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Thread: Precedence of Emergency Branches

  1. #21
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    Quote Originally Posted by d0nut View Post
    I don't really have a good answer for that question. There must have been a reason when the code required the separate branches rather than allowing everything on one ATS.

    Regardless, with all of the testing of the essential electrical system that is required, I would imagine that simultaneous failures of the utility and an ATS would be rather rare. I think that we in the industry spend so much time thinking about how to mitigate problems that we tend to overestimate the likelihood of the problems occurring. Arc flash events are rare events when you consider how much electrical equipment there is installed, but yet we spend much time and effort trying to mitigate the danger of an arc flash that I am sure I would greatly overestimate the likelihood of an arc flash occurring.
    That would be I guess a low probability high impact event- not likely to happen, but when it does the risk to life is high.

    Hopefully someone on here knows why code made the change.

  2. #22
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    1983 change? Found this:
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    Quote Originally Posted by mbrooke View Post
    Regarding the last part if more load meant greater chance of failure we would be talking about loading restrictions or restrictions on the max size equipment that could be used.
    But there are restrictions - big equipment goes on the equipment branch (only fractional horsepower motors are allowed on the critical branch), and only loads required for emergency use are allowed on the LS branch. There are other limits on what can be placed on each branch per 517. Also, its noted that the number of ATS's should be based on the load to be served, so they basically leave it up to the designer to decide when one ATS should become 2 separate ATS's. If you really want redundant branches to critical circuits, just install 2 separate ATS's.

    Quote Originally Posted by d0nut View Post

    Regardless, with all of the testing of the essential electrical system that is required, I would imagine that simultaneous failures of the utility and an ATS would be rather rare.
    Its not so much limited to an ATS failure. Having a breaker trip that supplies the ATS with em. power can also cause an area to be without power when the normal power is out. Or if a main breaker that's on the load of an ATS trips, that can also leave an area without power even when both the utility and generator are operational.

    I've heard that back in the 60's when backup generators for OR's became common, many engineers thought it was best to just put the entire OR on backup from a single ATS. I hear there were several blackouts in the OR's caused by either a tripped breaker or failed ATS's, while normal areas of the hospital where operating as normal under the usual utility power. Didn't take long for a code requirement for normal power to be added for critical care areas.

    Today that requirement still stands, unless the area is supplied by 2 separate critical branch ATS's.

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    Quote Originally Posted by steve66 View Post
    But there are restrictions - big equipment goes on the equipment branch (only fractional horsepower motors are allowed on the critical branch), and only loads required for emergency use are allowed on the LS branch. There are other limits on what can be placed on each branch per 517. Also, its noted that the number of ATS's should be based on the load to be served, so they basically leave it up to the designer to decide when one ATS should become 2 separate ATS's. If you really want redundant branches to critical circuits, just install 2 separate ATS's.


    Ideally, but many installations are code minimum. It should be a mandate that critical loads be split across 2 ATSs.


    Its not so much limited to an ATS failure. Having a breaker trip that supplies the ATS with em. power can also cause an area to be without power when the normal power is out. Or if a main breaker that's on the load of an ATS trips, that can also leave an area without power even when both the utility and generator are operational.

    I've heard that back in the 60's when backup generators for OR's became common, many engineers thought it was best to just put the entire OR on backup from a single ATS. I hear there were several blackouts in the OR's caused by either a tripped breaker or failed ATS's, while normal areas of the hospital where operating as normal under the usual utility power. Didn't take long for a code requirement for normal power to be added for critical care areas.

    Today that requirement still stands, unless the area is supplied by 2 separate critical branch ATS's.

    I think it boils down to code minimum. I hate to say it, but for many the NEC is simply a design guide. IE 2 random examples;


    Page 9 and going onward:


    https://www.vendorportal.ecms.va.gov...Q-1315-002.pdf


    Page 34 onward:

    https://www.dshs.wa.gov/sites/defaul...0-%20Final.pdf


    Both are real world (typical really) examples of code minimum where a failed critical ATS or critical branch will take out 90% of the critical care area receptacles and 100% of the critical loads in operation.

    BTW, thank you for the history
    Last edited by mbrooke; 06-20-19 at 05:53 PM.

  5. #25
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    Ie, a 400 and 800amps ATS, replaced with two 600 or two 800amp ATS. Each floor already has multiple critical risers and panels.


    Ideally moving a few conduits around in the basement between the life safety and the critical switchboard would add diversity. Patient booms and head wells fed from at least 2 critical panels if not already the case.


    Just food for thought- what if, why not- type deal.
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    At a large experimental physics lab at Stanford, backed up power was provided for those bits of equipment that would cause damage to the apparatus during an unplanned shutdown.
    Unfortunately the individual experimenters were responsible for their own decisions and the main breaker to the backup power network opened, guaranteeing damage for everybody without an actual outage

    Sent from my XT1585 using Tapatalk

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    Quote Originally Posted by GoldDigger View Post
    At a large experimental physics lab at Stanford, backed up power was provided for those bits of equipment that would cause damage to the apparatus during an unplanned shutdown.
    Unfortunately the individual experimenters were responsible for their own decisions and the main breaker to the backup power network opened, guaranteeing damage for everybody without an actual outage

    Sent from my XT1585 using Tapatalk
    Did you ever find out why the main opened?

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    Quote Originally Posted by mbrooke View Post
    Did you ever find out why the main opened?
    Simple overload. Too much equipment plugged in at one time. The sum of the branch breakers was greater than the back-up panel main. The design load calculations did not have each branch fully loaded at the same time.

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  9. #29
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    Quote Originally Posted by GoldDigger View Post
    Simple overload. Too much equipment plugged in at one time. The sum of the branch breakers was greater than the back-up panel main. The design load calculations did not have each branch fully loaded at the same time.

    Sent from my XT1585 using Tapatalk
    Thank you for this. ICU and OR panels add to over 1,500amps of branch breakers but only have a 200amp feed.

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