TT System High Ze leading to possible Zs fails

North88 · Aug 30, 2023

Doing a lot of EICRs at the moment and have came across what I'm sure is a common enough problem. In an off grid rural cottage I have a TT system, Diesel Generator that runs and charges batteries when they are below 10 percent or so.

Ra is about 150 Ohms... was about 3 ohms for Ze before main earth was disconnected from electrode.

Now table 53.1 tells me that a 500mA RCD would be acceptable in this instance, not that that would ever be the case as everything is protected by 30mA RCD albeit one main one for everything. ( An exception to this is the circuit from generator to Invertor IN circuit and Invertor OUT i don't believe is on an RCD. Invertor out is a 25A MCB which limits the entire supply which is a second 2 way DB in the shed that house the generator doing 2 sockets and then onto an 8 way main RCD board in the house.

My question is basically most mcbs 20 amp and above are failing max Zs readings ( In most instances not by much ).

Because it is a remote holiday let it has a very small amount of sockets and no kettle or any other appliances supplied and all heating cooking is gas/oil. I maybe have the luxury of sticking a couple of rings onto 10A mcbs in which case they pass but not sure if table 53.1 negates the need for this?

SO long as the RCD trips within 200ms can this be passed so long as deviations from BS7671 are noted?

Example readings are 16A MCB 2.5 Ohms which is 2.2 max and 32A MCB 3.5 Ohms which is 1.1 max. all other 6A circuits pass.

Appreciate advice as its not something i have dealt with too much over the years.

( Sorry for waffling on also cheers! )

timhoward · Aug 30, 2023

North88 said:
Doing a lot of EICRs at the moment and have came across what I'm sure is a common enough problem. In an off grid rural cottage I have a TT system, Diesel Generator that runs and charges batteries when they are below 10 percent or so.

I think you might have a TN-S system, the earth rod for the generator being the first T in TN-S not the second T in TT.

North88 said:
My question is basically most mcbs 20 amp and above are failing max Zs readings ( In most instances not by much ).

Well as Zs is the resistance of the circuit added to Ze, this is to be expected. So it comes down to considering line-earth faults at each point in the setup and working out what would happen (or not happen).

In my mind this boils down to:
1) confirming the generator earthing arrangements.
2) what is the type and rating of the fuse / device protecting the generator output
3) will a fault to earth before the first RCD clear and how long will it take
4) Is there 30ma RCD protection for every final circuit

Also, it may be that the generator output can be treated as a distribution circuit for disconnection time purposes.

davesparks · Aug 30, 2023

If the installation is being fed from a small generator or an inverter then, regardless of the Zs, the supply will not be capable of delivering enough fault current to achieve disconnection times.

If an earth fault occurs it is likely that the generator's engine would stall due to the sudden additional load before any fuse or MCB operates.

The generator should incorporate the necessary protection for it's output, this may not be obvious though as it can be built in to the internal control system.

North88 · Aug 30, 2023

davesparks said:
If the installation is being fed from a small generator or an inverter then, regardless of the Zs, the supply will not be capable of delivering enough fault current to achieve disconnection times.

If an earth fault occurs it is likely that the generator's engine would stall due to the sudden additional load before any fuse or MCB operates.

The generator should incorporate the necessary protection for it's output, this may not be obvious though as it can be built in to the internal control system.

Thanks for the reply mate, in normal operation the batteries/invertor would be supplying the installation so the generator stalling wouldn't stall as such. I think another visit to have a look at the invertor itself and inspect whether its control system offers any fault protection may be an idea?

North88 · Aug 30, 2023

timhoward said:
I think you might have a TN-S system, the earth rod for the generator being the first T in TN-S not the second T in TT.

Well as Zs is the resistance of the circuit added to Ze, this is to be expected. So it comes down to considering line-earth faults at each point in the setup and working out what would happen (or not happen).

In my mind this boils down to:
1) confirming the generator earthing arrangements.
2) what is the type and rating of the fuse / device protecting the generator output
3) will a fault to earth before the first RCD clear and how long will it take
4) Is there 30ma RCD protection for every final circuit

Also, it may be that the generator output can be treated as a distribution circuit for disconnection time purposes.

Cheers for the quick response I appreciate it. Yes I believe you are right and it is infact TN-S... in my head the earth electrode automatically meant TT but i have little experience of dealing with them and have spent a bit of time reading up.

I am going to go back out a look this week but I believe it was a C40 double pole MCB on the generator itself.

May need some advice on point 3 and all final circuits have 30ma RCD.

