Earth in a shed

Wearb · 03-03-2020 11:45am #1

I was servicing a boiler in a shed yesterday and noticed no continuity between L and boiler casing. While sorting this I noticed that the power from the consumer unit in the house to the shed was just 2 core and the shed earth taken from an earth rod at the back of the shed. Everything seems fine but I am left wondering if this is an acceptable way to do it?

10-10-20 · 03-03-2020 12:36pm

You mean "E and boiler casing"?
So no earth in the feed to the boiler, but there was an earth attached to the earth terminal which came from a local earth-point.
Was there an aux panel with RCD's in the shed, or was the boiler alone in having power in the shed?

03-03-2020 12:45pm

Wearb wrote: »

I was servicing a boiler in a shed yesterday and noticed no continuity between L and boiler casing. While sorting this I noticed that the power from the consumer unit in the house to the shed was just 2 core and the shed earth taken from an earth rod at the back of the shed. Everything seems fine but I am left wondering if this is an acceptable way to do it?

This arrangement is not acceptable. For a start the earth fault loop impedance would be very high even with the local earth rod. There should be a 3 core cable (L, N & E) from the main board to the shed.

Wearb · 03-03-2020 12:47pm

I got the earth sorted out. It was a corroded tag on the casing, probably caused by a slight condensate leak. I moved this connection to a dryer part of the boiler and fixed the leak. I suspect that the earth might even have worked under load but not show on a multimeter.

The shed has its own RCD and MCB's.

However it is the apparent disconnect between the house earth and the shed earth that I am concerned about. I know it's all the same earth in the long run, but just wondering if I should have written it up on service report as a potential fault. (I didn't as I didn't want to needlessly (perhaps) worry the old pair)

03-03-2020 12:53pm

Wearb wrote: »

I suspect that the earth might even have worked under load but not show on a multimeter.

Define "worked"

I would expect the earth fault loop impedance to be far too high. This will result in protective devices taking too long to trip or not tripping at all.

However it is the apparent disconnect between the house earth and the shed earth that I am concerned about.

As you should be. I would use a "wander lead" to measure the resistance between the shed earth and the MET in the main board. This will tell a lot.

I know it's all the same earth in the long run, but just wondering if I should have written it up on service report as a potential fault. (I didn't as I didn't want to needlessly (perhaps) worry the old pair)

What you have to consider is that it is a very poor connection to the "same earth" at the shed. An earth rod would typically provide a connection with a resistance of 100 to 200 ohms to earth.

Wearb · 03-03-2020 1:06pm

2011 wrote: »

Define "worked"

Point taken. I just meant that that it might have tripped the RCD under most common fault conditions.

I would expect the earth fault loop impedance to be far too high. This will result in protective devices taking too long to trip or not tripping at all.

As you should be. I would use a "wander lead" to measure the resistance between the shed earth and the MET in the main board. This will tell a lot.

Beyond my pay grade

What you have to consider is that it is a very poor connection to the "same earth" at the shed. An earth rod would typically provide a connection with a resistance of 100 to 200 ohms to earth.

Should I inform the householder. From a personal professional point of view i am probably covered as i would not be expected to investigate beyond establishing the earth presence with a multimeter.

.

03-03-2020 1:11pm

Remember RCDs wonderful and all as they are only provide supplementary protection, therefore they should be treated accordingly.

Yes, I would inform the home owner. The wiring is not compliant with the rules.
It would be best to get a REC to resolve this issue.

Wearb · 03-03-2020 1:41pm

2011 wrote: »

Remember RCDs wonderful and all as they are only provide supplementary protection, therefore they should be treated accordingly.

Yes, I would inform the home owner. The wiring is not compliant with the rules.
It would be best to get a REC to resolve this issue.

Thanks. I will do that.

Michelinextra. · 03-03-2020 4:51pm

This would be common enough in the UK but not here

Maybe it was an English spark that did the work

Michelinextra. · 03-03-2020 4:58pm

Wearb · 03-03-2020 5:14pm

I don't know who wired it. Rural house and looks about 30 years old. Shed, maybe newer.

