Author Topic: A small SAPS - DC Earthing/Bonding/Fault Protection  (Read 393 times)

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Offline OTG

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A small SAPS - DC Earthing/Bonding/Fault Protection
« on: March 11, 2017, 10:17:29 PM »
Hey Folks... I'm planning my next project. A small Stand Alone Power System (SAPS) for my remote Bore Pump. It will run a small 240VAC 800W submersible bore pump, and some 24VDC loads (lights, timer, maybe a DC surface pump in future, etc...).

I've drawn it up as attached (no masterpiece, but it should suffice for this discussion) and I'm happy with most things (e.g. panels, wire/breaker/fuse sizing, AC distribution panel) all spec'd out to relevant AS/NZS standards/CEC Guidelines. But there were several points where I found the standards lacking in detail &/or little help on the Interwebs.

So here goes... I'd appreciate any advice at all, especially from those with a 'downunder' perspective! If I need to clarify anything just let me know!

1. DC Combiners/Distribution - I've seen DC PV Combiners & Load Center's/Distribution Panels from Midnite Solar and Rainbow Power Company, but they tend to be pricey (Midnite) or hobbled together (RPC). I've used Auto/Marine blade fuse blocks before, but I want a more professional job this time round. Does anybody see any problems using;
 a. standard Surface Mount Enclosures (e.g. 8 or 12 pole) to hold DC MCB's/SPD's/Fuse Holders?
 b. the included Neutral Link to combine/distribute DC - loads. They copper bars seem 'chunkier' than many Auto blocks I've used. I think they're 63A 240VAC rated?
 c. the included Earth Link to ground DC SPD's?
d. a Single Phase Fork Style Insulated Busbar to string the DC + Sub Load Panel DIN mount fuse holders together? Again fairly chunky and they tend to be 63 to 80A AC 240VAC rated.
Or if you know of a better way to do this let me know!

2. DC SPD's - worth it or not? Thoughts/Suggestions/Experiences? I'm planning on Class II DIN MOV SPD's for the incoming PV, and the 24V loads. I'll probably do the 240V board too, but my focus is DC right now.

And before you ask - yes I've done the AS/NZS 1768 risk calculation and it was bugger all, but... My neighbours regulator was blow and I had a cheapie regulator on an exhaust fan pop by close lightning strikes. So thats why I'm considering this stuff.

3. Inverter Earthing - The Power Jack Inverter has the 240VAC outputs earth wired to a ring terminal thats screwed down inside the bottom half of the aluminium chassis. It also has a "CHASSIS GND" screw point on the powder-coated steel backplate that holds the DC power terminals. What should I do if anything with this GND point? For my money it wouldn't be very conductive for starters, and if I did anything I would have thought earthing the top half of the aluminium chassis to the AC earth would be more effective (I don't think the anodised? paint/coating would bond the two chassis halves very well).

4. DC Earthing/Fault Protection - I'm happy with bonding/earthing of PV array's as per AS/NZS 5033, but I found AS/NZS 4509 SAPS somewhat lacking with respect to the DC side of things in general. Basically all it has to say is this...

3.6.6 Battery overcurrent protection
The output conductors of the battery bank shall be protected against overcurrCllt, by HRC
fuses or d.c. rated circuit breakers, as follows:
(a) Where the battery bank is electrically floating (i.e. neither side of the battery is
earthed), protection shall be provided in both positive and negative battery leads.
(b) Where one side of the battery bank is earthed, protection shall be provided in the
unearthed battery lead.

I've never earthed SAPS batteries before. I'm perhaps overly cautious using HRC Fuses AND DC Breakers for the PV and Battery connections (the DC Main Distribution Panel does also serve as my DC lockout/emergency shutdown box) , and fuses breakers protect from overloads/short circuits, but what about DC earth leakages? Is there such a thing as a DC RCD/RCCB? Would a DC MEN link work? If not, would it be safer if I doubled down and fused both + & - for DC loads?