Generator to Invertor and Invertor to sub mains are not RCD protected I don't believe but I'm gonna have a proper study when i get back.

davesparks · Aug 30, 2023

North88 said:
I am going to go back out a look this week but I believe it was a C40 double pole MCB on the generator itself.

Often the generator MCB will have a shunt trip fitted so that the generator control panel can trip it should there be a fault. What make and model of generator is it? There may be earth leakage protection built in that isn't obvious at first due to it being inside the control panel.

North88 · Sep 2, 2023

davesparks said:
Often the generator MCB will have a shunt trip fitted so that the generator control panel can trip it should there be a fault. What make and model of generator is it? There may be earth leakage protection built in that isn't obvious at first due to it being inside the control panel.

Cheers Dave,

Haven't made it back out as of yet.... it really is the back of beyond but i will get passed this coming week and find that out.

North88 · Sep 13, 2023

davesparks said:
Often the generator MCB will have a shunt trip fitted so that the generator control panel can trip it should there be a fault. What make and model of generator is it? There may be earth leakage protection built in that isn't obvious at first due to it being inside the control panel.

Hi Dave the generator is a Lister Pelter T series nameplate 20001074TR2A08. Im not sure is there is earth leakage built in as of yet either but would I be right in saying that if it is charging batteries via the invertor then it would be the inverter that would need to have fault protection?

The invertor is a Outback Power VFX3024F single-phase Sine-Wave Invertor/Charger with 30 Amps AC Transfer switch.

There are 9 ( I believe and will confirm ) Rolls battery engineering Deep Cycle Series 4000 S-5550 Batteries in the system ( 6- Volt )

Appreciate the help for you guys on here as its not the type of thing I have a lot of experience with but interested to lay more about.

This particular issue is on a bit of a time scale and getting a bit of pressure but I won't sign to off until I have a better understanding of what is installed is safe.

Rockingit · Sep 13, 2023

Sounds to me as though this is a barrel of monkey's. You may have either a TN or floating earth from the generator, depending on size and quality and then again either a created TN or possibly even IT from the invertor.

Neither of these, as others have said, is likely to be able to produce enough raw energy to produce sufficient pfc for ADS, so your Ra is all but irrelevant. However, if all final circuits are protected by 30mA RCD then your max allowable Zs becomes the magic 1667Ω.
Ultimately in these situations what you're having to make a decision on is whether something is safe, not that it complies with scenarios never considered years ago. Engineering sense would suggest that the simplest way to make the system safe would be to contemplate making the whole situation IT - however frustratingly that's one thing you can't do according to BS7671 unless it's under supervision.

What I would say is that for this reason the vast majority of invertors have some very clever fault sensing circuitry within them to allow for these situations which makes both an overload situation unlikely and you have final cct protection via RCD's to limit touch voltages for humans. It can be very easy to go down a rabbit hole of overthinking things!

133.5 "Where the use of a new material or invention leads to departures from the Regulations, the resulting degree of safety of the installation shall not be less than that obtained by compliance....."

timhoward · Sep 13, 2023

Rockingit said:
You may have either a TN or floating earth from the generator, depending on size and quality and then again either a created TN or possibly even IT from the invertor.

In this case the inverter isn't IT, N is passed straight through it.
So whatever earthing the generator is will also apply to the inverter output.

[ElectriciansForums.net] TT System High Ze leading to possible Zs fails

Also of note is:

So hopefully the installation earth isn't only connected to the ground terminal of the inverter.
It's clear from the manual that the inverter is clever and can be configured to not overload a generator and disconnect the output if too much is drawn.

( https://www.energymatters.com.au/images/outback/FX-VFX-Manual.pdf )

With final circuits RCD protected, I'm struggling to see real-life issues with this setup.

Rockingit · Sep 13, 2023

timhoward said:
In this case the inverter isn't IT, N is passed straight through it.
So whatever earthing the generator is will also apply to the inverter output.
View attachment 110482

Also of note is:
View attachment 110481
So hopefully the installation earth isn't only connected to the ground terminal of the inverter.
It's clear from the manual that the inverter is clever and can be configured to not overload a generator and disconnect the output if too much is drawn.

( https://www.energymatters.com.au/images/outback/FX-VFX-Manual.pdf )

With final circuits RCD protected, I'm struggling to see real-life issues with this setup.

Trouble is, unless I'm not understanding the OP properly, the generator isn't running all the time anyway, so one has to question where/how the Neutral path is being derived - methinks possibly via some weird paths off the TT!!

timhoward · Sep 13, 2023

Rockingit said:
Trouble is, unless I'm not understanding the OP properly, the generator isn't running all the time anyway, so one has to question where/how the Neutral path is being derived - methinks possibly via some weird paths off the TT!!

