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Dick White

What Happens When A Relay Fails?

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Ok, the topic heading is the question of the day. I know the answer(s) generally, but have not personally experienced a certain extreme of the question, and I foresee a pissing contest about it up the hill in the near future, and want to be prepared. So, here is the rest of the scenario. The low voltage electronic controller activates a relay (several in some circumstances) to switch the high voltage heating coils on and off as needed. From time to time, said relays fail. Nothing unusual about that. If it fails in the open position, the heating coils get no electrical power and the kiln simply fails to heat up. If it fails in the closed position, the coils remain on full power and the kiln overheats. Again, I understand this. What I have not experienced, is what happens when a relay fails in the closed position and then you turn off the little toggle switch on the side of the control box? Having just built a controller from scratch with the new Bartlett Genesis board (love it BTW), I know that the little toggle switch simply turns off the feed to the transformer that powers the control board. It is the controller, through the relays, that turns the elements on and off. It would seem to me logically from all this that if the relay is broken in the closed position, the elements will remain powered without regard to the controller being on or off. In other words, the controller toggle switch is useless for turning off a kiln with a relay that failed in the closed position. For full safety, there needs to be another shutoff device upstream. Am I right or wrong?

 

Thanks

dw

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Correct. When a relay fails 'On', shutting off the power to the controller will not affect the relay at all. The mechanical contacts are welded together, and no signal (or loss of signal) from the controller can make them move. In my experience, only about 20% of relays fail 'On'. The rest fail off in one of 3 ways-

1. It totally stops working, but shows no physical signs of it being dead. This is the most common scenario. You have to use a meter to figure it out.

2. It stops working and totally fries out and melts part of the outer casing. This makes it easy to tell which relay is dead. It's kinda cool to see.

3. It sticks. When the controller sends out a signal it sometimes works and sometimes doesn't. This is a pain to diagnose, because it most often happens at high kiln temps when the controller is heated up. Usually when they get sticky they'll totally fail within a few firings. You have to do a program with a rate of 9999 and turn it on and off several times to try and get it to stick.

 

Here's what I have seen: When a relay fails 'On' in a 3 ring kiln, you may get some slight over firing in that particular section of the kiln at low fire temps. When firing to cone 5/6, that one section should not overfire, but rather just slow down the cooling of the kiln as a whole. In a 2 ring kiln firing to low fire temps, that section of the kiln can definitely overfire when going to low fire temps. I don't know what will happen in a 2 ring kiln at cone 5/6. Haven't seen it. 95% of the kilns I work on are 3 rings.

 

The odds of more than one relay failing 'On' at the same time are pretty slim. Like probably lottery odds slim. But it is possible in theory. This is why you should always check to make sure the kiln has shut of properly. The controller can't stop a fused relay.

 

If a relay sticks, the controller will know it and will put up an error code and stop the firing. I think you can use Output 4 on the controller to power a large main relay that feeds the other relays, so when the firing shuts down the main relay will kill power to the others. I would confirm that with Bartlett before trying it. Of course, in theory the main relay and the smaller relays could all fuse 'On'.....

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Okay, which leads me to ask another question?   ( great answer Neil)

 

Does the new Genesis controller indicate relay failure?  I know it does some read outs on consumption and other firing diagnostics. If not, kiln makers who follow this thread- something to put on your list.

 

Nerd

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Thanks Neil for the thorough explanation, particularly the probability estimates. You are confirming my logical beliefs. The underlying issue is not so much whether a kiln will overfire, but rather the potential for a technical pissing contest with a certain technician in our college facilities department, who has no f'n idea about kiln operations but loves to throw around industrial certification (or rather lack thereof) as a reason for locking out our kilns. We are currently defending to the college provost that we are not going to blow up the college with our gas kiln by using the manufacturer's designated shutoff valve to completely turn it off at the end of the firing despite our clear lack of AGA certification to even so much as touch that gas valve. We are winning this battle for now, but the larger war is just beginning as they are nearing completion of a new art building and they are having trouble getting their heads around mere artists (in particular the young blonde female head of the ceramics program) having any technical know-how. We have negotiated double gas valves for the new gas kilns (one we can use, one upline that only they can use), but I don't know where will end up with the electrics. Right now in the old kiln shed, we have access to a dedicated circuit breaker panel right there in the room to kill power to an errant kiln (or work on it to change elements, etc.; they are hard wired). But as we aren't certified licensed master electricians either, I can see them getting a bug up their arse and locking out the electrical panel. So we will need to argue for another external shutoff switch installed between the panel and the kiln, otherwise we might burn down the college if a relay fails.

