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viceroy

PV circuit breaker tripping!!

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I have 3180W of PV panels configured to output between 80v and 100V. At peak output the voltage is around 86V. The peak Wattage I see is 2800W, but usually closer to 2500W.

When I redid my system recently, I installed an Onesto 250/500VDC 6kA 63A double break circuit breaker. I had nothing previously and this one seemed the closest match to what my panels put out.

Since then, the breaker is tripping, usually late morning, and not every day.

Not only is this annoying, as I'm at work and can do nothing about it until I get home, but is wasting precious solar to charge my batteries.

Looking through ICC outputs, I can't see anything unusual. I've even lowered the PV charging amps in case there was some sort of overload causing the issue.

The attached image is from today when the CB tripped. As you can see, looking pretty normal until the sun suddenly disappears out of the sky.

1254507651_PVtripping.thumb.jpg.ddfdb958ce637f0d3a8938f969ffe0ae.jpg

Any ideas?

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4 hours ago, viceroy said:

Since then, the breaker is tripping, usually late morning, and not every day.

Hi Viceroy normally if a breaker trips, it is drawing more than it’s rating (In your case 63A) Or the breaker is faulty and trips pre-maturely. The fact that it trips late morning it is possible it’s due to it being close to peak pv power. The best way to check is to put a D.C. tong tester on that cable at the same time and check the actual current.

In the meanwhile you could reduce your pv size by pulling one a fuse link on one string and see over a 24hr period if it still trips. Or even pull two strings that might steer you in the right direction.🙂

 

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Had the same problem after installing Pylontech batteries . The high charging rate of the lithuim batteries caused heat transfer from the wires to the breaker . Never had 60 amp at the breaker before . Replacing the wires with 10 mm solved my problem  .

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7 hours ago, viceroy said:

I installed an Onesto 250/500VDC 6kA 63A double break circuit breaker.

They  trip once the current reach 42 amps, I took it up with the supplier, but no response yet. 4 strings and above trips the onesto. I  replaced all of them with NO-Ark 63A. Problem solved on all affected sites. 

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27 minutes ago, Jaco de Jongh said:

They  trip once the current reach 42 amps

I too have had a circuit breaker trip. The data says I was doing 30A at the time, and it is a 40A breaker. Now I know that under DC the breaker reacts differently, but usually it is the other way around. It needs more DC current to trip. That's because for most C-curve breakers, the thermal (slow) component reacts the same, but the magnetic component needs more current. You can see that already on the curves, if you have them, that while 5In (5 times rated current) is an instantaneous trip on AC, you need 7In with the same breaker on DC. See for example this Gewiss breaker (rated 55V per contact on DC) 🙂

Selection_479.thumb.png.c4025ebb6683c2920d6bbc8cc54c76f2.png

 

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The reason for needing a higher current, again, is probably because the inductance of the magnetic part has no effect under DC. The same thing happens with irrigation valves. They draw more current at 24VDC than at 24VAC (which they are designed for), they can in fact be damaged with DC, so you need to use a lower current. Probably the same thing here...

 

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6 hours ago, plonkster said:

while 5In (5 times rated current) is an instantaneous trip on AC, you need 7In with the same breaker on DC.

I note that 7/5 = 1.4 ≈ √2. I think something is responding to the peaks of the AC waveform.

@Chris Louw, interesting about the thinner cable affecting the breakers by heat conduction. Though of course, nothing less than 10 mm² should be used for 60 A continuous anyway.

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Thanks all.

Looking back through the history, it's tripped 4 times, 42A, 49A, 26A, and yesterday at 27A. Not a great record for one month. Definitely looking like it's faulty.
Breaker is warm to the touch when I've been there to catch it.

Going to play it safe and replace the breaker, and the wiring for that section.

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2 hours ago, Coulomb said:

Though of course, nothing less than 10 mm² should be used for 60 A continuous

This is the interesting part, 4 string coming in, 4 mm wire from each and then combined on top of the Isolator/Breaker and 16mm from there to the mppt and still tripping. I even tried 16mm busbar and even then it tripped. 

 

2 hours ago, Coulomb said:

I note that 7/5 = 1.4 ≈ √2. I think something is responding to the peaks of the AC waveform

Very interesting observation, I totally overlooked that possibility.  

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19 hours ago, viceroy said:

I installed an Onesto 250/500VDC

It looks like the voltage also have a big effect on the tripping curve, The voltage from pv is 80 - 100V and breaker designed for 250/500VDC that might be where the difference comes in. Other brand breakers with a lower voltage range might be a better option.

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4 hours ago, Coulomb said:

I note that 7/5 = 1.4 ≈ √2. I think something is responding to the peaks of the AC waveform.

That might well be so (and my guess of inductance might be wrong). The magnetic component of the breaker is the affected part, so it might well be that the magnetic flux correlates with the peak current and not the RMS value. It's been 20 years since I studied this stuff (Physics 144 I think it was called), so this is a total guess.

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4 hours ago, Coulomb said:

I note that 7/5 = 1.4 ≈ √2. I think something is responding to the peaks of the AC waveform.

@Chris Louw, interesting about the thinner cable affecting the breakers by heat conduction. Though of course, nothing less than 10 mm² should be used for 60 A continuous anyway.

The 6 mm cables was done by the installer also with AC breaker no panel fuses and battery fuses . It was working for 5 years no problems , also I must say it newer run at 60 A for more than 30 min . The problem only started with the lithium batteries . I learned a lot on the forum and know that everything is now correct after installing battery fuses , panel fuses , DC breakers and correct cables . Thanks

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Circuit Breakers can typically either have a thermal / magnetic or an electronic trip unit fitted. Some points to consider are:

1) Electronic trip units are far less affected by ambient temperature.

2) Thermal magnetic trip units typically have to be derated for ambient temperatures above 40 degrees C. (Eg. at 50 degrees, delta T = 10 degrees, 5% current derating)

3) Trip units are sometimes interchangeable for AC and DC circuit breakers. As @plonkster pointed out the thermal protection of the trip curves are identical since the bimetal strips are responding  to the RMS value. The instantaneous protection against short circuits are influenced by the "ferromagnetic phenomena" which in tun is affected by the circuit breaker design and the way the poles are connected. ABB have a coefficient called "KM" that the instantatneous trip value for AC has to be multiplied with to obtain the DC values. This coefficient varies between 0.9 - 1.3 

So coming back to @viceroyproblematic circuit breaker.

1) I would suspect that the breaker may have a high internal contact resistance which is causing  localised heating near the bimetal strip.   

2) Other factors such as undersized cabling (@Chris Louw), poor crimp connections, incorrectly tightened etc. will also cause localised heating resulting in a derating of the trip setting.  

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