03-03-2020 5:26pm

Michelinextra. wrote: »

This would be common enough in the UK but not here

This is not what is shown in the diagram you posted.

In fact this diagram shows what is often done in Ireland i.e. a sub distribution board fed from the main board with a 3 cores providing a phase, neutral and earth.

Edit: my bad. I just saw the note on the drawing explaining that the earth does not link both boards.
I’m not a fan of this arrangement to be honest.

Michelinextra. · 03-03-2020 5:58pm

2011 wrote: »

This is not what is shown in the diagram you posted.

In fact this diagram shows what is often done in Ireland i.e. a sub distribution board fed from the main board with a 3 cores providing a phase, neutral and earth.

Edit: my bad. I just saw the note on the drawing explaining that the earth does not link both boards.
I’m not a fan of this arrangement to be honest.

Me neither

Michelinextra. · 03-03-2020 6:53pm

https://professional-electrician.com/technical/stroma-certification-supply-chain/

A bit of a read-up on it there

Not sure why they did it here unless there was no neutralising present and they decided to do it

In that case they should have dealt with the neutralizing as part of the work

stickman1019 · 07-03-2020 12:59am

Essentially in this diagram they are converting the TNCS to an IT system locally.

This is the same arrangement allowed for in Electric Vehicle charging points in BS7671.

Think the school of thought boils down to not relying on the PEN when cabling is exiting a building.

There is a lot on info on youtube on this including the Amendment 1

The Amendment is available from the IET website on the below link for viewing

https://electrical.theiet.org/bs-7671/updates-to-18th-edition/

Michelinextra. · 07-03-2020 4:48am

stickman1019 wrote: »

Essentially in this diagram they are converting the TNCS to an IT system locally.

This is the same arrangement allowed for in Electric Vehicle charging points in BS7671.

Think the school of thought boils down to not relying on the PEN when cabling is exiting a building.

There is a lot on info on youtube on this including the Amendment 1

The Amendment is available from the IET website on the below link for viewing

https://electrical.theiet.org/bs-7671/updates-to-18th-edition/

Converting to TT for outbuildings

That's the method in the UK

Risteard81 · 07-03-2020 7:22am

As above it's converting to a TT system (not IT).

IT is generally illegal in the UK (except for specialist installations e.g. hospital operating theatres).

With EVSEs simultaneous contact between Earthing systems has to be considered, and now new O-PEN devices exist to allow use of TN-C-S. (It breaks the Earth connection as well as the neutral if loss of PEN is detected.) But that's all in the realm of BS7671 rather than ET101/I.S. 10101

07-03-2020 9:06am

In the event of something going wrong none of this matters.
What is important is the high impedance of the earth fault path.
I would not accept this whether I was in the UK or the ROI.

Bruthal · 07-03-2020 4:47pm

2011 wrote: »

In the event of something going wrong none of this matters.
What is important is the high impedance of the earth fault path.
I would not accept this whether I was in the UK or the ROI.

This earth rodding of shed could simply be another sds drill engineer thinking an earth rod is a fine solid earthing method, without any further thought on it.

stickman1019 · 07-03-2020 4:49pm

Risteard81 wrote: »

As above it's converting to a TT system (not IT).

IT is generally illegal in the UK (except for specialist installations e.g. hospital operating theatres).

With EVSEs simultaneous contact between Earthing systems has to be considered, and now new O-PEN devices exist to allow use of TN-C-S. (It breaks the Earth connection as well as the neutral if loss of PEN is detected.) But that's all in the realm of BS7671 rather than ET101/I.S. 10101

Apologies typo there it was meant to be TT.

Risteard81 · 07-03-2020 5:27pm

2011 wrote: »

In the event of something going wrong none of this matters.
What is important is the high impedance of the earth fault path.
I would not accept this whether I was in the UK or the ROI.

In a UK single phase supply essentially you would have had no choice, as other options were wholly impracticable (and often impossible).