So I'm keen to hear others thoughts/experiences with regards to DC Earthing/Fault Protection. Who floats or earths their batteries, + or -, why, pros/cons either way? How do you protect your DC Loads from overloads/surges and how do you protect yourself from possible DC earth leakages... if at all... maybe you're from the "bird on a wire" camp and prefer not to give those electrons a return path, through you?  :D


Why the 101 questions I hear you asking? Well I just want to ensure the ELV part is as safe as reasonably possible. Thats all. I'd rather not see smoke coming from that end of the paddock.  ;) Plus I think discussion is generally lacking on this stuff anyways, so it'd be good to have a chin wag about it.

I look forward to hearing what y'all have to say.  :)

Offline welshman

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Re: A small SAPS - DC Earthing/Bonding/Fault Protection
« Reply #1 on: March 12, 2017, 05:17:15 AM »
do NOT bond DC NEG with AC EARTH. Especially if you're going to bond AC EARTH and AC 'NEUTRAL'. The Powejack and solar charge controllers is the only protection a battery bank needs, also the chassis earth of the PJ is irrelevant if you have the AC EARTH, earthed.

You have two choices here.

You either leave AC ' NEUTRAL' Floating or you bond AC 'NEUTRAL' and AC EARTH at the PJ mainboard.

If you leave it floating, then you can only get a shock hazard from touching BOTH AC LIVE AND 'NEUTRAL' (LIVE).

If you bond the 'NEUTRAL' to the AC EARTH then you can get a shock hazard from touching ground/anything metal EARTHED and AC LIVE.

The choice is yours... But you can only fit a working RCBO if you do bond the EARTH and 'NEUTRAL'.

It's all swings and roundabouts. Because either way has it's benefits.

However after operating a similar system for some time, i decided to bond as i have a generator that has a EARTH/NEUTRAL bond, which made the generator housing live and caused a small shock to be felt off touching it.

BTW you can protect your DC side by not having anything connected to it that causes input overloads.

Offline oztules

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Re: A small SAPS - DC Earthing/Bonding/Fault Protection
« Reply #2 on: March 12, 2017, 03:39:08 PM »
This is not what you want to hear..... but this is no way to efficiently pump water in an agricultural setting.

The are now plenty of solar bore pumps that will easily overshadow your current wish system. Only require panels, controller, and three phase BLDC pump.. preferably screw type.......

And they are now cheap on ebay, and easily run 600 cows.


There are a few on the island now..... unbelievable performance.
You need a very different reason to use battery now days that brushless units are common.
One of those units pumps into a 120psi head@ over 30lpm, and fills a man made dam.

Just in case you had missed these developments.

For earthing I think Welshman is close enough.

Flinders Island...... Australia

Offline OTG

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Re: A small SAPS - DC Earthing/Bonding/Fault Protection
« Reply #3 on: March 15, 2017, 12:32:12 AM »
Thanks for the reply fellas.

I'll leave the PJ chassis earth alone.

I shared your sentiments re DC/AC bonding welshman, but I've seen schematics like those linked below which appear to do so and I wondered if I was missing something. Can anybody explain the DC side of these...?

Can you give an example of something on the DC side that can cause input overloads? Other then PV gone-wrong I cant think of anything else... I wont be charging with a generator either.

Why bond AC Earth/Neutral at the PJ Mainboard - as opposed to using a MEN link in the AC Dist Panel?
I assume to bond at the Mainboard is simply a matter of a short length of wire with ring terminals linking the AC -/Earth screw down terminals? My intent was to use an RCBO with MEN in the AC Dist Panel.

And yes I've heard some generator horror stories... yours had -/Earth bonded but no inbuilt RCBO? Far from ideal...  :P

Finally I couldn't agree more Oz - I'd love to go with a SBP! I'm a yuge fan of DC 'appliances' - fridge/freezer, lighting, love my DC MIG Ready Wedler!, etc.

But... the bore has an existing 240v pump which is doing the job, and I've already got the batteries/panels/inverter - so it's just the BoS bits and I'm done (more affordable for me in the short term at least). Plus 240AC will probably come in handy running transfer pumps, mowing the grass around the tank/bore, and maybe pumping from a nearby dam. So I figured I'd just go with a SAPS setup.

I'm curious about the bores on Flinders now - is the water brackish or does the strata (limestone?) prevent salt water ingress?

I must admit though, my innner Bill Mollison cringed at the thought of turning ground water into surface water... it's too precious a resource round my neck of the woods!  ;D