Why did power plug connector vaporize?

[Daniel G]

Harbor Freight Chicago Electric 10” sliding miter saw. 61972 product id. 5 to 10 years old.

 

Previously worked fine. Used it a lot. Today, when pressed trigger to run, made brief sound, then nothing. Blade turns freely by hand. But does not run when press trigger. Just sits there.

 

Disassembled trigger handle. Easy to do. Trigger presses on a simple switch (see picture), with hot lead in. Tested switch (with no leads connected) with multimeter. Infinite ohms when switch not pressed. Zero ohms when switch pressed. So seems switch functions perfectly.

 

At that point, not sure what to do. Figured it would not hurt to bypass switch. Removed two leads from switch. Plugged in saw. Touched leads together, holding by insulated part of wires. Saw briefly tried to start. Then electric smell. Then noticed that plug neutral prong had melted through (see picture). Most of prong remained in electric outlet.

 

Why did connecting the two hot leads have such an unexpected (to me) effect? I thought connecting the leads by hand would be the same as connecting with the switch by pressing trigger. What did I miss?

 

It seems there was a *big* current surge that melted the neutral plug prong. Any idea why that happened?

 

Any thoughts about what I might look at next to get the saw working again? Besides replacing the power plug.

[wingless]

Welcome to the forum.

 

That Harbor Freight 61972 tool consumes about 15A of current.

 

Normally all is good.

 

But, if there is a high resistance connection, either within the cord, such as near the plug, or in the receptacle / plug connection, then that 15A will also make heat at that point.

 

After properly replacing the tool plug, insert mostly into the same receptacle and carefully operate the tool (loaded blade / cutting) while measuring the voltage on the slightly exposed plug prongs to verify full voltage exists.

 

If not, also swap the plug. 

 

Another option is to just also safely swap the plug.

 

[Daniel G]

Thanks. Yes, I am going to swap out the tool plug, as I have no choice, the neutral prong is gone. It seems you are suggesting there was high resistance at the point where the prong vaporized. That makes sense, could explain why the prong melted, makes more sense than my idea of “current surge”. But any idea why 1) saw totally did not work with apparently good switch, and 2) had catastrophic failure of neutral prong when I bypassed the switch?

 

I guess here is one possible idea: Neutral prong was already broken, and did not complete the circuit when I was using the switch. Then later, when I bypassed the switch, the prong made a weak, high-resistance connection and was damaged even more, splitting off the prong. Before the prong vaporized, I measured the voltage coming in, and that was OK. But I did not measure that the neutral prong had continuity. Did not imagine there could be a break.

 

I will try replacing the plug, and see if saw then works. Let me know if any other ideas what happened.

[Daniel G]

Replaced the plug. Saw now works fine again.

 

So it seems did not originally function because neutral prong was already broken inside the plug, so there was an open circuit. During my troubleshooting I should have focused on the plug first. I assumed the plug was OK, it seemed OK, and I did not look at it.

[wingless]

Thanks for the update.

[Daniel G]

Thanks for helping figure this out. Your comments were very helpful.

[IssacPatel]

Oh, the mysteries of power tools! Sounds like a head-scratcher with that vaporized power plug. Kudos for diving into the disassembly and troubleshooting.  Bypassing the switch leading to an unexpected meltdown is quite the puzzle. Maybe there's something in the motor or wiring that's causing a surge? I'm no expert, but have you considered exploring options related to different types of plastic moulding for a potential replacement or upgrade in the plug department? Could be worth a look!