DC Breakers and Acceptable Operating Temperatures

Hoping someone can offer some insights. I have an 80V DC 100A circuit breaker installed in my system. I did a thermal imaging scan to check for hot spots and found everything looks good except for this breaker running slightly hot at around 42.9 deg C (verified with a thermometer) when carrying around 70A at 55.8V DC. This temperature increased and the highest I measured was 49deg C around 10 minutes later running similar currents.

My first thought was contact resistance but I checked the cable contacts and escutcheon (double pole for one cable) and the thermal image shows these areas are relatively cool. The heat seems to be building up inside the breaker body. Now the question is two-fold,

  • Is this an acceptable temperature/heat loss for an 80V, 100A breaker running at 55.8V & 70A?
  • Should I be concerned about thermal runaway at the max current of 85A for prolonged periods?

Attached some photos for clarity.

IMG_0028 - Copy

This doesn’t strike me as unusual.
What does is the high current in this circuit. I presume this is the combined PV power before it is fed into the inverter. Surely you can connect your panels in a series configuration that reduces this current?

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Ok that’s good news, so just to have some kind of benchmark what do you think typical max operating temperatures for a DC breaker should be when continuously running close to its rating, say 90A in this case? I don’t really have a frame of reference just trying to get a feel for it

As for the high current, it is on a Victron MPPT RS’ battery output between MPPT and the DC Busbar. The PV currents are much lower, around typical 10-12A.

@plonkster Your department…

Surely you can look at the CBI datasheet on their website?


Check here:

It suggest that Ambient Operating Temperature is -40°C to +85°C

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“highest I measured was 49deg C”

I wouldn’t be comfortable with continuous temperatures like this (at this low current)… Just feels “wrong” and the ambient temp rating is ambient not operating…

At some stage the breaker will trip or melt and I wonder how the heat is effecting the lifespan. What is the cable temperature? from the picture it looks like the positive is running hotter than the negative. Maybe this is caused by the breaker…

The Noark is rated at 500V and I run far higher current with almost no temperature issues (I’ll double check this later).

Thanks for the feedback everyone and the datasheets @Bobby , for some reason I wasn’t able to find it here https://cbi-lowvoltage.co.za/ kept going in circles, my bad.

@mmaritz thanks for the tip. I would like to try the Noark but as far as I know the Noark is polarized i.e. only intended for breaking current in one direction with source side v.s. load side always constant. Usually correct wiring indicated with those little + and - markings or sometimes arrrows. They are typically also higher voltage rated as opposed to low such as the 80V with this CBi. I use a similar breaker to the Noark in my PV String boxes since PV voltages will almost always be higher (source) (no current at night) and the MPPT RS gives galvanic isolation between PV inputs and battery output so I am assuming (hopefully correctly) reverse currents will never occur. I don’t have a parallel PV setup but two separate strings each feeding into its own MPPT on the MPPT RS so this breaker acts predominantly as an isolation device, overcurrent is redundant since limited by PV Isc

As for the CBI, I was hoping these temperatures are acceptable to avoid additional costs and rework but you confirmed what my gut tells me, high operating temperatures, even mild, is a sign of inefficiency and losses. So looks like I might have to try another breaker.

Is this breaker on your inverter/battery cables? If so then I wouldn’t use a breaker at all just a Keto DC disconnect… I may have misread that this is on the RS? Then Polarised is good…

No this is on the RS between RS and DC Busbar. To be honest, for battery protection I am only using the internal breaker on a Freedom Won battery to avoid any further contact resistances being introduced. I will probably be chastised for this since the Freedom Won breaker only breaks the live and many will say (including latest SANS draft I believe) both pos and neg must be broken but at the time of installation I read this in the Freedom Won manual and since we are not in Australia I acted accordingly :zipper_mouth_face:

4.2.1 DC Isolator
The LiTE range includes an integrated battery DC breaker/isolator that breaks the positive cable continuity inside the battery. This breaker, on all Home and Business models, is rated for a 36kA fault (short circuit) current. The system therefore does not require another DC isolator or breaker except where required by local regulations, for example in relation to conformance with the Clean Energy Council of Australia battery design Best Practice Guide, which states, “Should the internal battery isolator not offer isolation of BOTH the positive and negative terminals of the battery, an external isolator is required that can isolate both the positive and negative cables/terminals of the battery.”

Pity they don’t show the time temp graph in the data sheet…


This would have been extremely helpful

Ok - my batteries will be full before I can generate peak amps but currently:
Noark (last 60 mins) - started @ 17deg (ambient was the same) now @ 20 deg @ 23a (and ambient is 19deg)
Yesterday my peak was 41a (no temp reading) on the same breaker.

