DC fuses vs DC breakers

lazza · February 2012

HI forum

I have been looking at fuses and fuse holders to go between the panels and the Charge Controller and then the Charge Controller to the batteries. The system is at 24V and will have 6 panels in parallel (Isc 5.25A), which means I wish the breakers or fuses to be rated at 40A.

Problem. The maximum rated current for fuse holders for the 10x38 fuse size, is 32A. And i dont want to put the larger size fuse holders as I will be putting in 4 (2 on each side of the CC, as I wont be grounding the battery negative), and the bigger fuse holders and fuses (22x58 ) are unnecessarily large and use up too much space.

I saw these DC circuit breakers on the windsun website http://www.solar-electric.com/stfubr1.html Looks like they will do the job nicely... could I put in 4 of these instead of 4 fuses?

Cheers
Larry

BB. · February 2012

Re: DC fuses vs DC breakers

lazza wrote: »

I have been looking at fuses and fuse holders to go between the panels and the Charge Controller and then the Charge Controller to the batteries. The system is at 24V and will have 6 panels in parallel (Isc 5.25A), which means I wish the breakers or fuses to be rated at 40A.

If you have 3 or more panels in parallel, then each parallel connection should have a fuse rated for the maximum Isc*1.25 rounded up to next standard size (if your panels do not list a Series Protection Fuse).

So for your above panels, I would have expected a 7-8 amp or so series fuse/breaker per panel string.

Problem. The maximum rated current for fuse holders for the 10x38 fuse size, is 32A. And i dont want to put the larger size fuse holders as I will be putting in 4 (2 on each side of the CC, as I wont be grounding the battery negative), and the bigger fuse holders and fuses (22x58 ) are unnecessarily large and use up too much space.

You do not need another fuse/breaker on the solar panel input to the charge controller if the wiring is correctly sized for Isc-array (and the array is already fused). Also, fuses tend to not be good "switches". Some fuse holders have been shown to arc and catch on fire if a person just "pops" the fuse loose and does not remove ("Touch Safe" holders have been demonstrated to have this problem). One reason that breakers are not a bad solution (if not cheap) if you do need an on/off switch too.

Note another issue with many glass fuse holders. The higher the current rating, the lower the rated maximum interrupt voltage.

I saw these DC circuit breakers on the windsun website http://www.solar-electric.com/stfubr1.html Looks like they will do the job nicely... could I put in 4 of these instead of 4 fuses?

Yes, but if you are paralleling solar panel strings, you probably want to look at a combiner box with fuses or breakers.

Pre-Wired Combiner Boxes with MC connectors
Unwired Combiner Boxes

-Bill

vtmaps · February 2012

Re: DC fuses vs DC breakers

Two more things.

1) Many DC breakers are polarized Its not always intuitive which way to put them. For example If you want to put a breaker between the combiner box and the charge controller, you might need to put two breakers in series, with each breaker hooked up with opposite polarity. The breaker whose plus side is towards the combiner box is your PV disconnect and it will be disconnecting current flow from the PVs to the CC. The other breaker has its plus side towards the CC. Its purpose is to limit current from the battery flowing into a short circuit in the PVs or their wiring.

Likewise, the breaker in each series string of PV panels should have its minus side connected to plus side of the PV string. Its purpose is not to disconnect the string, but to limit current flowing backwards through a defective string from the other strings.

2) Many DC breakers must be in the upright position in order to work correctly.

--vtMaps

EDIT: The BOLD text above is sort of wrong. when there are two breakers in series between the panels and the CC, the second breaker is usually part of a GFP device. To limit current flow from the battery to to the CC or panels, it is necessary (and important) to have a breaker between the battery and the CC.

lazza · February 2012

Re: DC fuses vs DC breakers

HI , yeah, I will be putting a small fuse of 7.5A between each panel (in effect each panel is a series as the system is at 24V). The fuses/breakers between the panels and the charger are as much as a quick disconnect, as protection (always helps for quick isolation and fault finding). The other two will be to protect the CC from the battery.

So it looks like it may be a good solution these breakers, especially if I will be using them as switches at times.

Any idea how long shipping times are to Europe?

BB. · February 2012

Re: DC fuses vs DC breakers

None of us moderators here work for NAWS--So we cannot answer for them (all of us are volunteers too--just like you guys).

If you have business related questions for our host Northern Arizona Wind & Sun, please contact them here (webform or at below):

Northern Arizona Wind & Sun
4091 E Huntington Drive
Flagstaff, AZ 86004

Sales & Service: 800-383-0195 | 928-526-8017
Fax: 928-527-0729

Our office hours are from 8AM to 4PM, MST Monday through Friday. We are closed for major holidays. If you are in the Flagstaff area, please stop by our store and visit.

By the time shipping, duties, insurance, and tariffs are charged--You might be better off looking local. There are a few people from Spain/Europe here that may be able to give you some local supplier information.

-Bill

PS: For series connected panels, you only need one fuse/breaker per string, not one per panel (i.e., two panels in series for 24 volts still only need one Series Protection Device).

lazza · February 2012

Re: DC fuses vs DC breakers

HI Bill, thanks for the replies, ok i'll get in touch with them directly

The panels are actually 24V, hence I mentioned fusing each panel

Not linked to this thread, apologies but i'm still a bit dizzy re: Grounding.

Is it ok to ground the PV structure to the same common ground rod for the PE, Neutral of the system? (Referring to an off-grid TT type instalation)

I've read that if the PV structure is more than 15m then a separate grounding rod is recommended...

