Electrical Code Pertaining to Critical Battery Bank - Short Circuit Protection

Thread Starter

Tribute3

Joined Jul 12, 2021
7
Hi Everyone,

I'm looking to a reference to the NEC or CEC code that pertains to short circuit protection of Critical Battery Banks.

I have a critical (safety sensitive) 12VDC battery bank. The discharge current is high, nominal load at 12VDC is less then 20 Amps. Does the CEC or NEC mention that a fuse is required less then a certain voltage level? Am I allowed to let the bank operate unfused? Specifically looking for a CEC or NEC reference.

One thing I cannot have is a single point of failure (such as a fuse) preventing the bank from suppling the critical energy required.

Thanks for any feedback.
 

djsfantasi

Joined Apr 11, 2010
7,913
Have you considered that if a fuse blows, then you may have a problem which will destroy your load? Or possibly your batteries. There is a chance of a fire which destroys your lab or building.

Critical power is maintained through redundancy, not by eliminating safety measures.
 

Thread Starter

Tribute3

Joined Jul 12, 2021
7
Great reply, thank you. I have considered that. Assume in this case, we must we run the load even if it will destroy the equipment. I know that's a hard pill. The load is that critical...

Assume redundancy is not possible. We are at 12VDC, is there a code requirement that states a fuse or short circuit device is required? I've seen a code that states that its not required if the voltage is less then 30 VDC but the power output is also less then 100 VA. In this case the power output is greater then 100 VA which means fuse is required.
 

BobaMosfet

Joined Jul 1, 2009
1,896
as a follow-up i am pretty sure the circuits i am talking about here are classified as class 1 and 2 per the code.
In all cases, NEC codes will defer to safety of life over loss of power. There is no situation that can be foreseen that overrides this consideration. Your sole option, is redundant power-supplies and delivery mechanisms to the load in question.

You can obtain the code here:

https://www.nfpa.org/codes-and-stan...ds/list-of-codes-and-standards/detail?code=70
 

Lo_volt

Joined Apr 3, 2014
233
I can't see any situation where it would be safe to leave out fuses on a system like that. Maybe in a situation such as a starter motor, where the wire is heavy enough to handle a short across a battery but even that is risky. For example, a short on the leads to a starter motor may handle the short circuit current, but the battery may, over time, start to out-gas hydrogen, heat up or generally self destruct.

Does your system need to be battery powered via the 12v battery you are currently using?

To explain, "fail safe" usually applies to safety of life. Is there a way to extend that to cover your energy needs so that the need for a battery is eliminated? My point being that you've discussed nothing a "preventing the bank from suppling the critical energy required" making the battery necessary for your system to be fail safe. In my opinion batteries fail too often for them to be used in a "critical" function.

My question to you, then, is whether you can do the critical function with something other than a battery? You've not told us what this critical function is so we can't even make suggestions for alternatives. I think it's worth considering other ways to do it.

Lastly, BobaMosfet's suggestion for redundancy may be your only option. Probably not cheap but at least it gets you the power for your critical function.
 

Deleted member 115935

Joined Dec 31, 1969
0
I have only a minor knowledge of the codes for batteries and protection,
things like Lithium batteries have a requirement to have an over current protection circuit,
no voltage is mentioned,
I understand the reason for this is that the cell can go bang very spectacularly if over currented.

I'm guessing 12V your a lead acid type battery, which don't have the above restriction of going bang, they just bubble away.

I have worked on 48 v systems with many hundreds of amps,
they don't have fuses for the critical parts,
on the basis that the bus bars can take any current and its more important to keep the equipment going for a few more minutes even if after that the battery will be useless,

But they are in a "special" environment, where the loss of a fuse would be much worse than a battery boiling over.

interested to hear your answer
 

djsfantasi

Joined Apr 11, 2010
7,913
Great reply, thank you. I have considered that. Assume in this case, we must we run the load even if it will destroy the equipment. I know that's a hard pill. The load is that critical...

Assume redundancy is not possible. We are at 12VDC, is there a code requirement that states a fuse or short circuit device is required? I've seen a code that states that its not required if the voltage is less then 30 VDC but the power output is also less then 100 VA. In this case the power output is greater then 100 VA which means fuse is required.
I guess you missed the point of my reply. You’ve made clear that even if a fuse blows, your load needs to keep running.

I think the part that you don’t understand is if a fuse blows, your load certainly won’t work. But just as importantly, if you omit the fuse, YOUR LOAF CERTAINLY WON’T WORK!

In either case, you will lose all functionality. And possible worse, whatever your load is will fail and require maintenance. Such work will keep your system down for a significant amount of time. Your objective will not be met and perhaps your design decisions will increase the probability of failure.

Either way, you are screwed.

It doesn’t matter if you don’t have the room for redundancy. Because you are almost guaranteeing the failure of your system.

And perhaps, inviting major disaster from a fire, that will destroy your power source, your system and any building where the equipment is housed.

Good luck with that!
 

Thread Starter

Tribute3

Joined Jul 12, 2021
7
Thank you all for your replies. Post #7 from andrewm covers my situation. i.e. Lead acid battery bank in a situation where the battery is needed for a few more minutes even accepting the risk of bubbling the battery/ burning the enclosure down. Ultimately, I was looking for a code that shows this is acceptable.... which we cannot point to. I assume this past practice was based on old grandfathered rules / standards / military practices? Modern standards certainly don't indicate this is acceptable.

All your warnings have been taken into consideration. I will recommend that fuses are used to comply with NEC 240.4 / CEC 16-104. This will be a group decision so this original post was more to encourage a review of code to see if the run to failure approach is supported by code. Rest easy, I will make the argument to protect with fuses. :)

Thank you all for your inputs.
 

Thread Starter

Tribute3

Joined Jul 12, 2021
7
Another one... A motor for a fire pump is permitted to run without over current protection under certain circumstances. The building is already on fire... Keep the pump going at all costs.
 

djsfantasi

Joined Apr 11, 2010
7,913
Another one... A motor for a fire pump is permitted to run without over current protection under certain circumstances. The building is already on fire... Keep the pump going at all costs.
All costs include the pump motor quickly burning up and you’re left with no protection. There’s only one solution.

Redundancy.

I ran an e-commerce web site. We had five nines of reliability. Every component was at least 2x redundant. Some were 20x redundant. If management doesn’t want to pay for redundancy, they don’t want a fail safe system. Is total loss of the facilities worth the risk of having the protection systems fail? If the answer is yes, I’d start looking for another job. Because guess who’s not going to have one when a catastrophic failure occurs? No one in management I assure you.
 

djsfantasi

Joined Apr 11, 2010
7,913
Addendum: What it looks like is that you guys don’t understand if your protection devices fail, and you bypass them to keep everything running…

THEN your system will fail in a short period of time.

If your pump has an over current condition and you ignore it, the pump’s windings will burn up, melt, short out and you’ll be left with expensive boat anchors.

You can quote me to your bosses!
 

Thread Starter

Tribute3

Joined Jul 12, 2021
7
I get it.... I'm trying to do the right thing here. I.e. add protection and monitoring. I think there's a happy medium.
 

djsfantasi

Joined Apr 11, 2010
7,913
I get it.... I'm trying to do the right thing here. I.e. add protection and monitoring. I think there's a happy medium.
The happy medium is based on redundancy.

With out refundancy, you’re doomed to fail.

I’m retired IT professional for over 50 years. I’ve been a developer, infrastructure architect, operations manager and quality control. You need redundancy.

Protection and management is built upon that.
 
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