Now, in Amendment 1 to BS7671:2018 (only released on the 1st February 2020) there is an option for these new O-PEN devices on a single phase supply. Matt:e manufactures them, and as I stated they break the cpc connection as well as the phase and neutral. They are only coming onto the market now. I tried to source one with a Type B RCD but was told that it would be another few weeks/months before it's released and that pricing hasn't even been confirmed for it yet. Therefore I was left with the only option being to use the unamended BS7671:2018 (which can be done until I think July) and adopt a TT system for the EVSE installation. A simultaneous contact risk assessment needs to be carried out to ensure that exposed and extraneous-conductive-parts of the TN-C-S installation cannot be touched whilst charging the EV.

07-03-2020 5:34pm

You are missing the point. Physics doesn’t care about revaluations. If the casing of the boiler becomes live due to a fault I want the upstream MCB or fuse to disconnect the supply rapidly. If the earth fault loop impedance is too high this may not happen. As a result of this someone may be injured and no amount of quoting regulations that apply in foreign countries will be of any consolation to them.

Besides, this is the ROI not the UK.

stickman1019 · 07-03-2020 5:34pm

Risteard81 wrote: »

In a UK single phase supply essentially you would have had no choice, as other options were wholly impracticable (and often impossible).

Now, in Amendment 1 to BS7671:2018 (only released on the 1st February 2020) there is an option for these new O-PEN devices on a single phase supply. Matt:e manufactures them, and as I stated they break the cpc connection as well as the phase and neutral. They are only coming onto the market now. I tried to source one with a Type B RCD but was told that it would be another few weeks/months before it's released and that pricing hasn't even been confirmed for it yet. Therefore I was left with the only option being to use the unamended BS7671:2018 (which can be done until I think July) and adopt a TT system for the EVSE installation. A simultaneous contact risk assessment needs to be carried out to ensure that exposed and extraneous-conductive-parts of the TN-C-S installation cannot be touched whilst charging the EV.

Do you have a link to Single Phase product ?

I heard they did a 3Ph one.

Below video is worth a watch.
https://youtu.be/9GzEL3WqcrQ

07-03-2020 5:40pm

Risteard81 wrote: »

Now, in Amendment 1 to BS7671:2018 (only released on the 1st February 2020) there is an option for these new O-PEN devices on a single phase supply. Matt:e manufactures them, and as I stated they break the cpc connection as well as the phase and neutral. They are only coming onto the market now. I tried to source one with a Type B RCD but was told that it would be another few weeks/months before it's released and that pricing hasn't even been confirmed for it yet. Therefore I was left with the only option being to use the unamended BS7671:2018 (which can be done until I think July) and adopt a TT system for the EVSE installation. A simultaneous contact risk assessment needs to be carried out to ensure that exposed and extraneous-conductive-parts of the TN-C-S installation cannot be touched whilst charging the EV.

@Risterad: sorry I am only reading this part of your post now, my bad. Looks interesting. Can you please start a new thread to discuss this as it does not really fit here?

Risteard81 · 07-03-2020 5:45pm

stickman1019 wrote: »

Do you have a link to Single Phase product ?

I heard they did a 3Ph one.

Below video is worth a watch.
https://youtu.be/9GzEL3WqcrQ

https://www.cef.co.uk/catalogue/products/4873097-single-phase-voltage-monitoring-and-protection-unit

Risteard81 · 07-03-2020 5:46pm

2011 wrote: »

@Risterad: sorry I am only reading this part of your post now, my bad. Looks interesting. Can you please start a new thread to discuss this as it does not really fit here?

Sure thing. Will do that a bit later.

Bruthal · 07-03-2020 8:10pm

stickman1019 wrote: »

Do you have a link to Single Phase product ?

I heard they did a 3Ph one.

Below video is worth a watch.
https://youtu.be/9GzEL3WqcrQ

I seen that exact scenario happen in a group of houses before. One I was called to had over 300v L-N in it.

If the houses all had similar loads on, voltages would be normal in them. Cooker on in one and bulb on in another, and bulb gets most of the 400v between the 2 houses phases.

Michelinextra. · 07-03-2020 8:20pm

Risteard81 wrote: »

In a UK single phase supply essentially you would have had no choice, as other options were wholly impracticable (and often impossible).

