Precedence of Emergency Branches

mbrooke

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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.
 

steve66

Senior Member
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.

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.
 

mbrooke

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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/FBODocumentServer/DocumentServer.aspx?DocumentId=1217877&FileName=VA263-14-Q-1315-002.pdf


Page 34 onward:

https://www.dshs.wa.gov/sites/default/files/FSA/capital/17103.09 WSH Generator and ATS Report - Final.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 :)
 
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mbrooke

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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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GoldDigger

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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

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mbrooke

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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

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Did you ever find out why the main opened?
 

mbrooke

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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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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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ee
FWIW, here is 80's paralleling set up with automatic load shedding. Life safety takes precedence over critical it seems:

No question. Life Safety (ingress, egress, gas alarms) are just that--if they fail, doctors and nurses can't get in or out to help patients in locked areas, conduct evacuations or bringpatients into ICUs when be limited by locked doors or doors that cannot be manually opened without power. Gas alarms tell you when your oxygen, surgical suction or waste anesthetic gas evacuation is failing.

The paradox is that the critical equipment like mechanical ventilators, surgical equipment and so forth can and do fail, and that's actually more acceptable than Life Safety. For example, every ventilator currently in use (ignoring those in storage for the pandemic) in my local hospital has a built in backup battery, which is key, as sometimes they become unplugged or need to be moved within the room for a procedure. Also, every ICU nurse and respiratory therapist is trained to instinctively take the patient off the ventilator and "bag" them manually if there is any hardware issue (most often not power related!--these are modern computers), or if there is an obstruction or disconnection in the gas tubing circuit to/from the patient. Not a big deal--unless you can't open the door to get inside the ICU/room or there is no gas to bag the patient with an oxygen requirement with (which you wouldn't know was needed without an alarm system) or can't open the closet with the back up tanks, etc. Failure is the norm--it's all about the backup plan and detection (alarms, doors, lights, patient safety controls--not the actual processes which "do the work").

Even if both the LS and C circuits must be backed up, having the LS circuits separate and on with the most limited amount of equipment reduces the probability of overcurrent interruption, and other troubleshooting issues. Redundancy and simplicity drove that.
 
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mbrooke

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No question. Life Safety (ingress, egress, gas alarms) are just that--if they fail, doctors and nurses can't get in or out to help patients in locked areas, conduct evacuations or bringpatients into ICUs when be limited by locked doors or doors that cannot be manually opened without power. Gas alarms tell you when your oxygen, surgical suction or waste anesthetic gas evacuation is failing.

The paradox is that the critical equipment like mechanical ventilators, surgical equipment and so forth can and do fail, and that's actually more acceptable than Life Safety. For example, every ventilator currently in use (ignoring those in storage for the pandemic) in my local hospital has a built in backup battery, which is key, as sometimes they become unplugged or need to be moved within the room for a procedure. Also, every ICU nurse and respiratory therapist is trained to instinctively take the patient off the ventilator and "bag" them manually if there is any hardware issue (most often not power related!--these are modern computers), or if there is an obstruction or disconnection in the gas tubing circuit to/from the patient. Not a big deal--unless you can't open the door to get inside the ICU/room or there is no gas to bag the patient with an oxygen requirement with (which you wouldn't know was needed without an alarm system) or can't open the closet with the back up tanks, etc. Failure is the norm--it's all about the backup plan and detection (alarms, doors, lights, patient safety controls--not the actual processes which "do the work").

Even if both the LS and C circuits must be backed up, having the LS circuits separate and on with the most limited amount of equipment reduces the probability of overcurrent interruption, and other troubleshooting issues. Redundancy and simplicity drove that.

But can't these doors be manually opened? Second, how can you bag 30 or more patients in a pitch black ICU? Plus getting meds in under a controlled fashion? Or an OR with a patient under circulatory bypass? Heart lung machine? Remember that life safety only hits exit signs, hallways and stairwells. Not inviduals rooms or ORs. Stairwells are easier to nevagate with flash lights than dozens upon dozens of rooms.
 
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ee
You’re right. The corridor should at least be lit (517.32(A)) incidentally but yeah, its odd. I still think (but do not know the history for sure) that the motivation is to have the smallest set of conductors and loads so having every critical care room lighting on LS circuits would pervert that. It’s a fair proposal though. Virtually all of the other machines you mention can be self battery powered. For example unplugged and moved around for a while. No question if critical fails for more than a bit the stuff will hit the fan in any hospital. (and that is while LS and C are both emergency circuits.)
 
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mbrooke

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You’re right. The corridor should at least be lit (517.32(A)) incidentally but yeah, its odd. I still think (but do not know the history for sure) that the motivation is to have the smallest set of conductors and loads so having every critical care room lighting on LS circuits would pervert that. It’s a fair proposal though. Virtuslly all of the other machines you mention can be self battery powered. For example unplugged and moved around for a while. No question if critical fails for more than a bit the stuff will hit the fan in any hospital. (and that is while LS and C are both emergency circuits.)
But why not have two C circuits? I mean code specs two branches and two ATS... so why not take advantage of that and have both do 50/50 of essential services in any critical area? Require reasonable separation with 100% selective coordination would take care off most of the arguments against it.

Any sane doctors would laugh and wonder why the OR is pitch black, but the automatic sinks are the only thing that works (those are on the equipment branch).



Does the blood cooling during heart bypass have battery backup?
 
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mbrooke

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Seems like a good idea, although not code :). Maybe one day it will be. Or have critical lighting as its own. Complexity has its own downsides of course.

Yeah, all of the non-antique ECMO and bypass machines have integral batteries. For example, a discussion of that on the medical side:

You want to know a little secret? Outside the US, Mexico, and Canada hospitals typically have two or more independent, separated power systems with high level selective coordination that will feed all areas of the hospital 50/50 or 33/33/33 including stair wells. That way to total, catastrophic melt down or causal auto throw over failure will not black out any part of the hospital even under utility failure.

I personally like this concept way more.
 
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