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Old 14-04-2016, 19:14   #16
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Re: Breaker advice

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Originally Posted by sailorboy1 View Post
I think you are on the wrong track.

For the controller supply/panel output all you need is a fuse. If you connect the controller output to your main DC panel you need a 12V breaker sized to the smaller of the controller rating or the wire size you used. Other wise all you need is a fuse on the output.
This is all that is necessary. I still recommend a two-pole breaker. We drop out all electronics in bad weather. Lightening strike two years ago destroyed all of our gear EXCEPT the devices on 2-pole air gap toggle switches. Electronic ON/OFF switches are software devices and the neutral can back-feed the device. We have retro-fitted all devices so that 2-pole switches or breakers isolate them from the boat's grid, including VHS antennas.
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Old 14-04-2016, 20:04   #17
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Re: Breaker advice

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Originally Posted by Nicholson58 View Post
This is all that is necessary. I still recommend a two-pole breaker. We drop out all electronics in bad weather. Lightening strike two years ago destroyed all of our gear EXCEPT the devices on 2-pole air gap toggle switches. Electronic ON/OFF switches are software devices and the neutral can back-feed the device. We have retro-fitted all devices so that 2-pole switches or breakers isolate them from the boat's grid, including VHS antennas.
So you are saying you breaker both pos and neg, not just switch or break pos.

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Old 14-04-2016, 21:15   #18
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Re: Breaker advice

For lightning protection, yes, break both legs. Your DC system negative is probably bonded to your engine and then to sea water. Your rigging may or may not be tied into the same bonding system. So, if you get a lightning strike on your rig it is far more likely that the energy surge will reach your electronics through the negative leg rather than the positive.
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Old 14-04-2016, 21:23   #19
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Re: Breaker advice

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Originally Posted by Nicholson58 View Post
This may be true in high voltage DC but not at 12 to 50. Professional engineer with 43 years building custom machines. I have consulted with my 3 EE guys where we build testing machinery installed all around the planet. In this use, these Din Rail breakers will do nicely. BTW - they have a much larger gap than the tiny push button breakers in my DC panel. The open circuit rating is 600 volts - far greater than the DC breakers. Vendor's data also says AC or DC. I have also contacted our Allen Bradley/Siemens distributor. He confirms these are OK DC and Amps is amps.
I can throw pretty similar credentials back your way. If it's not DC rated it's not DC rated. The OP stated the 65VDC was too low, so he's already outside your 12-50VDC range. Where is the limit?

The AB Bulletin 1492 supplementary protection breakers, for instance, are rated at 48VDC per pole (so, if you break one pole you can reliably break 48V, if you break both poles you can reliably break 96V because you have doubled the gap length). Just because some rep tells you something else means nothing, there is a reason companies put these ratings on the breakers, and pay UL to test them, and perform testing...

The OPs proposed system is 79.5VDC. Another way to look at this, the maximum voltage at which most DC welders operate is 80VDC, and they can strike an arc at essentially zero current at that voltage. Same thing happens in your breaker.

The AB 1492 breakers, two-pole, would be fine in this scenario, but only because they are rated to 96VDC in that configuration. AC only rated breakers shouldn't be used.
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Old 15-04-2016, 07:26   #20
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Breaker advice

No breaker is required on the panel side. A switch on the input to the MPPT controller is a good idea. The concern about DC breakers/switches is not relevant to solar panels because the current can never exceed the panel's short circuit current. Thus a high current arc can never happen which is what drives the DC rating of a breaker or switch.

A fuse or breaker is definitely required where the MPPT controller feeds the battery. Ideally this fuse is very close to the battery. In this application the breaker or fuse must be rated for the voltage and very high short circuit current. Very few breakers are rated for this application due to the high DC current interrupting rating. Best to only use a fuse of the proper interrupting rating based on the battery bank size.

Circuit interrupting devices have 3 main ratings:

Opening current (I.e. Trip current)
Operating voltage both DC and AC
Maximum interrupting current (usually thousands to tens of thousands of amperes)

The maximum interrupting current is often ignored by DIY boaters because they don't understand the importance of this specification. A battery bank can deliver thousands of amps to a short circuit. The DC voltage rating is next most ignored specification but is also critically important. Ignoring these two ratings means that the device may not interrupt a fault thus causing a fire. There is more to it than just the number stamped on the handle.
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Old 15-04-2016, 08:07   #21
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Re: Breaker advice

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Originally Posted by Dsanduril View Post
Sorry, but that's just not true. Voltage is critical in sizing DC switches, breakers, and fuses. With AC power the voltage goes to zero 50 or 60 times every second, extinguishing arcs. With DC this doesn't happen, and the gap a DC arc can jump is directly correlated with the voltage/power flowing when it is opened. The higher the voltage the greater the arc length. So opening a DC breaker (switch, fuse) can just result in an arc across the contacts. .


It's important when dealing with electricity to use components within the ratings. If a switch or breaker says 32 volts DC, don't use it in a circuit with a higher voltage.
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Old 15-04-2016, 08:18   #22
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Re: Breaker advice

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No breaker is required on the panel side. A switch on the input to the MPPT controller is a good idea.
Right, was looking at a switch, but it seems that dc din rail breakers are about the cost of a DC switch. Or less

Was the reason I asked about just putting in a 'regular 120V wall switch' in blue plastic box. Someone said they did just that.

I just want the ability to turn off output of panels to controller without having to take the wires off the terminals. For me, this is in a large cavity under a berth, so no chance of accidentally hitting the switch.

If I wanted to break both +/-, home depot has DPST switches for $5, if just +, 99c.
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Old 15-04-2016, 08:32   #23
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Re: Breaker advice

The 120V wall switch will work ok in series with the solar panel.
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Old 15-04-2016, 19:12   #24
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Re: Breaker advice

Breakers arrived at the house today. In the end we went with the Midnite Solar option, that is rated at 150VDC. Will update after I install this weekend.
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Old 16-04-2016, 06:53   #25
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Re: Breaker advice

IRT interrupting DC current, I ran across this little video. Test Voltage is a little above the level in this discussion, but does illustrate the added burden with switching DC systems.

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