DC disconnect switch - on positive or negative

New to renewable energy? Have questions? Here's a great place to ask them and view information about wind and solar power basics.

Re: DC disconnect switch - on positive or negative

Postby Warpspeed » Wed Apr 30, 2014 7:11 am

That all sounds quite logical.
I still prefer isolating both sides of a relatively high voltage system, whatever else is connected to it for electrical safety reasons alone, particularly if its mounted above a metal roof.
Warpspeed
Solar Crusader
Solar Crusader
 
Posts: 435
Joined: Fri Jan 03, 2014 3:18 pm

Re: DC disconnect switch - on positive or negative

Postby Gordon-Loomberah » Wed Apr 30, 2014 7:26 am

Make sure that the Notify me when a reply is posted box is checked below your reply, which it is by default, and emails will be sent to your registered email address each time a new post its made.
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5763
Joined: Thu Sep 24, 2009 9:41 pm
Location: Loomberah NSW Australia

Re: DC disconnect switch - on positive or negative

Postby SolarPhil » Wed Apr 30, 2014 9:25 am

Hi

If the open circuit DC array voltage if over 120v DC, then the array needs to be wired in the same fashion as a grid feed system at LV and should be installed by an electrician.

Because the solar regulator is effectively wired to the battery and if there is an upstream fault in the array wiring and a fault occurs in the solar regulator, you could get a battery driven short which needs to be diconnected by an MCB. If a DC switch is used, you may not get sufficient protection. I always use double pole DC rated MCB's for solar array wiring in DC coupled circuits rather than DC switches.

As for earthing the battery negative, most off grid installers in Victoria don't earth the battery negative so a double pole switch is used to ensure that the battery is completely disconnected from the system when the switch is activated.

Phil
SolarPhil
Solar Supporter
Solar Supporter
 
Posts: 2
Joined: Tue Apr 08, 2014 7:33 pm

Re: DC disconnect switch - on positive or negative

Postby dmatter261 » Wed Apr 30, 2014 5:23 pm

Hi,

Double pole MCB would mean even the grounded conductor , in this case - ve of my solar array gets disconnected when the MCB is open. Would this create any problems wrt safety as the grounded conductor is being opened ?

Its interesting to hear that battery negative isn't grounded.Don't the battery installation manuals make it mandatory?

N btw, my notify me option is ticked every time I post a reply however I'm still not receiving mails. Surprising :?:
dmatter261
Solar Supporter
Solar Supporter
 
Posts: 9
Joined: Mon Mar 24, 2014 4:52 pm

Re: DC disconnect switch - on positive or negative

Postby Warpspeed » Wed Apr 30, 2014 5:45 pm

dmatter261 wrote:Hi,

Double pole MCB would mean even the grounded conductor , in this case - ve of my solar array gets disconnected when the MCB is open. Would this create any problems wrt safety as the grounded conductor is being opened ?

Exactly the opposit in fact.
It is current flow that electrocutes you, and for current to flow you need a complete circuit.

If you totally isolate a power source, accidentally touching either terminal individually will not complete a circuit through which current can flow.
But if you solidly ground one side, touching the other side will complete the circuit......
Warpspeed
Solar Crusader
Solar Crusader
 
Posts: 435
Joined: Fri Jan 03, 2014 3:18 pm

Re: DC disconnect switch - on positive or negative

Postby dmatter261 » Wed Apr 30, 2014 7:45 pm

Yes,you are right in case of ungrounded systems where neither the negative or positive array terminals are grounded. But this depends on the type of inverter chosen because ungrounded systems use transformerless ones.

Since my inverter has a transformer and the manufacturer even recommends grounding I am forced to ground the negative of the solar array. In such a case I was wondering whether I could do with a single pole MCB, only on positive.

However, as you said, since the risk of electrocution is more in case of grounded systems, its a safer to go for an ungrounded one. The next time around I should probably select such an inverter.
I should mention here that I live in India and the issue I faced with a transformerless inverter suppliers is that the features didn't include an in built charge controller and MPPT. For me I felt an all in one device is so much easier to handle and install instead of separately integrating them.
Looks like its always win some and lose some.
dmatter261
Solar Supporter
Solar Supporter
 
Posts: 9
Joined: Mon Mar 24, 2014 4:52 pm

Re: DC disconnect switch - on positive or negative

Postby davidg » Wed Apr 30, 2014 8:26 pm

dmatter261 wrote:Yes,you are right in case of ungrounded systems where neither the negative or positive array terminals are grounded.

The Australian Current regs require the PV modules and array frames/railing be grounded not the Neg or Pos of the array, regardless of whether it is transformer based or not. (Although for off-grid ELV based array system this is not really all that important) I would suggest it is still done this anyway though.

dmatter261 wrote:But this depends on the type of inverter chosen because ungrounded systems use transformerless ones.

