire IS the task (as stated) and is a confusing task as numerous things come into account: Load current, environmental (ambient) temperature where the wire is routed, forced or natural convection movement of the air adjacent to the insulation, wire cross sectional area, wire material resistance, insulation temperature rating, wire bundling or lack thereof, raceway confinement or lack thereof........

I think I'd go with the "replace with the one that you had".....specified value for that tractor. If it blows then worry about it. Course just because it is an 8v lamp doesn't necessarily mean it will draw a proportionally larger current than a 6V. Depends on wattage consumption when at full luminance for a given applied voltage. If the 6v was a 35 W rated bulb at 6v and the 8v is a 30w rated bulb, you haven't increased your current much (17%) so questioning your fuse size is not an issue.

Fuse applications are usually designed with a safety margin to allow for impulse currents, like motor starting, "lamp ratings" where the resistance of the wire cold is like 1/10 what it is when the filament is glowing. Course you have different types of fuses also, like ultra fast blow for sensitive electronic circuits, normal blow which means a couple of things, slow blow, and breakers with predetermined load/time ratings.

Other thing on a fuse is the overload. Speed and value of the overload work hand and hand. A short high current spike could get by without blowing the fuse if the temperature it generates fits the envelope of the fusing material. A slight overload over a long period of time can cause it to blow if it exceeds that envelope.

On an SAE and AWG type, to name a couple, normal blow, glass envelope fuse, it's easy to see what kind of overload you experienced: If the wire is melted, like solder looks as it is changing from the solid to molten state, you had a long term slight overload. If there is a big gap between the pieces of wire and splattering on the glass, you had a high, short duration overload, like something shorted directly to ground (power return).

Littlefuse is a major mfgr. of fusing devices and their www has a lot of helpful info.

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