I'm not the most experienced person here, but I just went thru the same issue with an outboard. My guess is that you have a dead short in your wiring harness. Check exposed wiring first, particularly spots where the wiring is subject to chafing. You might find a 12V positive wire exposed and making contact with the chassis.
Lewis

Have you recently jump started the battery and maybe connected the two batteries with crossed polarity between the 2 batteries (like.. negative to positive instead of negative to negative)?
Or, have you had the battery out of the machine and possibly installed it with reversed polarity?
If you have done either of those actions, you may have ruined the charging diode or the voltage regulator.
If you have committed neither of those sins, it is likely that your key switch has shorted internally and is shunting the battery input circuit (feeds the key switch) directly to ground inside the switch.
Disconnect the key switch harness from the key switch and see if the fuse still blows.
If it still blows then, physically trace the battery feed wire that carries current to the key switch to find a possible short where the wire has chafed against a metal surface.

Thanks for the input I will check the key switch. I have not committed the two sins, what happened I was mowing and I stopped cut the PTO off put on the hand brake got off the mower for a couple off minutes and then I heard like the mower was increasing rpm's then mower died. Seem like the mower got really hot. When I got back on and tried to start nothing so I checked the fuse, it was blown replaced it and it blew has soon as I put it in with key switch off.
I will check the switch and report back.

Agree with mownie about a possibly bad key switch-

I do have one alternative scenario however-
I'm kind of shooting in the dark here-

IF this tractors lights only operate when the engine is running, possibly the alternator has shorted?

I don't have a schematic, but "SOME" tractors with this type charging system have the DC output of the alternator wired directly to the B terminal of the key switch.
Since the B terminal is directly wired to the battery + (fused wire), it basically a dead short.

Disconnect wiring plug to the engine to test that.

BTW- What's a "cub cadet rat 50"?

This post was edited by bill_kapaun on Sat, Sep 28, 13 at 11:49

It is a typo I meant to put cub cadet rzt 50. I unplugged the key switch and checked I am still getting sparks when I tried to plug the fuse. Then I unplugged the PTO switch same thing, then I unplugged the electric clutch same thing, with all three switch's unplugged if try and put in a fuse it start sparking. I have looked for burnt wires or bare wires I just don't see any. Thanks for the input. Greatly appreciated.

Found a schematic at the link below.
Last page.

With the plug to the key switch disconnected, there should be no connection to anything from the fused wire.
Therefore-- there has to be a short in the wire between the fuse and the key switch plug.

(easy to say from my key board)

Here is a link that might be useful: Manual

There is a short somewhere between the fuse holder and the ignition switch. Did Bill say that already? :^)
The wire is Red with a black tracer.
Be suspicious of any place where it looks like a harness might be squeezed by, rubbing, or bent sharply around any metal surface.
If you see any of the mentioned conditions, you may have to physically cut into the harness over wrappings to inspect the wiring inside.
Be really careful if you decide to start opening a harness, you don't want to create more problems by hacking ruthlessly.
When I have to open a harness I use a #11 X-acto knife and slice with the cutting edge facing AWAY from the harness bundle so I am only able to cut the tape and not the wires underneath. Then I peel the tape away. Many harnesses these days have plastic split loom covering the wires and if cap unravel the tape you can remove the loom to inspect everything inside.
Sometimes shorts can develop between a hot wire and a ground wire inside the harness if the harness gets "mashed' really hard or crushed tightly for a long time.
Searching for shorts can be a daunting task sometimes.

I had a similar problem on my Cub (2166).
Turned-out to be a short in the wires going to the headlight (they're mounted to the hood, so got mangled in the mechanism).
Because the switch passes the "headlight on" position before engaging the starting mechanism, the fuse would blow whenever I attempted to start the tractor.

Hope your fix is as easy.

Thanks for all the input have not had time to look at in the past few days. I will post what ever outcome. Thanks again.

Dwinters

***"Because the switch passes the "headlight on" position before engaging the starting mechanism, the fuse would blow whenever I attempted to start the tractor."***
I'm not being critical about sharing various problems that members might have had, but the one described herein would not be the case in dwinters55 issue.
dwinters55 states that his fuse blows as soon as a fuse is plugged into the fuse holder........not associated with any movement of the switch.
But I do commend gewf631 for exposing the issue he had because the unique conditions described by him could certainly be a head scratcher/mind strainer if one was not aware of the link between the headlight circuit and the starting switch layout. Kudos :^)