greg_SA
Expert Member
That is true, but you then have neutral currents flowing on the earth wiring between you Eskom supply (DB) and the inverter input when in bypass mode.
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Axpert invertors
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That is true, but you then have neutral currents flowing on the earth wiring between you Eskom supply (DB) and the inverter input when in bypass mode.
I wouldn't say that is true.
https://en.wikipedia.org/wiki/Stray_voltage
Voltage (known as potential) means that there is a measurable difference in potential between two points.
As with everything in nature, these two points want to be at the same potential, so current will flow between them to bring them to the same potential
At above ±50v the potential is high enough to overcome the resistance of your body
The higher to potential, the more current can flow through your body and the more energy (joules) is transferred for a given time.
So the higher the potential, the quicker energy (jules) can flow from one point to another. The flow of that energy we measure in amps.
When it comes to shock and actual death the amount of energy (joules) is the most accurate measure of what is dangerous.
So even if you have a very high potential (volts), there will only flow as much energy as available (joules) at a given current (amps) before the two points reach the same potential.
Frequency also plays a role: 50/60Hz also happens to be the perfect frequency to cause your heart to lose its natural rhythm, therefore death by cardiac arrest is the most typical mode of death for mains electrocution. Whereas death by direct current will typically be due to joule heating (aka burning) and typically requires quite a bit more energy for you to die.
Keep in mind that if a multimeter measures a voltage between two points, there is some path that will allow these two points to reach the same potential if connected.
Last but not least, stray voltages can often be of low joule energy and therefore touching will bring them to the same potential without noticeable effect to the human because of the low amount of energy that passed through them.
Again, you are creating a dangerous circuit. Your earth wire to the inverter becomes a parallel conductor back to the Eskom transformer.
Electricity takes the path of least resistance. By paralleling you decrease resistance. Therefore, this new magical bond you've created will mean your home neutral and earth are sharing the neutral current back to the transformer. Bonded by your inverter wire.
You are connecting a wire between earth and neutral, the fact that it runs all the way to your inverter just makes it more dangerous. (and it is in contravention to SANS I should point out)
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Yes, that's the theoryMany people don't worry about E/L on this type of inverter output, because the only way you can shock yourself is if you touch both the live and neutral, and that it relatively unlikely.
If your neutral happens to touch a ground, become grounded, etc. you also get electrocuted if you touch the live wire
It is the reason that ground/earth wires were introduced in the first place. It became evident that if one of the wires of an electrical installation touches ground (as is therefore at the potential of the earth), touching the other conductor would cause electrocution.
It turns out insulation failure is common (hell when I rewired my place I found 3 places where insulation was inadequate)
Live (or live wires in 3p) and neutral is determined by which was bonded to ground after a transformer.
In 3phase if you bond any tap (to ground) other than the center tap you would have live wires with some very high potential
After passing through an isolation transformer (any transformer with galvanic isolation, eg. primary and secondary, not auto transformer).
Whichever leg (wire) you bond to ground becomes your neutral wire and is at the potential of earth at that point (which is the potential we mere humans live at)
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greg_SA
Expert Member
If your neutral happens to touch a ground, become grounded, etc. you also get electrocuted if you touch the live wire
Sure, but that still means two things have gone wrong which is unlikely... Earth-neutral short and touching live wire.
But I agree with everything you say
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Getafixx
Active Member
Sure, but that still means two things have gone wrong which is unlikely... Earth-neutral short and touching live wire.
But I agree with everything you say
My thinking is that I'd rather want the power to trip immediately when someone touches a live wire, even if the floating output means they won't get a shock. If someone has touched a wire there is a chance they may touch both live and neutral next. Which means you need E/L which means you need to bond the neutral and earth when the inverter is running. Since I don't use PV I'll probably go for the relay approach using the supply power on the coil. Seems the most sensible thing for now
Chris Hobson
Member
My thinking is that I'd rather want the power to trip immediately when someone touches a live wire, even if the floating output means they won't get a shock. If someone has touched a wire there is a chance they may touch both live and neutral next. Which means you need E/L which means you need to bond the neutral and earth when the inverter is running. Since I don't use PV I'll probably go for the relay approach using the supply power on the coil. Seems the most sensible thing for now
Why not a RCCB that detects any difference in current flow between live and the neutral. In a perfect circuit they should be the same. Then you do not have the hassles with bonded earths etc. If safety is your primary concern then a RCCB is the safest thing I can think of. The hassle is that even a brand new iron can cause a RCCB to trip. They are much more sensitive than a EL as not all faults will result in current flowing in your EL whereas any small fault in your wiring will cause a RCCB to trip.
