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Single Phase installation from a 3 Phase Board


JohnMarc
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I have a 3 Phase board in my house (I only installed the 3 Phase to supply our 3 Ph instant hot water systems) and would like to know is there any reason why I could not dedicate one of the Phases to my 5Kva invertor and batteries. (won't need the hot water system during shutdowns as I have installed a system that automatically kicks it over to gas via solenoid valves)

I have managed to have all of the critical supply wired to the one phase and would like to have some thoughts as to wether I am missing something here that does not allow me to do this. I have things like laundry and workshop etc on the other two phases so reasonably well balanced on Escom supply but when it switches off I want to operate just a single phase ..... Am I missing something that does not allow me to do this?

Appreciate your thoughts

Stay safe and healthy 

 

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I can’t see a problem with feeding only one  of the three phases from a inverter, I have done similar before with a single phase generator feeding a three phase installation during load shedding. We had a manual change over switch, and switched off all  non-essential circuit breakers before powering the critical loads. I actually looped all three phases to have generator power but just manually switched off all three phase breakers and non-critical loads.

The only problem is if you have three phase loads with three phase motors, those need to be switched off or wired so that they can’t be started during inverter power. 

You would need a either a manual or auto change-over switch and can’t be any grid tie system. But I would say is doable. 🙂

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Hi

I agree with Gerrie. You would need to have a manual change-over switch and switch off all non essential circuits to keep the power under 5Kva.

You could also split up you essential circuit further and put the critical items on that circuit  under 5Kva which can run off the inverter continually  and switch over automatically.

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7 hours ago, JohnMarc said:

Am I missing something that does not allow me to do this?

You can do that without any problem. 

1 hour ago, Gerrie said:

You would need a either a manual or auto change-over switch and can’t be any grid tie system.

 

12 minutes ago, Peter Topp said:

You would need to have a manual change-over switch and switch off all non essential circuits to keep the power under 5Kva

Non of this is needed. If your critical loads are under 5kva and on one phase, you can install a Hybrid inverter on that phase without any other components.  

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1 hour ago, Jaco de Jongh said:

Non of this is needed. If your critical loads are under 5kva and on one phase, you can install a Hybrid inverter on that phase without any other components.  

If a change-over switch is installed it should be quite easy to get a COC as it is than a off-grid system. But I don’t know if it would be as easy to get a COC for a grid tied system without a change-over switch.

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Thanks for the responses, really appreciate this discussion.

Somehow to me (in my admittedly absolute ignorance) the installation of a change-over switch sounds like an an unnecessary overkill considering the switchover capabilities of the Invertors.  I am worried about adding potential redundancies into a system unless entirely necessary/required. I want to mitigate risks in the system as I will be away for extended periods (ha ha especially after this "house arrest" we are all having to endure, the bush is calling me). I will check if required for COC requirements.  

A further question if I may....... DC fuse as well as DC breaker ??? Do I need both, does the fuse add a layer of protection (or visa versa) that the breaker doesn't? Once again just want to be risk averse and do not want to add "unnecessary" potential failure points unless absolutely required.

Stay safe and healthy

 

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16 hours ago, Jaco de Jongh said:

Not sure I understand Peter, the previous advice to the OP was to install a manual changeover, and I can not see why its needed?

It's for a three phase install where you want all three phases to be backed up (partially). Then you have a changeover switch that throws all three phases on parallel (changing it into a single-phase install), and then all three phase loads have to be disconnected as well (especially if they have a neutral connection, in which case they can be damaged).

If all your essential loads are on one phase (typical on a farm, for example, where the main house would be on one phase only), then simply install the inverter on that phase. That precisely what happens in the city anyway. The supply in the street is 3-phase. You're connected to one of them, and you install your inverter to that one phase 🙂

 

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47 minutes ago, plonkster said:

It's for a three phase install where you want all three phases to be backed up (partially). Then you have a changeover switch that throws all three phases on parallel (changing it into a single-phase install), and then all three phase loads have to be disconnected as well (especially if they have a neutral connection, in which case they can be damaged).

If all your essential loads are on one phase (typical on a farm, for example, where the main house would be on one phase only), then simply install the inverter on that phase. That precisely what happens in the city anyway. The supply in the street is 3-phase. You're connected to one of them, and you install your inverter to that one phase 🙂

 

I am actually liking the idea of the changeover switch that turns the three phase into a single phase...... I had no idea this was possible, please elaborate.....

