Neutral/earth bond for inverters

[Justloadit]

Correct me if I am wrong here, BUT the Earth/Neutral bond is only required when in Islanding mode(Incoming Power fail), not as a permanent connection.
Hence the relay pulling in during islanding mode, making the Load Neutral referenced to Earth in order that the ELU can pick up an earth fault. When the power from the grid is available the Neutral is joined to Earth by the power supplier either at the Transformer or DB box depending on the type of Earthing System for the respective site.

[Derlyn]

I've been following this post with interest, but I still do not understand why, when an inverter is involved, is there so much interest in how much current is flowing in the neutral earth bridging link.

On an installation without an inverter, no thought is given to how much current is flowing through the neutral earth bridge. We are only interested in the earth and neutral loop impedances and the elevated voltage between neutral and earth. Why is this ?

Another question I have is . .. can the amount of current flowing through the neutral earth link affect the operation of an earth leakage relay negatively. I've always understood that the earth leakage unit checks the current between the live and neutral conductors and trips if the difference between these two is greater than the rating of the unit, ie 30mA for the most common units. The earth leakage therefore doesn't care how much current is flowing through the neutral earth bridge. Am I correct in my line of thinking ?

[Derlyn]

Another question we should be asking ourselves is ...... why is the neutral and earth bonded ? Has it got anything to do with the operation of the earth leakage relay ?

The answer is NO.

The neutral and earth is bonded in order to bring them to the same potential so that should a live conductor touch earth it would be the same as that live wire touching neutral, causing what we would refer to as a dead short. Should this happen, the current must be high enough to trip the overcurrent protection. This is why we do the earth and neutral loop impedance tests. These tests are done to ensure that the impedance is low enough to cause a current of at least twice that of the main breaker rating to flow should that happen, ensuring that the main breaker will trip. This has got nothing to do with the operation of the earth leakage relay.

[GCE]

Another question we should be asking ourselves is ...... why is the neutral and earth bonded ? Has it got anything to do with the operation of the earth leakage relay ?

The answer is NO.
.

The answer is yes
If your neutral earth is not bonded the earth leakage will not trip - Try it .

There are 2 normal faults that we get called on with inverters - LED lights still glowing slightly even though light switch is off and Earth Leakage not tripping - Both when grid fails and inverter suppling power.

The neutral needs to reference to earth

[Justloadit]

On an installation without an inverter, no thought is given to how much current is flowing through the neutral earth bridge. We are only interested in the earth and neutral loop impedances and the elevated voltage between neutral and earth. Why is this ?

Usually not a concern, as the power company would ensure that the correct wire size would be used for this connection, however, when the inverter is used, the Earth/Neutral bridge wire size is suited for the respective DB, and not for a fault current which may occur on the transformer feeding side, which could vaporize your Earth/Neutral bridge, as the full transformer current could flow through it.

Another question we should be asking ourselves is ...... why is the neutral and earth bonded ? Has it got anything to do with the operation of the earth leakage relay ?

The answer is NO.

Yes and No. If there is no Neutral Earth bond, then under a mild fault condition, such as a human touching live and neutral and earth simultaneously, there is no equipment that could detect this and trip to save the human from electrocution, unless there is a reference to earth, which is accomplished by the Earth/Neutral bond.
Secondly by not referencing one of the transformer output wires to Earth, which then gets referred to as Neutral, the other connection then would be referred to as Live, there is nothing preventing a potential difference being charged up between the floating wires of the transformer to earth, and at some point in time could be thousands of electrostatic volts between earth and the transformer secondary, which in turn could be potentially dangerous for a human to touch.

By adhering to the regulations stipulated, it has made the secondary side of the transformer a safer piece of equipment.

[Derlyn]

If what you are saying is true, then an isolating transformer is not that safe after all.

Will respond further later when power is back on. Cellphone battery almost klaar. 👍

There are some points that need further discussion and debate.

[Isetech]

What made me test the current on the bridge, we were fault finding an inverter, I had disconencted the earth wires to do a few test and I got one of those wake up calls touching an earth wire. You should never get shocked when touching an earth wire.

I've been following this post with interest, but I still do not understand why, when an inverter is involved, is there so much interest in how much current is flowing in the neutral earth bridging link.

On an installation without an inverter, no thought is given to how much current is flowing through the neutral earth bridge. We are only interested in the earth and neutral loop impedances and the elevated voltage between neutral and earth. Why is this ?

Another question I have is . .. can the amount of current flowing through the neutral earth link affect the operation of an earth leakage relay negatively. I've always understood that the earth leakage unit checks the current between the live and neutral conductors and trips if the difference between these two is greater than the rating of the unit, ie 30mA for the most common units. The earth leakage therefore doesn't care how much current is flowing through the neutral earth bridge. Am I correct in my line of thinking ?

[Isetech]

The catch is what happenes if the relay fails, this is where The old SABS would have stepped in and verified all senerios to identify if there was apotential hazard.

We have a potentially dangeorus installation if there is permanant bond and the same if the relay fails.

Who should be responsible for creating a safe environment for the public, the manufactuer or the electrician?