Now, in Amendment 1 to BS7671:2018 (only released on the 1st February 2020) there is an option for these new O-PEN devices on a single phase supply. Matt:e manufactures them, and as I stated they break the cpc connection as well as the phase and neutral. They are only coming onto the market now. I tried to source one with a Type B RCD but was told that it would be another few weeks/months before it's released and that pricing hasn't even been confirmed for it yet. Therefore I was left with the only option being to use the unamended BS7671:2018 (which can be done until I think July) and adopt a TT system for the EVSE installation. A simultaneous contact risk assessment needs to be carried out to ensure that exposed and extraneous-conductive-parts of the TN-C-S installation cannot be touched whilst charging the EV.

Can you expand on this a little , the O-PEN devices and the TT/TN-C-S

Maybe it's the drink again bit I'm not quite following it

Bruthal · 07-03-2020 8:43pm

Risteard81 wrote: »

https://www.cef.co.uk/catalogue/products/4873097-single-phase-voltage-monitoring-and-protection-unit

How does it see a difference in N to E potential if connected to the neutralized house?

stickman1019 · 07-03-2020 8:48pm

Michelinextra. wrote: »

Can you expand on this a little , the O-PEN devices and the TT/TN-C-S

Maybe it's the drink again bit I'm not quite following it

I will try to assist the best I can.

The attached and below video from about 5:00 onwards will give you a understanding of the dangers of an Open PEN.

https://www.youtube.com/watch?v=FowcjLoMDUI

The next video is for the un-ammended version but still fairly relavent goes through in fairly good details the obstacles around the compliance and the only real option (local earth rod) before the the Matt-E products come on the scene.

https://www.youtube.com/watch?v=q87H7aIujjA

Bruthal · 07-03-2020 8:51pm

2011 wrote: »

@Risterad: sorry I am only reading this part of your post now, my bad. Looks interesting. Can you please start a new thread to discuss this as it does not really fit here?

Fairly simple device anyway, sort of like a voltage operated ELCB.

Very easy to make up a voltage detector circuit with a few components and get them to operate the contactor.

I am curious how it detects any voltage in a neutralized system if it is not measuring between N and an independant earth point.

Maybe i am missing something obvious.

Edit: It detects a drop in L-N voltage probably, when the N into the house fails. (L-N voltage reduces to a level depending on earth rod impedance and house load, bigger load = lower voltage L-N = higher touch voltage on N and E)

I wonder how it handles neutral failure if the main neutral to a mini pillar fails, in which case the L-N voltage can increase or decrease in a single phase house, as now we have multi single phase loads with no neutral (floating star point) . The L-N voltage here can go above 230v. So if it went to 330v in a house, the neutral is displaced 100v.

So the o-pen unit would have to detect voltages higher or lower than the expected 230v +/- tolerance between L and N, if that's what it measures, which seems likely. A time delay to eliminate voltage dips needed perhaps.

Another DIY microcontroller ADC application, and a simple one at that.

Risteard81 · 27-03-2020 4:19pm

2011 wrote: »

Besides, this is the ROI not the UK.

YOU were the one who mentioned the UK, so it's disingenuous now to claim that it's not relevant to the thread. YOU made it relevant. And so my response is completely and absolutely valid and true. The FACT is that you wouldn't have had a choice in a UK installation, so you would have had to have accepted it whether you liked it or not.

"Quote:
Originally Posted by 2011
In the event of something going wrong none of this matters.
What is important is the high impedance of the earth fault path.
I would not accept this whether I was in the UK or the ROI."

Reply: "In a UK single phase supply essentially you would have had no choice, as other options were wholly impracticable (and often impossible).

