Large sparking from testing radiator fan directly on 12V battery?

[MadeFromMetal]

I had a radiator blow a week ago and went to test a electric radiator fan on the battery:
It was jammed with a piece of the fan shroud. Removed the plastic and it worked with a D battery. But needed to test with my Jeeps 12V battery, to see if it went full speed.

I used a 10AWG OFC primary wire to touch + wire to battery, the negative fan wire is bolted to frame bolt. As soon as I touched it, sparks flew and the fan jerked out of my hand like it had high current surge.

Why does this happen when touching a wire to battery post but not when it's properly wired before turning on switch?
How dangerous is testing the fan this way and what way do you recommend testing it?
My power circuit probe can only do 7A before it shuts off (safety feature).

 

[Nigel Goodwin]

You don't test a car radiator fan in your hand - you're lucky you didn't lose any fingers!. Bolt it in place, and then try it.

 

[picbits]

I measured a car radiator fan pull around 40A on startup, thats a big surge of current and yes, you will get sparks and a fair bit of inertia from the fan / blades

 

[Nigel Goodwin]

I measured a car radiator fan pull around 40A on startup, thats a big surge of current and yes, you will get sparks and a fair bit of inertia from the fan / blades

Still, there's no need to be able to count to ten on your fingers?

 

[crutschow]

As soon as I touched it, sparks flew and the fan jerked out of my hand like it had high current surge.

What makes you think your hand is a proper mount for a high-powered fan?
And yes, there is usually a high current surge when voltage is suddenly applied to a stalled motor.

 

[bob4bvm]

Old thread but i found your replies so amusing, had to comment

When i was a kid i did the same thing with a starter motor from a V-8
Needless to say the bendix slammed forward with a bang and the 20# starter leapt out of my hands, leaving a nice laceration
I could write a book of my history of such escapades, the final chapter being a treatise on why i am still here
Too many episodes to be mere coincidence !
Cheers
Bob

 

[ZipZapOuch]

Old thread but i found your replies so amusing, had to comment

When i was a kid i did the same thing with a starter motor from a V-8
Needless to say the bendix slammed forward with a bang and the 20# starter leapt out of my hands, leaving a nice laceration
I could write a book of my history of such escapades, the final chapter being a treatise on why i am still here
Too many episodes to be mere coincidence !
Cheers
Bob

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