There doesnt seem to be much concern about either method being used, if it was that dangeorus, the DOL would have stepped up and issued a statement. So for now just do what you feel is safer.

With a 60/40 vote, its clear that nobody really has the answer.

Until someone is seriously injured, nodody will commit to the issuing a formal statement.

The answer is yes
If your neutral earth is not bonded the earth leakage will not trip - Try it .

There are 2 normal faults that we get called on with inverters - LED lights still glowing slightly even though light switch is off and Earth Leakage not tripping - Both when grid fails and inverter suppling power.

The neutral needs to reference to earth

[Isetech]

IF you beleive your method is the safer option, feel free to share your test results. Like someone mentioned on another platform, if you want to find out if your statement is false, put it on an open platform and people will correct you.

This is an open public platform, feel free to share your opinion. No invitation or approval required.

Lets get the facts right so that people can create a safer environmant for the public.

[Justloadit]

If what you are saying is true, then an isolating transformer is not that safe after all.

Will respond further later when power is back on. Cellphone battery almost klaar. 👍

There are some points that need further discussion and debate.

The word safe needs to be defined here.
Safe as far as an earth fault of the primary side ELU - No.
Safe as theoretically you can touch either wire and Earth on the secondary side - Yes.

There are times that the use of an isolating transformer is desirable, and the majority of cases it will be safe, since there usually is only one specific piece of equipment connected to it, so the chances of static build up are small. Such as a heater element in a humid area. However if there is electronic equipment generating high frequency switching, then invariably there will be a static charge build up simply because of the system dynamics. What I usually do is connect a very large value resistor between one wire of the isolating transformer secondary and supply/equipment Earth such as 1 mega ohm, then the build up of static charge will be discharged via this resistor, and still maintaining the isolation and could loosely be referred to as Neutral. I usually do this when I am working on certain Mains powered equipment, and need to do fault finding with sophisticated equipment, the Earth/Neutral as a reference can be extremely dangerous under these conditions as one tends to focus on the fault finding and not quite on what one is really doing when connecting up test probes while being powered.

[Derlyn]

@Justloadit & GCE.

I still disagree that the earth leakage relay references to earth.

The million dollar question is ... how does it reference to earth when it itself does not have an earth connection ? It only has live and neutral connections.

The fact that the earth leakage relay does not trip with the tester if the bond is not there is due to the fact that the tester cannot create a leakage current without that bond. That is, in my opinion, a design fault in the tester. As a matter of interest, the earth leakage will also not trip with the tester if the neutral and earth are reversed on the socket outlet.

HOWEVER

Should a person shock down to earth irrespective of whether there is a neutral earth bond or not, the earth leakage WILL trip because it will detect the imbalance of currents between the live and neutral.

[Derlyn]

On an installation where the lights are not on earth leakage, take a lights neutral and connect it to earth leakage neutral bar and switch on.

What happens ?

The earth leakage trips. Correct?
Is there an earth fault ? No
Has it referenced anything to earth ? No.

All it has done is detected the difference in live and neutral currents through it.

The above will happen whether there is a neutral earth bond or not.

[Justloadit]

@Justloadit & GCE.

I still disagree that the earth leakage relay references to earth.

The million dollar question is ... how does it reference to earth when it itself does not have an earth connection ? It only has live and neutral connections.

The fact that the earth leakage relay does not trip with the tester if the bond is not there is due to the fact that the tester cannot create a leakage current without that bond. That is, in my opinion, a design fault in the tester. As a matter of interest, the earth leakage will also not trip with the tester if the neutral and earth are reversed on the socket outlet.

HOWEVER

Should a person shock down to earth irrespective of whether there is a neutral earth bond or not, the earth leakage WILL trip because it will detect the imbalance of currents between the live and neutral.

This is not how it works.
Effectively if you take your Clamp tester, and test Live current, it should read exactly the same if you measure the Neutral current. Now if you take a very sensitive clamp tester, and place Live and Neutral through your clamp meter, there should be zero Amps because the two currents will cancel each other out. Now if there is a small current flowing either from Live or Neutral to Earth, provided there is a bond connection before your measurement point, there will be an imbalance in the current flowing Through Neutral and Live, and you clamp meter if sensitive enough would measure this imbalance. The ELU works the same way, but the imbalance causes the breaker to trip. If there is no Earth Neutral Bond before your clamp or before the ELU, then there will be no imbalance, as there will be no current flowing from Live and Neutral to earth, as there is no return path, and therefor will not trip.

Here is a video which explains it with pictures.

You can do a Google search for other videos which goes deeper into the theory

[Derlyn]

@ Justloadit

What do you say about the light scenario I mentioned, where there is no leakage current to earth, yet the earth leakage trips ?

[GCE]

@ Justloadit

What do you say about the light scenario I mentioned, where there is no leakage current to earth, yet the earth leakage trips ?

It will trip as you have more current flow back on the neutral then you have going out on the live.


With an unbridged neutral earth point - The neutral does not know what earth is and so nothing will happen - there will be no flow of current