article question (fuses/circuit breaker)

mshen11

in this article...

http://www.solar-power-sales.com/sol...ycombiner.html

assuming the panels provide 15amps max and the breaker is 60amps... if the combiners contain 15amp breakers, why do they have a fuse between the combiner and charge controller if that wire is seized properly (unless you want to put more than panels)?

in general when do you use breakers and when fuses? why not just use breakers if they are better?

Cariboocoot

Re: article question (fuses/circuit breaker)

"the page can not be found" - so that's a bit of a problem.

Generally speaking, fuses are used instead of breakers because they are less expensive. If it is a part of a system, such as a PV array, that doesn't represent a lot of current and doesn't need a 'heavy' disconnect then fuses are the usual choice. On high-current circuits, such as battery feed to inverter, the choice may favour a DC breaker over a fuse & disconnect. Such breakers are expensive, as it is more difficult to interrupt DC than AC (it jumps a bigger gap, to put it simply).

Circuit protection gets put in wherever there's likely to be a need, and sometimes this can seem redundant. For instance if you have multiple battery banks feeding one inverter, there may be a fuse per bank even though the connection of all banks is continuous. Likewise, multiple inverters off one bank may have a fuse/breaker per inverter so that each device can be isolated from the power source separately should something go wrong with one of them.

In the example you cited, it sounds like true redundancy: the fuses per panel would protect the circuit and the 60 amp breaker is mainly a disconnect (but allows for additional panels in future). This is not necessarily wrong.

The important thing to remember with any circuit is that the weakest link (fuse/breaker) is what should give out should an over-current condition exist.

mshen11

Re: article question (fuses/circuit breaker)

sorry this is the link

http://www.freesunpower.com/project1.php

so... fuses are to protect wires, not the devices (inverter, charge controller, panels, etc...) themselves? if i do the calculations out exactly on how much each wire will get, then fuses/breakers are not a "must-have" but a "nice-have" (in another words if you are tight on budget and time and have a few hundred watts worth of panel setup can you delay putting in breaker/fuses if the wire sizing is done correctly)?

also... the placement of breakers/fuses - is it as close as possible to the battery for ALL cases (inverter, controller, etc...)?

Cariboocoot

Re: article question (fuses/circuit breaker)

"Free Sun Power" - very misleading. As we all know, this "free" power ends up costing around $1 per kW/hr over the life of the system (+/- due to local variations).

You are right: fuses are to protect devices. But wiring too is a "device", and you don't want it acting as a fuse. Fuses blow is a short location; wire tends to fry along its length when over-loaded and sets the insulation on fire, burns down the house ...

Fuses/breakers are definitely a "must have" just for that reason. If you buy a cheap inverter and it shorts out internally you don't want it burning the place down. Fuse blows, power is gone, effects of short are minimized.

You don't necessarily need them on a PV array, if it's small. You definitely want them on the battery to inverter connection, as that is where the most current will flow. And there is sometimes a problem with AC breakers on inverter systems: the inverter will "fault" before the breaker will trip because breakers require over-current for a length of time before shutting off; inverters tend to shut down instantly upon being over-loaded.

A really good source of information for what fuses/breakers to use in the instruction manuals of the respective equipment you plan to buy. Most reputable companies have this available on-line these days, so you can look them up, download it, and read it. This gives lots of practical insight into a particular product. It is a given that the companies know what they're talking about, so if they say "use a 125 Amp fuse" it's correct.

That said, there are a number of disreputable companies out there who sell over-rated junk and ill tell you anything (or nothing) in order to get you to buy it. Before you spend the money on a particular product, ask here if anyone's had experience with it. One answer will be "Do NOT buy BZ charge controller".

Cariboocoot

Re: article question (fuses/circuit breaker)

BTW - the diagram shown on that link page is a correct install. There is one fuse between the array and the charge controller, another between the controller and the batteries (different voltage/current on the in/out of the CC), a breaker between the batteries and the inverter, and the inverter's AC out fed into a distribution panel. It also shows a shunt and installed battery meter. We can assume there are breakers built-in to the back-up generator, so all circuits are properly protected.

dwh

Re: article question (fuses/circuit breaker)

mshen11 wrote: »

so... fuses are to protect wires, not the devices (inverter, charge controller, panels, etc...) themselves?

