'Offgrid' Hybrid Inverter Ground and AC breaker question

[ddd]

Hi


I have a hybrid inverter that accepts AC input (from mains), and DC inputs (from solar panels and batteries). This is to be installed in an outbuilding as an ‘offgrid’ system (except for the AC input, which obviously is connected to the grid).


The inverter’s manual states that it needs to be connected to a permanent grounded wiring system; and that it needs to have a 32A AC breaker.


So first question: is the outbuilding’s own AC breaker (installed by a pro electrician, although not sure of the Amp rating) good for the purpose? I would be connecting the inverter’s AC input to a wall socket. As it is, I could switch off the outbuilding electrics from its own AC breaker, or from the mains board in the house. Do I need a third AC breaker between the wall socket and the inverter?


Regarding earthing the inverter, I asked the online shop about the possibility of a ground loop if I installed a separated ground point for the inverter (loop between the inverter’s ground and the main house ground) . They suggested that I run a cable to the same earth point as the main house. The problem is, this is a huge pain for all sorts of reasons, so I’d like to check if there is any other possibility. Can I just not use the ground provided by the AC input? Any other options? How safe is to have a ground loop if a lighting hits etc with all the gear in between the two ground points?


System:

• 4 x 275W solar panels
• Iconica Hybrid 3000W 24V pure sine wave inverter
• 6 x 100Ah AGM deep cycle Photonic Universe leisure batteries

Many thanks!

dd

 

[ericmark]

There are many earthing systems but it can be split to TN and TT for most.

TN means supplied by electric supplier, TT means an earth rod.

Distance is important you don't want to be able to touch two diffrent earthing systems at same time.

 

[flameport]

an ‘offgrid’ system (except for the AC input,

Offgrid means no connection to any AC grid power.
if it is connected, then it's not an offgrid system.

I would be connecting the inverter’s AC input to a wall socket.

No.
Inverters connected to AC must be permanently wired with appropriate isolation and protection devices. Not via any plug and socket arrangement.

Regarding earthing the inverter,

The options are:
a - use the same earthing system as for the rest of the electrical installation including the house
b - use it's own entirely separate earthing system
c - switching between a and b depending on whether it's connected to AC power or not.

 

[ddd]

There are many earthing systems but it can be split to TN and TT for most.

TN means supplied by electric supplier, TT means an earth rod.

Distance is important you don't want to be able to touch two diffrent earthing systems at same time.

The Instructions don't specify which type. But if I install a second rod connected to the Inverter, then a ground loop will be formed when I connect the Inverter's AC Input, like this guy explains on this video.

 

[ddd]

Offgrid means no connection to any AC grid power.
if it is connected, then it's not an offgrid system.


No.
Inverters connected to AC must be permanently wired with appropriate isolation and protection devices. Not via any plug and socket arrangement.


The options are:
a - use the same earthing system as for the rest of the electrical installation including the house
b - use it's own entirely separate earthing system
c - switching between a and b depending on whether it's connected to AC power or not.

All the examples of this Inverter that I can find online are connected to AC via a wall socket. See this

or this

Option A is a pain, as the house rod is far from the outbuilding and there is lawn and concrete to dig a cable through etc.
Option B equals ground loop.
Option C. That's a good possibility, if I there is an easy way of disconnecting from the earth rod at the Inverter's end. Can a simple switch do that?

 

[ericmark]

The Instructions don't specify which type. But if I install a second rod connected to the Inverter, then a ground loop will be formed when I connect the Inverter's AC Input, like this guy explains on this video.

The first thing one notices is he talks about ground not earth, remember some areas of USA have a really high amount of electric storms.

I was employed for around 4 months doing very little else than installing earth rods, each had to be 8 Ω and I would need between 2 and 8 rods, it was a large site, and where the site connected to national grid there was a resistor around the size of a car so current between to two earth systems was limited current wise, this is a problem when using earth rods and the DNO earth, if your earth is too good, under fault conditions a massive current can flow, but in general a single earth rod is only around 50 Ω which is good enough to ensure an RCD will trip but not good enough to allow a massive current to flow.

The only way with domestic that a earth can allow enough current to flow to cause damage is when some one like a radio ham has made a earth mat for his aerial system which is connected to the DNO earth, the earthing arrangement shown for a cattle shed could also cause a problem, but that is hardly domestic, we no longer heat our homes by having cattle in the lower floor.

We do look as a voltage gradient however, what we don't want is for an animal (this includes human) to get any significant current flow between their limbs. 25 volt it seems is enough to kill a cow, so looking at around 25 volt per meter limit, unless fenced off, most likely animal to have problems is a dog, as even where I live we don't tend to have cows wandering our streets. If you live in Colwyn Bay you may get some goats in your garden.

This has been a problem with EV charging points, although caravans and boats are not permitted a TN-C-S supply, in the main fire regulations mean the caravan is at least 2 meters from a building, and it is unusual for a building to have earthed parts outside, I do question the idea of charging a car near to domestic accommodation due to fire risk, but as it stands one can even park the car in an integral garage, which seems foolhardy, even for petrol or diesel cars.

The idea is an electrical installation is designed, not simply this is the way I have always done it, so I will continue doing the same even if requirements have changed. The idea is we use our brain and knowledge and assess the risks, 230 volt is potentially dangerous, but we can minimise the risks.

