Precedence of Emergency Branches

mbrooke

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Why does the Life safety branch take precedence over the critical branch? And why call it critical when it is truly life supporting over all else? Id think ventilators would need power first and before all else.



Hospitals require a large amount of emergency power because lives depend on the ability of many services in the hospital to remain functional at all times. As described previously, code requires that the emergency power is segregated into four different branches. When the generator starts up, the power branches are started sequentially to prevent the generator from being overloaded by all systems starting at once. The life safety branch starts first, followed by the critical power, the equipment, and finally the X-ray branch. Now that the X-Ray is served by a UPS, there will be no problems with starting this last.
 

packersparky

Senior Member
Location
Wisconsin
Why does the Life safety branch take precedence over the critical branch? And why call it critical when it is truly life supporting over all else? Id think ventilators would need power first and before all else.
IMO the life safety branch is focused on supplying loads that are needed to evacuate the building when it is not safe to stay in the building. If a hospital is on fire the patients on life support will need to be evacuated and connection to life support systems would not really matter.
 
IMO the life safety branch is focused on supplying loads that are needed to evacuate the building when it is not safe to stay in the building. If a hospital is on fire the patients on life support will need to be evacuated and connection to life support systems would not really matter.
That's also my thinking. Needs of the many (to move around and maybe get out) over needs of the few.
 

charlie b

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Seattle, WA
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Electrical Engineer
IMO the life safety branch is focused on supplying loads that are needed to evacuate the building when it is not safe to stay in the building.
That notion had never occurred to me. But looking at the list of the only things that are allowed to be on the LS branch, in 517.32(A) through (H), it makes sense. Thanks for sharing.

 

mbrooke

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IMO the life safety branch is focused on supplying loads that are needed to evacuate the building when it is not safe to stay in the building. If a hospital is on fire the patients on life support will need to be evacuated and connection to life support systems would not really matter.



But I don't see a fire today getting that large to the point where 1,500 patients need to be evacuated.


That notion had never occurred to me. But looking at the list of the only things that are allowed to be on the LS branch, in 517.32(A) through (H), it makes sense. Thanks for sharing.


What year did the code mandate the separation of the life safety and critical branch? And idea of the substantiation? I remember old hospitals just mixed the two branches- at least in practice.
 

steve66

Senior Member
Why does the Life safety branch take precedence over the critical branch? And why call it critical when it is truly life supporting over all else? Id think ventilators would need power first and before all else.
I've wondered about that a lot also. Only the LS branch has to comply with the stricter requirements in 700. So its obviously considered more important.

IMO the life safety branch is focused on supplying loads that are needed to evacuate the building when it is not safe to stay in the building. If a hospital is on fire the patients on life support will need to be evacuated and connection to life support systems would not really matter.
Hospitals are usually designed with a "defend in place" strategy for a fire event, since some patients can't be moved.

So I would probably add to that statement that the LS branch is also to provide things like emergency lighting needed to assist fire fighting and emergency operations.

BTW, I'm not sure, but I think ventilators are more commonly powered by compressed medical air. So it makes sense to have the Med Gas Alarm panels on the LS branch.

I've heard tale of a contractor that cut a medical air line, and the havoc that wrought. Nurses had to use the masks with the squeeze bulbs on them to keep patients breathing.

Also, would guess any electric ventilators would probably also have battery backup.
 

mbrooke

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I've wondered about that a lot also. Only the LS branch has to comply with the stricter requirements in 700. So its obviously considered more important.



Hospitals are usually designed with a "defend in place" strategy for a fire event, since some patients can't be moved.

So I would probably add to that statement that the LS branch is also to provide things like emergency lighting needed to assist fire fighting and emergency operations.

BTW, I'm not sure, but I think ventilators are more commonly powered by compressed medical air. So it makes sense to have the Med Gas Alarm panels on the LS branch.

I've heard tale of a contractor that cut a medical air line, and the havoc that wrought. Nurses had to use the masks with the squeeze bulbs on them to keep patients breathing.

Also, would guess any electric ventilators would probably also have battery backup.

Honestly, it would actually make sense if hospitals just required two equal branches. Failure of an ATS or branch results in all critical load being dropped or all life safety load being dropped. Mandating a few receptacles on normal power is silly when a blackout is causing it.


Regarding ventilators they are electric powered, but use piped medical gas (oxygen / air) instead of the air inside the critical care area.
 

packersparky

Senior Member
Location
Wisconsin
But I don't see a fire today getting that large to the point where 1,500 patients need to be evacuated.

