AC breaker does not break

I have a system that has been online for a couple years. It has worked very good. I had a temporary outlet off my AC distribution panel while I was building my cabin. I have recently finished the AC distribution system and installed the breakers. One of the circuts has a short. The AC distribution panel circut breaker does not break. Instead the inverter buzzes and goes into error. A thought is I do not have enough grounding to handle the problem. But then again not sure. Anyone understand this problem. Thanks


  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    Re: AC breaker does not break

    What size is the inverter and breaker?

    I am going to guess that the inverter can not deliver enough power to trip the breaker. Most inverters will shut down if it is loaded passed its max load, just like you described.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
    Re: AC breaker does not break

    There can be several issues...

    A circuit breaker, will (sort of) open when carrying rated current ore more (carrying rated current may open in minutes or hours or never--carrying 2x rated current it will open in seconds or minutes--there are many types of breakers, designs, and rating types).

    So, assuming the circuit breaker is OK (usually they fail to stay closed, or false open), then you may have something limiting the current.

    The first possibility is the Inverter itself. It may not be output full current because:
    • 1. Insufficient battery size/charge (measure battery voltage)
    • 2. Insufficient cable size to inverter (measure DC voltage at input to inverter)
    • 3. Inverter is unable to output enough current to trip the breaker (15 amp breaker at 115 volts = 1,725 watts)
    • 4. Inverter may "fold back" the current electronically if exposed to a short circuit... Many power supplies (don't know about inverter specifically) are designed, that when they "experience a short (too much current), they stop trying to output 100% current and cut back to 10% current (as an example). It is possible that the inverter is "folding back" is output faster than the breaker can trip.
    • 5. A "dead short" may not be really a "dead short"... With long wiring runs of (relatively small diameter) wire (or poor connections somewhere) actually does not have a low enough resistance to trip the protective devices. So, if you have a long wire run (for example) its total resistance is not enough to pull 15 amps when shorted at the far end. Or the "short" may be a staple through the insulation--if the inverter is left on long enough--could cause smoke/fire at staple site.
    • 6. A short may between Line and Neutral or Line to Earth Ground. In older construction (50+ years old?); ground wires were a smaller gauge than the supply wires. So--it was possible that a Hot to Neutral short would trip a breaker, but a Hot to Ground short had too high of resistance to trip a breaker (open ground to cold water pipe, relying on conduit to supply safety ground, etc.).
    Is this what you were looking for?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: AC breaker does not break
    n3qik wrote: »
    What size is the inverter and breaker?

    I am going to guess that the inverter can not deliver enough power to trip the breaker. Most inverters will shut down if it is loaded passed its max load, just like you described.

    I have 2 outback VFX 3524 3.5 kw 24v inverters. It is a 20amp breaker
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: AC breaker does not break

    Just some points to ponder:
    What is the load that is causing this short that won't trip?
    Never overlook the obvious; that breaker may be defective.
    There are three different types of AC breaker; standard, ground-fault, and arc-fault. The latter reacts instantly to the slightest perceived short. They are more expensive, of course.
    A basic breaker is bi-metalic circuit which heats up under current and 'bends', tripping the switch off. It is necessary for the current overload to exist for some time before the breaker will trip. It's possible your circuit nears this point, draws down the inverter voltage, lowers the current, cools the breaker, and starts all over again.
    In short, not all breakers or loads are the same, especially not where an inverter is the power source.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: AC breaker does not break

    Once, we had direct short on a buss protected by a 30 Amp AC breaker. The inverters simply shut off -- the breaker did not trip. In isolating the fault, needed to initialize the inverters. The Xantrex inverters made four short-duration buzzes, and again shut down, and again the breaker did NOT trip. I think that with modern SW inverters, it is very difficult to trip traditional AC breakers on the output of the inverter(s). In the case of direct shorts, the inverters simply give up and wait for one to clear any faults. Am certain that in the case of an overload of a circuit (say at 150, or 200% of the breaker rating) the breaker will trip in time.

    There was a large "weld spot" at the point of the fault, in a traumatized outlet box, on a temporary drop cord. The inverters delivered a lotta current for a short time until the inverters gave up. The breaker was not designed to react very quickly.

    I was happy with this behavior, but at times one might wish that a breaker would trip instead of the entire power system going down, but this was the reality with my system. YMMV. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.