DC disconnect switch - on positive or negative

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Re: DC disconnect switch - on positive or negative

Postby davidg » Thu May 01, 2014 8:07 pm

dmatter261 wrote: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?

Yes, that's the normal of doing a battery bank, eg floating above ground on both the positive and negative.

dmatter261 wrote:Even if I wanted to avoid a bonded earth system at all, I still have to right?

No..............It is typical to see a not bonded earth system and it is typical to switch and isolate both positve and negative of batteries.

dmatter261 wrote:Do Australian standards allow for non-bonded earth systems despite having an inverter with a transformer

Yes, they most certainly do.
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Re: DC disconnect switch - on positive or negative

Postby dmatter261 » Fri May 02, 2014 3:53 am

Warpspeed wrote: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.


Do I understand correctly that if I wish to work on my battery I open my battery double pole isolator even though my battery negative is grounded ? Or should I use a single pole one only , just on the positive ?
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Re: DC disconnect switch - on positive or negative

Postby Warpspeed » Fri May 02, 2014 7:18 am

dmatter261 wrote:
Warpspeed wrote: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.


Do I understand correctly that if I wish to work on my battery I open my battery double pole isolator even though my battery negative is grounded ? Or should I use a single pole one only , just on the positive ?


It depends on the battery voltage.
If the battery voltage is very high, say over 300 volts dc, then I certainly would not want to be topping up cells, taking hydrometer readings, or checking and cleaning the bolted battery connections at the +300 volt end of the string, with the other end solidly grounded.
Such high voltage battery systems do exist.
There is a real beauty installed at the Sydney Opera House.
It provides 240 volts dc for the incandescent house lights through a custom diode voltage dropper regulator which it was my job to design.

At the time, it was the largest battery installation in the Southern Hemisphere, and probably still is.
If all three incoming phases fail, the house lights immediately switch over to direct dc battery power.
Its a very big and crude dc UPS for the emergency lighting.

Cannot remember exactly, but the discharge requirement was somethging like 800-1,000 amps for two hours minimum.
Our company (Saft Nife) supplied the flooded Nicad batteries, although I was not personally involved with that side of it.
My part involved the two 100 amp battery chargers, and the rather special voltage regulator for the incandescent lamp load, and the controls and alarms for the whole system.

If its a low voltage system you can do it pretty much any way you want, without much risk.
But at higher voltages, electrical safety becomes increasingly important.
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Re: DC disconnect switch - on positive or negative

Postby dmatter261 » Tue May 06, 2014 7:26 pm

Ok now its a little clear to me.
For higher voltages its always better to disconnect the positive and negative irrespective of the negative being grounded for safety reasons. It doesn't matter much for lower voltages. I'm assuming the same applies to solar arrays as well.
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Re: DC disconnect switch - on positive or negative

Postby davidg » Tue May 06, 2014 9:33 pm

dmatter261 wrote:For higher voltages its always better to disconnect the positive and negative irrespective of the negative being grounded for safety reasons.

Actually I would always regardless, I have worked on battery banks that floated over 100V above ground when connected and active. Double isolation even for just 48V, 72V or 96VDC is a way better option. ;)
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Re: DC disconnect switch - on positive or negative

Postby Warpspeed » Wed May 07, 2014 8:04 am

Grounding one side of the battery is always good practice in a working system.
For maintenance, electrical safety comes first, and complete isolation of whatever you are working on from everything else is always a prudent step.
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Re: DC disconnect switch - on positive or negative

Postby dmatter261 » Fri May 09, 2014 7:05 pm

Thanks everyone for the replies !
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Re: DC disconnect switch - on positive or negative

Postby davidg » Fri May 09, 2014 7:23 pm

Warpspeed wrote:Grounding one side of the battery is always good practice in a working system.

Not always, for instance if a system is hit by lightning you can kiss the batteries goodbye if they are grounded on one side (Bonded Earth), will cook/blow them up them very effectively, floating they will have a good chance of surviving, just one example. I'm making the assumption that a proper ground protection system for the framing system is in place for the array.
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