If I can find a way to automatically switch off the feed to the three phase geysers everything else in fact is on single phase. So a far simpler solution is possibly the 3 Phase changeover switch scenario assuming I am able to switch the instant hot water systems off. Am i right I would simply do that with a three phase contactor ..... but then what would happen when the power comes back on....is there a delay in the switchover process back to three phase. Hell I hope I have made sense here......please forgive my ignorance ..... 

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2 minutes ago, JohnMarc said:

Hell I hope I have made sense here......please forgive my ignorance ..... 

Haha don't worry man!

Is this geyser fed direct from your DB? In that case it shouldn't be that difficult to wire and automatically switch it.

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1 hour ago, plonkster said:

It's for a three phase install where you want all three phases to be backed up (partially). Then you have a changeover switch that throws all three phases on parallel (changing it into a single-phase install), and then all three phase loads have to be disconnected as well (especially if they have a neutral connection, in which case they can be damaged).

That, I am aware of, but its unnecessarily complicating the setup. The OP needs to feed one phase with a 5kv unit after connecting all his essentials to that phase. Why install a changeover and then go switch off all three phase appliances and non essential loads. 

The answer to his original question:

On 2020/04/18 at 2:06 PM, JohnMarc said:

I have managed to have all of the critical supply wired to the one phase and would like to have some thoughts as to wether I am missing something here that does not allow me to do this. I have things like laundry and workshop etc on the other two phases so reasonably well balanced on Escom supply but when it switches off I want to operate just a single phase ..... Am I missing something that does not allow me to do this?

No nothing is stopping you from doing it this way, and it would be the easiest route by far.......... 

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2 hours ago, Jaco de Jongh said:

No nothing is stopping you from doing it this way, and it would be the easiest route by far.......... 

I missed the part where he said the relevant loads are already on the same phase. My bad 🙂

3 hours ago, JohnMarc said:

I am actually liking the idea of the changeover switch that turns the three phase into a single phase...... I had no idea this was possible, please elaborate.....

OK, it is important that you understand this. In a three phase system you have loads that are themselves three-phase, and then you also have single-phase loads.

A single-phase load will have two wires (and earth of course) connected to neutral and one of the phases. Two wire colours generally brown and blue.

A three phase load will have three (or four) wires. It will be wired to all three phases. Three colours, red, yellow and blue. And in the case where the load also has a neutral connection, there will be a black wire for that.

Now... if you have no three phase loads, that is if all your loads are single phase loads between one of the lines and neutral, then you can use a switch to change your install into a large single-phase supply. You essentially disconnect L1, L2 and L3 from the upstream grid connection, and tie them together into a large L1+Neutral.

Now when you do this, all your single phase loads will work (as long as you keep it below 5kVa), but three-phase loads will not work. What is more, the three-wire 3-phase loads suffer no ill effects at all (all three wires are at the same potential), but the 4-wire appliances may suffer ill effects (because they see 230VAC between phase and neutral but without the required phase shift).

To dumb it down, a real 3-phase connection has 230VAC between each phase and neutralm and 400V between the phases themselves. A "fake" 3-phase connection that can run single-phase appliances have 230VAC between phase and neutral but 0V between the phases.

Now to add a bit more interesting info: In North America it is common to have split-phase supplies. You have L1, L2 and neutral. L1 to neutral measures 120V, L2 to neutral measures 120V and L1 to L2 measures 240V. The phases are 180° apart. Your big appliances (water heater, drier, etc) are 240V appliances. All the smaller stuff are 120V (for historical reasons). The houses are wired such that about half the plugs are on one phase and the other half on the other phase. Now many houses will use a single-phase backup generator/inverter to power the whole house by simply bridging L1 and L2 together (and of course disconnecting from the grid), but the moment you do this, the 240V appliances top working, because now they have 0V across them. In N-America this is fine because mose 240V appliances don't connect the neutral, they don't depend on having two actual 120V phases. They are in fact 240VAC single phase appliances 🙂

 

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1 hour ago, plonkster said:
1 hour ago, plonkster said:

I missed the part where he said the relevant loads are already on the same phase. My bad 🙂

OK, it is important that you understand this. In a three phase system you have loads that are themselves three-phase, and then you also have single-phase loads.

A single-phase load will have two wires (and earth of course) connected to neutral and one of the phases. Two wire colours generally brown and blue.

A three phase load will have three (or four) wires. It will be wired to all three phases. Three colours, red, yellow and blue. And in the case where the load also has a neutral connection, there will be a black wire for that.

Okay so I am thinking the best way would be to give a clearer picture of what I am actually dealing with..... I am really hoping the attached picture is self explanatory ......