"Now, in Amendment 1 to BS7671:2018 (only released on the 1st February 2020) there is an option for these new O-PEN devices on a single phase supply. Matt:e manufactures them, and as I stated they break the cpc connection as well as the phase and neutral. They are only coming onto the market now. I tried to source one with a Type B RCD but was told that it would be another few weeks/months before it's released and that pricing hasn't even been confirmed for it yet. Therefore I was left with the only option being to use the unamended BS7671:2018 (which can be done until I think July) and adopt a TT system for the EVSE installation. A simultaneous contact risk assessment needs to be carried out to ensure that exposed and extraneous-conductive-parts of the TN-C-S installation cannot be touched whilst charging the EV."

27-03-2020 4:54pm

Risteard81 wrote: »

YOU were the one who mentioned the UK, so it's disingenuous now to claim that it's not relevant to the thread. YOU made it relevant.

Ok

And so my response is completely and absolutely valid and true.

.... and what response would that be?

The FACT is that you wouldn't have had a choice in a UK installation, so you would have had to have accepted it whether you liked it or not.

Physics does not care which country we are in. Electricity will follow the paths of least resistance / impedance and will divide up accordingly. So if the net sum of the return paths is high impedance the protective device may not operate within the required time frame or at all.

Simply put:
Something like a metal casing on an enclosure becomes live due to a fault, a good earth path causes the fuse / MCB to operate almost instantly, injury / property damage / fatality etc averted. This is what matters not politics, geography, copy and pasted text.

Risteard81 · 27-03-2020 5:10pm

2011 wrote: »

Ok

.... and what response would that be?

Physics does not care which country we are in. Electricity will follow the paths of least resistance / impedance and will divide up accordingly. So if the net sum of the return paths is high impedance the protective device may not operate within the required time frame or at all.

Simply put:
Something like a metal casing on an enclosure becomes live due to a fault, a good earth path causes the fuse / MCB to operate almost instantly, injury / property damage / fatality etc averted. This is what matters not politics, geography, copy and pasted text.

I didn't suggest that phsyics is different.

But your claim that you wouldn't allow a TT installation for an EVSE installation in the UK is flawed given that it would have been your only option in the overwhelming majority of cases. BS7671 is not identical to ET101 or the soon to be implemented I.S. 10101 for that matter.

And there are valid reasons for some of the provisions in BS7671. Regardless of whether we consider them to be the right approach or not there is reasoning behind the repudiation of TN-C-S installations externally in circumstances such as this.

My preference would be for TN-S installations over TN-C-S - but unfortunately there are cost issues with that approach (and obviously an Earth could become faulty without it being evident due to the Neutral remaining intact, which doesn't happen with a PEN conductor). I like to see Earth electrodes with all system types, but that is not the UK approach. (It's not prohibited either.)

As for my response, it was written under the original message - the response which you claimed was irrelevant because we weren't discussing the UK (despite the fact that you brought up the UK and that was why I responded to it). It was also quoted under my reply a couple of posts above.

27-03-2020 5:25pm

Risteard81 wrote: »

I didn't suggest that phsyics is different.

Than Ohm's law? Because that is the only physics I am referring to.

But your claim that you wouldn't allow a TT installation for an EVSE installation in the UK is flawed given that it would have been your only option in the overwhelming majority of cases.

Which claim?

BS7671 is not identical to ET101 or the soon to be implemented I.S. 10101 for that matter.

No one suggested otherwise.

And there are valid reasons for some of the provisions in BS7671.

No one suggested otherwise.

Regardless of whether we consider them to be the right approach or not there is reasoning behind the repudiation of TN-C-S installations externally in circumstances such as this.

I kind of like them myself although a broken neutral can cause obvious issues.

My preference would be for TN-S installations over TN-C-S - but unfortunately there are cost issues with that approach (and obviously an Earth could become faulty without it being evident due to the Neutral remaining intact, which doesn't happen with a PEN conductor).

Yup.

I like to see Earth electrodes with all system types, but that is not the UK approach. (It's not prohibited either.)

So do I, great to see we are in agreement

As for my response, it was written under the original message - the response which you claimed was irrelevant because we weren't discussing the UK (despite the fact that you brought up the UK and that was why I responded to it). It was also quoted under my reply a couple of posts above.

Sure if you want me to say it is relevant then consider it done

Risteard81 · 27-03-2020 5:29pm

2011 wrote: »

What is important is the high impedance of the earth fault path.
I would not accept this whether I was in the UK or the ROI.

