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 Post subject: Operation of Manual Transfer Switche Under Load?
PostPosted: Wed Apr 02, 2014 12:37 pm 
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Location: Utah
Any thoughts Pro/Con of operating a 400V 3-Phase 1200A Manual Transfer switch and how to safely implement this?

Thanks!


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PostPosted: Wed Apr 02, 2014 2:09 pm 
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1. Has an arc flash hazard analysis been performed for this switch?
2. If so, was it an actual study or are you using the tables in NFPA 70E?
3. If using the tables, how do you know you are meeting the parameters of the table?

The above will allow you to select the proper PPE for this task. Since it is a manual transfer switch, it should be operated without any load on it to be safe.

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PostPosted: Wed Apr 02, 2014 2:21 pm 
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There is no arc flash hazard analysis presently as this is still in the design phase, so my question should have been more specific to this. Thanks!


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PostPosted: Thu Apr 03, 2014 7:54 am 
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wbd wrote:
Since it is a manual transfer switch, it should be operated without any load on it to be safe.


Why??
There are many load break rated manual transfer devices.
I don't know why you would want to slam 1200A of load onto a source, but hey that is what happens with a regular ATS during retransfer.


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PostPosted: Thu Apr 03, 2014 3:02 pm 
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Whenever possible it is preferred to transfer without loads, but certainly not a requirement. All switches are rated for a number of mechanical cycles, and electrical cycles under certain conditions such as under rated load or under maximum interrupting load. The number of cycles gradually decreases in this order. So for instance a switch might be rated to 10,000 mechanical cycles, 1,000 cycles under normal rated load, and only 1 cycle under maximum interrupting capacity. Also, some switches are not designed to break loads (non-LBS).

Note that these parameters are much easier to find with utility grade gear than others but the data is there if you look hard enough for it. For the record I've put in a lot of manual switches. Depending on conditions it is often easier and more reliable to go find an available generator or welder that works than to maintain a dedicated, fixed backup generator. I use an ATS though when it's a dedicated generator.


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PostPosted: Fri Apr 04, 2014 3:47 am 
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Yes, ATS will transfer under load but it is faster than a person manually operating a transfer switch. I think is safer to transfer with as much load off.
Question to Kimo: Does the transfer operation happen prior to generator starting or are the loads operating, generator operating and then transfer switch operating? You substitute second source for generator here.

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PostPosted: Fri Apr 04, 2014 6:58 am 
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Not sure on how they actually plan on implementing. This is for a large dining facility, overseas, with refrigeration. There will be a 2000A main that will feed a 1000A DP for refrigeration and a 600A DP for general power/HVAC/lighting/etc. Each DP will have a transfer switch for a possible generator. If the facility is going to be down for an extended period they will bring in mobile generators. 1st Priority is the 1000A DP for refrigeration so food does not spoil, 2nd is the 600A DP if they decide to cook and serve. So, fast switching time is not an issue and hence why the do want/require an ATS. My understanding of how you would normally bring up a generator (as how it was normally done while I was in the Navy), start-up the generator (un-loaded), bring-on-line, and then apply loading. I would hope the procedure would be similar for this installation, but if someone does not turn off the refrigeration units prior to bringing the 1000 DP on-line, there is the potential of a large inrush of current as all the compressors are starting up simultaneously. My preference in this would be more inclined to have remote operation for this application, but I am not sure they even want to spend the money for this.


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PostPosted: Fri Apr 04, 2014 7:12 am 
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wbd wrote:
Yes, ATS will transfer under load but it is faster than a person manually operating a transfer switch.

Most load break rated Manual Transfer Switches operate as quickly as an ATS and typically much faster than M-T-M throw over schems using separate devices.

Based on the most recent information, I see nothing in the OP's system that could not be powered down so that the transfer is done no-load.


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PostPosted: Fri Apr 04, 2014 7:14 am 
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Kimo wrote:
... there is the potential of a large inrush of current as all the compressors are starting up simultaneously.

The compressors don't have a start button? Just an ON button?
Maybe a small relay timer in their control circuit when they see power would be enough to not all start at the same time when power comes back, with a different setting for each (staged). Just an idea.


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PostPosted: Mon Apr 07, 2014 7:00 am 
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Does your manual TS have in-phase monitoring? If so, will it be enabled? Because you might be transferring power with both sources operating and have motor loads, consider writing a method of procedure and or doing the transfer with the power off. If that's not always possible, see if you have in-phase monitoring in the switch. The idea behind in-phase monitoring is to prevent damage the equipment connected to the sources, it's also to prevent tripping breakers or fuses. ASCO says the inrush with an out of phase transfer can be very high.
The following comes from ASCO's website
"An In-phase monitor is a motor load transfer feature that monitors the voltage, frequency and phase angle of the two sources. It ensures the two sources are within predetermined phase angle before the ATS controller initiates transfer of the load to the alternate source. The transfer is completed in a safe window to limit the motor inrush current to be equal to or less than normal motor starting currents"


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PostPosted: Tue Apr 08, 2014 3:44 pm 
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Just looking at your ratings, honestly, good luck getting arc flash down to reasonable levels. Strongly suggest that you consider putting in more transformers and distributing the load out more. For 480 V three phase, it is difficult to do much of anything with arc flash once you exceed around 1500 kVA. Once you go to a higher voltage level though (2300 or 4160), I'm not sure where the cutoff is since I've successfully kept it down to reasonable values up to 10,000 kVA which is the largest transformers that I've worked with in recent memory, even with very low impedances.


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