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markus_m2

Fuses vs. CBs

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Hi guys,

Everybody seems to protect the DC side of their installations (battery to inverter and PV to inverter) with fuses rather than circuit breakers...

I know fuses generally interrupt the flow of power quicker than a CB - is this a requirement / preferred on the DC side?

Any other reason why fuses are preferred over CB's?

Cheers, Markus

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51 minutes ago, markus_m2 said:

I know fuses generally interrupt the flow of power quicker than a CB - is this a requirement / preferred on the DC side?

The purpose is to protect the cable, and since both a fuse or a breaker will interrupt the current before the cable sets something on fire, there isn't really a reason to pick fuses over a breaker other than cost. When you go over 100A, breakers get expensive quite quickly, but fuses, and fused disconnects, are not that bad.

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Thanks Plonkster, so it really only comes down to cost?

The space saving, reduced complexity and ease of troubleshooting capability the CB has over the fuses may just justify the added costs :)

 

Any recommendations on DC circuit breakers? Looking for a 100A Battery "disconnect" CB  (correctly DC rated)

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7 minutes ago, markus_m2 said:

The space saving, reduced complexity and ease of troubleshooting capability the CB has over the fuses may just justify the added costs :)

Of course, if you frequently need to disconnect it, or you expect it to trip/fuse a few times a year, then go with the breaker. I've had my Mersen disconnects in for years now and still running on the same fuses.

8 minutes ago, markus_m2 said:

Any recommendations on DC circuit breakers? Looking for a 100A Battery "disconnect" CB  (correctly DC rated)

When you consider how readily available the Jean Müller KETO disconnects, or the Mersen ones are... compared to the hunt for a breaker, that might also answer your question 🙂

ABB makes really nice DC breakers. But they cost 4 times as much as the fused disconnect.

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Appreciate @plonkster

Sho, that ABB breaker is indeed a bank-breaker 😂

What about something like

  • Hager  -  Says it is "Compatible with DC Voltage Application", although it doesn't give much more detail on it's DC compatibility  - it's rated voltage is 230V AC, considering the battery will be call it 55V max DC (Pylontechs) I'd think this breaker should be good?
  • Schneider Electric C120N series - specifically mentions rated operating voltage <= 250V DC
  • Lovato - rated 60V DC. Not sure of the brand Lovato though?  (last thing I want to do is install some cheap crap)

 

Edited by markus_m2

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CBI has a pretty decent DC circuit breaker range, thats what I'm using for my system, the prices did not seem that outrageous. They're also a 'local' company, which is nice.

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Hi

From my experience in using fuses especially on the PV to inverter side there is a caution. Dc produces a large arc when connecting and disconnecting when the PV,s are live. 

If you do not open the fuse holder fast enough the arc can set the fuse holder alight (comes from personal experience). A dc circuit breaker will have a fast switching action which will limit the arc and is safer. I still use fuses but I am now more careful.

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Indeed! For a demo of AC vs DC switching have a look this clip on YouTube:

AC versus DC load breaking comparison with a knife switch

 
Edited by Richard Mackay
grammar!

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Guys, please indulge my ignorance and learning process:

Looking for a CB between Pylontech batteries and goodwe inverter.

Comparing the following two:

  1. Single Pole and Neutral MCB 100A (230 VAC - 60 VDC)
  2. Double Pole MCB 100A (400 VAC - 125 VDC)

What's the difference between 1P+N compared to 2P? I suspect this point alone will sway it towards the more expensive option 2?

Also, does 60V DC leave enough "room" (I suspect it should), or would you rather overspec with 125V of option 2?

 

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46 minutes ago, markus_m2 said:

1P+N

I'm not sure that there is a common terminology that goes across manufacturers, but 1P+N usually means it is a two pole breaker, and that the live side has overcurrent protection while the neutral side has only a switch. The two handles are tied together, so that an overcurrent event switches off both sides.

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My last deciding factor in CB vs fuse - 

I know one shouldn't mix AC and DC cables in the same trunking or DB, what about two pairs of DC cables with different voltages?

In this example, can one install into one single sub-DB the cables and circuit breakers for the PV (350VDC) AND for the battery (48VDC)?

 

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Posted (edited)

Something I don't see mentioned is the breaking capacity rated in kA.  The kA rating determines how many instantaneous amps the circuit breaker can quench without arcing.

