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Author Topic: DC breakers/fuses  (Read 4507 times)
stephendv
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« on: June 23, 2014, 07:42:42 AM »


I'm helping a mate with an off-grid install on a budget and I'm not sure about which DC breakers/fuses/switches to use for the main battery bank.  Install is around a Victron C1600/24V, so they'd need about a 150A protection.  A single outback 175A DC breaker goes for 80+ euros which is too dear.  However, 2 x 80A breakers in parallel would be about 30 Euros which is doable- but it's not a neat solution as I'd have to superglue or find some other way to make sure the breakers operate together.
The Victron has a surge rating of 3000W and the owners will only be using washing machine + fridge (not bought yet), so do you think a single 125A outback breaker would be good enough (it goes for just over 30 euros and would solve the switch and protection problem in 1 unit).

Another option is simply a Fuse + disconnect switch, this is the cheapest solution so far, thinking of using a 150A mega-fuse (like in the victron itself) and one of these disconnect switches: http://www.ebay.es/itm/Desconectador-de-Bateria-150A-pico-400A-con-Llave-y-Tapa-Terminales-Coche-4x4-/321390605416?pt=LH_DefaultDomain_186&hash=item4ad45f7468&_uhb=1

Thoughts?
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billi
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« Reply #1 on: June 23, 2014, 08:43:37 AM »

Ola Stephen

Perhaps here http://www.fraron.de/en/power-inverter-accessories/fuses-fuseholders/


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stephendv
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« Reply #2 on: June 23, 2014, 08:53:26 AM »

Thanks billi!  That looks like the perfect solution.
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Justme
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« Reply #3 on: June 23, 2014, 04:41:05 PM »

Dont do it.

I have them as a simple disconnects on my DC PV strings.

I would not dream of putting more than about 20 amps through them.

Some of mine are 300amp.
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stephendv
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« Reply #4 on: June 23, 2014, 04:43:18 PM »

Hi Justme, why not??

Is it breakers in general that you don't like as disconnects, or that specific model? 
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billi
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« Reply #5 on: June 23, 2014, 04:50:53 PM »

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I would not dream of putting more than about 20 amps through them.

then report to the supplier , i would say    Wink




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Eccentric Anomaly
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« Reply #6 on: June 23, 2014, 06:02:21 PM »

Am I right in thinking that those “disconnect” switches that Stephen links in the first post are really isolator switches; that is, they can carry high current and they can safely separate systems but they should only be switched off when there is no (or little) current flowing?

OTOH, I can't see the point of a 150 A breaker which can't break considerably more than 150 A. Maybe it'll only do it a few times but I think repeated operation at full current could be avoided in this application as normally you'd turn the inverter off or at least disconnect its load before manually triggering the breaker.
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nowty
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« Reply #7 on: June 23, 2014, 06:36:49 PM »

I use a fork lift truck battery fuse, about £20.



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stephendv
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« Reply #8 on: June 23, 2014, 06:44:11 PM »

I use a fork lift truck battery fuse, about £20.

And to disconnect the battery from the inverters?
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stephendv
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« Reply #9 on: June 23, 2014, 06:48:25 PM »

OTOH, I can't see the point of a 150 A breaker which can't break considerably more than 150 A. Maybe it'll only do it a few times but I think repeated operation at full current could be avoided in this application as normally you'd turn the inverter off or at least disconnect its load before manually triggering the breaker.

Yeah, from my perspective I want something that will work as a protection device in the event of a short circuit, so it must be able to break thousands of Amps.
The other nice-to-have feature is to be able to isolate the battery from the rest of the installation occasionally, which I'll do as you describe, only under low or no load.
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billi
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« Reply #10 on: June 23, 2014, 07:20:01 PM »

what are u on about ?

 ok   my setup is fused  like Stephends second  approach     fuse and disconnect switch

I have my megafuses ( that never blew)  and a disconnect   switch


So  surely i would follow my  given link,  nowadays , cause it seems much easier  in some installs


Quote
Am I right in thinking that those “disconnect” switches that Stephen links in the first post are really isolator switches; that is, they can carry high current and they can safely separate systems but they should only be switched off when there is no (or little) current flowing?
Please tell me  if thats important !  Cause i switch my PV of/on  , when ever i want
« Last Edit: June 23, 2014, 09:01:37 PM by billi » Logged

1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
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« Reply #11 on: June 23, 2014, 08:04:06 PM »

I guess he means that most pv panels have a little sticker on them that states that they should never be switched off under load,
                         However,If I have to switch mine off during the day,I throw a tarp over the array and then disconnect.
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billt
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« Reply #12 on: June 23, 2014, 09:11:48 PM »

I'd guess that he means that DC breakers are harder to design and make than AC breakers. (Properly designed solar string DC isolators tend to be more expensive than similarly rated AC isolators.)

A battery isolator is a safety device; you may want to disconnect the battery in an emergency when it is delivering power and you cannot remove the load by any other means. If the battery isolator is not designed to break the full load at the system voltage then it may not work at all and weld itself closed.

Your choice. If you are satisfied that the load can be removed by another means so little current is flowing through your switch then use the cheap option. I'd go for the expensive but reliable option.
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Eccentric Anomaly
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« Reply #13 on: June 23, 2014, 09:33:58 PM »

Quote
Am I right in thinking that those “disconnect” switches that Stephen links in the first post are really isolator switches; that is, they can carry high current and they can safely separate systems but they should only be switched off when there is no (or little) current flowing?
Please tell me  if thats important !  Cause i switch my PV of/on  , when ever i want

Dunno, that's only a semi-rhetorical question. My understanding is that those isolators are not rated to break at full load current but a) I'm not sure and b) I too would be interested to know at what current they can safely break.

As Billt says, they could weld shut. However, they have to break to draw an arc so that doesn't seem like an immediate threat to me. Either they'd open but not suppress the arc so continue to pass current and eventually overheat or, I'm guessing, they'd more likely be pitted by arcs draw, get higher resistance and not work well or run warm subsequently.

Of course, there's the difference between what they're rated for and how they work in practice. Maybe Billi's switches work fine because he doesn't turn his PV off often in full sunlight (why would he)? The actual switch rating would be defined on the assumption that it would be operated often.

It's a matter of what works vs what an electrician can actually sign off.

I guess he means that most pv panels have a little sticker on them that states that they should never be switched off under load,

Switching panels off under load is fine. PWM controllers do it multiple times per second. The MC4 connectors aren't rated for disconnection under load - a slightly different matter.
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nowty
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« Reply #14 on: June 23, 2014, 11:28:34 PM »

And to disconnect the battery from the inverters?

Shutdown the inverter and unscrew the fuse.  fingers crossed!

In an emergency, take a sledge hammer and smash the fuse cos replacing the fuse will be the least of my problems. sh*tfan
Actually I did think about putting in an isolator but decided that an isolator might cause more problems that it solves with the large currents supplied from the battery.
I also decided to remotely fit the fuse away from the battery housing as a fuse blowing could act as a perfect detonator to any explosive gases.
« Last Edit: June 23, 2014, 11:30:20 PM by nowty » Logged

11kW+ of PV installed and 54+ MWh generated.
Lithium battery storage of 50+ kWh.
Hot water storage of 15+ kWh.
Heat storage of 15+ kWh.
6kW Ground source heatpump.
EV BMW i3 (another 30+ kWh's of storage).
250,000+ litres of water harvested from underground river.
Home grown Fruit and Veg.
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