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Yup Nerd, but you will notice in the picture of that device there is a lock eye on the handle. Uncertified artists should not be allowed to operate a college-owned master on-off switch, only the manufacturer's switch on the kiln (which, as we just discussed has a higher probability of burning down the college than our unauthorized use of a switch on the main).  We will get there, and that will probably be the preferred solution, but I wanted to be absolutely certain of my position that for the electrics the nominal manufacturer's on-off switch is insufficient for total control and they will need to provide some alternate capability if they can't deal with allowing us uncertified hacks to flip a breaker once in a while if/when needed.

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I have been looking into the safety features also. The Bartlett V6-CF Technical Manual provides some relevant information. I'll share what I have learned in case it is helpful. I have not implemented anything yet, so I don't have any real world experience on this issue. 

 

I cannot find a technical manual for the Genesis controller, but its wiring diagram does have the same outputs relevant to the relay/power safety issue, so it may function in the same way as the V6-CF (of course, check with Bartlett and your local electrician before implementing or relying on anything said here). 

 

As Neil pointed out above, the controller does have an output that can be used as a safety to power a relay between the line power and the switching relays. Appropriately enough, they call it the "Safety Output". (Note that "Output 4" is something different, and it may be used to run a fan, alarm or extra kiln section). 

 

Bartlett says the Safety Output "powers on at the beginning of a firing and off at the end of the firing". This would seem to be a great solution, except that the controller will not always end a firing if a relay goes bad. Bartlett is probably thinking of "safety" in this context as keeping power out of the element circuits (even with a stuck switching relay) except while actually firing - that is a good thing and it could prevent electrical shock to people in certain circumstances. And, I think that is the crux of Dick White's issue anyway (energized elements while kiln switch is off, as distinguished from overfiring). Again, as Neil points out, that assumes the safety relay is itself not stuck on. 

 

So, even using the safety output/relay could allow continued firing with switching relay(s) stuck on. The controller would have to sense an out of limit temperature rise to stop the firing (I don't think the controller senses bad relays, or does it?). If it did so, and the safety was in use, that should shut off line power to the switching relays (and therefore to the elements even if a switching relay is stuck on). However, line power would still be running in the kiln up to the safety relay (electrical shock to humans and equipment shorts, and resulting fires, still possible). 

 

Adding that safety output/relay would help with Dick White's original concern: If you turn off the kiln "power" toggle switch, the controller would lose power and would not send output to the safety relay, and therefore the circuit would end there and cut off line power to the switching relays and the elements down the line (again assuming the safety relay is not stuck on along with one or more of the switching relays). Basically, we are just adding another relay into the circuit based on the probability that both the safety relay and one or more other relays will not be stuck on at the same time. But, that probability is not certainty. Maybe we could add two safety relays :) :)

 

Interesting note, here's how Bartlett describes this safety system: "Safety Relay. Optional. Not Installed on most Kilns".

 

Bottom line? I think Dick White's conclusion is correct. To be absolutely certain and in total control (and safe) you do need a line shutoff mechanism prior to the kiln.  

 

Sorry for the long-winded post. I was thinking this through as I was writing. Hope it helps. Comments and corrections welcome.  

 

--Edward

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They can lock you out of the panel, and you can turn off a kiln.  The ole double  gas shut off principle...

 

NOTICE the large On and Off stickers for those without certifications.

 

Nerd

 

Hey Nerd I have one of these on my pot shop supply line-100 amps off my 200 amp main house power. I use it to switch on power when my generator is powering the system as my transfer switch is only a 20 amp circuit for shop lights and wheels.

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I have one exactly like that running to my small kiln! I also have a larger rated one going to my larger kiln that looks a bit different but same principle.

 

A stuck relay is one of the reasons I always try and check the video camera I have pointed at the kilns around the time they are due to shut off. Once I see the CPLT flashing I go down and record the final readout, turn off the kiln power switch, then the wall mounted shut off switch. The shut off boxes on the wall are always off unless the kilns are running. But then I tend to be a bit paranoid about these things.

 

I hope you get your college on your side. My husband worked in the university system for 30 years, and I hear your pain, they had the weirdest rules about stuff.