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greg_SA
Expert Member
In general, when we are talking about E/L, we actually mean RCCB. All my "E/L" are RCCBs in my DB. For an RCCB to work, you need neutral bonded to earth at the power source, which is the idea behind adding the relay.
Getafixx
Active Member
Getafixx
Active Member
So I've just added my relay in the cavity underneath the inverter. Works like a charm. Only thing is when I switch back from battery mode the earth still floats for about 2 seconds before the inverter goes back to bypass mode. But I'm OK with that.
I have tested the whole rig on and off batteries using an ELCB/Polarity tester (Toptronic TEL1TLB). Without the bond it complained that there is no earth and then the earth leakage test fails too. Now the tester is happy that the E/L (RCCB) works correctly on and off batteries.
I have tested the whole rig on and off batteries using an ELCB/Polarity tester (Toptronic TEL1TLB). Without the bond it complained that there is no earth and then the earth leakage test fails too. Now the tester is happy that the E/L (RCCB) works correctly on and off batteries.
greg_SA
Expert Member
Awesome
Glad it works... gonna hook mine up soon. What relay did you use? Part number / link?Getafixx
Active Member
Awesome
I just walked into Yebo electronics in Bellville and ended up with this:
http://www.fort777.co.za/index.php?main_page=product_info&cPath=269_270_425_428&products_id=8653
It's a complete overkill and you have to solder your wires on the pins and add heat shrink to isolate. But works fine. It sits in the space right under my unused PV terminals. The other possible spaces are right under the fans.
greg_SA
Expert Member
Cool, thanks. That's cool - any relay will work fine.
Look on pg23 of the PDF / connecting neutral to earth when UPS/Inverter running from batteries.
Cahier Technique Schneider Electric no. 114 / p.21
http://www.google.com/url?sa=t&rct=...=YdgYnRTGYLWqywvUj25ODg&bvm=bv.99804247,d.d2s
Note that we don't use TT but TNC-S but the same idea for the relay to recreate the earthing scheme
Cahier Technique Schneider Electric no. 114 / p.21
http://www.google.com/url?sa=t&rct=...=YdgYnRTGYLWqywvUj25ODg&bvm=bv.99804247,d.d2s
Note that we don't use TT but TNC-S but the same idea for the relay to recreate the earthing scheme
Basil959
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You don't need to solder on to the base you can just buy the matching 11 pin relay base which has screw terminals.
Getafixx
Active Member
I did, but it makes the relay too large and then it doesn't fit.
As I say there are other relays better suited for the job but i could get this one off the shelf. Relay base, anyone?
greg_SA
Expert Member
One other thing that I wanted to ask... Is your inverter supplied by an E/L circuit, or directly from the "mains"?
I'm not sure what you mean by the "Eskom transformer", but I assume you refer to the Live and Neutral Mains connection to your premises and where the Earth and Neutral connections are bonded, just before the E/L unit?
OK, your explanation makes sense. What would be the best method achieving this 1) safely and 2) legally? I assume it would be the relay way. Since the SANS diagram you've posted does not include the relay and since the E/L in that diagram will in any case not trip in case of a earth fault when Eskom is down and the inverter supplies the load, will this circuit with the relay be CoC'd or will the person certifying the installation be confused all over again?
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Your earth and neutral are not bonded at the panel in SA. Not normally at least.
We use TN-C-S earthing system:
The relay is probably the simplest way and you don't need a lot of knowledge to do it correctly
You also won't void any warranties.
The SANS diagram doesn't go into detail, it is just an overall diagram.
SANS does say that your earth and neutral should be bonded at the generator output (before the change over, not after, it makes a big difference).
This would be the equivalent to that.
I can't comment on the person doing the install, each person will respond differently. If it is explained however, I think they'll agree it is safer than a floating earth
Your earth and neutral are not bonded at the panel in SA. Not normally at least.
We use TN-C-S earthing system:
OK, this is new to me. I'm not an electrician, but every (single phase) installation I've came across, had only neutral and live entering the premises and the neutral and earth going to the main db was bonded inside the outdoor "meter box". That is where the earth was also connected to the earth rod.
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That is a TN-C-S install, sorry I thought you meant the panel in the house. That is a no-no.