DC Isolators vs DC Circuit Breakers

Hi all,
I’ve finally bitten the bullet and purchased my solar panels (25 x 450W Trina), my hybrid charger-inverters (2 x MPP PIP5048MKX) and 3 x 48V 200AH LifePo batteries (Blue Carbon). Now I’m trying to finalize a wiring plan and need some guidance.

Specifically I am trying to understand if installing 2P DC breakers on each of the 3 strings of PV panels (11, 7, 7) are enough protection for the charger-inverters or if its also necessary to install (expensive) DC isolator(s)

I read on an internet blog that this may not protect against one panel in a string shorting out and frying the breaker contacts. Another bloke on a Youtube (claims to be a Aussie solar installer) even claimed that breakers can catch fire and strongly recommended using rotary style DC isolators on each string.

Would appreciate your expert advice and guidance.

Thanks

Hi @chewface

I wish I could remember, it’s been too long since I was across this detail. I think the regulations in Australia changed a while ago to require a switch on the roof and the ground - not necessarily for each string I don’t think, but up to a certain voltage (so you might be able to parallel the two string of 7 I’m assuming). 4p 1000V DC breakers are still being sold for solar installers in Aus, so I’m assuming they can be used, which would indicate they haven’t started too many fires! I have personally seen a good quality DC isolator goes up in smoke, so isolators may be safer (I don’t know), but you certainly still need to be careful with wiring them.

DC isolators shouldn’t cost much more than breakers though. You can get decent 1200V ones is Aus for <USD$20 and 1500V for <USD$30.

I don’t think you can go too wrong if you follow the Australian regulations, because they are pretty strict, so hopefully someone with a better understanding of the regs can weigh in here.

Cheers
Marty

I recommend you use 2P DC breakers since they are current-limiting devices, unlike DC isolators. (Unfortunately in Australia we have to use rooftop-mounted DC isolators which are very problematic due to water ingress and other issues)
If you use a 2P DC breaker on each string in a DC junction box then you can easily isolate each string if required for testing and a faulty string will not draw too much current from the other strings resulting in a catastrophic failure.

Note: You should always use current limiting devices (breakers or fuses) when you are using more than 2 strings in parallel

Thanks very much for your advice.
I have another (sort of) related question. The installation manual for the MPP Solar 5048MKX charger-inverter recommends installing a “current limiting device” between the MKX and the battery bank (600AH LifePo in my case).

I bought a 250A DC breaker (Lovadon lPM1Z 250/2300) for this purpose. Unfortunately it did not come with any installation diagrams or instructions. I suppose their assumption is that the wiring should be obvious(?) It has 4 screw terminals labelled 1-4) with #1 #3 at the top of the breaker and #2 & #4 on the bottom. The schematic on the breaker label designates @2 & #4 as the “load side”,

My question is how to wire the breaker? Is the “Load” the battery bank or the charger-inverter? It’s confusing to me because the MKX is sometimes charging the battery (so is the “source”) and other times drawing power from the battery (so is the “load”).

Or perhaps is this breaker not suitable for the job? If not, then should I just install 250A line fuses?

Your further advice would be much appreciated.

Is this the breaker?
image

With the screw terminals numbers - 1 goes to 2, and 3 goes to 4.
It looks like it is polarised so positive and negative will need to be connected to the correct terminals. Can you get a close up of the diagram?

I personally use ZJ Beny DC isolators for battery and cable protection because they are NON-polarised and the pos and neg can be used on either side…

Hi jason,
Yes the photo you sent is my product/model. Attached is a close-up of the schematic diagram on the label.

I’m unclear what you mean by “polarized” ? I was given to understand from another ‘expert source’ (on “DIY Solar Power Forum”) that dc breakers are “bi directional” … which suggests it doesnt matter about which side you connect the load?

Does the the direction of the arrows on the breaker schematic mean that the positive connection flows from screw terminal #3 to #4… so the +ve wire from the battery connects to #3 and the ‘LOAD’ ( assume the inverter in my case) connects to #4? Similarly the -ve wire from the battery connects to #2 and the inverter to #1? If so this is even more confusing cos the schematic seems to show the “LOAD” side corresponding to screw terminals 2 & 4?

Hope you can untangle my brain on this one!
Thks!

I already bought and paid for the Lovadon DC breakers and they were not cheap so am stuck with them unless they just won’t work in my application… fingers crossed!

YES, this is correct.

Polarised breakers only function correctly when used with the correct polarity. In Australia (where I am located), the industry regulator banned polarised breakers many years ago due to installers mixing up the polarity and the breakers not working correctly.

If you are using this breaker then I would assume that the load is the inverter. The potential fault current will come from the battery which is the source.

Hi Jason;
I found a discussion paper on the wiring issues with polarised dc breakers (see attached) and think I now understand. However, if the polarised breaker is designed to work (only?) in the direction of current flow i.e. towards the ‘load’ then does this mean that there is no protection when the current flow is reversed, as in the case when a hybrid charger-inverter charges the battery instead of drawing power from the battery?

I’m getting dizzy from thinking about this :wink:

Thanks for your help

forgot the attachment. here it is…

(Attachment Correct-Wiring-of-Double-Pole-DC-Breakers-LVL2-131210-v2.pdf is missing)

I understand your concern but there will not be an issue with the current flowing from the inverter to the batteries. A large "fault’ current that could potentially trip the breaker can only come from the batteries. The inverter will not be able to produce enough current to trip the breaker

Hi Jason,
OK sounds logical. Thanks for all of your help. I’ve learned a lot from your site. :slightly_smiling_face:

Take care and keep up the great work. You guys are really helping to make solar more accessible to the average DIY’er …and make a meaningful difference to our Mother Earth.

Brgds,
William (chewface)

Hi Jason,
If you install Life powerplus 4833 P battery’s,they have already a DC circuit breaker included,is’t fine to use a DC isolator only then are must you still install a separate breaker
Thanks
Marco

@Marco, it is highly recommended that the battery system has a “main” battery isolator point (breaker or fuse) to disconnect the battery system in the event of an emergency. It’s also required to protect the main battery cables and inverter in the event of a short circuit in the DC bus.

The battery circuit breakers will protect each battery, but if you have a large battery bank then the potential (peak) fault current is huge (250A per battery).

Thanks Jason,
that make sense for the security
It’s good that we can find this knowledge and big help here from you and the community,so that we can find the right solutions