This is where you mentioned that you would not accept a TT system.

Unless you are suggesting that you would be prepared to accept a TT installation for an EVSE installation in a domestic premises with a sub-1 Ohm impedance. That's not a realistic or affordable solution in the overwhelming majority of cases.

27-03-2020 6:53pm

Risteard81 wrote: »

This is where you mentioned that you would not accept a TT system.

Thanks.

You see part of my job is to design, sign off on and inspect electrical installations. So in my case in the context of the OP’s initial post I personally would not design or sign off what the OP has described.

Why? For a verity of reasons such as the obvious challenges of achieving a sufficiently low EFLI value. For me it is best to bring a CPC from the MET to the shed. A local earth rod at the shed connected to a sub board in the shed wouldn’t hurt either. This is a domestic installation with a garden shed let’s not get carried away

Now you may want to take a different view and you may well be able to achieve this in a compliant manner, if so good luck to you. Each to their own.

Risteard81 · 27-03-2020 7:54pm

2011 wrote: »

Thanks.

You see part of my job is to design, sign off on and inspect electrical installations. So in my case in the context of the OP’s initial post I personally would not design or sign off what the OP has described.

Why? For a verity of reasons such as the obvious challenges of achieving a sufficiently low EFLI value. For me it is best to bring a CPC from the MET to the shed. A local earth rod at the shed connected to a sub board in the shed wouldn’t hurt either. This is a domestic installation with a garden shed let’s not get carried away

Now you may want to take a different view and you may well be able to achieve this in a compliant manner, if so good luck to you. Each to their own.

That's fine, but you claimed that you would insist on this also for a BS7671 installation. Yet you haven't explained how you would do this in a way which actually complies with BS7671. And that was my point.

BS7671 requires a completely different approach to ET101.

27-03-2020 8:27pm

Risteard81 wrote: »

That's fine, but you claimed that you would insist on this also for a BS7671 installation.

No I didn’t.

What I did was describe what I would do, what I would accept and I explained why.

I would be very interested to know what your views on this scenario:

Just like the OP described, a house and a garden shed. The shed is wired from the house with a phase and a neutral. There is no CPC between the house and the shed. The shed contains a boiler. The boiler develops a fault resulting in the enclosure becoming live. The enclosure is insulated reasonably well from earth because the only connection to earth is via a local earth rod. The earth rod makes a poor connection because the soil is very stoney and it has not rained in weeks. Other factors such as corrosion don’t help with the connection to earth. The result is the MCB doesn’t trip as the current flowing through it is well below the rated current. All that is left is the RCD which as you know only provides supplementary protection. The conductive enclosure remains live if the RCD fails to operate (not unheard of) just waiting for someone to touch it. All because of a high earth fault loop impedance.

Are you stating that you prefer the above to my alternative? What would you actually do to make the installation safer? Rather than just pointing to a standard or pasting something in can you actually describe what you would do as I have?

Risteard81 · 27-03-2020 8:35pm

2011 wrote: »

No I didn’t.

What I did was describe what I would do, what I would accept and I explained why.

I would be very interested to know what your views on this scenario:

Just like the OP described, a house and a garden shed. The shed is wired from the house with a phase and a neutral. There is no CPC between the house and the shed. The shed contains a boiler. The boiler develops a fault resulting in the enclosure becoming live. The enclosure is insulated reasonably well from earth because the only connection to earth is via a local earth rod. The earth rod makes a poor connection because the soil is very stoney and it has not rained in weeks. Other factors such as corrosion don’t help with the connection to earth. The result is the MCB doesn’t trip as the current flowing through it is well below the rated current. All that is left is the RCD which as you know only provides supplementary protection.

Are you stating that you prefer the above to my alternative? What would you actually do to make the installation safer? Rather than just pointing to a standard or pasting something in can you actually describe what you would do as I have?

