Re: [RE-wrenches] Current limiting fuse at battery bank

[Allan Sindelar]

Dan, That's right about the unprotected DC + main. Trace used the inverter GJ, so turning off DC loads required turning off the DC to the inverter - which meant reprogramming it afterward. And it used a #10 tap pigtail. Stoopid. If I have several DC loads I have sometimes fed the DC + buss through a DC Main breaker as you write (I think), then the output is bussed to the rest of the load breakers. I don't understand the self-destruct with PV. Modern MPPT controllers won't exceed safe voltages. I don't do wind as a rule, so what is at risk of self-destruction? On the Class T question, I'm with you. If we accept that a GJ-type 50,000 AIC breaker (per Midnite) is used, and a 10,000A AIC panel mount or DIN rail CBI breaker will handle battery fault current at the disconnect, then what exactly would adding a class T catastrophic fuse accomplish? Seems to me it protects only the cables and the buss bars in the disconnect enclosure (e.g. E-Panel or equivalent) ahead of any breakers. By Code it oughtta be in conduit (sch. 80 in New Mexico) anyway. And the inside of the disconnect enclosure is for Qualified Personnel only, so we oughtta know what we're doing in there. In all my years I have once seen a class T fuse do its job, when an an early SW4024 failed internally. But that was in 1995 and the Boltswitch pull-out was the sole overcurrent protection. It worked perfectly (can anyone visualize flames in the inverter when I did a quick test override?). I guess I'm missing the basic point of why we'd add a class T fuse on the batteries anyway. I have put class T fuses on each parallel string of a large battery bank before bussing them together. To me that makes sense, either in the event of major cell failure or at end-of-life of the batteries, when one of two or three strings might be handling all of a big load profile. But in general, isn't the fuse just redundant to the breakers while adding complexity and more failure points? Unlike you, so far I haven't lost any sleep over this. Max, you say ALWAYS good practice, but I don't get it. What am I missing? Thanks, Allan Allan Sindelar al...@sindelarsolar.com NABCEP Certified PV Installation Professional NABCEP Certified Technical Sales Professional New Mexico EE98J Journeyman Electrician Founder (Retired), Positive Energy, Inc. 505 780-2738 cell   On 10/23/2015 8:30 AM, Dan Fink wrote: Esteemed Wrenches; This creeps me out too and sometimes causes lack of sleep. The Midnite Solar power panel install manual shows no OCPD on in DC + main from battery bank to power panel. Easy to wire the main DC breaker to control those conductors, but then you have a "self destruct" switch if system shutdown and restart is not done in the proper order, IE PV and wind turbine shut down first before battery bank. For a class T fuse on the battery bank, it's a big $ ordeal because it has to be outside the battery box in an enclosure per NEC.  So, recently I've just been making sure the conduit from DC battery mains is sound up to the power panel. The only issue I can see is mice chewing 2/0 cable inside the battery box -- which means the box wasn't designed right to exclude rodents, or the owner has a lazy cat. I'd feel much  better if there was an OCPD NEC legal to attach right to the main battery DC + and wire from there to power panel, but I don't think that exists. Dan Fink Adjunct Professor, Ecotech Institute IREC Certified Instructor™ for:  ~ PV Installation Professional ~ Small Wind Installer Executive Director, Buckville Energy NABCEP / IREC / ISPQ Accredited Continuing Education Providers™ 970.672.4342   On Thu, Oct 22, 2015 at 11:26 PM, Mbalchowsky <m...@seesolar.com> wrote: It's ALWAYS good practice to install a current limiting fuse at the battery bank, if fact it's very poor practice not to. We're working with a lot of larger lithium ion phosphate manganese systems and I would not install them without it. As far as working on someone else's install, make your  contract very clear as to what the customer can expect as far as your liability when working on an existing system. The more detail, the better..... On Oct 22, 2015, at 7:25 PM, Jerry Shafer <jerrysgarag...@gmail.com> wrote: I agree, fuse it Jerry On Oct 22, 2015 3:26 PM, "Tom Ruscitti" <trusci...@taitem.com> wrote: On battery based systems I always install a Class T fuse as close to the battery bank as possible. 690.71(C) is a little vague in requiring a current limiting fuse only where the available short-circuit current from the battery bank exceeds the interrupting ratings of the inverter breaker. Inverter breakers vary in their interrupting ratings and battery banks are big current sources when things go wrong, so it's just a sensible best practice. My question is about service work on systems we didn't install. We're quoting the second job this year for battery replacement where there is no current limiting fuse installed. It's always a tough question to judge what extra work might be required when you touch another contractor's system, but does anyone have an opinion about recommending or requiring the installation of a fuse at the battery box as part of the battery replacement? Thanks, - Tom -- Tom Ruscitti

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