Slide2.thumb.jpeg.f097f73d14dac2050c37e249b98aa620.jpeg

 

 

 

DB.jpg

Edited by JohnMarc
wrong picture
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The photo with descriptions looks good, normally I would fit a change over before the main switch but the position you have indicated will be perfect due to the 3ph water heaters that need “not” be on inverter. This way you would “not” need to switch off water heaters when you switch over to inverter power. 

That blue phase jumper would need to move like you indicated. It would also be necessary to split the neutral (for inverter loads away from Eskom neutral) Get a four pole change over switch.

 You can leave water heater neutrals on Eskom neutral bar as is. That way the heaters can still work on Eskom supply while your inverter loads are off-grid with their inverter neutral.

To wire all three phases to inverter simply  fit 2 x bridge pieces on change over switch at inverter inlet-side from L1 to L2 to L3 and inverter live just on any of those three points. Hope it makes sense 🙂

 

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2 hours ago, plonkster said:

Now to add a bit more interesting info: In North America it is common to have split-phase supplies. You have L1, L2 and neutral. L1 to neutral measures 120V, L2 to neutral measures 120V and L1 to L2 measures 240V.

I know I’m off topic but this reminds me almost how hospital socket outlets in ICU’s and operating theaters work, they use a isolating transformers. It has a L1 and L2 and centre tapping. L1 to L2 measure 230V but L1 or L2 to centre tap measure 115V.

The socket outlets are wired live-pin to L1 and neutral-pin to L2,  If you measure L1 or L2 to earth pin in measure 115V but between live and neutral pin it measure 230V. The nice thing about this system is you can touch L1 or L2 and earth and not get shocked because it is a isolated supply. Just don't touch L1 and L2 as that will bite😄

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Thanks so much Gerrie, really appreciate your concise response. Can you possibly elaborate on the neutral  ..... there is no neutral feed to the water heaters...certainly not that I am aware of.... only three core cable connecting to heaters..... but a good point you raised, would the neutral from changeover switch connect to all three neutral bars, effectively bridging them, would this not have an impact on the earth leakages on the individual phases under normal operation? 

Edited by JohnMarc
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4 hours ago, JohnMarc said:

Does anyone out there have a wiring diagram showing the operation of a changeover switch from 3 phase to single phase, battling to find one on the net. Well battling to find one that makes sense to me.......

May I ask what you would hope to achieve by doing this. You want to install a 5kva unit. The geyser alone will be 9kw at best. but I think its a three phase geyser, so it wont even work in any case. 

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26 minutes ago, JohnMarc said:

would the neutral from changeover switch connect to all three neutral bars, effectively bridging them, would this not have an impact on the earth leakages on the individual phases under normal operation? 

The neutral from changeover switch still go to earth leakages (as is) bottom of earth leakage go to neutral bars (also as is) you don’t have to change anything there just basically the changeover switch will feed either eskom neutral or inverter neutral.

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17 minutes ago, Jaco de Jongh said:

May I ask what you would hope to achieve by doing this. You want to install a 5kva unit. The geyser alone will be 9kw at best. but I think its a three phase geyser, so it wont even work in any case. 

Hi Jaco, the geysers will not be on inverter they will be on Eskom only,  but all the other single phase loads will be able to work from inverter as long as JohnMarc keep loads under 5KVA. 

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29 minutes ago, Jaco de Jongh said:

May I ask what you would hope to achieve by doing this. You want to install a 5kva unit. The geyser alone will be 9kw at best. but I think its a three phase geyser, so it wont even work in any case. 

Hi Jaco if you go back a few posts of mine I said I did not want to run the 3 Ph hot water systems during a shutdown, as I mentioned they automatically switch over to gas with the use of solenoid valves, normally open but in a shutdown they redirect the water through the gas geysers automatically activating them. I don't have a particularly high usage house etc. so 5 KVA should be ample. I would like to avoid reinventing the wheel if possible and effectively switch from # Phase to a single phase supply. 

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39 minutes ago, JohnMarc said:

there is no neutral feed to the water heaters..

If the water heaters have no neutral then you do not need to worry about that, but you still need to split the Eskom and inverter neutral as they must be isolated from one another ( the changeover actually do that). If loads are on inverter the inverter neutral must be used or if loads are on Eskom than Eskom neutral must be used.

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17 minutes ago, Gerrie said:

The neutral from changeover switch still go to earth leakages (as is) bottom of earth leakage go to neutral bars (also as is) you don’t have to change anything there just basically the changeover switch will feed either eskom neutral or inverter neutral.

Aah yes got it thanks a lot

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