DC disconnect switch - on positive or negative

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DC disconnect switch - on positive or negative

Postby dmatter261 » Wed Apr 02, 2014 9:20 pm

Hi everyone,

I am designing a small 3kW off grid solar PV system and I am a bit confused about the DC disconnect switch.

I need to select a DC disconnect switch. Now instead of this I am planning to use an MCB of an appropriate rating. Is this alright? The negative terminal of the array will be grounded.
And if yes , should the MCB be only on the positive terminal of the array or both positive and negative. I have seen many designs where the DC disconnect switch is placed only on the positive terminal. What is the reason for this?
I also understand that many battery disconnect switches have disconnect on the positive as well as negative terminals. Is solar PV array anyway different ?

Thanks a lot in advance!!
Akella
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Re: DC disconnect switch - on positive or negative

Postby Tracker » Thu Apr 03, 2014 2:17 pm

the thing with normal mcbs is that they only work correctly with 240v AC...
so they won't work normally with DC..

the first thing is to know the planned battery voltage... I assume that it is 48 or less and the PV panels will be presenting an unloaded voltage of perhaps 90v..

at these low voltages, most CBs will work happily as a switch, provided the CURRENT is appropriate..

so..... what battery voltage..

just a starter, I would be switching both leads for both the PV and the BATTERY..
eg. ... 3000w at 50v is 60a...
a quad 63a and use two poles each for the Post and Neg, would be great..
..
.
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Re: DC disconnect switch - on positive or negative

Postby SolarPhil » Tue Apr 08, 2014 9:00 pm

Hi Akella

May I suggest that you use a double pole, non polarised, DC MCB in both the positive and the negative lines of the PV array.

The Noarc brand is inexpensive and works well and has a DC rating of 250v per switch

It is usual these days to use a MPPT solar regulator which gives at least a 15% improvement in output over and above a simple switching regulator.

The Battery disconnect is generally a double pole ceramic type HRC fuse - this arrangement has been standard in all of my off-grid systems and of many other off-grid installers and also grid backup systems.

As far as Battery voltage is concerned, 24v for small systems which use an inverter up to 3kw and a 48v battery for 5 - 7 kw inverter/chargers

I hope I've answered your concerns

Phil
Last edited by Gordon-Loomberah on Tue Apr 08, 2014 10:21 pm, edited 1 time in total.
Reason: removed excessive quoting
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Re: DC disconnect switch - on positive or negative

Postby Warpspeed » Wed Apr 09, 2014 10:01 am

A circuit breaker is not going to achieve very much in a constant current circuit.

Even if you dead short all your solar panels, the "fault" current is not going to be very much higher than the maximum normal full rated operating current.
As we size our solar collector wiring for minimum reasonable voltage drop anyway, the wiring will easily carry whatever maximum current the panels can produce totally shorted out, without any risk of overheating or damaging the wiring.

The wiring to solar panels therefore needs no overcurrent protection, there is simply no possible way any overcurrent damage can occur.
But you do need to be able to completely isolate the panels, for safety and practical reasons, and a big suitably rated two pole isolation switch will do that.

By all means use a circuit breaker if you wish, especially if you already have one, but it really achieves nothing more than an ordinary isolation on/off switch would achieve.
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Re: DC disconnect switch - on positive or negative

Postby Keith B » Wed Apr 09, 2014 10:48 pm

I think that you have missed the point of having a circuit breaker on the solar panels. It is there for two purposes
1. It is an isolator to disconnect (under no-load conditions) the panels whilst working on the system. Depending on the configuration there could be several hundred volts generated by the panels and this could be fatal if you think that you are working on extra low voltage (ELV).
2. It is to break the circuit under full load or fault conditions. Each panel may only produce a few amps but in a parallel or series/parallel situation this could be considerably more.
The circuit breaker must not only be capable of carrying the normal current but also breaking that current under fault conditions and dissipating the energy released, even if it may only be a few amps.
A suitably rated AC MCB will carry the current without any problem but may completely fail when being used to switch off even a quite small DC load. The reason for this is in the design of the arc extinction devices built in to the circuit breaker.
AC 50 Hz types of MCB rely on the fact that every half cycle the current falls to zero, i.e. 100 times every second. The arc is relatively short lived and is cooled by a device known as a Jacobs Ladder which consists of a series of metal plates adjacent to the contact.
If used on DC however the arc does not stop each half cycle and is magnetically attracted to the Jacobs Ladder where it continues to grow until it has burnt away sufficient material to widen the gap to the point where it can no longer be maintained. Could be that it gets wide enough just after your house catches fire.
A DC breaker usually uses magnetic fields which create a motor effect to drive the arc off the contacts and into an extinction cooling device.
Having said all of that it could be that you would never have any problems using an AC MCB but if you do, it could be a disaster.
Multi pole DC breakers can be purchased direct from China on AliExpress at prices similar to locally purchased AC types. Is it worth the risk
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Re: DC disconnect switch - on positive or negative

