Grounding system for standalone off-grid electricity

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dyllos

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#315551 24-Jul-2024 22:06

Hi all

Am preparing to get our off grid system installed and was wondering if it needs to be using the MEN system (ie neutral and earth connected in the switchboard) even if it will never be connected to the grid? I ask because we have a relatively new tiny house that we bought and I noticed that the neutral and earth are not connected in its switchboard. Is this because it is not necessarily grounded the same way a house is?

pih

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#3264151 24-Jul-2024 22:11

Sounds like you need the services of a qualified electrician...

dyllos

20 posts

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#3264155 24-Jul-2024 22:25

is there really something wrong with being prepared before getting someone in??

SomeoneSomewhere

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#3264159 24-Jul-2024 22:39

Yeah, get a qualified electrician who's experienced and has carefully read the standards and regulations, because they're a conflicting mess. Can't claim to have done so recently.

Broadly speaking, you have two options:

IT (AKA floating; stands for insulated-terra).
- This is what a portable generator, inverter, or isolating transformer delivers unless specifically earthed.
- You can touch any wire in any one place, or suffer any one fault without either damage or harm occurring, and everything continues to work.
- If you have two faults, all bets are off and an RCD probably won't help unless you have multiple RCDs and the faults are on separate RCDs. Any kind of large-ish residential system is always going to develop a fault somewhere pretty fast.
- We use it on really small systems like generators and isolating transformers because it's hard to have two faults when you're checking everything regularly and only using one or two tools.
- We also use it on big critical systems like diesel-electric ships and locos, because it's only got like a dozen parts (a few generators, a few propulsion motors, a few transformers to lower voltage) and you have fault detectors. This lets you keep operating if a fault occurs.
Some kind of earthed system (TN-S/TN-C-S/MEN)
- The difference is arbitrary with a single supply and single dwelling.
- Protection should pick up any fault and disconnect it quickly, but you'll still feel a shock or get blast marks.
- We now specify that there should be only one earth-neutral link in each separate earthing system. It might be compliant to have the earth-neutral link at the inverter, and your 'main switchboard' in the house is effectively a sub-board with a separate neutral and earth feeding it.
  - There's a requirement in the regs that the MEN link be in the main switchboard, but you get into the weeds of 'main switchboard' and I'm not sure whether it applies to standalone supplies. 3000 is perfectly happy for you to put the MEN link in the supply transformer.

Rikkitic

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#3264251 25-Jul-2024 13:19

RCD circuits require separation of neutral and earth.

Plesse igmore amd axxept applogies in adbance fir anu typos

johno1234

2808 posts

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#3264267 25-Jul-2024 14:15

Good explanation here:

Understanding the MEN system, AS/NZS3000

SomeoneSomewhere

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#3264272 25-Jul-2024 14:49

Rikkitic:

RCD circuits require separation of neutral and earth.

Not quite.

RCDs will trip if there is a path around them, and earthing conductors must go around them.

The normal MEN system has neutral and earth connected upstream of the RCD. Therefore, any neutral-earth connection downstream of the RCD gives you a path around the RCD which it will detect and trip.

If you don't have a neutral-earth connection upstream of the RCD, that inadvertent neutral-earth connection downstream of the RCD won't be detected as a fault.

In a system with no neutral-earth connection, there is no real distinction between phase and neutral - it's just two live conductors. As such, one of the rules I forgot is that with isolated supplies, all switches must be double pole. That means your breakers and you need double pole switched sockets. I think even light switches, but I'm not certain on that.

dyllos

20 posts

Geek

#3264504 26-Jul-2024 08:59

SomeoneSomewhere:

Yeah, get a qualified electrician who's experienced and has carefully read the standards and regulations, because they're a conflicting mess. Can't claim to have done so recently.

Broadly speaking, you have two options:

IT (AKA floating; stands for insulated-terra).