NB: Resistance causes a rise in temperature so maybe a crimp isn’t tight or a screw is loose? Check all those as well.

From the datasheet, the temperatures are probably OK. Looks like around 1 mOhm on resistance. Which gives 6.4W dissipation @ 80A. In an enclosed space, that would easily reach 40C.

Fuses operate a lot hotter when operating close to their limits.

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Yes, I was about to say look at the temp graph. That usually tells you how much the breaker derates based on temperature. Most breakers are designed to handle 80% of their rated current continuously, so whatever temperature the chart shows for 80A on this breaker, that should be fine.

A quick google suggests it is fine for breakers to run up to 60°C.

For overcurrent protection this is sufficient. The FW battery has a very decent built-in breaker. This breaker is however only in the positive line. South African regulations require both sides to be isolated, so an isolator is needed that breaks both positive and negative. Most people use the Keto/Mersen units as above simply because it is the cheapest option that gets the job done.

Titbit to share:
Never switch off a Victron MPPT when under load.

Recently I was asked if I had a DC Breaker between the MPPT and the battery.

No, I replied. That is batshit crazy on Victron MPPT’s. (Don’t know the RS …)

Email my preferred Victron supplier/repairer.

Indeed, do not put a DC breaker between MPPT and batts. Fuse or a Keto fuse between MPPT and Batts will suffice.


If you switch off a breaker, even Keto Fuse, that sits between MPPT and batteries, it is not good at all when done under load.

Switch off the panels first, then pull the Keto fuse holder/pull the fuse, if you want to do it properly.

DC Breaker may lead to someone thinking it can be done causing potential damage to the MPPT when the current from MPPT to the system/batteries is switched off under load.

ALWAYS switch off the panels first!

You make a very good point, feel stupid for not thinking of that. Most of the Victron schematics such as this one also show a fuse not a breaker, in this case incorporated in what looks like a Lynx distributor. Will be changing to a Keto as soon as I get a chance.
13860-SmartSolar_MPPT_RS-pdf-en.pdf (397.9 KB)

@mmaritz @plonkster @justinschoeman (hope I didnt miss anyone) thanks for your input on temperatures, helps a lot to know in what ballpark temperatures should be, Forums like this are often more helpful than suppliers themselves.

Just a last question for @plonkster, apologies if it is a tedious one to answer. I am aware of the regulations regarding breaking the positive and negative of the battery but seeing as Freedom Won was willing to state in their instructions as per above it is not required unless enforced by local regulations I always wondered about the reasons for this. Could you perhaps provide some insights regarding the potential fault scenarios under which breaking only the positive is a problem? Does having a negative grounded battery (system grounding) vs. ungrounded battery play a role here?

The Victron schematic attached for instance only shows a fuse on the positive in the Lynx distributor but I know Victron’s wiring unlimited (p. 65) states the negative of the battery should be grounded. However when I asked a Freedom Won technician about this he said it is not necessary to ground the Freedom Won’s pos or neg. I have also seen most solar installers not grounding the battery. So this has been a matter of confusion for me for a long time.

I have heard this too, but then (I think it was Will Prowse) did some experimentation trying to damage an MPPT ( and I think it was a Victron), by repeatedly doing precisely this, and nothing happened.
I have also done this numerous times on Outback MPPTs.

So, here is my hypothesis on “Why?” and why I believe you are OK if things have been done right.
A few people sail close to the wind regarding their open-circuit PV voltage.
They get away with it because when the MPPT kicks in, it pulls the voltage down to Vmpp.
Vmpp happens to be below the maximum voltage threshold the MPPT can handle, so all is right in the world.
Now, if the PV supply side is opened to the MPPT, the MPPT is not exposed to the excessive Voc. However, should the output side of the MPPT be opened first, the input voltage will suddenly shoot up to Voc.
Depending on how this has been spec’d, this Voc could damage the MPPT.
That is my rationale, why MPPT damage and doing this are linked together in people’s minds.

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Luckily my string Voc is well within the RS’s specs running Voc(0 deg C) at around 267V, only 5 panels per string.

When I want to know how good things work, or not, I ask a repairer of said equipment. :slight_smile:

Surprisingly, in repairer’s words, Victron does handle it very well, but it is not advisable at all, for similar reasons you deduced.

I know nothing about anything, but when I read ground and batteries, I think of a battery in a vehicle/boat/ship, not in a house. Ever.