LuxPower - emergency backup enabling

[Swwils]

If you comply, the earthing arrangement of most EESS in island mode where the installation has a low voltage public supply connection is always TN-S.

Please look at the requirements (the link), the earthing arrangement and source of supply in island mode.

[Oldgreybeard]

If you comply, the earthing arrangement of most EESS in island mode where the installation has a low voltage public supply connection is always TN-S.

Please look at the requirements (the link), the earthing arrangement and source of supply in island mode.

How can any supply that has a locally provided earth electrode possibly be TN-S?

If there is a local earth electrode that is carrying the earth fault current then the supply is either TT, or a hybrid form of PME where the local earth electrode forms an additional intermediate that can be used in the event of the distributor provided path to earth becoming broken.

Any supply where the earth fault current passes through a local earth electrode to ground, rather than passing through the distributors cable back to the generator end, is really a form of TT.

Even Wikipedia (not always the best source, but in this case it's right) makes the definitions very clear: https://en.wikipedia.org/wiki/Earthing_system

International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.

The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):

"T" — Direct connection of a point with earth (Latin: terra)
"I" — No point is connected with earth (Latin: īnsulātum), except perhaps via a high impedance.

The second letter indicates the connection between earth or network and the electrical device being supplied:

"T" — Earth connection is by a local direct connection to earth (Latin: terra), usually via a ground rod.
"N" — the earth connection is supplied by the electricity supply network, either separately to the neutral conductor (TN-S), combined with the neutral conductor (TN-C), or both (TN-C-S).

and specifically for TN-S (my added highlight):

TN−S
PE and N are separate conductors that are connected together only near the power source.

If there is an earth electrode at the consumer end that is used to carry the earth fault current then the installation is definitely not TN-S.

[sharpener]

Disagree.

The local earth electrode is essential for the case where the battery system is operating in UPS, EPS or island mode (terms vary), since the supplier's earth cannot be relied on.

In this situation the inverter is the "generator" (Form A1-1 describes it as the "Proposed Additional Generating Unit").

By virtue of the mandatory neutral-earth bond relay the neutral is grounded at "the generator", so the local arrangement is then as @Swwils says TN-S. This is irrespective of the supplier's earth, which may or may not be present and is in any case not relevant because no current either load or fault can be taken from the grid, since whenever the bond relay is closed the anti-islanding relay is open.

The local earth electrode is required whatever the existing earthing arrangements. If they are TT then one will already be present, but its suitability still has to be verified (<200 ohms).

[Oldgreybeard]

Disagree.

The local earth electrode is essential for the case where the battery system is operating in UPS, EPS or island mode (terms vary), since the supplier's earth cannot be relied on.

In this situation the inverter is the "generator" (Form A1-1 describes it as the "Proposed Additional Generating Unit").

By virtue of the mandatory neutral-earth bond relay the neutral is grounded at "the generator", so the local arrangement is then as @Swwils says TN-S. This is irrespective of the supplier's earth, which may or may not be present and is in any case not relevant because no current either load or fault can be taken from the grid, since whenever the bond relay is closed the anti-islanding relay is open.

The local earth electrode is required whatever the existing earthing arrangements. If they are TT then one will already be present, but its suitability still has to be verified (<200 ohms).

Actually you are agreeing! This is what I've been saying all along, the installation MUST be TT when the local earth electrode is providing the earth fault current path in the event of a failure in the distributors side wiring. There is no local earth electrode for a TN-S supply, although under the latest amendment to the regs there may be one for a TN-C-S/PME supply (and TN-C-S and PME are subtly different - in this case it is definitely PME if there is an added consumer side earth electrode paralleled to the distributors protective earth).

You cannot run a battery back up system with just a TN-S earthing system, there must be a local earth electrode for that case and that means the installation is no longer TN-S but TT (at least for the EPS case).

[sharpener]

Disagree.

In this situation the inverter is the "generator" (Form A1-1 describes it as the "Proposed Additional Generating Unit").

Actually you are agreeing!
You cannot run a battery back up system with just a TN-S earthing system, there must be a local earth electrode for that case and that means the installation is no longer TN-S but TT (at least for the EPS case).