BS7671 states that an impedance above 200 Ohms may not be stable for an electrode, as it can vary by a factor of about 8 depending on weather etc. (And to ensure disconnection with a 50V fault for a 30mA RCD would require an impedance not exceeding 1666.66 Ohms.) Typically you would like the impedance to be much lower than this of course.

In fact the last one I did I achieved around 8 Ohms. The one before that was around 40 Ohms. So yes - these are still relying on the RCD for fault protection, and of course RCDs are not perfect devices. So TT systems are not my preference (except where they can reliably be brought below around 1 Ohm in order to operate protective devices such as fuses and MCBs).

But for an EVSE installation there are real potential dangers having a large metallic object outside sat on top of four large rubber tyres if the PEN conductor were to fail whilst someone is in contact with true Earth potential outside and with the vehicle chassis.

Bruthal · 27-03-2020 9:34pm

Risteard81 wrote: »

But for an EVSE installation there are real potential dangers having a large metallic object outside sat on top of four large rubber tyres if the PEN conductor were to fail whilst someone is in contact with true Earth potential outside and with the vehicle chassis.

Similar for a metal shed down the garden. Tyres probably wont make a whole lot of difference, depending on conditions.

Risteard81 · 27-03-2020 9:36pm

Bruthal wrote: »

Similar for a metal shed down the garden. Tyres probably wont make a whole lot of difference, depending on conditions.

Tyres do make a difference - they completely isolate it from true Earth potential.

Bruthal · 27-03-2020 9:38pm

Risteard81 wrote: »

Tyres do make a difference - they completely isolate it from true Earth potential.

As I said, depends on the scenario.

Get a metal box. Earth rod into ground. Phase onto metal box. Metal box is now at 230v. So is earth rod, and a small area of soil around it.

Not very far from earth rod, the earth is at true earth potential, simple to visualize as the area rapidly becomes infinite very quickly. Pools of potential and all that.

Car likely to be on concrete, so the ground around it will within a very short distance, be at true earth even if the car was on its rims.

But, again, depends on the scenario as to the exact details.

The tyres do of course completely insulate the car from the ground, so might have some bearing in some setups, depending on impedance of items displacing neutral etc. Likely minimal though in a lot of cases, but tyres are a factor for sure.

27-03-2020 9:40pm

Risteard81 wrote: »

BS7671 states that an impedance above 200 Ohms may not be stable for an electrode, as it can vary by a factor of about 8 depending on weather etc.

Where do you think they selected the 200 Ohm figure from?

(And to ensure disconnection with a 50V fault for a 30mA RCD would require an impedance not exceeding 1666.66 Ohms.) Typically you would like the impedance to be much lower than this of course.

Yes, in fact that is my argument. If we take the 50V value you provide and use the 1666.66 Ohm value you also provide we can see that only just over 3 mA would flow. Although enough to trip a functioning RCD that trips with a 30 mA differential but nowhere near enough to trip what is most likely a 10A B Type MCB.

In fact the last one I did I achieved around 8 Ohms.

That is very impressive, but immediately I can tell that this far more than a single domestic earth rod. I remember having to get an earth grid down to 5 Ohms and it took a substantial number or earth electrodes, horizontal bare copper tape too. I remember the longer than normal earth rods being at least 150 Ohms each to earth.

What method did you use to test the resistance? We finished by using a grip on device (like an ammeter).

So yes - these are still relying on the RCD for fault protection, and of course RCDs are not perfect devices.

I'm glad to see you agree.

So TT systems are not my preference (except where they can reliably be brought below around 1 Ohm in order to operate protective devices such as fuses and MCBs).

Good luck with that!

But for an EVSE installation there are real potential dangers having a large metallic object outside sat on top of four large rubber tyres if the PEN conductor were to fail whilst someone is in contact with true Earth potential outside and with the vehicle chassis.

Nothing to do with this thread as the OP is discussing a boiler in a garden shed however this can be dealt with simply enough with under voltage and over voltage protection, its really not that complicated. What people fail to realise (but I am sure you know) is that the fault you describe lead to neutral displacement which can cause the voltage on the "neutral" to exceed 230V.

So I ask again:
Are you stating that you prefer the above to my alternative? What would you actually do to make the installation safer?