That is correct - fuses and/or breakers are to protect the wire. The fuse or breaker must always be rated less than what it will take to overheat the wire.

if i do the calculations out exactly on how much each wire will get, then fuses/breakers are not a "must-have" but a "nice-have" (in another words if you are tight on budget and time and have a few hundred watts worth of panel setup can you delay putting in breaker/fuses if the wire sizing is done correctly)?

You can do anything you want...the question is whether it's smart to do it.

With 200w of panel putting out 17v, the amperage is 11.7. Are you saying you can afford all this stuff but can't justify the cost of a 15a fuse?

If there's a short, and there is no fuse, then the power will continue to flow until the copper in the wire melts. By the time that happens, your roof might well be on fire.

Even if, as you say, the wire is "properly sized"...

Imagine that you have a 2000w inverter. It's drawing say 150 amps from the battery (at full load). So let's say you use 0 size wire (one ought) from the battery to the inverter. That wire is rated at 150a. Great.

Then there is a short, and now the wire is pulling EVERYTHING THE BATTERY CAN GIVE. It's not pulling 150a, it's pulling maybe 600a or 800a. You have just created a *gigantic* electric ignitor.

0 size wire has a 8.25mm copper conductor - basically just a hair less than the diameter of a 9mm bullet. That wire isn't going to melt easily, it's going to take a long while and a LOT of heat. Enough heat to ignite whatever flammable material is anywhere near it.


If you are going to put this out in a field somewhere then maybe...but if you are going to put it on your house then DO NOT neglect the fusing.

also... the placement of breakers/fuses - is it as close as possible to the battery for ALL cases (inverter, controller, etc...)?

Generally, it's considered best practice to put the fuse/breaker nearest the source of the power in order to protect the "greatest length" of wire.

So for the "PV -> charger" it would be best to place the fuse as close to the PV as possible (usually the combiner box). For "charger -> battery", then close to the charger, and for "battery -> inverter" then close to the battery.

john p

Re: article question (fuses/circuit breaker)

That is correct - fuses and/or breakers are to protect the wire. The fuse or breaker must always be rated less than what it will take to overheat the wire.

This is not really true .
Example A person uses #4 wire to connect a 300w inverter to a battery..Now that wire would take a lot of power to heat it up..and a 100a fuse/circuit breaker would do its job eventually ..but its way too big for the inverter if the fault is in it.
the fuse is really there to stop the power from entering the damaged device/wire.If the fuse /circiut breaker operates then no current can flow from the solar panels,, to charge controller or from batt to device...The fuse /circuit breaker shold only be rated just above the surge rating of the device.
ie if battery to 300 w inverter 12v v the fuse should be max 50a that allows for 600w surge draw of inverter..the wire connecting them should be rated at that as a minimum.
in any well wired system the likelyhood of the wires from solar panels to charge controller becoming short circuited is very low..as they should be routed in conduit.also wire from battery/ies to inverter very unlikely as very short wire. but fault in inverter can cause big drain from battery and cause damage..

Circuit breakers are in most cases better than fuses as fuses eventually fail because of stresses in the fuse wire when starting a heave load.

mshen11

Re: article question (fuses/circuit breaker)

thanks for all the reply. i asked a person this quesition another place, specifically why in his case fuses were used instead of breakers... his reply was breakers sometimes fail in places with alot of heat.

can someone add to that statement?

mike95490

Re: article question (fuses/circuit breaker)

On a roof top in a PV combiner box, a fuse costs $1, and rarely fails. A $20 breaker in the same situation, when cooking in the sun, may trip off, on a toasty hot day.

Sometimes fuses fail too, but replacements are only a buck or 2.

Cariboocoot

Re: article question (fuses/circuit breaker)

Anything will fail given enough heat.

Breakers actually work by heat: they contain a bi-metallic element that heats up with current flow. If it heats up enough (due to excess amperage) the element bends and flicks the switch off. Prolonged exposure to high ambient temps can change the properties of the metal and may cause it to not respond as per spec's.

This is why you generally see inexpensive fuses on PV's, up on the roof where it's very hot. They don't react so much to ambient temps. Fuses will fail over time due to 'fatigue'; the closer they are run to maximum current, the sooner they will fail. But they're much less expensive than breakers.

Breakers are usually used only at the battery because they provide both circuit over-current protection and a convenient disconnect for the inverter. They may also be used at the charge controller for the same reason. DC breakers are expensive, and some people prefer the lower-cost option of fuses and a separate disconnect switch.