There are some good guides like BS 7671, but it is not law, even if it can be used in a court of law. There is even a guide to the guide. But solar panels and EV charging points are some thing relatively new, yes we had fork lifts and milk floats, but the charger was static, so only up to 70 volt DC was supplied to the vehicle and this went through an isolation transformer. Having 600 volt batteries is new. As is having the charger built into the vehicle.

The problems with DC freezing RCD's is also relatively new, my only RCD's have no icon or any other marking to say type AC, A, F, B, or S although S was around when they were made. The government has been pushing LED lights, but has not pushed the warning that using them may cause a RCD not to work. Well lets face it, the government have also been pushing the take up of smart meters showing a guy clapping his hands switching the lights on/off, that is clearly not going to happen, they seem to have lost the plot!

You may be able to have a on/off grid system, I would look at caravans and narrow boats, both are some times plugged into a electric hook-up, but also can run off grid. There are special inverters to supplement shore supplies, which do grid tie, so a 4 amp supply can run items needing 13 amp. Both caravan and boats tend to be made of metal, so there is a real danger when boarding or alighting that our legs can form a connection to true earth and supply earth, I noted mobile homes have also been considered the same with amendment 2 and TN-C-S supply it not allowed. Interesting as to if a mobile home is covered by building regulations, I suspect not, as to landlord laws on inspection and testing also not sure.

But you are doing some thing specialised and so you really need a specialist to advise on site.

 

[bernardgreen]

The only way with domestic that a earth can allow enough current to flow to cause damage is when some one like a radio ham has made a earth mat for his aerial system which is connected to the DNO earth

and when the local water supply is metallic and the incoming metallic water pipe is bonded ( for safety ) to the DNO "Earth"

 

[ericmark]

and when the local water supply is metallic and the incoming metallic water pipe is bonded ( for safety ) to the DNO "Earth"

This is true if only one property is bonded to gas or water the services may be unable to take the current from other properties from the same transformer, but if all properties are the same they will share the load.

I would agree if we have PME the M stands for multiple, and if there are enough earth rods fitted by the DNO for example one every three properties, then even if the PEN is lost, the current through any earth electrode should be low enough not to cause damage.

The problem arises when there are not enough earth electrodes to share the load. And all the power goes through a gas pipe, melts it and sets the gas alight. There should be insulating sections to stop gas pipes acting as an earth electrode, but instead of blaming lack of isolator, earth wire wrong side of isolator, no multiple earthing etc, the fire is often blamed on copper theft.

 

[SUNRAY]

and when the local water supply is metallic and the incoming metallic water pipe is bonded ( for safety ) to the DNO "Earth"

This is true if only one property is bonded to gas or water the services may be unable to take the current from other properties from the same transformer, but if all properties are the same they will share the load.

I would agree if we have PME the M stands for multiple, and if there are enough earth rods fitted by the DNO for example one every three properties, then even if the PEN is lost, the current through any earth electrode should be low enough not to cause damage.

The problem arises when there are not enough earth electrodes to share the load. And all the power goes through a gas pipe, melts it and sets the gas alight. There should be insulating sections to stop gas pipes acting as an earth electrode, but instead of blaming lack of isolator, earth wire wrong side of isolator, no multiple earthing etc, the fire is often blamed on copper theft.

Here's one I found earlier:
https://www.diynot.com/diy/threads/a-real-moan-tonight.552183/
Ex telephone exchange for excellent local earth arrangement, bonded to metal water supply, metal theft and a stupid metal box added to make it really dangerous for me.

 

[flameport]

All the examples of this Inverter that I can find online are connected to AC via a wall socket.

Those examples are not in the UK, and are dangerous lashups.

An inverter cannot be connected via a plug and socket, as if unplugged the pins of the plug will remain live until the inverter detects the loss of AC and shuts down.
If a fault occurs on the AC circuit it's connected to, the protective device will disconnect the AC within the required time, but that circuit will remain energised until the inverter detects the AC loss, which will result in the required disconnection times not being met.
Hard wired into it's own dedicated circuit is the only option.

, like this guy explains on this video

Everything in that video should be ignored.

Before any decision can be made - what type of earthing does the property have now?

 

[plugwash]

I don't think the product the OP is talking about is a grid tie inverter. It seems to be a combination inverter/charger controller of the type that may be used on say a narrowboat.

 

[ddd]

Those examples are not in the UK, and are dangerous lashups.

An inverter cannot be connected via a plug and socket, as if unplugged the pins of the plug will remain live until the inverter detects the loss of AC and shuts down.
If a fault occurs on the AC circuit it's connected to, the protective device will disconnect the AC within the required time, but that circuit will remain energised until the inverter detects the AC loss, which will result in the required disconnection times not being met.
Hard wired into it's own dedicated circuit is the only option.


Everything in that video should be ignored.

Before any decision can be made - what type of earthing does the property have now?

Ok, understood. looks like the AC input is out of the equation then.
Main house has a wire going straight into the outside concrete.

 

[ddd]

I don't think the product the OP is talking about is a grid tie inverter. It seems to be a combination inverter/charger controller of the type that may be used on say a narrowboat.

Indeed, it's an 'all in one' inverter charger, with the added possibility of an AC input in case batteries or PV run out.