This is what a portable generator, inverter, or isolating transformer delivers unless specifically earthed.
You can touch any wire in any one place, or suffer any one fault without either damage or harm occurring, and everything continues to work.
If you have two faults, all bets are off and an RCD probably won't help unless you have multiple RCDs and the faults are on separate RCDs. Any kind of large-ish residential system is always going to develop a fault somewhere pretty fast.
We use it on really small systems like generators and isolating transformers because it's hard to have two faults when you're checking everything regularly and only using one or two tools.
We also use it on big critical systems like diesel-electric ships and locos, because it's only got like a dozen parts (a few generators, a few propulsion motors, a few transformers to lower voltage) and you have fault detectors. This lets you keep operating if a fault occurs.
Some kind of earthed system (TN-S/TN-C-S/MEN)

The difference is arbitrary with a single supply and single dwelling.
Protection should pick up any fault and disconnect it quickly, but you'll still feel a shock or get blast marks.
We now specify that there should be only one earth-neutral link in each separate earthing system. It might be compliant to have the earth-neutral link at the inverter, and your 'main switchboard' in the house is effectively a sub-board with a separate neutral and earth feeding it.

There's a requirement in the regs that the MEN link be in the main switchboard, but you get into the weeds of 'main switchboard' and I'm not sure whether it applies to standalone supplies. 3000 is perfectly happy for you to put the MEN link in the supply transformer.

Thanks for the reply - it's good to expand my understanding. Can I check then; does this mean that the Tiny House is inherently less safe/less protected becuase it doesn't have the 'extra' MEN system involved? it kind of relies on wherever the power is originating from to have MEN protection? i.e. through the Earth cable of the supply cable?

and I am assuming it's only 15A supply because it's the caravan connector underneath ? or can it be higher? I wonder why the breakers and switches are rated much higher than 15 or 16A - the main switch is 63A and the 2 RCDs are 40As each?

Can I ask what kind of faults "always going to develop" in a large-ish residential system? what is typical/common? swapping of neutral and live, rodents chewing cable, ? or is it electronic faults in appliances themselves?

Thanks

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richms

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#3264536 26-Jul-2024 09:49

dyllos:

Thanks for the reply - it's good to expand my understanding. Can I check then; does this mean that the Tiny House is inherently less safe/less protected becuase it doesn't have the 'extra' MEN system involved? it kind of relies on wherever the power is originating from to have MEN protection? i.e. through the Earth cable of the supply cable?

and I am assuming it's only 15A supply because it's the caravan connector underneath ? or can it be higher? I wonder why the breakers and switches are rated much higher than 15 or 16A - the main switch is 63A and the 2 RCDs are 40As each?

Can I ask what kind of faults "always going to develop" in a large-ish residential system? what is typical/common? swapping of neutral and live, rodents chewing cable, ? or is it electronic faults in appliances themselves?

Thanks

There is no link as it will probably be downstream of an RCD, so cant have any link or it is a fault.

There was a thing many years back when campgrounds started to have RCD protected outlets in them and people had old caravans that did have the link in them and just tripping it as soon as plugged in. All I know is that most people just removed the link in the antique distribution board and left it out which meant other issues when back on a different install. Thats why they are supposed to have electrical WOFs to check that dumb stuff has not been done like that.

Richard rich.ms

SomeoneSomewhere

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#3264632 26-Jul-2024 11:47

A caravan is always assumed to be a sub-installation, and we (now) always put the one-and-only MEN link at the most upstream switchboard. Not finding an MEN link doesn't mean that you're not on the MEN system.

In this context, a 'fault' is a live conductor (phase or neutral) contacting ground. Could be miswiring, rodent damage, a spade through a cable, moisture ingress, or a variety of other things.

The 40 and 63A ratings of switches and MCBs are just a safe operating limit like the rating on a cable or plug. There should be an MCB that enforces the 16A plug limit.

I would need to double check the connectable installation rules, because I'm pretty sure it's meant to be an MCB as a main switch to enforce that overload protection, and double pole breakers.