fuse between inverter and load
hello
Registered Users Posts: 9 ✭
hello
i have a question on off grid system
if i have a system like this :
panels
charge controler
battery
inverter
load
normaly ,i do put a fuse between battery and inverter ,but do i need to put ac breaker or fuse between inverter and load
and if a suppose that a short circuit happen on load ,what fuse will shut down the first between battery and inverter or the one between inverter and load
i have a question on off grid system
if i have a system like this :
panels
charge controler
battery
inverter
load
normaly ,i do put a fuse between battery and inverter ,but do i need to put ac breaker or fuse between inverter and load
and if a suppose that a short circuit happen on load ,what fuse will shut down the first between battery and inverter or the one between inverter and load
Comments
Welcome to the forum Hello!
Fuses/breakers are to protect wiring from high current power sources.
The primary source of high current will be your battery bank +/- bus.
So, any (typically positive) wires that leave the battery + bus should have a fuse or breaker sized for the loads and wiring capacity.
You used about an AC inverter--So, the generic calculations for a 12 volt 1,200 Watt AC inverter (running at full rated load) would look something:
1,200 watt * 1/0.85 inverter eff * 1/10.5 inverter cutoff * 1.25 NEC derating for wire/fuses = 168 Amp branch fuse/breaker/cable minimum rating
For each circuit that leaves the + battery bus, the fuse/breaker/wiring needs to be rated as above. You can use NEC or other standard (marine for example) to then pick the wire that meets those requirements.
DC fuses/breakers are a bit more difficult to find, and can have issues between 12/24/48 volt rated battery systems.
Fuses may be cheaper to install, but many high current DC fuses are not cheap--So, many times, it is simply "better" to use a DC Circuit Breaker instead. You get the protection of a fuse, plus a handy "on/off" switch in one unit.
Rather than typing about generalities--As your questions for your specific needs. It will be less confusing.
Besides fusing the DC battery bus--You may have fusing requirements for your solar array (typically three or more solar panel/strings in parallel usually need a fuse/breaker per string). On the AC output side--It depends. For example, the minimum AC curcuit in the US is rated for 15 amp on 14 AWG wire... If you have a 300 Watt AC inverter--You really do not need a fuse/breaker. If you have a 2kWatt+ inverter, then you may need a fuse/breaker per branch circuit.
-Bill
thank you
i need to know ,if i use a 2kwatt inverter ,and i use a fuse/breaker after the inverter ,did this breaker will shot down before the dc fuse in battery or not? because if a short circuit happen on the load side ,this means that the it will short circuit the battery and the inveter can be damaged? because if the ac breaker work that means that the short circuit current from the battery will passe throw the inverter to the load side ,and can damage the inverter?
Sad to say this but circuit protection, CB's of Fuses, are NOT there to PROTECT inverters, appliances, etc, but to protect the wiring and PREVENT a FIRE
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(from an electrician's/NEC perspective) that is mostly the way it is. There are a few pieces of equipment that the NEC requires to be protected from overload by an overcurrent protection device: The most common one is panel boards (408.36). Transformers require protection also but this OCPD can be up to 250% of the transformers nameplate in certain situations. Motors: occasionally fuses are used to protect the motor from overload, but usually the OCPD is there to protect the conductors and other means are employed to protect the motor itself.
In stand alone systems the conductors of the the inverter output circuit are sized based on the output rating of the inverter, and these conductors are suppposed to be protected by an OCPD "at the output of the inverter". Usually the NEC is quite specific as to the exact location and length of conductors until protection for taps and transformer secondary conductors but this is a little vague.
Regarding your question about what happens in the case of a fault, I think that most of the time the inverter will detect the fault and shut down before the OCPD trips. I think it would depend on the impedance between the fault and the inverter, the trip curve of the OCPD, whether the conductors and OCPD were sized for continuous load (125% rule), and the inverters tolerance and scheme for dealing with overloads. Maybe others have practical experience with this but my guess is the inverter will shut off before any OCPD's trip.
In general, fuses and breakers have pretty slow trip times (minutes to fractions of a second for serious overloads). For electronics, they can fail in microseconds--And it may be over voltage, not current, that causes a component failure.
As said by others, the over current protection by inverters is going to be much faster than any breaker/fuse. Fuses/breakers are there to prevent the wiring from catching fire if the failure is (for example) a dead short (fairly common failure mode for many electronic components).
