Midnite breaker polarity

Hairfarm

Greetings everyone,

So, I'm using Midnite breakers for my off-grid system, three 15 amp breakers for each PV string, a 63amp breaker going into the charge controller and an 80amp breaker going out of CC.

I found this comment on Midnites forum:

"The MNEPV din rail breakers do have a polarity. The + on the bottom side of the breaker means it wants to be hooked up to the highest potential. That will be the PV+ in a combiner or to the battery + when connected to the output of the controller. Now you may think that the output of the controller is at a higher potential than the battery plus terminal. Wrong! When a controller fails it shorts battery + to battery -. The battery+ terminal is the real highest potential when the controller fails. That condition is when you need the breaker to trip. I have found that unless you are at the top end of the voltage rating and at very high current, these din rail breakers trip just fine even hooked up backwards. Do it right though. It doesn't cost any more."

What does this mean in a nutshell? That the + wire needs to go toward the largest potential power source? This would mean that these are uni-polar and not dual-polar right?
Can someone please explain this to me?

thanks,

Hairfarm

inetdog

Re: Midnite breaker polarity

Hairfarm wrote: »

Greetings everyone,

So, I'm using Midnite breakers for my off-grid system, three 15 amp breakers for each PV string, a 63amp breaker going into the charge controller and an 80amp breaker going out of CC.

I found this comment on Midnites forum:


What does this mean in a nutshell? That the + wire needs to go toward the largest potential power source? This would mean that these are uni-polar and not dual-polar right?
Can someone please explain this to me?

thanks,

Hairfarm

It means that the breakers are asymmetrical in their current interrupting capability, because a DC arc has to be disrupted somehow, and that is sensitive to the direction in which the current is flowing. That in turn means that instead of looking at the direction in which current is flowing normally, you have to look at which way the current is flowing when the breaker is called on to interrupt it. A breaker between panels and CC or between CC and battery is very unlikely to have to trip based on a short at the battery side. The current from the panels and the current from the CC are limited to a value smaller than the rating of the breaker. You do have to consider it if you intend to use the breaker frequently to disconnect the panels while the sun is on them.

But if the short is on the panel side and the CC has also failed, then the battery voltage could be looking back into a short circuit (or near short circuit). In that case the breaker will have to be able to interrupt the DC arc while the current is flowing towards the panels. In that case, the battery + is the higher potential and the short circuit is the lower one. This is the opposite of what happens during normal operation (where V panel greater than Vbat and current is flowing from panel + toward battery +)

Cariboocoot

Re: Midnite breaker polarity

The condensed version:

The breakers are there to protect the wiring in the event one of the panels fails and becomes a load (short) rather than a source. They need to be installed with that eventuality in mind, even though it seems to be backwards of what you'd normally do.

Nice thing about fuses; no polarity to worry about.

Photowhit

Re: Midnite breaker polarity

In a nutshell, on the Solar panel side of the CC connect the + side of the breaker to the + of the array (or toward the array) on the Battery side of the CC + should be toward the battery.

Hairfarm

Re: Midnite breaker polarity

Thanks for the description guys. I realized that I should probably wire the + toward the largest power potential but I just needed it verified.

So why doesn't Midnite just make a dual polarity breaker? I know they exist.

Cariboocoot

Re: Midnite breaker polarity

Hairfarm wrote: »

Thanks for the description guys. I realized that I should probably wire the + toward the largest power potential but I just needed it verified.

So why doesn't Midnite just make a dual polarity breaker? I know they exist.

Not as common as you think, but MidNite is working on it as the NEC regs will require it soon.

Photowhit

Re: Midnite breaker polarity

Hairfarm wrote: »

So why doesn't Midnite just make a dual polarity breaker? I know they exist.

They exist in panel mount breakers, but DC Din rail breakers, as far a MIdnite or rather Robin sourced them or had them made for Outback and now Midnite. One of the threads at Midnite speaks to the, rule change, "2014 NEC will require dc breakers to be non polarity sensitive" and Robin off in Africa? seeing if the people who make the breakers can meet the new guidelines.

tallgirl

Re: Midnite breaker polarity

Hairfarm wrote: »

Thanks for the description guys. I realized that I should probably wire the + toward the largest power potential but I just needed it verified.

So why doesn't Midnite just make a dual polarity breaker? I know they exist.

It has to do with how the arc is suppressed when the contacts are opened under load. An arc isn't just electricity, it's super-heated gases that have made the air more electrically conductive. If the arc isn't suppressed properly, the air can continue to conduct electricity to places it shouldn't.

This is not an easy problem to solve ...

BB.

Re: Midnite breaker polarity

One method to suppress the arc is to use a permanent magnet next to the "arc path". When the arc forms, its magnetic field is the same polarity as that of the magnet--Which forces the plasma in the arc chute (or where ever it is "safe" for the arc to dissipate into).

If the arc is the "wrong polarity", the arc is pulled towards the magnet and can cause all sorts of problems (mainly a sustained arc that simply melts down everything in the breaker).

Depending on the breaker design, they will also be orientation sensitive... The arc plasma wants to rise--So a wall mount vs flat mounted or side mounted breaker could all have different internals.

-Bill

mtdoc

Re: Midnite breaker polarity

For a nice demo of current arcing and the principles discussed by Bill and Julie , have a look at THIS video by Frantone of her Jacob's ladder.8)

BB.

Re: Midnite breaker polarity

I still have my 15kVolt @~1,000 watt neon sign transformer from the old Ham's brewery sign near downtown San Francisco.

Makes one wicked Jacob's Ladder.

I think I was about 12 years old when our neighbor (an iron worker that took down the sign) gave it to me (yes--my Dad let me play with it as long as I was careful--Must have been, I am still here:roll:).

-Bill "Zap" B.