What kind of switch will mechanically break a circuit in response to a 12V signal

dgfoster · Apr 29, 2021

I have finished designing and building a small heat-treat oven 5 feet long, 16 inches wide and 12 inches high for stress relieving cast iron castings I make. It uses an excellent Bartlett kiln control board that switches a 4000W to a16ga Kanthal A heating element. I am using an electromechanical relay to control heating driven by the board. It works fine. But, as an added safety feature I would like to add a switch triggered by a 12V signal from the board that occurs should an over-temperature condition occur.

The board procvides a pin which will output 12VAC (easily converted to DC with a bridge rectifier if needed) in the event of an over-temperature condition. I am thinking an electromechanical circuit breaker-like switch would be ideal. So that when the 12V signal occurs the switch opens and has to be reset mechanically for current to once again flow through the heating element. I imagine such a switch exists. But, not knowing a proper name for such makes finding it difficult. I suppose once someone tells me the proper name I will feel like a dunce (I actually already feel that way for being clueless), but I can live with that.

The reason for the added switch is to take care of the situation where the primary relay fails closed and power is delivered uniterupted to the heating coils leading to meltdown or fire. Yes, I know the EM relays usually fail open. But....

Denis

Robert R · Apr 29, 2021

It is called a shunt triggered circuit breaker.

There is a additional circuit built into the circuit breaker that can be powered by a low voltage low current signal that will cause the trip. The circuit breakers are expensive when purchased new. The used and surplus units are inexpensive on ebay.

Shunt Trips - Shunt Trip Circuit Breakers - Relectric

https://www.eaton.com/content/dam/e...ns-for-undervoltage-release-shunt-trip-an.pdf

There are also inexpensive thermal snap action switches available that require a push to reset operation once a over temperature has been reached. The snap action switch can be wired in series with the furnace contactor coil.

sfriedberg · Apr 29, 2021

You could also use a suitable power relay (contactor) with 12VAC coils. Put the relay contacts "downstream" of the controller board, as you don't want the 12V signal from the board to cut the power supplying itself.

[Added in edit] Oops, I missed the "must be reset manually" requirement.

Milland · Apr 29, 2021

I hope this isn't a dimbulb answer (electronics is not a strength), but wouldn't a normally-closed relay of sufficient capacity serve this need? As long as it doesn't see a 12V signal, it's closed completing the heating circuit. At triggering it opens the circuit.

No worky? Not reliable enough if the contacts are arced closed for some reason? Probably a solid-state method that wouldn't have this issue.

[Missed it by that much...]

nc5a · Apr 29, 2021

Denis, I believe I understand what you are after but from a controls standpoint it doesn't make much sense. Clearly you want protection from a runaway furnace but there are other ways. Plus doing what you want may not be as easy as you think, depending on your temperature controller of course. For instance not all outputs like your 12 vac can drive a mechanical relay. And a relay would be one of the switches of choice to shut things down when something goes wrong. You could of course use a solid state relay or a logic output from your controller if one is available. But this can get complicated. And as Robert pointed out a shunt trip breaker is an option although sometimes problematic with spurious trips.

Without knowing what you have and how it's configured it's hard to give you reliable guidance. Plus from your basic description it appears you have an on/off control, is that correct?

sfriedberg · Apr 29, 2021

nc5a said:
For instance not all outputs like your 12 vac can drive a mechanical relay.

I had to deal with just this situation a while ago. Multichannel remote control (think garage door opener) with 12V output signals not adequate to drive the coils on the relays I needed to use. (And I'm glad I checked, and didn't burn up the remote control board.) Standard solution: small control relay with coils sufficiently tiny to be driven from the remote control receiver, whose contacts were sufficiently large to drive the power relay coils.

dgfoster · Apr 30, 2021

Thanks to all for the above comments some of which hit the mark and some not so much. From my initial read, the second post suggests the type of breaker I hoped existed. I want to not depend on continuous appropriate output from the board to actively provide protection once the over temp situation occurs. I want a mechanical switch to trip that passively remains open and needs physical override by me before current flow through through the heating elements can can resume.

Using a relay to boost current to maintain an open relay would sort of do what I want, but lacks the once-tripped-die-cast setup that I desire.

I'll spend a little time reading about shunt breakers.

Denis

Billtodd · Apr 30, 2021

Typically, you would use the controller output to drive a relay . The relay's normally closed contacts would form part of the power contactors Stop circuit , which is a normally closed (NC) loop holding the main contactor closed. The NC loop would include an emergency stop switch and the normal off switch and any other saftey interlocks.

Bill

thermite · Apr 30, 2021

dgfoster said:
I have finished designing and building a small heat-treat oven 5 feet long, 16 inches wide and 12 inches high for stress relieving cast iron castings I make. It uses an excellent Bartlett kiln control board that switches a 4000W to a16ga Kanthal A heating element. I am using an electromechanical relay to control heating driven by the board. It works fine. But, as an added safety feature I would like to add a switch triggered by a 12V signal from the board that occurs should an over-temperature condition occur.