Postby Tracker » Thu Apr 10, 2014 7:13 am

dmatter261 wrote:I am designing a small 3kW off grid solar PV system and I am a bit confused about the DC disconnect switch


Over a week since the question was asked, and no response from the writer... :roll:
Helpful comments made, and requests for clarification, and no response.... kinda rude. :!:
..
.
Retired Engineer and keen PV experimenter - Always ready to learn and share.
2 x CMS2000 (fan cooled) GCI and SE 170W panels
1.7kW First Solar/Outback Island circuit - Peak Replacement Power
Governments won't save the world :-) They will just TAX it :-(
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Re: DC disconnect switch - on positive or negative

Postby Clemo » Fri Apr 11, 2014 12:57 pm

SolarPhil wrote:
The Noarc brand is inexpensive and works well and has a DC rating of 250v per switch


From what I can gather the Noark's have a max DC voltage rating of 1000v @ 3amp and Max of 300v @ 32a per pole.

The appropriate isolator will need to be rated for both the correct Voltage and current, that will depend on how your array is configured, may required string fuses too.

PS Noark are not a quality isolator IMO, but yes they are cheep!
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Re: DC disconnect switch - on positive or negative

Postby Warpspeed » Fri Apr 11, 2014 1:21 pm

While all of the above is true, it needs to be put into a bit of context.
Below about 50 volts, just about anything that will carry the rated current is going to work just fine carrying dc.

Up to maybe 100 volts dc, you may also be pretty safe, UNLESS either the dc current being interrupted is fairly high, there is a lot of inductance in the circuit, or the contacts are in constant repetative operation.

Car ignition points come to mind, they can burn out pretty fast, compared to say the contacts in the starter motor solenoid.

Above about 100 volts, interrupting the current in dc circuits starts to quickly become very hairy.

All the evils Keith mentioned above ^^^ definitely come into play, and you may need to take some professional advice on selecting components for a dc switch, dc power contactor, or dc circuit breaker.
Even special fuse types are recommended for interrupting fault currents in higher voltage dc circuits.

For the majority of us here, standard 240v ac rated stuff will work just fine below 50 volts, and with a bit of thought below perhaps 100 volts, for fairly infrequent circuit isolation if the current is fairly low.
Arc suppression is one issue, another is contact errosion.
Ac contacts are generally tungsten, dc contacts pure silver.
Dc systems are definitely very different to ac systems as far as switching high currents on and off goes.

However, for us low voltage guys, there is no need for paranoia about all this.

But if you are building a really big high voltage high power system, some good professional advice on component selection will be well worth seeking.
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Re: DC disconnect switch - on positive or negative

Postby Clemo » Fri Apr 11, 2014 3:57 pm

For all we know, it could be AC coupled, IE higher DC voltages.

As stated, best to seek professional advise/design.
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Re: DC disconnect switch - on positive or negative

Postby dmatter261 » Wed Apr 30, 2014 2:36 am

Hi everyone,

Thanks for the responses.
Terribly sorry about the delay :( :( :( :( , I was travelling and neither did I receive a mail that I have got replies though I seemed to have followed this post :?

After I posted the ques, I read quite a bit about DC disconnect switches and a lot of literature(esp NEC) suggests that for systems with inverter grounded , the negative of solar array should be grounded as its isolating the AC and DC. It also mentions that there shouldn't be any sort of disconnection on the grounded conductor.
However for systems with transformerless inverters , i.e. without any isolation of AC and DC sides, DC disconnection should be provided on positive and negative.I was planning to go ahead with a single pole DC MCB because my inverter has an transformer.

@Tracker - I was planning to use a DC MCB anyway so I guess the issue with no zero crossing wouldn't arise as Keith B suggested.

@ Solarphil - I have seen many systems where battery negative is grounded. however why is it that we still disconnect both positive and negative there while the same logic doesn't apply to solar panels.

@Keith B - Is it so that the response time for DC MCB s is that slow!! :o .. Where can I get curves for this? I was checking ABB S800PV S and M . Curves for disconnection switch tripping isn't given though for string protection MCB ( PV S ) it has been given.

@Warpspeed - Our system DC voltage( DC Vmpp at STC) is about 122V and battery operating voltage( there is a charge controller before battery ) is 96V.

@Clemo - I am not sure what AC coupled in solar array means. Could you elaborate?

And so sorry again everyone for my delay.!! I'll make sure I don't depend on mails and open this page frequently henceforth!!! :)

Akella
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