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Old 12-01-2012, 12:11 PM   #21
Rarian
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Sorry but I must disagree. Unless your control panel falls into your boil kettle, what is the reason for killing the feed? If it did fall into the BK, the gfci would take care of that anyway. Killing the main contactor will kill all power leaving the CP and that is where the issue would be. I think using both methods is smart. If I had to chose one, my e-stop would kill my main contactor and it does.
For what it's worth, if I was only going to do one it would definitely be to cut the main contactor. The GFCI shunt is there as secondary to this, and stemming from some of my formal training to over-engineer everything.

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An OH - By the way - The 1 amp fuse I placed in the diagrams is only there to insure that the wiring is done in a proper way. Once completed and tested, it is no longer needed. (It is a initial don't fail on me plan - once done? Not needed.!)
Thanks P-J, I understand this, at the same time I'd rather keep it inline as redundancy. It's not harming anything being there, and in all likelihood will never blow but if I needed it to, I'd enjoy having it stop more than an amp going across that line.
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Old 12-01-2012, 12:21 PM   #22
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But, if the 1A fuse is blown, your E-Stop is just a nice plastic red button that does nothing.

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Old 12-01-2012, 01:08 PM   #23
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I agree with lschiavo.

Using a safety device as a control is a poor practice. Doesn't seem to be much different than jamming needle nose pliers into an outlet to trip a breaker so you can work safely on the circuit.

Also (and I am not a licensed electrician so my interpretation may be wrong) the NEC seems to prohibit intentionally using the ground as a current carrying conductor.

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Old 12-01-2012, 01:14 PM   #24
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...
Also (and I am not a licensed electrician so my interpretation may be wrong) the NEC seems to prohibit intentionally using the ground as a current carrying conductor.
I suggest that you read the code that applies to the building wiring when it was installed. You might learn something.
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Old 12-01-2012, 02:18 PM   #25
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I suggest that you read the code that applies to the building wiring when it was installed.
I'll do that.

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You might learn something.
I'm always willing to improve my understanding.

Here's something to improve yours.

I am an Industrial Controls Tech. I maintain, repair and upgrade control systems like these every day. I showed your E-stop circuit to our Electrical Design Engineering group. Degreed Engineers with collectively over a century experience. Every one of them said it was a bad design.

Will it work? Probably.

Is it a good way to implement an E-stop? No.

Is an E-stop needed? No. Electrical failures happen too quickly and the E-stop button is located next to whatever disconnect you have anyway.

Don't get me wrong. The schematics you have provided are otherwise fine and I appreciate the free service you have provided to members of this forum.

If you want a detailed explanation of why the engineers didn't like your circuit I will be happy to have one of them contact you.
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Old 12-01-2012, 06:49 PM   #26
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I'm an engineer (but I haven't worked on circuits like this for many years) and my objection is that such a system fails 'on'. You want a safety system to fail 'off'. Using the E-stop button to unlatch a contactor fails off and is the way that such controls are usually set up.

One way to do things is have the key switch send line AC to the controllers and a 24 V transformer. The secondary (24V) of the transformer would be connected to a series chain of push buttons, terminal blocks (with jumpers) and the contactor coil. The first push button (START) is NO has an auxiliary contact on the contactor wired across it. The other push button is NC. When you push the START button the contactor is energized thus closing the auxiliary contact and latching the contactor on. It stays on until you push the NC button which is the STOP button or one of the links on a terminal block is removed. Remote STOP (NC) buttons can be attached to the terminal blocks and daisy chained all over the place e.g. if your setup is in the basement and you smell smoke in the kitchen you can mash a STOP button at the top of the basement stairs. Other normally closed safety switches (e.g. liquid level switch or ground fault/N-G fault detection circuit) can be connected to another terminal block. Any thing which fails disables the system. It can't be turned on until the fault is cleared. And after the fault is cleared the system can only be turned on by pushing 'START'. An added benefit here is that the safety loop is low voltage - same as your doorbell and furnace thermostat.

The code does prohibit use of the ground conductor as a current carrying conductor. It does allow the use of the neutral as a grounding conductor in grandfathered installations. Not sure I see the relevance of that to this discussion here unless it refers to the fact that the ground carries the faux fault current in the 'fool the GFCI' arrangement.

With START/STOP push buttons and a contactor I don't see the need for the other two contactors.

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