Kiln issues, blowing fuses, power calculations through the lines

D

Don Nguyen

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Oct 4, 2011
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I didn't want to keep cluttering the thread, but I've got this wiring set up from the sticky: http://www.bladeforums.com/forums/showthread.php/985651-Heat-Treat-Oven-Wiring-Schematic

I've quintuple checked it; everything looks correct and make sense as far as connections go. I'm running:


  • 220v
  • Measuring 16ohms across my elements,
  • thus calculating that I'm running at ~14A ([SUP]220[/SUP]/[SUB]16[/SUB]=~14, right?).
  • Using 10A MDL slow-blow 250v fuses through L1 and L2.
  • From it was said about each line carrying about half the total amperage (in that thread), I figured 10A fuses for each line would be sufficient.

The kiln is working and powering the elements, but the fuse on L1 gets hot and pops if it sends power longer than 5 seconds.

Is my basic power calculation wrong, or is there something else going on?
 
I think if you are pulling 14 amps you are going to need to use 15 amp fuses, two 10 amp fuses won't allow you to pull 20 amps all it will do is show you which fuse is weaker.

Another way to think about it, AC stands for alternating current which means the current flows one way, then it flows back the other way, with your setup you are using 14 amps in both directions
 
What John said is correct, except you should only load overcurrent protection to 80%. 80% of 15amps is 12amps so go to 20amp fuses.
 
That's what I was thinking too. Looking at the schematic it doesn't make sense for the current to split.

Thought I'd ask before I do something wrong and make something explode :)
 
On my last post to you about the fuses, I started to ask why you had 10 amp fuses if the current was 14 amps, but figured you had two coils that ran on 120VAC each and were using a 240V line to supply it.
If the 10 amp fuses are in series, then one or the other will blow at 14 amps. If parallel, then they will share the current and be fine. What your are describing is series. Install a 20 amp fuse on each leg.

One fuse would work, but there are electrical reasons for the two fuses.
 
I'm running it in series; it makes sense to me now. I will get 20A fuses now. Thanks guys. Sad to say, but I just learned more about electrical than I did through a whole semester of it :)

Now, a few questions that I've been wondering about. I read in that thread that the dual SSR is to prevent the elements from turning on due to accidental contact, but isn't that what the door switch is for? Does it still go live even with the switch?
 
Its like a redundant safety feature.

I read somewhere a while back, That when an ssr goes bad, it's likely to be in the on position, having a door switch gives you one more layer of protection against that scenario.

Having one ssr fused and one working would be the same as building your oven with 1 ssr, you will always have 120volts in the system, with no way to tell unless you touch the coil.
 
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The biggest reason is that with only none SSR, the coils are always live tio ground. the door switch will only remove the low voltage power to the SSR. The line voltage will be live all the time.

Explained in words;
1) Line 1 and Line 2, SSR on line 1 only . When PID is OFF and/or the door open the SSR is OFF. The coils will not be heating, seems safe, right?....If you touch the heating coils and are grounded you will get a serious shock, as the path from line 2 to ground is always present.
2) Line 1 and Line 2, SSR on Line 1 and SSR on Line 2. When the PID is OFF both SSRs are OFF and there is no voltage in the heating coils. If door is opened, the power to both SSRs is interrupted and the coils are inert.
 
Disregard my last post in part with stacy's better explanation.

When I was putting my oven together my door switch was going to kill the power directly at the L1 and L2 source, not the low voltage supplying the ssr's, my oven was built with low cost coming first, safety second ;0) sort of... after the cycle finishes I unplug the oven and then open the door, no conglomeration of switches in the world is safer than that, I mean other than curiosity there is no reason at all to open the oven after starting the cycle right? ;0)
 
I have a double door switch, both phase and 0 contacts are broke when the door is open. You don't have a shock hazard when you touch the coils with a pair of pliers. I don't like 2 SSR's since they are prone to failure. A good extra safety feature is connecting an extra normal relay to the alarm port of your PID. You set the alarm 100 degrees above your set point or at 5 degrees above your oven's maximum. When a SSR fails, and keeps making contact, temp will rise, but the safety relay will switch it of when a maximum temp has been reached.
 
Running 220 volts at 15 amps through a limit switch on the door isn't a great idea. Redundant disconnect relays and alarm controls are much better, as VZ noted.

As far as SSRs going bad, I have never seen one go out that was properly installed on a heat sink. Even better are the ones with integral heat sinks built in.
 
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