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Old 07-12-2012, 12:51 AM   #11
P-J
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Originally Posted by bdjohns1 View Post
Semantics, but per NFPA 79, that's not true (see below). Emergency Stops are discussed in a section there called "Control Circuits and Control Functions".



+1 for alien.

I know what P-J's getting at with his E-Stop design, but I agree with alien - GFCI devices have a certain lifetime as well, and I'd wager that it's at least a couple orders of magnitude less than a contactor. I did a little Google research, and I've found nothing to say that it's prohibited under the NEC to deliberately introduce a ground fault, it's generally considered to be a Bad Idea.

We had a whole thread on this a few months ago - unfortunately, it looks like you'd have to buy a copy of UL Standard 489, but it appears that UL certification for a GFCI breaker only requires a rated life of >50 fault trips:

http://www.homebrewtalk.com/f170/gfi-breaker-pricing-cant-right-295194/

A little other research turned up NFPA code 79, which is specific to industrial electrical design. You don't have to follow Code 79 in your home for sure, but I'm pretty sure that a commercial operation would/should be following this as part of their design.

The one relevant quote I found in there in talking about equipment grounding was:

(14.1.1 basically says that connections should be secure.) According to that reference, grounding conductors are only allowed to be connected for the purpose of grounding. Since the resistor/fuse trip method introduces a deliberate current to ground from a hot wire, that point where the hot wire is connected via fuse and resistors to the ground conductor is the specific violation.

Reading more of NFPA 79 suggests that the resistor/fuse method further violates their definition of an emergency stop, specifically requirement (3) of section 9.2.5.4.1.1 which states "Reset of an emergency stop circuit shall not initiate a restart". Most versions of the resistor/fuse design I've seen do not satisfy that requirement - you reset the E-Stop and the GFCI, and the power is automatically restored to your PIDs (and therefore your SSRs) and your pumps unless you switched them all off.

A modified version of Kal's safe-start addition to his brewery would satisfy the requirement, as the main power contactor relay can't be re-energized if any of the switches for PID power or pumps are in the on position - the only position that allows the contactor coil to be energized is with all of the pumps/PIDs off and the key switch on. In other words, it requires definite action on the part of the operator to re-energize outputs after an E-Stop. Another HBTer provided a simple working schematic for a NFPA79 compliant E-Stop here: https://www.circuitlab.com/circuit/5zd3fg/holding-contact/ - combining that simple setup with the ice-cube relay and NC contacts on all of the switches from Kal's design would meet the requirements. Once you reset the E-stop, the ON button doesn't function until all of the other switches are off.

Another important point of distinction is that an E-Stop is not supposed to prevent you from getting electrocuted inside a control panel. Lockout/Tagout (LOTO) is a separate issue. I work in an industrial food manufacturing setting, and the rule is that if you're opening up something electrical to do work, you isolate the power upstream and lock it out (on our systems, by putting a lock on the circuit breaker, local disconnect, etc). If you just hit an E-stop for a system, that'll prevent you from turning any outputs on. To open an energized panel after hitting an E-Stop, you either have to throw the main disconnect switch (which kills everything in the panel back to the point where the wires enter the enclosure, or if authorized to work in an electrical enclosure, use the bypass switch to open the door without turning off the disconnect (usually a screw that has to be turned while pulling the door open).
Thank you for all of your BS...

This prompts me to question all of my efforts in trying to help the E-Brew community. I draw diagrams for simplicity and ease of build. I stay well within the requirements of NEC. My mission has been (was) to allow build concepts to be conveyed to those intensely interested in how to do 'stuff'.

I'll have to really think on this before I ever attempt to draw another diagram......

Thank you for that. You might have saved me hundreds of hours every month....

However: I declare BS on you as usual...
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Old 07-12-2012, 01:48 AM   #12
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Thank you for all of your BS...

This prompts me to question all of my efforts in trying to help the E-Brew community. I draw diagrams for simplicity and ease of build. I stay well within the requirements of NEC. My mission has been (was) to allow build concepts to be conveyed to those intensely interested in how to do 'stuff'.

I'll have to really think on this before I ever attempt to draw another diagram......

Thank you for that. You might have saved me hundreds of hours every month....
Dude, learn to take constructive criticism. This isn't personal.

I know your designs pretty much square up to the NEC. My brewery design is 90% based on your schematics. Your work hasn't been for nothing, and it's certainly been appreciated by myself and other brewers. I'm just saying that your E-Stop method can be improved upon for not much money. $12 for another button, <$20 for the relay, and a few more spade connectors and contact blocks. If you can't accept that feedback, maybe you should

Since we're talking missions, mine is to provide brewers with useful information about industrial process control and food safety topics relevant to brewing practice and equipment design. Why? My day job is as an engineering manager at a very big cheese plant (and if you know dairy science and zymurgy, you'd know that there are a lot of similarities). We're automated to ridiculous levels (so ridiculous that I use a couple spare Allen-Bradley PLC modules as bookends).

How big is very big? Well, it takes the milk output of about 15-20,000 cows every day.

You call BS? I'm the one who's cited to nationally accepted standards. Show me something from the NFPA or some other industry association that says a relay-based E-Stop design isn't correct.

Put up or shut up time, chief.
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Old 07-12-2012, 01:53 AM   #13
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Oh, and the bookends? Here's a picture:

image-1776521535.jpg  
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Old 09-20-2012, 07:30 AM   #14
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@PJ, like many I have learned a lot from your schematics. Yes I am going to do it my way. My way happens to be close to the recommendation made by bdjohns1.

Quote:
Originally Posted by bdjohns1 View Post
Another HBTer provided a simple working schematic for a NFPA79 compliant E-Stop here: https://www.circuitlab.com/circuit/5zd3fg/holding-contact/ - combining that simple setup with the ice-cube relay and NC contacts on all of the switches from Kal's design would meet the requirements. Once you reset the E-stop, the ON button doesn't function until all of the other switches are off.
FWIW I used a very similar relay-based sytem with momentary ON and E-stop on the PID controller for my smoker. The only difference is that I used a DPDT relay so that I could control a 15A heating element with switches rated for lower power.

I detect no BS in this thread, only a useful discussion of safety.

But perhaps I should have started a new thread rather than revive an old one, you're right about that.
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Old 09-20-2012, 10:06 AM   #15
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Oh, and the bookends? Here's a picture:
Haha, only problem with that is it looks like the books should be helping the PLCs stay standing up!
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Old 09-21-2012, 04:28 AM   #16
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Ole PJ acting the angry drunk again. Must have drank a little too much of the home brews again.

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Old 09-22-2012, 06:19 PM   #17
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Quote:
Originally Posted by mattd2

Haha, only problem with that is it looks like the books should be helping the PLCs stay standing up!
True. I should be using some spare 3" tri clamp pneumatic valve bodies with such weighty texts. Safety first and all
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Old 09-26-2012, 04:41 PM   #18
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Oh, and the bookends? Here's a picture:
I have to ask. If you got spare modules to use as book ends do you have a spare thermalcouple module you can let go cheap or trade? I need one for my allen bradley build.
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Old 09-27-2012, 02:13 PM   #19
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Ask Maxim for a sample of the MAX6675?

http://www.maximintegrated.com/datasheet/index.mvp/id/3149

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Old 09-27-2012, 09:30 PM   #20
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Thanks for the info. I should be getting the kit from them in a few days. It looks like you can only use one thermocouple while using this. Did you use these for your build? I'm looking to use 4 thermocouples in my build. Not sure this will work.

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