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2 Chiller - redundancy

Question asked by chiefns830 on Aug 12, 2019
Latest reply on Aug 16, 2019 by chiefns830

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I have two, large commercial chillers. One will operate at any given time. The designer is attempting to use an ATS to switch between which chiller will operate. I'm told that there will be a "remote switch" inside the building that will activate the ATS to switch over to the other chiller.

I'm asking if this is a listing violation. An ATS is designed to switch between two power sources and one load. I don't believe it is listed to switch between two loads, from one power source.

Q1 - Do you agree that what I'm attempting to describe is a problem?

Q2 - Wouldn't they require a 3-pole, double-throw switch for this purpose?

Any comments would be appreciated.

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11 Replies

Steve Benton Aug 12, 2019 1:34 PM
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Nick, I have seen situations where an ATS has been used to switch between two loads, most commonly between large sump pumps. One is set up as normal with a failover by the ATS if the primary pump fails. Also, in many industrial applications the ATS is is breaker based or contactor based. At the location where I work we have both types installed.
I hope that this helps.
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j_w_hines Aug 12, 2019 3:20 PM
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I called 2 suppliers and both said ATS UL listed is as you said, transfer between 2 power sources. Steve added that it could be contactors with a controller. If larger chillers, they should have a sequence and cutting power is probably not a great idea. Suggest you ask for manufacturer literature for proposed switch and the chillers.
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chiefns830 Aug 12, 2019 9:58 PM
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I will keep everyone posted as this plays out. I'm still not convinced it is possible, but we will see what they come up with. Maybe there is some way...
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graceyj20 Aug 15, 2019 9:52 AM
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I've seen a manual transfer switch for this type of setup several times. It's a transfer switch designed for that use and not an ASCO or Ruselectric typical switch. Alternately, you could use control lockouts.
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fourseng Aug 16, 2019 9:52 AM
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I have witnessed and extended a single source of power to large motors in the past where the HVAC engineer
specified an "Alternator" [ best word I could re-call ] where the control device sensed the shut down of the
motor and automatically switched to the "Other motor". I believe their is such equipment available for this operation
that also would be "Listed" for such an operation.
I hope this helps.
Have A Great Day
Harvey Peel, PE
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david.heiliger Aug 16, 2019 11:15 AM
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If Installed to NFPA 70; 705. 12 A & B. I believe it is still the same principle, with motors as too panelboards first then to load. And disconnection (NEC 705.12 (B) overcurrent protection is on the load side of either side of ATS switch? Though if the unit is hard start and not a soft start (VFD) I assume a starting contactor will carry the arching from the starting load. And that remote ATS time delay so it does not try and switch over under a load or switch back and forth during monetary power loss, causing arching on contacts. One chiller would be on normal open contacts while the other on the closed contacts. It probably has, safety's circuits that are ran through SCAD system and / or PLC controls, to prevent this. Though we had six 4K ton chillers (Carrier 17 DA's) on 4160. We spent, Millions on new VFD's too soft start these chillers and only run them up to only 70% before we brought on another chiller, then bring them both down to 35% operation level for energy savings. Motors that only run up to 70% cuts energy consumption down to almost half. That’s why we would not run at full speed. Plus starting load is high. The chiller on hard start, required us calling the power company before starting the unit to increase the voltage 10% until it got to full speed before opening vain’ s to add load. It dim lights all over the area, when starting. All the other chillers on old VFD's were not reliable and expensive to maintain, and cheaper to replace with new drives. The money spent in this equipment paid for its self in savings in three and a half years. Though whatever the voltage, we found between power spikes or during heavy loads, the utility company could not always provide a good sine wave or power factor, thus causing the drives and motors to work harder and fail. So past the step down transformer we had another transformer to clean the power sine wave and provide a steady power factor needed for large motors with drives. A lot depends on how much you have to spend.
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rcramereleceng Aug 16, 2019 11:24 AM
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Perhaps there are no restrictions from the ATS OEM in reference to this however, it opens up a plethora of problems.

No matter how many signs, bells and whistles there are, this is not a normal installation. Perhaps everybody from the get go are on-board with this cleaver (miss)use of a component - there will undoubtedly be new hires or Contractors from the outside servicing the equipment - usually in a rush - and possibly complicating any problems that come up down the road.

Per the Code, the Power (or Line side) is installed on the top and the Load side goes on the bottom … with the exception of … (come you Code guys, what are the exceptions?) … unless the Power comes in from under ground and so on. THEREFORE, the ATS has to be installed upside down. Hmmmm, just flip the Label over - good to go - I don't think so. Let's put up some more signs, bells and whistles.

Next issue is a Control function problem. The ATS operates via loss of Power, therefore (THEREFORE) the Remote Control Switch used to swap Chiller selection has to always have power to the ATS Control Module. While the normal Controls Designer might not see this a problem, this is huge. Often enough, work is done on Equipment where something doesn't go quite right and there's an inadvertent interruption on the Power - Chiller swaps. Or, there's a Power interruption from the Utility Company, perhaps during a storm, and the Chiller swaps.

Even if you have convinced yourself there's no problem (yet), the out-of-the-box parameter selection on the ATS keypad will need to be modified no doubt. This is not a problem until it becomes a problem (haha). Say for example replacing it for some reason down the road.

What ever happened to KISS (Keep It Simple & Straightforward). The way to control this goes back a hundred years. The original application for swapping Power started with Pumping applications where a "Duplexing" system was developed. Another name was mentioned by Fourseng, an Alternator, or an Alternating Relay. This has been so long out of use that modern terminology confuses it with an Alternator in Automobiles - not the same thing. An Alternator Relay is also known (and sold as) a Ratchet Relay. These Relays 'alternate' or change state literally using a mechanical ratchet. The key to this feature is that it remains in it's last controlled state until told to change (mechanically or electrically). So if Power is lost and then restored, it powers back up with the last state it was in before power loss.

So what's a mother to do? The problem is that the Designer was trying to cut costs of having (2) honker Contactors. My solution is thinking outside the box by utilizing a Reversing Motor Starter Combination. Still (2) honker Contactors BUT, one MMS (Manual Motor Starter). The Control Coils of the Contactors are interlocked AND the Contactors can be 'Mechanically Interlocked' whereby only one OR the other Contactor can be engaged.
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- fourseng @ rcramereleceng on Aug 16, 2019 11:50 AM
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  I clarified that the word "Alternator" was a past description and a path
  for discussion; participation on the exchange is to help others NOT to be
  ostracized!
  Harvey Peel, PE
  Have A Great Day
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david.heiliger Aug 16, 2019 11:29 AM
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NEC 430.13 (A) & (B) Arching principle under load.
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- chiefns830 @ david.heiliger on Aug 16, 2019 7:26 PM
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  Never mind, I was looking at the wrong chapter.... : (
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chiefns830 Aug 16, 2019 7:26 PM
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Hi David, I'm using 2017. I don't see that reference, did they move it... or omit it?

fyi - I still did not get any information back on this. We will wait and see how they attempt to handle. The only thing I can come up with is a manual, TPDT safety switch. But maybe a manufacturer makes something for remote (automatic) operation. Example:
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2 Chiller - redundancy

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