 

T

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Thanks to all for the helpful information (and sympathy). The safety relay is interesting, but probably not going to be useful. The new kilns will probably not have them factory-installed and we can't/won't install them ourselves (as the college won't allow anything cobbled together by an end-user, reasonably so). My main concerns are two-fold - 1) I need something easily and fully accessible for immediate shutoff by an amateur in case of a misfire of whatever nature. We can't wait for an authorized certified licensed professional from the facilities staff to come in from wherever (and they go home at 4pm) to turn off the flaming mess. And 2) I need some means for taking a kiln offline while not in use or performing maintenance. Their current argument that we amateurs aren't licensed to touch that valve doesn't cut it.

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I totally forgot about the safety output! Good call, StarJupiter! The circuit board on the Genesis is identical to the V6-CF, so yes the wiring diagrams and all that will work for both. There's no reason you can't hook up a safety relay externally. That would be easy enough to do, especially since it's running on low voltage.

 

In a zone control system, the controller will easily recognize if a relay is stuck 'On', because that section will overheat very quickly. The controller doesn't necessarily directly know the relay is stuck, but it recognizes the symptoms and puts out an error code specific to that, E-2. In a single zone kiln it will take longer for it to recognize that theres a stuck relay if the relay is not in the same zone as the thermocouple, but eventually it will figure it out and put up the error code and shut it down. However, even if it doesn't recognize it, it's very unlikely that the section will over fire at cone 5/6, and at low fire it will only overshoot a bit if at all, especially if the safety relay is in use and shuts it down when the other sections get to temperature.

 

As for burning down the school, it's not going to happen due to the kiln getting too hot. The electrical system will give out before the bricks melt down. There are two ways for a kiln to burn down the school, and neither are actually the fault of the kiln-

1. Something combustible is left too close to the kiln and it catches fire.

2. There's a wiring problem and the outlet or wiring catches fire. If you hard wire and use conduit, it's not really an issue.

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Ok, the topic heading is the question of the day. I know the answer(s) generally, but have not personally experienced a certain extreme of the question, and I foresee a pissing contest about it up the hill in the near future, and want to be prepared. So, here is the rest of the scenario. The low voltage electronic controller activates a relay (several in some circumstances) to switch the high voltage heating coils on and off as needed. From time to time, said relays fail. Nothing unusual about that. If it fails in the open position, the heating coils get no electrical power and the kiln simply fails to heat up. If it fails in the closed position, the coils remain on full power and the kiln overheats. Again, I understand this. What I have not experienced, is what happens when a relay fails in the closed position and then you turn off the little toggle switch on the side of the control box? Having just built a controller from scratch with the new Bartlett Genesis board (love it BTW), I know that the little toggle switch simply turns off the feed to the transformer that powers the control board. It is the controller, through the relays, that turns the elements on and off. It would seem to me logically from all this that if the relay is broken in the closed position, the elements will remain powered without regard to the controller being on or off. In other words, the controller toggle switch is useless for turning off a kiln with a relay that failed in the closed position. For full safety, there needs to be another shutoff device upstream. Am I right or wrong?

 

Thanks

dw

I have seen the same problem happen in a college studio, where a student fired a kiln on a Friday and didn't come back until Monday and a relay had stuck on.  Luckily only one section kept firing and didn't do much damage.  Before electronic controllers the old Dawson safety shut off with timer would have shut the main power off.  There use to be an option to buy a kiln sitter with timer, with a kiln that had an electronic controller.  This could satisfy the University.

David

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Late to the party and I didn't read everything in the entirety :) but it sounds like in order to satisfy some concerns from higher up, you need better shutoffs in case of emergency.

 

In a setting with multiple users, the kiln should be wired to a shutoff/disconnect box like pictured above, from there it gets power from the main panel.  This way the technician/staff can lock out the electrical panel for whatever reason and everything will be completely dead.  When they want to give access, they tun on the breaker and leave the shutoff/disconnect box to kill power in case of emergency, the user will notify the staff of emergency shutoff so they can lock/tag out the equipment.

 

I've had relays fail in both directions, but I never really saw any bad problems from a relay getting stuck closed (powered on).  In those instances (WAY fewer than relay stuck open/off) the kiln just held temp longer as it tried to cool down.  You kinda notice when your control box says complete, yet you still hear power going through the box.  My kilns are plug-in, so I just pull the plug or flip the breaker.

 

 

 

On the gas kiln, you can get a large main gas solenoid or manual lockout valve that can be used to lockout the gas supply upstream.  Since kiln most likely has electronic components, you can also simply have an electrical shutout to keep non-authorized users from tinkering - can't really start up the kiln without power.  I never rely on the gas solenoids when shutting down my kilns, I ALWAYS close the burner and pilot valves manually afterward.

 

I like the idea of an external timer on the gas solenoid - but in this setting being at a school they may want an actual lockout device.

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