As it seems to be designed for the scenario that it is being used for, I wonder (hope!) if the the N is common to primary and secondary sides of the inverter

North88 · Sep 14, 2023

timhoward said:
In this case the inverter isn't IT, N is passed straight through it.
So whatever earthing the generator is will also apply to the inverter output.
View attachment 110482

Also of note is:
View attachment 110481
So hopefully the installation earth isn't only connected to the ground terminal of the inverter.
It's clear from the manual that the inverter is clever and can be configured to not overload a generator and disconnect the output if too much is drawn.

( https://www.energymatters.com.au/images/outback/FX-VFX-Manual.pdf )

With final circuits RCD protected, I'm struggling to see real-life issues with this setup.

Hi Tim,

I managed to get hold go the guy that set up the generator and invertor and he confirmed that the AC output Neutral has been connected the ground terminal. He also advised that the only protection the invertor provided was anti-surge san that was to protect the invertor itself and would not disconnect the load side feeding the house.

The invertor has a max output of 3Kw and when the generator ( 9Kw) kicks in it feeds the house while the invertor charges the battery. Im lead t believe then that the only protection the generator and invertor provide is a C40 and B32 respectively.

Everything is on 30mA RCDs thereafter with only the B32 mcbs failing in regards to Zs ratings (which can be downgraded if need be).

Does it make any sense to try and add another earth electrode outside the shed housing where the elements may help a reading or does the output of the generator/invertor mean these Zs readings can be overlooked?

North88 · Sep 14, 2023

Rockingit said:
Sounds to me as though this is a barrel of monkey's. You may have either a TN or floating earth from the generator, depending on size and quality and then again either a created TN or possibly even IT from the invertor.

Neither of these, as others have said, is likely to be able to produce enough raw energy to produce sufficient pfc for ADS, so your Ra is all but irrelevant. However, if all final circuits are protected by 30mA RCD then your max allowable Zs becomes the magic 1667Ω.
Ultimately in these situations what you're having to make a decision on is whether something is safe, not that it complies with scenarios never considered years ago. Engineering sense would suggest that the simplest way to make the system safe would be to contemplate making the whole situation IT - however frustratingly that's one thing you can't do according to BS7671 unless it's under supervision.

What I would say is that for this reason the vast majority of invertors have some very clever fault sensing circuitry within them to allow for these situations which makes both an overload situation unlikely and you have final cct protection via RCD's to limit touch voltages for humans. It can be very easy to go down a rabbit hole of overthinking things!

133.5 "Where the use of a new material or invention leads to departures from the Regulations, the resulting degree of safety of the installation shall not be less than that obtained by compliance....."

Hi Rockingit appreciate you taking the time to help. To my understanding when the generator runs it would be TN......S? When the invertor is running the installation via the batteries the output neutral in connected to the ground terminal of the invertor. Would the magic 1667 only apply to TT?

Furthermore would it be acceptable to state that the Ra and exceeded Zs readings are to be overlooked as in my opinion the invertor/generator outputs negate these and 30mA RCDs cover the installation?

Thanks again.

Rockingit · Sep 14, 2023

North88 said:
Hi Rockingit appreciate you taking the time to help. To my understanding when the generator runs it would be TN......S? When the invertor is running the installation via the batteries the output neutral in connected to the ground terminal of the invertor. Would the magic 1667 only apply to TT?

Furthermore would it be acceptable to state that the Ra and exceeded Zs readings are to be overlooked as in my opinion the invertor/generator outputs negate these and 30mA RCDs cover the installation?

Thanks again.

It's what we're here for!

So, the generator requires an earth regardless of what's downstream of it - BS7430 is the guidance on this but you'll find an easier to digest summary in section 2.4 of the OSG.

The 1667Ω max Zs is applicable to any circuit* downstream of a 30mA RCD irrespective of TT / TN status because that makes for safe touch voltages (search on other threads on here for that).

ADS basically doesn't work in this situation - if you do some rough maths a 9KW generator is capable of producing 38A (if we ignore some finer technicalities which happen incredibly quickly then diminish). Consider then the time/current curve of your C40 breaker and it's fairly obvious that will NEVER trip because even if you had a Zs of 0.001Ω (pfc = 230kA!) you can't get more than 38ishA out of the supply. However, an eg B6 device will activate for obvious reasons - so long as the fault current remains active long enough for the time curve.

*there are some special exceptions to this that don't apply here

TT System High Ze leading to possible Zs fails

North88

timhoward

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davesparks

North88

North88

davesparks

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Rockingit

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timhoward

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North88

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