High Limit Controller in Electric heating application - can the PLC do it?

Have a customer asking if they can eliminate a High Limit controller from an electric furnace they're building since we now have PLC controls. This is the high-limit controller they plan on replacing:

https://www.honeywellprocess.com/library/support/Public/Documents/51-52-25-122.pdf

I've been Google searching and can't find anything on any kind of requirement to have a separate high limit controller. It doesn't look like this is a safety device since it's easily programmable. It's probably a good idea from a design standpoint to have it in case of a PLC malfunction, but I can tell them that and they could easily say "we're not worried about that."

Is anyone aware of a regulation requiring a high-limit controller on an electric heating application like this? I could see it on a gas-fired furnace, but electric is a bit of a different animal.

RussB said:

Sure it can but it shouldn't.

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That's where I'm coming from. I basically told them I am unaware of any regulation that requires them to use it, however I recommend against using the PLC because if it were to malfunction there would be nothing to prevent the equipment from overheating.

FactoryTalktotheHand said:

That's where I'm coming from. I basically told them I am unaware of any regulation that requires them to use it, however I recommend against using the PLC because if it were to malfunction there would be nothing to prevent the equipment from overheating.

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I believe that NFPA 86 (2015) section 8.18 speaks to this. Where there is not a specific "this does not apply" I think it is wise to interpret the document as "this *does* apply"

While I am not seeing direct language that says that it is stand-alone, there is fairly clear language of independency. For instance 8.18.2.9 "The operating temperature controller and its temperature-sensing element shall not be used as the excess temperature limit controller." I think it is fair to say that the PLC is the "Operating Temperature Controller" so to use it as the High Limit would be inconsistent with this section.

There are other statements that make it sound like a stand-alone device. They speak of it as an Excess Temperature Limit Interlock. They say it shall indicate its setpoint and that it shall be equipped with temperature indication. Yes, a PLC with an HMI can do this, but even if we don't accept the previous paragraph as the controlling language, I think that you are setting yourself up for trouble if you try to use semantics as your argument for not having a stand-alone High Temperature Limit.

Yes, this is my opinion, but for what it's worth, I can tell you that the company I work for would not put in a system without a stand-alone High Temperature Limit regardless of the heating source.

A High Limit device has to be FM (Factory Mutual) approved.
https://www.fmglobal.com/assets/pdf/fmapprovals/3545.pdf

In our use of a high limit...it is a standalone instrument that drives a relay.
The relay has 3 functions.....
It activates an under-voltage coil in the circuit breaker to kill main heating power.
It disables the output from the plc going to the heating contactor.
It does feed an input on the PLC...... for alarming purposes.

Electric or gas....they can both go over temperature.

NetNathan said:

A High Limit device has to be FM (Factory Mutual) approved.
https://www.fmglobal.com/assets/pdf/fmapprovals/3545.pdf

In our use of a high limit...it is a standalone instrument that drives a relay.
The relay has 3 functions.....
It activates an under-voltage coil in the circuit breaker to kill main heating power.
It disables the output from the plc going to the heating contactor.
It does feed an input on the PLC...... for alarming purposes.

Electric or gas....they can both go over temperature.

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Be it F.M. or any other external body, the ultimate approval/disapproval will come from the Authority Having Juristiction. In this case, I have always been told that, in lieu of a knowledgable AHJ, the insurance underwriter then sets the terms, hence F.M.

I think, at one time, the LIMIT CONTROLLER had to LATCH the overtemp condition, or, at a minimum, need to be RESET when the condition cleared. That could be old information.

But, I do agree, that just because you CAN do a thing does not mean that you SHOULD.

jdbrandt said:

Be it F.M. or any other external body, the ultimate approval/disapproval will come from the Authority Having Juristiction. In this case, I have always been told that, in lieu of a knowledgable AHJ, the insurance underwriter then sets the terms, hence F.M.

I think, at one time, the LIMIT CONTROLLER had to LATCH the overtemp condition, or, at a minimum, need to be RESET when the condition cleared. That could be old information.

