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3-Phase Heating with Single Phase GFCI

 
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Kevin59




Joined: 03 Aug 2012
Posts: 1047
Location: Fort Collins, CO

Drinking: Imperial Brown Ale

Working on: Oatmeal Stout, IPA


PostLink    Posted: Fri Nov 20, 2015 8:32 pm    Post subject: 3-Phase Heating with Single Phase GFCI Reply with quote


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Been a while since I've had time to post here, so I thought I'd come back with something a bit off-the-wall. I'm looking at putting together a 1 bbl setup to use as a pilot line at our brewery when we open next year, and I want to use the 208V 3-phase power for the heating elements in the BK. But since I'll be wanting to use it in advance at home I want to use (3) 6000W/240V "single phase" heating elements in the BK, allowing me to use 2 of them at home on my 2 separate 240V/30A GFCI circuits, or all 3 of them in a 3-phase delta configuration at the brewery. I'm including an image and PDF for reference, so here I go with my crazy ideas.

I will have (2) 3-phase 208V/50A outlets at the brewery whose primary purpose is to drive (2) 15kw 3-phase elements in our 400 gal HLT. When running the pilot system one of those outlets will be used for the 1 bbl pilot setup. Full disclosure - the outlets are NEMA L15-50 so just 3 hots and ground. Those 3 hots are L1, L2 and L3 in the diagram. Neutral "N" will come from a 120V line into the "enclosure" shown in this diagram. Both the 208V 3-phase and 120V single are coming from the same sub-panel, and the neutral line will not be used for anything other than the neutral reference for the (3) 240V/30A GFCI breakers shown. I have not doubt that such a configuration isn't "code" per se, but my bigger question is does this work. So to summarize the enclosure itself it will have two power cords, one L15-50 for the 3-phase power in and one L5-15 for 120V hot/neutral with only neutral being used.

The small diagram on the right edge shows the effective circuit I'm trying to build. Each element has a resistance of 9.6 ohms (240V * 240V / 6000W = 9.6 ohms). So the leg/phase current will be 21.7A (208V / 9.6 ohm), and the line current will be 37.5A (SqRoot of 3 * 21.7A).

So here's where things get really wonky... I'm trying to use a 30A 3-pole w/ neutral GFCI breaker to implement GFCI protection for each of the element circuits. The neutral output from the breaker is not connected which I know is fine, since the GFCI circuitry will look for a balance between the two hot lines and be "happy" if they're balanced. Or I guess I should say I believe that should be fine.

Why am I doing this? Because purchasing a true 3-phase power GFCI protection device from Bender would cost a bloody fortune (~ $1500) as compared to (3) 3-pole GFCIs.

One thing I'm already wondering about is whether I should use a 3-phase SSR to switch the 208V 3-phase coming into the panel, or use 3 SSRs, 1 each on each heating element leg. The latter would be nearly identical to how we switch the elements in the TEB panel and so the GFCI should not trip. But I wonder if a 3-phase SSR on the front end could somehow create an imbalance during switching?

Please fire away, ask questions, point out oversights on my part, etc. This is a work in progress, but since this is the best electric brewing forum on the planet...

Cheers!

Kevin



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Yo_Pauly




Joined: 30 Nov 2014
Posts: 42
Location: New Richmond, WI


PostLink    Posted: Mon Nov 23, 2015 9:00 pm    Post subject: Reply with quote

After contemplating this for a while I've convinced myself that the DIY "3-phase" gfci you have proposed won't work and here's why: The breakers you plan to use are the same as the ones most home brewers have feeding their 30A control panel, correct? With standard 240v house wiring L1 and L2 are 180 deg out of phase and the sum voltage is always zero (or perhaps stated another way, L2 acts as the return for L1 and vice versa and the currents in these two legs are always balanced;) however, with 3-phase the legs are 120 deg out of phase, so the sum voltage at each of the gfci breakers will never be zero. Therefore, return current for L1 will be split between L2 and L3, Etc. This imbalance I believe will immediately cause the gfci's to trip. Now, I'm no electrician, and I encourage you to pose your question to the larger electrical community elsewhere on the web. Good luck. please post back here anything you learn along the way.
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Kevin59




Joined: 03 Aug 2012
Posts: 1047
Location: Fort Collins, CO

Drinking: Imperial Brown Ale

Working on: Oatmeal Stout, IPA


PostLink    Posted: Mon Nov 23, 2015 9:53 pm    Post subject: Reply with quote

Hmm.. Very interesting and I'm thinking you might be right about that... I will definitely do some more digging into this, and go back and see what I can re-remember from my power classes 38 years ago....