See above.

dmatter261 wrote:Since my inverter has a transformer and the manufacturer even recommends grounding I am forced to ground the negative of the solar array.

See above, what you are talking about is a bonded earth system and that is completely style of earthing system, it opens a different can of worms.

dmatter261 wrote:In such a case I was wondering whether I could do with a single pole MCB, only on positive.


Ideally AVOID a bonded earth system, unless you really understand the consequences of doing a system that way, in which case it would typically be the NEG that is bonded and the CB / MCB would only be in the POS of the battery system.
SPS Standalone Power System/Hybrid specialist - Store the Sun!
SELECTRONIC SPMC482-AU, 8.2kW's of Arrays
Selectronic Certified AC-Coupled Fronius 8.2kW Inverter
1900Ah 48V Bank

An OTT Genset for a house.
PVOutput Stats

Sparkys light up your life :)
User avatar
davidg
Solar Crusader
Solar Crusader
 
Posts: 2102
Joined: Sat May 05, 2012 7:38 pm
Location: Dandenong VIC- *Really don't like Power Retailers at all.

Re: DC disconnect switch - on positive or negative

Postby Keith B » Wed Apr 30, 2014 10:19 pm

Response time for DC breakers on DC would be much the same as for AC breakers on AC given similar ratings and curves but an AC breaker on DC will respond quite differently. After the initial trip point has been reached, which depends on current and time, the current will continue to flow across the arc until it has been fully extinguished. An AC breaker will not extinguish a DC arc until there is sufficient gap between the contacts. This could be greater than the physical fully open distance between the contacts once the surrounding air becomes ionised by the arc. A bit like a DC welder which will maintain the welding arc at very low current much easier than an AC welder.
Keith B
Solar Supporter
Solar Supporter
 
Posts: 11
Joined: Wed Jan 08, 2014 7:42 pm

Re: DC disconnect switch - on positive or negative

Postby dmatter261 » Thu May 01, 2014 2:30 am

I probably should have used the word bonding/functional grounding.
Earthing of the metallic frames/equipment I would certainly be doing irrespective of neg and pos bonding.

Anyway thanks a lot for the responses. I'm clear about the DC disconnector switch now.

Sorry if I am deviating slightly from the topic of original post but I have more questions.I'd post the same in another thread if its not ok, but here goes.

If my inverter has galvanic isolation provided through a transformer, is there any way I can still go ahead and not bond the negative of the solar array? Even if I wanted to avoid a bonded earth system at all, I still have to right?
Do Australian standards allow for non-bonded earth systems despite having an inverter with a transformer?

Akella
dmatter261
Solar Supporter
Solar Supporter
 
Posts: 9
Joined: Mon Mar 24, 2014 4:52 pm

Re: DC disconnect switch - on positive or negative

Postby Warpspeed » Thu May 01, 2014 8:07 am

Basically you bond one side of the system (usually negative) to ground to prevent the whole system from floating up to potentially high voltages that could break down insulation.
The most logical place to do this is at the dc input to the inverter.

The system only needs to be grounded at one point, and in fact MUST only be grounded at one point to prevent stray currents from circulating through the ground system.

To the inverter we connect two power sources, the battery, and the solar panel array.
And we fit a suitably rated double pole isolating switch to each.
If both isolating switches are open, there will be zero power to the inverter, and even though the inverter is still grounded, it will be completely powered down and electrically safe to work on.

A double pole isolator at the battery will then completely isolate the battery from both the load and the ground connection.
It is then "slightly" safer to start poking around the battery, especially a high voltage battery if it is well enough insulated from ground.
If you want to be really sure, connect a really big grounding clip directly onto the particular battery or cell you are working on.
The end cells could then rise to dangerous voltages, but the cell you are working on will be solidly at ground potential.

Same with the solar collectors.
Fit a double pole isolator to completely disconnect the solar panels from both ground and the possibility of any reverse battery feeding.
If its a high voltage system, use a big earth clip between the metal roof you are standing on, and one side of the particular solar panel you plan to work on.
A further refinement would be to fit multiple isolators to a very high voltage system, at various points in the series connected chain of panels to break it up into low voltage sections.

Electrical safety should obviously be designed into the system, but a big part of it is also understanding how the system works, and what can happen if you come into direct electrical contact with any particular part of it.
Warpspeed
Solar Crusader
Solar Crusader
 
Posts: 435
Joined: Fri Jan 03, 2014 3:18 pm

PreviousNext

Return to Q&A - wind and solar power basics

Who is online

Users browsing this forum: Majestic-12 [Bot] and 5 guests

new solar power specials
cron