Also it is very devious, some circuit breakers rate rated for AC and DC.  They usually quote the AC kA breaking capacity, but the DC rating is much, much lower.  For example, I had bought an overpriced Gewiss breaker that claimed 10kA AC but is rated at 3kA for 30volts DC.  When I connected it to the battery circuit just as a disconnector you could already see the problem.  Just turning it on caused the circuit breaker to get arc from the inverter capacitors charging up.

To determine the breaking capacity required, you need to calculate the total circuit impedance and know what your current source can supply.  But for batteries, the instantiation current is very high (even if for only a very short time)

Long story short but for batteries, a DC circuit breaker is not sufficient most of the time.  Not unless you spend a fortune.

The average DC circuit breaker, breaking capacity: 3kA-6kA.  Average fuse breaking capacity 80-200kA.  The two aren't even slightly comparable.  So it is most definitely not down to just price.

Just so we are clear, even Eskom and utilities world wide still use fuses for their supplies (yes for AC which is a LOT easier to break than DC).  They just also use a circuit breaker.  For the same reason.  It is called cascading.  If you have a fuse behind it all, you can rely on the circuit breaker for most situations but a dead short is taken care of by the fuse.

Edited by Gnome

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On 2020/02/28 at 7:26 PM, Peter Topp said:

A dc circuit breaker will have a fast switching action which will limit the arc and is safer. I still use fuses but I am now more careful.

I was surprised of some heat that i noticed on my pv fuse holders and even the fuses  itself during some maintenance work on my pv combiner box. It is not excessive heat but I wasn't comfortable with it, I think the clamps holding the fuse in-place might have something to do with it as they don't push in tightly, so I decided to get some dc breakers, I didn’t ask for a specific brand so they sent me onesto make. It’s the first time I use their dc breakers and used to be more than happy with their older type ac breakers, that I use to love. I must say I was bit disappointed when I’v put a bit of force on the positive side terminal and it looked like the breaker wanted to open up like a muscle. But they are in now, and will monitor it and see if it’s better than the fuses. Problem I had with fuses is you never have a spare when you need it and it’s sometimes not easy to get,  so you end up sticking any metal object in their to get it running again.

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1 hour ago, Gerrie said:

I was surprised of some heat that i noticed on my pv fuse holders and even the fuses  itself during some maintenance work on my pv combiner box. It is not excessive heat but I wasn't comfortable with it, I think the clamps holding the fuse in-place might have something to do with it as they don't push in tightly,

Are they are IEC type fuse holders (with that chamber you insert the fuse into and then it is rotated into position)?

I don't dislike these but their connection terminals (clamp type) are not great. What I've learnt is not to terminate the wiring with  a lug or whatever. Simply strip the insulation back and  insert the bare wire and screw tight!  (And don't parallel wiring in these terminals!)

The bigger stuff uses eye lugs crimped onto the cables and these are bolted onto the fuse lugs (no fuse holder)

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1 hour ago, Richard Mackay said:

Are they are IEC type fuse holders (with that chamber you insert the fuse into and then it is rotated into position)?

The brand is SWAN 32A and they do have an IEC marking. The current measured through one fuse was 9.1A and it had 15A fuses. I dont know if the fuses generally generate some heat.

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They do. The fuse holder will be made of a high temperature plastic, non combustible... (in the good old days they were white ceramic)

It's always a good idea to check one device to another. You will have 2 fuses (minimum) as I understand it: one in the positive and the other in the negative wire. These should be a similar temperature. If one is a much hotter then you have a problem.

Having a contactless thermometer (IR) is a great tool because you don't really wanting to be poking your fingers around the equipment.. Temperature is an excellent indication of the condition of electrical switch-gear and batteries etc...   

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39 minutes ago, Gerrie said:

I dont know if the fuses generally generate some heat.

Oh they do. If the fuses are operating close to capacity (which they usually do, they are sized around 20% higher than peak current), they are so hot it's uncomfortable to touch (should you remove them from the holder). It's a thermal device, it is designed to get hot, and it is designed to melt through after some time... it will get hot 🙂

 

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Posted (edited)

Noark make a nice relatively inexpensive DC breaker . Note when wiring up a DC breaker - most are polarised and wiring it up incorrectly is akin to using a AC breaker. The catch cage is on the wrong side and the magnetic field send the arc in the opposite direction.

Look  here.

Edited by Chris Hobson

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Posted (edited)
20 hours ago, Chris Hobson said:

Noark make a nice relatively inexpensive DC breaker

Who sells these?? (Livecopper do not even list them!)

Edited by Richard Mackay

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