The board procvides a pin which will output 12VAC (easily converted to DC with a bridge rectifier if needed) in the event of an over-temperature condition. I am thinking an electromechanical circuit breaker-like switch would be ideal. So that when the 12V signal occurs the switch opens and has to be reset mechanically for current to once again flow through the heating element. I imagine such a switch exists. But, not knowing a proper name for such makes finding it difficult. I suppose once someone tells me the proper name I will feel like a dunce (I actually already feel that way for being clueless), but I can live with that.

The reason for the added switch is to take care of the situation where the primary relay fails closed and power is delivered uniterupted to the heating coils leading to meltdown or fire. Yes, I know the EM relays usually fail open. But....

Denis

Dozens of ways to get to the same end, of course. The current to be interrupted is not trivial, but neither is it massive.

Your need as a "function" ?

... Is first-cousin to a common "magnetic starter" for a(ny) heavy load-motor control... with provision for an add-on remoted external E-Stop. That's the entry point for your alarm signal to be applied. As it can be a series "loop"? Any of MANY devices can be arranged to open it ... and shut 'er down.

"No reset after (either of) source power OFF or command OFF... until a human COMMANDS it be done " .. more than one actual "E-Stop" on the list.

.....is what "starters" already do. Motors are highly inductive loads. The primary contacts will be good for that.

I mention this because they have very well-understood load rating ranges.

That reduces need of any "Engineering". Just select the appropriate size or larger-than.
Even the 'brand" no longer matters. They have to meet the NEMA ratings.

And because they are not terribly costly in the used market, already boxed and ready to wire-in.

gustafson · Apr 30, 2021

Wire a self holding relay. Like when you emo a machine and have to hit the enable button.

Consider using an ssr rather than mech relay for control, better for switching multiple times

thermite · Apr 30, 2021

gustafson said:
Wire a self holding relay. Like when you emo a machine and have to hit the enable button.

Consider using an ssr rather than mech relay for control, better for switching multiple times

I've been using Crydom's & cousins since the dawn of the 1970's. I like them. I use them wherever I can. I even keep a goodly stash of several types - DC as well as AC - always on hand.

This is not an application where I would use one.

- SSR's are compact. Their heat-sinks, where wise, are bulky. "Heat" is in the area, after all. It's a FURNACE!

- it isn't MEANT to switch "often".

- it is an EMERGENCY over-ride or fall-back to an existing control that "normally" cycles the heater constantly to hold a set-point. THAT unit needs high-longevity, this unit needs high reliability.

SSR's have high reliability.. attractive in normal service.

They also have failure-modes in ABnormal conditions that are somewhat less attractive. They are Semiconductors. Semiconductors can fail, shorted. Too easily, actually.

This isn't a situation so dire as to require a "crowbar" circuit, nor explosive links in the conductors...

... but it DOES need to provide a high confidence level that it will not fail to act according to plan if/as/when called upon to open the circuit.

NEMA-rated "Motor Starters" from "the usual suspects" amongst major-makers actually have a rather more solid track-record at "JFDI" than not.

Is this really a wheel that needs a great deal of re-invention, R&D, or durability, life-cycle, or extreme stress vetting?

When all of that and more has already been done BY the major-makers for us as part of the normal development of a mass-market item in VERY long years of service all over the planet?

Put the "motor starter" at the head of the power feed.
Integrate the inverted 12V fault signal input.
Integrate one or more E-Stops.

HAVE a vanilla situation easily documented and maintained.
Even understandable by a third-party on "first sight".
Just isn't any real mystery to it.

dgfoster · Apr 30, 2021

I think I get what Bill is suggesting above and think that solution would do what I need. The “oral” schematic is as follows:

Proper rated motor starter—->Normally closed relay (also driven by over-temp pin of board)—->Existing EM Relay for furnace heater coils(also driven by board standard control signal)

With this configuration, should the over-temp condition occur, the board drives the NC relay open, then motor starter switch opens and will only reclose mechanically. That would do it.

So now I have learned about shunt-triggered circuit breakers (one option) and a use of more familiar components to form a good second option. Nice!

Denis

thermite · Apr 30, 2021

dgfoster said:
I think I get what Bill is suggesting above and think that solution would do what I need. The “oral” schematic is as follows:

Proper rated motor starter—->Normally closed relay (also driven by over-temp pin of board)—->Existing EM Relay for furnace heater coils(also driven by board standard control signal)

With this configuration, should the over-temp condition occur, the board drives the NC relay open, then motor starter switch opens and will only reclose mechanically. That would do it.

So now I have learned about shunt-triggered circuit breakers (one option) and a use of more familiar components to form a good second option. Nice!

Denis

Yup. The lighter-than-mains "aux" contact that holds-in the contactor actuator coil for its own HEAVY main contacts only needs that one break the circuit for OFF. It normally routes via an "NC" pushbutton.. marked "OFF'!