Bruthal · 27-03-2020 10:01pm

2011 wrote: »

Nothing to do with this thread as the OP is discussing a boiler in a garden shed however this can be dealt with simply enough with under voltage and over voltage protection, its really not that complicated. What people fail to realise (but I am sure you know) is that the fault you describe lead to neutral displacement which can cause the voltage on the "neutral" to exceed 230V.

I would imagine the N voltage compared to earth is a Max 230v with displacement. But you can have phase to neutral of well above 230v alright.

Under and over voltage protection is how O-PEN devices detect Neutral failures on the ones not using an independent true earth.

The Voltage Phase to Neutral reduces on Neutral failure coming to the house depending on the house loads, but can increase if it is a Neutral failure to a minipillar for example, depending how the loads are divided over the star connected houses with no neutral.

So if the Neutral is pulled up to 230v or close in a house with a shower running while the other houses have nothing on, the N in the other houses will be up to 400v from their respective phase.

And now the car in the driveway might be at or close to 230v via MET, and so would the steel garden shed etc.

There might be something to be said for under/over voltage protection to disconnect the whole house.

There would want to be a time delay so spikes dont trip the O-PEN device. Id assume the under and over voltage would want to be a decent bit outside the upper and lower tolerances of supply.

27-03-2020 10:17pm

Bruthal wrote: »

I would imagine the N voltage compared to earth is a Max 230v with displacement.

Yes, fair point.

But you can have phase to neutral of well above 230v alright.

Which could melt a few devices :eek:

Under and over voltage protection is how O-PEN devices detect Neutral failures on the ones not using an independent true earth.

That is the device I was thing of, couldn't think of the name.

The Voltage Phase to Neutral reduces on Neutral failure coming to the house depending on the house loads, but can increase if it is a Neutral failure to a minipillar for example, depending how the loads are divided over the star connected houses with no neutral.

I remember seeing a similar issue at work. Neutral broke on a supply to a distribution board, some loads suffered from over voltage, others from under voltage. Every time the load changed the "neutral" shifted and the voltages changed.

So if the Neutral is pulled up to 230v or close in a house with a shower running while the other houses have nothing on, the N in the other houses will be up to 400v from their respective phase.

And now the car in the driveway might be at or close to 230v via MET, and so would the steel garden shed etc.

That is an interesting way of putting it.

There would want to be a time delay so spikes dont trip the O-PEN device. Id assume the under and over voltage would want to be a decent bit outside the upper and lower tolerances of supply.

Fair point, I hadn't thought of that.

Bruthal · 27-03-2020 10:24pm

2011 wrote: »

I remember seeing a similar issue at work. Neutral broke on a supply to a distribution board, some loads suffered from over voltage, others from under voltage. Every time the load changed the "neutral" shifted and the voltages changed.

Yea as I had posted about before, I seen it on the parents road with one of their neighbors, 310v in the house. Most items in the house were ruined. Neutral failure supplying the minipillar neutral bar.

Bruthal · 28-03-2020 7:19am

2011 wrote: »

. That is an interesting way of putting it.

Yes the shower is more an example of the effect of a large load in one house with none in the others. An actual shower would stop in that scenario. But a cooker with a few rings on would stay on, in terms of maintaining a loading.

0lddog · 28-03-2020 5:26pm

Bruthal wrote: »

Yea as I had posted about before, I seen it on the parents road with one of their neighbors, 310v in the house. Most items in the house were ruined. Neutral failure supplying the minipillar neutral bar.

Lols !

This was explained to us over fifty years ago in a 1st yr eng lecture.

We all paid attention thanks to the preamble which was along the lines of

" this is how to blow up your neighbours TV "

Bruthal · 28-03-2020 5:41pm

0lddog wrote: »

Lols !

This was explained to us over fifty years ago in a 1st yr eng lecture.

We all paid attention thanks to the preamble which was along the lines of

" this is how to blow up your neighbours TV "

Yes, make sure you turn on everything on the cooker, but keep away from the bonded metal:)

Earth in a shed

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