Solid State Relay - Voltage

The 3-pole isolator Negative3 describes could be operated with a micro switch set up like a machine guard that would activate when the furnace body elevates or is otherwise opened. Sorry if that's already been covered, there's a lot to digest here.

Pete

 

Pete, that would work with a contactor for sure, the coil control voltage going through the microswitch.

 

So if I'm reading this correctly, the take aways from this thread are this: make sure your metal kiln parts are grounded and that you have killed the power to the SSRs before coming in contact with the coils. Right?

 

I think there is a big difference between changing a bit or a blade using a device that has a mechanical on-off switch, a fully insulated motor coil (unless it develops a fault), and a grounded metal case, and reaching into a furnace that has exposed and uninsulated conductors, so I don't follow your analogy.

480 volt (and higher voltage) circuits actually kill far less people than 120 volt circuits.
It is the heart fibrilation that starts with current above 10 mA that gets people.
The higher voltages cause the heart to contract violently, and generally it will restart after that (witness the shock machines they use to revive patients having a heart attack).

I can only say what I would do, but I have been shocked enough to know that it is wise to turn things off when you have exposed conducting parts, or potentially conducting parts.
No slight intended, that is just my feeling.

Ground fault circuit breakers and receptacles save a lot of people because you can often have stray and unexpected currents/voltages.

 

Yes In a nutshell however a little more in-depth.

 

Move the LED legs to the inside of the third pole of the switc/contactor (end of the coil) fused if desired.

 

If you want to reinvent the wheel slightly you could use mercury based contactors made for heating applications: http://www.plasticservices.com/techtips/contactors.htm

These are made to handle continuous switching created by heat control circuits.

No leakage on these bad boys, and I couldn't even fathom the number of cycles it takes to kill one of these. To put their durability into perspective. In my seven years at a former employer that used said devices. I never replaced a mercury based contactor that I installed. ...and again, those contactors cycled continuously on a 24/7 basis.

 

Monitor over time. What do the new coils measure, resistance wise, versus an old/used coil?

 

Kelly,

Just recently joined the forum so just replaying. We build Turbine warming systems at work and have used all types of SCRs/SSRs. These are SUPPOSED to 'leak'. We usually provide 240v to them and see real close to that on the output side when not energized. They key is there is no significant amount of current when 'off'.

My Kiln and Furnace are both electric and use variable output SCRs controlled with 0-10v. Each draws just short of 50A at 240V. If there was any current when 'off', I'd be in a world of hurt!

 

Just happened upon this thread. When I was building my kiln controller I read that when SSR's fail they can fail closed as well as open. Not the subject of the thread but still something to consider. Or maybe someone can correct my memory. I wired my kiln control through my kiln sitter so it will kill the kiln on over temperature (I was running 30 hour dryouts and could not watch it full time). The only SSR failures I've had was inrush to a cold coil popping the SSR on excessive current.

 

Yes, they can fail closed (ON). They don't fail often - at work we use good quality ones and have never had a failure other than the associated electronics (which on our SCRs is replaceable). Ours run 24/7/365 at 240V/50-70 Amps. Typically cycle a few hundred times a day.

I have a $10 Chinese SCR on my 3D printer bed - I did include a thermal 'trip' switch (from a hot water heater) to kill the power if the SCR get's stuck on.

If you have an electric kiln in the house (garage), some type of safety is in order. Mine is outside, worst that can happen is it melts down my $75 kiln. Odds are the breaker would blow or the heaters would melt down first.

 

I wired my kiln so that at maximum current, the wire and circuit breaker see less than 80% current flow of the respective device/wire rating, so in other words I can run my kiln at maximum current indefinitely without either the circuit breaker or the wire overheating.
A thermal overload would be a good idea if you use less than rated wiring or breaker.

For most electrical devices, wiring and wiring terminations are only rated at 80% of their nameplate rating if used continuously.

Edit
I have used a lot of "Mercoid" mercury pressure switches in a control setting, and they do basically last forever without problems, but have not used them to switch power circuits. I will have to look up their maximum voltage and current setting, and whether they can be had in multi poles.

 

My kiln will get to 2,200F or more and I figure that would shorten the life of the coil at least. My SSR is rated higher than the kiln current nameplate but it sure gets a big inrush when the coil is cold.