Any NUMBER of OTHER "NC' goods can be placed in that same series lead. E'Stop. relay. "etc'.

And it isn't a BAD idea to have a drop-out on utility-mains fail function at the very head entrance of the power for the ENTIRE RIG ...in any case.

Pretty close to a "freebie", then, to be able to cut power from any of SEVERAL 'safety enhancing" points Annnd to know in advance the heavy contactor's guts are rated for it.

And that a Square-D, Allen-Bradley, or Arrow-Hart starter is of a tribe as has been known to just do its damned JOB... for 80 years in not uncommon service.

You said 4 KW of heater as the load? Nothing special as far as "Starters" go atall. it won't be "pure" resistive load. But damned close. So it won't have nearly as nasty an "on interruption" arc as a motor has - and BFD, because the contacts are BUILT to manage that, year-in, year-out, already..

Wire and forget? Well. test it, too!

Then go melt arn!

Or is it: "Go pound sand!"

You'd have to know the foundry bizness?

Rugged individualists are such hard-asses they actually ENJOY "pounding sand!"

Pittsburgh-area schools, my watch, we kids actually did it for real in moldmaking class:

"Pounded our drag of Michigan Green".

Nowadays?

They'd figure you was talking about scoring debating points in a political campaign!

dgfoster · Apr 30, 2021

thermite said:
You said 4 KW of heater as the load?

Yup, 4Kw.

Denis

thermite · Apr 30, 2021

dgfoster said:
Yup, 4Kw.

Denis

May as well "OWN" the process of knowing how to select them, then?

Convert kw to hp

Nema Starters

For a motor, the "thermals" or protective devices need to be a close match. More than one type to those, but "most" are capable of being independently selected and/or swapped to "fine tune" starters built in ranges with a wide span to a narrower specific load-motor's needs.

Some starters use wise-ass electronics ones, not all of them adjustabe.

Some have none, rely on external-to-the-starter fuses or circuit-breakers.

Your rig's actual needs? Not enough information.

But "free" final answers never put no meat on a brain-rib, so...
Plenty more tutorials and toolsets out there!

blcksmth · Apr 30, 2021

Seems like the shunt tripped breaker is the most straight forward solution. They are used in passenger elevator systems so that a fire/heat sensor in the elevator control room will power the entire system down - even during an annual inspection mode of a fire alarm system.

Bob
WB8NQW

TDegenhart · Apr 30, 2021

By all means a shunt trip breaker. This is a job breakers are designed for.

Tom

dgfoster · Apr 30, 2021

One question that occurs to me is does anyone have experience to KNOW how much current is required to hold "on" a magnetic contactor. If they are designed to run a motor, at idle the motor will be drawing at least a couple amps of power. But in my application the only load on the contactor between segments of powering the heating coils would be perhaps milliamps to power the board and pilot light. So, I am suspicious the engineers might have designed the contactors to ignore such low power noise and open up. So from a computer logic point of view the circuit is closed (milliamps flow) powering the board and pilot light when the coils are not powered. But from a robust engineering point of view that power level might be "noise" and considered an open circuit.

I can see the wisdom of the shunt breaker advocates, BTW. I am just in the gathering data phase right now and trying to fill in some holes in my knowledge.

Denis

Robert R · Apr 30, 2021

dgfoster;3750028 The reason for the added switch is to take care of the situation where the primary relay fails closed and power is delivered uniterupted to the heating coils leading to meltdown or fire. Yes said:
This potential fault can be eliminated by replacing the existing contactor with a mercury type double pole relay. These are often used in furnace applications that are controlled by cycling the power on/off to regulate power input to the heating coils. The advantage of the mercury relay is that it can operate for millions of cycles without damage and the wet contacts cannot weld closed. If the electrical coil operating the mercury plunger fails the relay will fail open.

Mercury Relay - Hg Relay - Normally Open - 2 Pole - 35 AMP - 24 VAC Coil

dgfoster · Apr 30, 2021

Robert R said:
This potential fault can be eliminated by replacing the existing contactor with a mercury type double pole relay. These are often used in furnace applications that are controlled by cycling the power on/off to regulate power input to the heating coils. The advantage of the mercury relay is that it has near infinite cycle life and the wet contacts cannot weld closed. If the electrical coil operating the mercury plunger fails the relay will fail open.

Mercury Relay - Hg Relay - Normally Open - 2 Pole - 35 AMP - 24 VAC Coil

Now isn't that interesting. No more welded contacts----ever. The only reason this would not drop right into my control box is that the over-temp output is 12v DC and the coil wants 24VAC. And it looks like virtually all of these mercury switches use 24VAC or 120VAC for the coil. Yes, I could use another 12VDC relay to drive the 24VAC holding circuit as there is a center-tapped 12-24VAC transformer already included in my control box.

Denis

What kind of switch will mechanically break a circuit in response to a 12V signal

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