Building a heat treatment oven - questions

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ckdexterhaven

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Feb 16, 2019
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Edit: I've edited the name of the thread, as I will probably have some more questions and so I don't have to make new threads for every little question.


I am planing the build of a heat treat oven and have a safety related question related to the wiring set up.

Most of the detailed instructions and their wiring diagrams which you can find are for the US and it's hard to find a lot of info on single phase 230V set ups.

I want to include a main power switch and also a door switch and was basically thinking to wire it like this:

43072486om.png

(fuses are missing here)

This way, whenever I cut the main power switch or open the door, there is no power on the heating element and safety is ensured.

However, all wiring diagrams I was able to find on the net, wire it the other way around: The phase (hot wire) is directly connected to the heating element and neutral goes through the SSR.

For example:

43072501ab.jpeg

(source: https://www.indieknives.com/blog/knife-material-matters-heat-treat-oven-control-box/ )


In this case, when the switch is on, the coils have power on them, so I cannot safely open the door without completely turning off the PID.


Basically, they are all like this. This post gives an explanation (he had first done it the "safe" way): https://www.bladeforums.com/threads/diy-heat-treat-oven-question.622841/#post-6523780

He claims that this protects the SSR as it would burn faster than the fuse.

Can anyone chime in if it's okay to run the hot phase through the SSR and if my first diagram is okay?

Thanks!
 
Last edited:
After a bit of more reading, I can answer my own question: it's fine to run the hot wire through the SSR.

Anyway, maybe this post will help someone else...
 
It is a loop circuit. Both the wires will be carrying current when the power is ON. Your plan to have the door switch interrupt the low voltage DC line to the SSR is correct.
 
A little update, I was made aware that it's not ideal to have the door switch interrupt the low voltage DC line to the SSR as SSRs do fail and that could end bad if it happens in the wrong moment. Because SSRs have no mechanical switch inside when they get damaged on a high voltage side there will be no physical break in the circuit.

So it's better to put the limit switch (door switch) in between hot wire and the coil, as shown in this diagram:

43083685qy.jpg

source of this diagram is this recommended video:




On another note, has anybody tried using the Chinese cheaply available Type S thermocouples? I've seen them in quite a few builds...
 
If an SSR fails it usually does not transmit current., I don't know of anyone ever having an SSR fail in the closed position.
The best safety method is to interrupt the DC voltage to the SSR. If you wish to disrupt the actual current to the coils directly, you will need an additional SSR. Putting the door switch in the high current loop to the coils is not a safe method.
 
Thank you for your reply, Stacy. Could you explain why putting the door switch in the high current loop is not safe? (The guy in the video claims the opposite.)
 
ckdexterhaven said:
Thank you for your reply, Stacy. Could you explain why putting the door switch in the high current loop is not safe? (The guy in the video claims the opposite.)
Click to expand...
You can get a mechanical switch that can handle the current, and that is the safest option in my opinion. When I built my oven, I could not find an appropriate switch locally and used a switch to interrupt the line to the SSR. I am comfortable with the level of safety my setup provides, but if you can find a switch to interrupt the power instead of the control signal, I think it would be safer in the unlikely event that the SSR fails in the closed position and you don't notice that the elements are still under power when you open the door.
 
It places an open wire carrying a current of 15 amps or more right at the door. This is an area that gets intense heat when the door is opened. The wires will have to be kept away from the hot surface of the kiln, too.

I'm not saying it could not be done with a power switch, just that it is not nearly as simple as with a limit switch.

If you want to have the safety feature of the door switch and keep it simple, install two SSRs in series. The DC trigger line goes through the door switch before going to both SSRs. SSRs. This is called a Redundant fail Safe Circuit. The chance that both SSRs would fail in the closed position at the same time is virtually non-existent.

There have been past threads where the SSR failed due to not being installed properly on a heat sink., but as far as I could see in checking past threads, none failed in the closed position. The heat sink is an important part of an SSR circuit. People wrongly assume that if the SSR is rated at 40 amps running 15 amps through it will barely heat it up. This is not so.
 
I've had SSRs fail, 2 in a year. both times they blew the fuses I had installed and all power was cut off, I have fuses on the neutral and hot leg before they go into my main on-off switch, and I have 2 switches, the main switch on the hot leg of 240V and a second one on the 32vDC SSR wiring for turning the coils off when I open the oven. I think this setup is simple enough, safe, and protects the components. ( I stole all this from wiring guides on this forum, I didn't come up with it)
 
I have certainly known SSRs to fail "closed". Even good ones (Crydom). Many years ago I was very nearly killed when working on a circuit I'd been told was dead (by an old hand who supposedly knew the plant). The DC switching feed was indeed dead, but the 400V through the (Crydom) SSR was not. The power was cut, after what seemed a very long time, by the 100mA RCD (GFCI) back at the distribution board incomer. 100 mA at 230V is not at all nice when you are the only ground path.

When I build HT ovens, they get a 30 mA RCD in the control panel and the door switch goes in the hold-in circuit feeding the coil of an NVR contactor upstream of the SSR: my ovens get used either by me (and I quite like living) or by folk I like enough to build an HT oven for. I'd rather keep them alive too. The NVR circuit drops out the contactor as soon as the door switch goes open-circuit. The contactor breaks both the Live and the Neutral (here in the UK and most of Europe, industrial users get 3 phases at 400V phase-to-phase and 230V Phase-to-Neutral, with the Neutral bonded to ground at the transformer. Domestic users get one phase and Neutral for 230V. The single Hot is at 230V to ground on a domestic supply).

I use Mineral Insulated type N thermocouples. These are base metal (Nickel alloy) thermocouples that were developed to overcome the "drift" that occurs with type K thermocouples at temperatures above about 1000 degC (1832 degF). The Omega proprietary high-temperature sheath material is very good and probably worth it if you treat some of the fancier stainless steels that need the higher temperatures. I usually use type 310 stainless steel sheaths which are good to 1100 degC, 2012 degF for regular use. Type N tops out at 1300 degC (2372 degF).
 
Thanks Tim, I talked to my friend and he actually meant an NVR contactor (in the center of the diagram titled "Relais") but had forgotten the name.
 
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