-Bill
i need to know if the inverter will shot down before the ac breaker fuse shot down when the short circuit happen in the load side,what will be the job of the ac fuse/breaker ?
Transistors, ICs and other semiconductor devices are designed to blow out quickly to protect expensive fuses.
More seriously, it depends on the design of the inverter output circuit:
Some of them will limit the power output (specifically lowering the voltage to keep the current within limits, or else just shutting down). In that case the fuse or breaker will never blow and if the short circuit is generating heat you could have a fire unless monitoring at the inverter shuts the whole thing down.
Others will not have that sort of immediate current limiting, having only a thermal detector to deal with long term low grade overloads and/or high ambient temperatures. For that type of inverter, see my first characterization above.
If you have an inverter with 30 amp output and connect a 14 AWG branch circuit--The 14 AWG branch circuit would have a 15 amp breaker on it to protect the 14 awg wire from getting 30 amps (from the inverter) if there is a short/overload on the 14 AWG branch circuit.
Exactly what is your AC inverter and the loads you want to run?
-Bill
but if the ac breaker shot down to protect the 14 awg , when a short circuit happen in the load side after the 15amp ac breaker ,the short circuit current will go from the battery throw the inverter ,the problem is that the short circuit current of the battery is very big and it is a dc cuurent this means how can the ac breaker shot down befor the dc breker or the inverter?
or that mean that the ac breker is to protect against overload and not short circuit?
In a typical overload of ~25% too much current, it will take seconds to minutes for a fuse/breaker to trip.
And the current on the input is proportional to the output of the inverter... For a 12 VDC input and 120 VAC output inverter:
15 Amps @ 120 VAC => ~150 Amps @ 12 VDC
Because Power = Voltage * Current... If your voltage is 10x higher, the current is 1/10 as much. So, you would have a 15 Amp fuse on the AC output and a 150 Amp fuse on the DC output.
The 150 Amp fuse protects the DC wiring. And the 15 Amp fuse protects the AC wiring. Both fuses/breakers are needed because there are different types of failures (wiring short circuits, failure inside the inverter, etc.) that can cause different fuses to blow depending on the exact failure.
-Bill
thank you all for your help
I have portions of my house powered by an off-grid solar/battery/inverter setup. I had been considering various methods to use fuses or breakers on the output of the inverter. Not so much for the protection of wires (inverter will shutdown if overloaded anyway), but rather being able to isolate parts of the system when working on it. Also, if one area has a failure, it doesn't shutdown the entire system. The breaker for that section should pop first.
I lucked out and a coworker brought me a Paneltronics breaker panel. He was refurbishing his boat, and didn't need this anymore. It had some corrosion, so I took it all apart, cleaned it up, and installed two more breakers. Has the capability to hold 6 breakers, but only came with 4.
I have fuses actually mounted on the battery terminal, which are the only protection for the battery cables feeding the bus bars inside the garage.
I also added a DC circuit breaker between the solar panels, and the charge controller. It has a built-in switch, allowing the solar panels to be disconnected from the system.
I'd rather reset a breaker, or replace a fuse, than deal with a fire!
-Bill
I bought mine from here: http://www.bestboatwire.com/blue-sea-systems-5191-fuse-block-for-terminal-fuse
There's two parts to the terminal fuse blocks. The fuse block itself (link above), and the fuse (below).
http://www.bestboatwire.com/blue-sea-systems-5187-terminal-fuse-200-amp
These fuses are available in 30-300A. Since it mounts directly to the battery, it protects the all the wiring downstream if properly sized.
--vtMaps
Yes in each string between batterys
It can be tricky picking a fuse size for individual strings. Both charge and discharge currents are not likely to be divided evenly among the strings. If the fuses are too large, they fail to protect. If the fuses are too small, then the uneven current flow may blow one fuse leading to a cascade of blown fuses... when one blows, the current must go through the remaining battery strings which get overloaded and blow their fuses. It happens fast and expensively.
One reason to avoid parallel batteries is that adding all the fuses and connections to a system adds potential points of failure to the system. Parallel batteries are an unstable system to begin with, and adding all of the (necessary for safety) extra connections just makes it less stable.
--vtMaps
Also, can you provide a picture or a wiring diagram of how you have these batteries interconnected?
Also, do you know what a DC clamp ammeter is? You need one if you are going to be running parallel strings of batteries.
--vtMaps
--vtMaps