What year did the code mandate the separation of the life safety and critical branch? And idea of the substantiation? I remember old hospitals just mixed the two branches- at least in practice.
Hospitals are intended to be "defend in place" facilities, but in worst case scenarios it may be needed to evacuate a part or all of the building. The loads supplied by the life safety branch help facilitate evacuation.

http://www.paho.org/disasters/index.php?option=com_content&view=article&id=1745:hospitals-dont-burn-hospital-fire-prevention-and-evacuation-guide&Itemid=924&lang=en


Article 517 first appeared in the 1971 NEC and required separate branches. Before that there was just an article called "Flammable Anesthetics"
 
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mbrooke

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Hospitals are intended to be "defend in place" facilities, but in worst case scenarios it may be needed to evacuate a part or all of the building. The loads supplied by the life safety branch help facilitate evacuation.

http://www.paho.org/disasters/index.php?option=com_content&view=article&id=1745:hospitals-dont-burn-hospital-fire-prevention-and-evacuation-guide&Itemid=924&lang=en


But worst case such a rarity that it must be weighed against the higher probability of loosing a critical feeder or system.
 

d0nut

Member
Location
Omaha, NE
Why does the Life safety branch take precedence over the critical branch? And why call it critical when it is truly life supporting over all else? Id think ventilators would need power first and before all else.
Both the Life Safety and Critical branch are the same priority. Upon loss of power, the generator must start and pick up both branches within 10 seconds. Neither branch may be shed. The Life Safety branch feeds the loads necessary to evacuate the hospital. The Critical branch feeds the loads necessary to keep people alive in the hospital. The branches are separated to increase reliability. The more loads you add to any branch, the greater the chance that something will fail.
 

mbrooke

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Both the Life Safety and Critical branch are the same priority. Upon loss of power, the generator must start and pick up both branches within 10 seconds. Neither branch may be shed. The Life Safety branch feeds the loads necessary to evacuate the hospital. The Critical branch feeds the loads necessary to keep people alive in the hospital. The branches are separated to increase reliability. The more loads you add to any branch, the greater the chance that something will fail.
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.
 

d0nut

Member
Location
Omaha, NE
True. I guess I wasn't clear with what I was thinking. If you limit the loads to only certain loads as stated in 517 and NFPA 99, you will reduce the chance that other items may be added in the future that compromise the system, such as a large motor with an inrush that may trip an upstream circuit breaker. The life safety branch and critical branch serve such important purposes that we are limited to only specific loads that can be added.

From a practical standpoint, the actual loading on those two branches is somewhat limited as well based on the permitted loads, conductor lengths, building size, etc. I rarely end up with a life safety system over 100A, for example.
 

mbrooke

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True. I guess I wasn't clear with what I was thinking. If you limit the loads to only certain loads as stated in 517 and NFPA 99, you will reduce the chance that other items may be added in the future that compromise the system, such as a large motor with an inrush that may trip an upstream circuit breaker. The life safety branch and critical branch serve such important purposes that we are limited to only specific loads that can be added.

From a practical standpoint, the actual loading on those two branches is somewhat limited as well based on the permitted loads, conductor lengths, building size, etc. I rarely end up with a life safety system over 100A, for example.

But, picture putting 50% of the critical on the life safety and 50% of the life safety on the critical. One ATS or branch fails. The nurses has 24 out of the 48 outlets still energized at critical care beds and infinite life support bassinets.
 

d0nut

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Location
Omaha, NE
But, picture putting 50% of the critical on the life safety and 50% of the life safety on the critical. One ATS or branch fails. The nurses has 24 out of the 48 outlets still energized at critical care beds and infinite life support bassinets.
Take a look at 517.18 and 517.19. Category 1 and Category 2 spaces are required to be fed from both a normal and critical branch (or two critical branches from different transfer switches). If there is a failure of the utility, you will still have power at the locations from the critical branch. If there is a failure of the critical ATS, you will still have power from either the normal branch or the second critical branch. The NEC typically provides for one failure, in this case loss of a utility or loss of an ATS, not loss of a utility and loss of an ATS. Protecting against two failures gets more difficult and much more expensive.
 

mbrooke

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Take a look at 517.18 and 517.19. Category 1 and Category 2 spaces are required to be fed from both a normal and critical branch (or two critical branches from different transfer switches). If there is a failure of the utility, you will still have power at the locations from the critical branch. If there is a failure of the critical ATS, you will still have power from either the normal branch or the second critical branch. The NEC typically provides for one failure, in this case loss of a utility or loss of an ATS, not loss of a utility and loss of an ATS. Protecting against two failures gets more difficult and much more expensive.

Yes, however I would think an ATS failure is likely to happen orders of magnitude more during a utility outage then that during normal operation.


Thus, if code forces the cost of two ATSs- why not make an allowance to take advantage of such?
 

packersparky

Senior Member
Location
Wisconsin
Yes, however I would think an ATS failure is likely to happen orders of magnitude more during a utility outage then that during normal operation.


Thus, if code forces the cost of two ATSs- why not make an allowance to take advantage of such?
There is an allowance. 517.31(B) allows the use of multiple transfer switches for each branch. If the designer is worried about the failure of the critical branch, it can be designed with multiple critical branches. So in your previous example, the receptacles could be powered by different critical branches.
 

d0nut

Member
Location
Omaha, NE
Yes, however I would think an ATS failure is likely to happen orders of magnitude more during a utility outage then that during normal operation.


Thus, if code forces the cost of two ATSs- why not make an allowance to take advantage of such?
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.
 
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