No, I am disputing the definition of the eventual position in EPS mode.

From the POV of the consumer unit it looks like TT, but because you have your own generator and the neutral is earthed at this generator then the installation as a whole is TN-S.

As you yourself have quoted:

(i) "N" — the earth connection is supplied by the electricity supply network, either separately to the neutral conductor (TN-S).

In EPS mode you are your own "supply network"

(ii) TN−S PE and N are separate conductors that are connected together only near the power source.

Which they are!

[nowty]

For a laugh,

Lets take a TT system and lets have a short circuit to the earthed casing. Because there is no N to E bond at the installation the fault current has to go through the local earth rod T and through the earth to the supply earth rod T at the supply end transformer. Hence TT.

TT System



Now lets look at the same fault with an EPS enabled at the same installation. The external supply is now disconnected and because there is now a N to E bond at the installation the fault current does not even go through the earth rod anywhere. Instant short circuit back to the EPS, don't look like TT to me anymore, looks like some sort of TN.................s ?

[Oldgreybeard]

Disagree.

In this situation the inverter is the "generator" (Form A1-1 describes it as the "Proposed Additional Generating Unit").

Actually you are agreeing!
You cannot run a battery back up system with just a TN-S earthing system, there must be a local earth electrode for that case and that means the installation is no longer TN-S but TT (at least for the EPS case).

No, I am disputing the definition of the eventual position in EPS mode.

From the POV of the consumer unit it looks like TT, but because you have your own generator and the neutral is earthed at this generator then the installation as a whole is TN-S.

As you yourself have quoted:

(i) "N" — the earth connection is supplied by the electricity supply network, either separately to the neutral conductor (TN-S).

In EPS mode you are your own "supply network"

(ii) TN−S PE and N are separate conductors that are connected together only near the power source.

Which they are!

I think we are at cross purposes here, and that stems from looking at definitions in isolation, rather than as a system. The EPS is, as it's name implies, an Emergency Power Supply. I don't believe that the earthing arrangements for any emergency power supply should fundamentally change the definition of the whole installation.

If, say, someone has a TN-S or TN-C-S/PME installation, then the earth is provided by the distributor normally. That part doesn't change at all if an EPS is installed, the installation will still be TN-S or TN-C-S/PME when the EPS is NOT operating. This must change when the EPS is switched over, as the earth conductor provided via the supply (be it TN-S or TN-C-S/PME) could well disappear, leaving no earth fault current path.

For that case there MUST be a local earth electrode. The regs allow that to either be separate from the incoming supply, or a recent amendment allows that local earth electrode to be added in parallel to the distributor's earth (but ONLY for TN-C-S/PME I believe).

What the IET seem to be arguing (and what is the core of this non-disagreement!) is that if the distributor side is completely ignored (i.e. we assume there is nothing coming from the distributor at all) then the EPS operates as TN-S. The problem I have with completely ignoring whatever the distributor provides, when it comes to describing the totality of the installation, is that it is potentially confusing, possibly dangerously so.

My argument is that we should not change the description of the whole installation just because there is an infrequently used EPS installed. A TN-C-S/PME system should still be referred to as a TN-C-S/PME system, even if there is a local intermediate added to provide a local earth fault current path when the EPS is running and the distributor's cable may be broken.

This argument could be applied to our installation, which is TT, in that if the logic of ignoring the whole installation is followed, then when in grid mode our supply is TT, when in EPS mode it is TN-S. Using the very narrow definition the IET have opted for may be pedantically correct, but is potentially going to cause confusion.

[Swwils]

The code of practice literally introduces this exactly for prosumer electrical installations and the operating modes for electrical energy storage systems; it offers additional clarity especially when for example; an installation with a PME earthing arrangement for the grid connection, but separate TT earthing arrangements for outbuildings, or electric vehicle charging equipment comes along. There is a major change along these lines:

The maximum acceptable earth electrode resistance for installations operating TN-S, or in TT systems where earth fault loop impedance is not restricted to a lower value, is 200 Ω. (previously recommended a maximum of 200 Ω only for systems below 10 kVA.)