Notice the placement of fuses/breakers is always near the current source: @ panels, @charge controller, @batteries, @distribution box.

mshen11

Re: article question (fuses/circuit breaker)

at what temperature does breakers become flakey? an enclosed unventilated garage/attic in hot texas weather could probably exceed 130F (guess)?

also for fuses. what are the recommended ones that are common and interchangeable w/ the holders? where would you buy them locally? HD/Lowes dont carry DC fuses; car stores have it only up to 30 or 40 amps but no holders

[DC breakers are expensive, and some people prefer the lower-cost option of fuses and a separate disconnect switch.]

what is an example of a separate disconnect switch?

Cariboocoot

Re: article question (fuses/circuit breaker)

Working backward through your questions:

Disconnect/bank switch - http://store.solar-electric.com/basw1300amp.html

Fuses/breakers - http://store.solar-electric.com/cipr1.html

Automotive fuses can be used as long as current & voltage ratings aren't exceeded. As in a "12 V" array which is actually 17 V * 30 Amps = 510 Watts. (The engineers here will please correct me if I've done the math wrong. Again.)

As for at what temperature breakers start to break down, it would depend on the manufacturer's specs for their product. They generally have an operating range that exceeds anything your likely to find in ambient conditions, even in Texas (-40 to +185 F for example). It is more accurate to say that repeated or prolonged exposure to temperature extremes will shorten the life of any component.

Elsewhere you asked about shunts, so I'll give you a basic explanation: they're a kind of "fixed resistance" that is put in circuit where you need to measure some aspect such as current. A meter is attached on either side. Most of the current flows through the shunt, but a controlled and proportionate amount flows through the meter. This means you can accurately measure the DC load on your batteries without having to put 250 Amps through a digital multi-meter (which is impossible).

Nice things, these battery meters - http://store.solar-electric.com/metersmonitors.html

cb1000rider

Re: article question (fuses/circuit breaker)

mshen11 wrote: »

in general when do you use breakers and when fuses? why not just use breakers if they are better?

Article is AWOL.

Fuses are cheaper. They require replacement when blown. They are not as temperature sensitive as breakers.

Breakers are temp sensitive, more expensive, and can be easily turned on/off to test system components.

BB.

Re: article question (fuses/circuit breaker)

Fuses are temperature sensitive too... And breakers may be magnetic type (not very sensitive to temperature) or thermal (are sensitive to temperature)...

Fuses are certainly cheaper and more available at the high DC Interrupt current ratings. CB are more costly and not as available.

Fuses + switch (if needed) is probably more cost effective than using a CB as both a current interrupter and as a switch.

Neither device should be operated near its maximum rating or there can be false tripping (or even shortened life). Normally, NEC requires circuits to be loaded at 80% maximum current of their wiring/breaker ratings.

-Bill

crewzer

Re: article question (fuses/circuit breaker)

Both circuit breakers and fuses are suitable overcurrent protection devices. However, either needs special consideration for PV array combiner purposes.

A key issue is sizing. Fuses are typically sized at 156% of series STC Isc, but 100% continuous duty circuit breakers must only be rated at 125% of STC Isc. (ref: NEC 690.8 )

Fuses are not as cheap as they appear to be for this application. The issue is that both sides of the fuse holder may be energized: one side from the assigned string, the other side from the other strings wired in parallel.

The solution is the so-called "touch-safe" fuse holder. These holders are designed to inser/remove a fuse while protecting the user from the exposed contacts. These fuse holders can be installed in PV combiner boxes such as those from OutBack and MidNite,

See: http://store.solar-electric.com/mnts.html

However, the combined cost of the fuse holder and the high-voltage fuse is about the same as a comparable DC-rated circuit breaker, if not more. Also, the fuse holders are a bit wider than the circuit breakers, so a PV combiner box accommodates fewer circuits.

Accordingly, I like DC-rated circuit breakers, which can easily be used as switches to disconnect and isolate individual array strings. Temperature doesn't appear to be a problem. The CBI breakers are rated for at least 60 C (140 F) applications. They're also available in a broad range of current ratings appropriate for PV array applications.

See: http://store.solar-electric.com/pspv.html
and: http://store.solar-electric.com/mnepv.html

Finally, two parallel strings really don't require OCPD's for each string, and three parallel strings may not.

See: http://www.nmsu.edu/~tdi/pdf-resources/CC125.pdf

HTH,
Jim / crewzer

P.S. I recommend that automotive-type fuses and related hardware (including marine) not be used. They're typically rated at 30 VDC max, and they're not UL listed for PV applications.