BB. · February 2012

Re: DC fuses vs DC breakers

Yes, the AC Neutral and DC ground wires should all go to the same ground rod. If you need more rods (poor soil), run the cable from that rod to the extra rods (and to metal cold water pipe, other connections as need/per code). The AC and DC ground wires should only meet at one place, the ground rod.

Lightning does not travel very well down the grounding cables (lightning is a high(er) frequency wave form--if you are familiar with transmission lines and Radio Frequency energy). So local grounding of the metal solar panels/mounting frames is recommended for lightning grounds. You should run a cable from the lightning ground rod to the main home ground rod--This allows any short circuits to have a solid safety ground path through wiring (don't use the soil as the only safety ground path).

-Bill

Vic · February 2012

Re: DC fuses vs DC breakers

Couple of things:

2. Personally, I would place a main PV input Breaker very near the CC. This would be the a main disconnect for emergencies or other reasons. Many Combiners are on the roof for Off-Grid systems. THis is not very convenient in the caae of an emergency. And, also personally, I would try to avoid switching DC Breakers that are carrying load currents. Some CCs have an "off" mode that would remove the DC current from the PV input lead.

2. The most important DC breaker for the CC is the Battery lead. Here is a link to the Midnite SOlar Forum, where the correct polarity of this batt breaker is discussed:

http://midnitesolar.com/smf_forum/index.php?topic=113.0

Good Luck, Vic

lazza · February 2012

Re: DC fuses vs DC breakers

Ok thanks for your replies.

So the verdict is yes, to put in a separate ground rod for the PV metal structure, but also pass a wire from the PV ground rod to the House Common Ground Rod... what size should this wire be? I am thinking about 16mm2 (that's AWG #6)

As for Circuit Breakers, points taken on board, thanks

BB. · February 2012

Re: DC fuses vs DC breakers

In the US, I believe that 6 awg is the smallest bare copper wire that is allowed to be buried (smaller cable may be too easy to damage).

The other think to look at is the maximum current that the ground interconnect cable would be allowed to carry in the event of a short circuit. If I recall correctly, that is 200 amp maximum service panel before a larger wire would be required (US code, your code may differ).

If I was designing this for non-solar array (a piece of telecommunications gear for example), I would also calculate the total resistance of the worst case (typically either 1/2 trip or round trip) of a short circuit between hot and safety ground. The resistance should be low enough to trip the protecting circuit breaker (you don't want the wire to become an electric heater:roll:).

For solar panels which are naturally current limited, this calculation is sort of redundant. A shorted solar panel will still only output around Isc (short circuit current), unlike a lead acid battery bank which can output hundreds to thousands of amps into a short circuit.

-Bill

niel · February 2012

Re: DC fuses vs DC breakers

i'm not too sure on how the nec views it, but i would go #6 or better just simply because a degree of corrosion will occur on the wire reducing its effectiveness some. the extra thickness won't do much to stop breakage as only depth or other protective measures will help stop the wire from inadvertently being broken.

machineman · March 2012

Re: DC fuses vs DC breakers

vtmaps wrote: »

Two more things.

1) Many DC breakers are polarized Its not always intuitive which way to put them. For example If you want to put a breaker between the combiner box and the charge controller, you might need to put two breakers in series, with each breaker hooked up with opposite polarity. The breaker whose plus side is towards the combiner box is your PV disconnect and it will be disconnecting current flow from the PVs to the CC. The other breaker has its plus side towards the CC. Its purpose is to limit current from the battery flowing into a short circuit in the PVs or their wiring.

Likewise, the breaker in each series string of PV panels should have its minus side connected to plus side of the PV string. Its purpose is not to disconnect the string, but to limit current flowing backwards through a defective string from the other strings.

2) Many DC breakers must be in the upright position in order to work correctly.

--vtMaps

I have a question about this. So if the Breaker is polarized, + side being on the positive source voltage, what happens to the current flowing the other direction or source voltage is coming from the other direction? Does the reverse flow without any protection like it was a solid wire?

vtmaps · March 2012

Re: DC fuses vs DC breakers

machineman wrote: »

I have a question about this. So if the Breaker is polarized, + side being on the positive source voltage, what happens to the current flowing the other direction or source voltage is coming from the other direction? Does the reverse flow without any protection like it was a solid wire?

good question... I don't know about all types of DC breakers. The ones I do know about and use would trip from overcurrent in either direction. The problem with DC current (in general) is that it may be difficult to extinguish the arc when a connection is opened, and polarized breakers are designed to extinguish the arc in one direction only.

I am happy to share what little I know about DC breakers with you... hopefully someone more knowledgeable will chime in and tell us more of the story.
--vtMaps

rgk1 · March 2012

Re: DC fuses vs DC breakers

vtmaps, Thanks for the heads up about the breakers needing to be installed vertical. I have been thinking about an expansion that would include breakers in a horizontal position. I could be wrong, but it seems like I remember reading somewhere that these types of breakers are hydraulic.

This is a bit of info grabbed from the Midnite website:

"All DC breakers are polarity sensitive. When dealing with the high fault currents associated
with batteries, it is very important to get it right. The “+” sign on our din rail mount breakers and
the “Line” side of our panel mount breakers are intended to be at the highest potential during a
fault condition."

stephendv · March 2012

Re: DC fuses vs DC breakers

lazza, you can source these sorts of things closer to home from Nigel (user nigtomdaw on this forum): http://www.prismsolar.eu/shop/index.php?_a=category&cat_id=10&page=1

DC fuses vs DC breakers

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