But, I do agree, that just because you CAN do a thing does not mean that you SHOULD.

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NFPA requires a Manual Reset High Temperature Interlock.

Someone on here - I forget who - once put it this way:

"What would the headlines say?"

If something goes catastrophically wrong - even if it's completely unrelated to what you just did - what do you imagine will be on the evening news? Keeping in mind that to the general public (and the media) the difference between an actual safety device and a pseudo safety high limit controller is irrelevant.

If you can envision "new information has come to light that in the days before the blast a critical safety controller was removed from the furnace..."

Personally, I like to sleep at night

JHarbin said:

For instance 8.18.2.9 "The operating temperature controller and its temperature-sensing element shall not be used as the excess temperature limit controller."

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Would you interpret this to mean that the high limit controller will have its own sensing element?

Doug-P said:

Would you interpret this to mean that the high limit controller will have its own sensing element?

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Yes.

I concur completely with JHarbin's statements and interpretation of NFPA 86.

Furnace manufacturers that do firing rate control and furnace logic control with PLCs always use independent high limit controllers where each controller has a separate temperature sensor from the control temp sensor in order to comply with the NFPA safety standard.

Using the same controller for temperature control and as a high limit safety controller, even with separate temp sensors violates the safety concept of not allowing for a common-mode failure where a controller fault or failure could render it incapable of controlling the high limit.

FactoryTalktotheHand said:

I could see it on a gas-fired furnace, but electric is a bit of a different animal.

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Personal story

30 years ago, it was about my 3rd week as a salesman for temperature controllers.

A fairly new customer had started manufacturing electric heat ceramic kilns to complement their distribution business supplying ceramic clays, glazes and whatnot. He'd had an RFQ for an industrial sized kiln and needed an SCR for time proportional control.

I asked about what he was using for a high limit controller and whether he was using a shunt trip circuit breaker to interlock the SCR.

He blew the high limit off as a 'costly accessory', despite my tutoring on NFPA regs (even back then).

He bought the SCR and about 3 weeks later I get into the office on a Monday morning and there's frantic message from the customer - needs to see me immediately, if not sooner.

I drive over and find out that the kiln had been completed late the previous week and they'd turned it on to dry out the fire brick and had left it running over the weekend. The SCR had failed and SCRs fail in the unsafe ON state. The kiln's steel shell had been glowing red hot when the first employees arrived Monday morning. The kiln was total loss, except for the SCR unit which was repaired under warranty (it was in a separate enclosure that was sitting on the floor several feet from the kiln).

The closest building wall was slightly scorched and the shop order papers that had hung on the kiln wall were just ashes on the floor. The owner was stunned to realize that had the kiln not been in the center of fairly open work area, away from the building walls and work benches, that his building could have burned down from radiated heat from the run-away kiln.

Needless to say, he put high limits and shunt trips in his control panels after that.

Electric heat is just as subject to NFPA 86 as gas heat is. Electric contactors can fail closed, just like SCRs can failed closed circuit.

A couple weeks ago, Bernie mentioned that "Ohms Law is not suggestion". Neither are high limits on heating equipment.

Dan

You can likely use a safety PLC and meet code but it would be more expensive than a limit controller so I fail to see the point.

Don't even think about it with a non safety PLC.

I had a furnace completely melt down inside.
The customer was using an alarm in the chart recorder for the high limit (wrong).
To save paper in the recorder they turn it off when therre are no parts in the furnace.
I probably don't need to explain any further.....control system failed and furnace went to probably about 3200F and it was only rated for 2500. It literally melted the molybdenum element bars....which is what finally stopped it from heating.

i would recommend a separate not electric system with separate sensor like a simple thermostat.
if it is activated the CBR same as OCR should break and must be resetted manualy.
nfpa iec and insurance do demand it.
do the same in rooms with equipment, even one simple light in an environment chamber has burned down the complete chamber (siemens plc repair center germany had this.