Thanks Yo Pauly.
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Yo_Pauly




Joined: 30 Nov 2014
Posts: 42
Location: New Richmond, WI


PostLink    Posted: Mon Nov 23, 2015 10:03 pm    Post subject: Reply with quote

It "might" work if the loads were perfectly balanced and the elements were all firing at the same time, but in the real world I wouldn't expect that to be the case.
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Kevin59




Joined: 03 Aug 2012
Posts: 1047
Location: Fort Collins, CO

Drinking: Imperial Brown Ale

Working on: Oatmeal Stout, IPA


PostLink    Posted: Mon Nov 23, 2015 11:37 pm    Post subject: Reply with quote

Yo_Pauly wrote:
It "might" work if the loads were perfectly balanced and the elements were all firing at the same time, but in the real world I wouldn't expect that to be the case.


An interesting read on the topic perhaps... My brain hurts now...

http://www.screenlightandgrip.com/html/481_GFCI_Workshop.html

About 20% of the way down the page:

To understand how Single Phase 220V GFCIs (like the one pictured above) work, think back to the Open Neutral display you may have seen at one of our IATSE Local 481 Electrical Training Seminars. If you recall it consists of a board with three rows of lighting bulbs - each row consisting of ten 100W light bulbs. Each row of bulbs is powered by a different leg of the service (L1, L2, L3), they all share in the Neutral. For the first part of the demonstration the Neutral is lifted. Rather than going out, the bulbs all stay on because the current of one leg (L1) returns on another leg (L2). In other words, absent a Neutral the current on L1 returns on L2 and visa versa the current on L2 returns on L1. It doesn't matter that they are not 180 degrees out of phase because the return current of L1 cancels the going current of L1 and likewise for L2 since they both pass through the CT.

That's just one "leg" of my proposed idea. Like you said it might work with all 3 legs but even with the SSRs driven by the same control signal perhaps not...
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Kevin59




Joined: 03 Aug 2012
Posts: 1047
Location: Fort Collins, CO

Drinking: Imperial Brown Ale

Working on: Oatmeal Stout, IPA


PostLink    Posted: Tue Nov 24, 2015 6:36 pm    Post subject: Reply with quote

Yo_Pauly wrote:
It "might" work if the loads were perfectly balanced and the elements were all firing at the same time, but in the real world I wouldn't expect that to be the case.


Yo Pauly,

Just to further the discussion as it were, imagine a 3-phase 120V/208Y load center than can provide 120V single phase, 208V 3-phase and 208V single phase. I wonder (and I'm Googling to check) if a 2-pole 208V GFCI breaker is available? Looking at the Eaton product line it seems like do, which would imply this configuration could work... Maybe...?
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Yo_Pauly




Joined: 30 Nov 2014
Posts: 42
Location: New Richmond, WI


PostLink    Posted: Wed Nov 25, 2015 6:12 am    Post subject: Reply with quote

Following the Amazon link for the recommended 30 Amp 2-pole 240V gfi breaker I found this information posted for a Siemens QF-230 gfi:

"Siemens GFCI circuit breakers are UL Listed and CSA Certified as Class A devices. Current imbalances of 4-6 milliamps or more between load conductors will cause the ground fault sensor to trip the circuit Breaker. Note: A load neutral is not required on the circuit. However, the white line neutral (pigtail) must be connected to the panel neutral for the device to function. The Siemens 2-pole GFCI circuit breaker can be installed on a 120/240-volt AC single phase, 3-wire system, the 120/240-volt AC portion of a 240/120-volt, 3-phase, 4-wire system, or on a 208Y/120-volt, 3-phase, 4-wire system. When installed on these systems, protection is provided for 2-wire, 240-volt AC or 208-volt AC circuits; 3-wire, 120/240-volt AC circuits. ..."

"So you're sayin' there's a chance!"

Let me reiterate that I'm out of my league on this subject. However, I still feel nuisance tripping may be an issue. Be safe. Get expert opinions from multiple sources, if possible. I'd wager a bet that even they might disagree!
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Kevin59




Joined: 03 Aug 2012
Posts: 1047
Location: Fort Collins, CO

Drinking: Imperial Brown Ale

Working on: Oatmeal Stout, IPA


PostLink    Posted: Wed Nov 25, 2015 1:02 pm    Post subject: Reply with quote

So I created an account over on Mike Holt's Forum to discuss this as well. That seems to be an all-things-electrical place to go. I've had one response that basically says it will work as long as the breakers are designed for use on a 120V/208Y panel, so your Siemens info (thanks by the way!) substantiates that.

I guess time will tell if I pursue this solution.

Cheers!

Kevin
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huaco




Joined: 05 Apr 2012
Posts: 1506
Location: Burleson Texas


PostLink    Posted: Wed Nov 25, 2015 2:25 pm    Post subject: Reply with quote

Yo_Pauly wrote:
"So you're sayin' there's a chance!"


Sorry, this is off topic... But I saw your Dumb & Dumber quote here... I'm a Dallas Cowboys fan and THIS is exactly how I'm feeling right now. The NFC East is SO bad this year! lol
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