So in those kinds of systems the designer should ensure the values of both the consumer earth electrode for the island mode earthing arrangement, and the TT earth electrodes, are low enough to ensure operation of protective devices in the event of a fault to earth in the TT part of the installation.

Again this is all in the link.

[Oldgreybeard]

The code of practice literally introduces this exactly for prosumer electrical installations and the operating modes for electrical energy storage systems; it offers additional clarity especially when for example; an installation with a PME earthing arrangement for the grid connection, but separate TT earthing arrangements for outbuildings, or electric vehicle charging equipment comes along. There is a major change along these lines:

The maximum acceptable earth electrode resistance for installations operating TN-S, or in TT systems where earth fault loop impedance is not restricted to a lower value, is 200 Ω. (previously recommended a maximum of 200 Ω only for systems below 10 kVA.)

So in those kinds of systems the designer should ensure the values of both the consumer earth electrode for the island mode earthing arrangement, and the TT earth electrodes, are low enough to ensure operation of protective devices in the event of a fault to earth in the TT part of the installation.

Again this is all in the link.

And yet again we are in agreement!

The issue for me is over the pedantic nature of the IET in insisting that the system needs to be re-named whenever the EPS engages. Without wishing to cast aspersions at any trade, I know for a fact that there is already a lot of confusion surrounding the detail of earthing systems, and although it is pedantically correct to refer to a TN-S or TT (but not TN-C-S/PME) system becoming TN-S when the back up system is running and the earth fault current can only pass through the local earth electrode, it would seem to me to make sense to not have to double label installations but to refer to the normal grid supply earthing arrangement as being the one used to describe the installation, with the backup system earthing being only referred to in the context of the EPS.

TN-C-S/PME poses an interesting question for those with an eye for the fine detail. Because the regs now allow an additional intermediate to be installed in parallel with the incoming protective earth (or more accurately the PEN) then it isn't really TN-S when in EPS mode, simply because there may well be multiple earth fault current paths back through all the other PME earth electrodes, right back to the grid transformer.

Strictly speaking (and we are engaged in a debate about this very fine detail) it isn't TN-S, because the earth current fault path may well not be through the local generator earth electrode at all, there is a chance that the PEN remains intact in the event of a fault and so the earth fault current may well preferentially flow back to the supply generator earth electrode, rather than the higher impedance local electrode. No idea what name to give this arrangement, but it should, strictly speaking, have another T in front of it to recognise that dominant earth fault current path back through the grid supply earth conductor.

When all is said and done, if the installation needs a local earth electrode (as all back up/emergency power supply systems do) then to me it makes more sense to just convert the installation to TT (for the pedants, "TT in grid power mode"). My thinking is this:

• It's not unknown for the potential on the incoming PEN conductor to rise to an unsafe voltage in the event of a distribution fault.

• When this happens the additional local earth electrode isn't going to be much help, as its impedance will probably be too high to allow any significant fault current to flow to earth that way, without the touch voltage rising to an unsafe level.

• For the TN-C-S/PME case, unless the parallel intermediate local earth electrode has an unrealistically low resistance to earth then it doesn't make much sense to retain any connection between the incoming grid PEN and the installation PE.

• Converting the whole installation so that it is TT (in grid supply mode) removes any risk of importing a high voltage via the PEN, or the lesser issue of having unwanted earth fault currents from outside the premises circulating through the local earth electrode.

• Doing this only involves adding a Type S RCD to the incoming supply, a small price to pay (IMHO) for removal of the PEN fault risk, and disconnecting the neutral PE link on the grid supply side

[sharpener]

TN-C-S/PME poses an interesting question for those with an eye for the fine detail. Because the regs now allow an additional intermediate to be installed in parallel with the incoming protective earth (or more accurately the PEN) then it isn't really TN-S when in EPS mode, simply because there may well be multiple earth fault current paths back through all the other PME earth electrodes, right back to the grid transformer.

Difficult to see how fault current could flow back to the transformer earth, as although there may be an earth path there cannot be a live or neutral conductor going thereto as the relay in both of them will be open.

But I do agree with you that converting the whole installation to TT (on grid) is much the best way forward. Fortunately I never got round to converting mine to TN-C-S in the first place as proposed by WPD some years ago.