Melted bilge pump

[relamb]

Before my fuel system issues (see separate thread) when I headed to the boat this week there was water in the bilge and the pump was not working.
I used a manual pump to pump it down, and discovered my bilge pump was a total meltdown.   The top motor housing was melted to a blob, with a hole blown in the side like it had exploded.  Fortunately the fuse had blown before anything caught on fire.
Any idea of the cause?  I presume either the float stuck and it ran so long it overheated, or possibly the motor locked up somehow which caused the meltdown.
I replaced  the pump, and everything else seemed fine including the switch.
I've replaced a lot of bilge pumps, but never seen anything like this.  It didn't ice up and lock up, the boat's in FL.  Maybe there was just a short inside the motor windings.
One note, there was a blown 15amp glass fuse in the fuse holder.  The pump had stamped on the housing "use 8 amp fuse". I would have thought a 15 amp fuse would blow before it could melt the whole motor housing.  Guess not.

[MomentSurf]

Definitely sounds like a short in the pump or an overload (possibly debris caught in the pump causing a high impedance). 

I would never suggest going higher than the recommended fuse size...putting a 15amp fuse in allowed the fault to burn longer and hotter than an 8amp fuse would have.   If the pump was simply clogged or stuck, the 8amp fuse might have blown before such catastrophic results. 

[wes]

If the fuse was installed in the main DC panel, it is intended primarily to protect the wiring, not the pump or other device connected at the end of the wire. So 15 amps is typical. Protecting the device itself, if necessary, is usually done with a separate fuse located near the device. My bilge pump has such a "local" fuse a few feet from the pump.

[Tim Gardner]

Wes is correct, the 15 amp is to protect the wires connected to the fuse at the block.  However, if it is a dedicated circuit (meaning only the bilge pump is connected to the fused circuit), one could replace the 15a fuse in the fuse block with an 8a fuse to protect the pump.  This would result in one less point where corrosion could take its toll and possibly prevent the bilge pump from operating when one needs it to.

JM2¢

TG

[relamb]

Somebody had replaced the local fuse near the pump with a 15 amp.
I suppose it would be a good idea when acquiring a boat, to go through and check all of the fuses and wiring!
Who knows what may have been done to an almost 30 year old boat.
I found a pair of wires in the bilge that had a live 12v feed, connected to nothing.

[moonlight]

That's called, and caused by, a locked rotor condition.  It's a well known phenomenon amongst the rarer marine electricians. 
Upon closer inspection you may have found a piece of wire tie-strap or fish hook locking the rotor.  Bilge pumps are rated for proper fusing for exactly this reason; the starting and running amperage load is greater than the locked rotor load.  So there's a spike when the motor starts, and another spike when it siezes; if this doesn't blow the appropriate fuse then it sits there and gets real hot, real quick because there's no cooling water flow around.  they're also REQUIRED:
"22.5.8    No  external  temperature  shall  exceed  302°F  (150°C)  when  operating  at  120%  of  nominal  voltage  in  an  ambient temperature of 140°F (60°C) for seven hours."
but that doesn't mean your condition didn't last longer than 7 hours!

[redfishnc]

Never too old for a lesson. thanks/

[relamb]

Here's a picture of the meltdown....