PME problem. Please Advise.

[coys2006]

Hi All,

I'm after a some advice please on a job I recently carried out for a own peace of mind.

I was asked to fit a new dual rcd consumer unit to a property with a PME incoming supply. Before I changed the board I carried out an earth loop impedance test and measured 0.1 ohms which I was happy with. I fitted the board but when I tried to do an rcd test my megger meter didnt like it. I then placed a short between earth and neutral to simulate a fault and the rcd still didn''t trip. At this point I thought the rcd's may be faulty and replaced them. They still didn't trip. I also noticed that I could feel a small current across a socket and a light switch as I turned them on. This set the alarm bells ringing as there seemed to be no earth. I proved this by installing an earth stake and connecting it directly to the consumer unit. I then tested the RCD's and they functioned correctly.

My main questions are:

Why would the customer not have noticed this small current before I changed the board?

How can my tester give me an original Ze of 0.1 ohms with no earth? is it because the earth and neutral are the same potential?

With the leakage current I could feel at the switch and socket, should it not have tripped the Rcd when I installed the earth stake?. They were no longer present after this.

Lastly, after I spoke to somebody from EDF who were not the most helpful, I installed a new earth link between the head and cu. Can you tell me if this is entirely necessay.


If anyone can help me understand this I would really appreciate it as this is the first time I have encountered these problems with a PME system.

Thanks

Coys2006

 

[riveralt]

You say that there is PME coming into the house but you don't say whether or not there is a connection between the PME and the main earthing terminal or perhaps the PME direct to the consumer unit. (This is the responsibility of the supplier) If there is a main earthing terminal is this connected to the consumer unit? (this is the responsibility of the householder).

The supplier should have told you what the earthing connection was and if they were prepared to install an earthing system or not. If they say yes then the customer would have to pay for it to be fitted. If not then the custome would have to have a TT system fitted.

Not sure of the legality of connecting your own what I assume is either a TNS or TN-C-S connection.

What was the Ze reading before you changed the board?

 

[RF Lighting]

I was asked to fit a new dual rcd consumer unit to a property with a PME incoming supply.

You chose to install a dual RCD I presume?

Before I changed the board I carried out an earth loop impedance test and measured 0.1 ohms which I was happy with.

Where did you do this test?

Was it a Ze in isolation?

I fitted the board but when I tried to do an rcd test my megger meter didnt like it.

What meter did you use?

What did it say?

Did you try and retest Ze?

I then placed a short between earth and neutral to simulate a fault and the rcd still didn''t trip

Really? Why would you do this?

Didn't you try pressing the test button?

At this point I thought the rcd's may be faulty and replaced them

Really? What both of them?

They still didn't trip

No surprise there

I also noticed that I could feel a small current across a socket and a light switch as I turned them on. This set the alarm bells ringing

Thats reassuring

as there seemed to be no earth

Sounds like it

I proved this by installing an earth stake and connecting it directly to the consumer unit

Not really

I then tested the RCD's and they functioned correctly

Wouldn't a Ze test have been more apropriate?

Why would the customer not have noticed this small current before I changed the board?

Was it there before you changed the board?

How can my tester give me an original Ze of 0.1 ohms with no earth?

It depends. Maybe there was an earth. Maybe you were testing between the wrong things. Maybe your tester is faulty. who knows.

is it because the earth and neutral are the same potential?

No. If there's no earth then it's not at the same potential as anything

With the leakage current I could feel at the switch and socket, should it not have tripped the Rcd when I installed the earth stake?.

No. That's induced voltage, not leakage current.

Do you understand why RCDs are installed and how they actually work?

They were no longer present after this

No they wouldn't be.

Lastly, after I spoke to somebody from EDF who were not the most helpful, I installed a new earth link between the head and cu.

What was wrong with the existing one?

Should you be interfering with DNO property?

Can you tell me if this is entirely necessay

Probably not, and even if it is, it is not for you to do.

Did you confirm the prescence of or install main protective bonding to this property?

How old was the cutout?

How do you know it was a PME supply?

 

[coys2006]

Hi,

The head is in box outside with the consumer unit on the other side of the wall. There was a 10mm cable from the neutral bar in the head to the cimnsumer unit. I thought this may have been broken so I placed my own link using 16mm cable and it made no difference. It now has an earth rod installed with a 16mm earth direct into the consumer unit. A 10mm cable from the board to an earth block and a 16mm from the earth block to the neutral bar in the head. (This link is what I was unsure of).

The original reading of Ze was 0.1 ohms.

 

[electronicsuk]

If the property really does have a PME service, I find it hard to believe that the issues you describe could be caused by a fault on the supplier's side. After all, N and E are linked at the supply head, and we know the installation is working, so it's fair to assume that there is indeed a neutral present there. That doesn't leave a lot of places for a faulty earth connection to hide other than somewhere between the MET and consumer unit.

I assume that the test buttons on the RCDs worked? After all, the majority of RCDs do not use a functional earth, so the test button should work regardless of the presence of a good, low-impedance earth. However, your tester has rightly alerted you to a potential fault.

Are you really, really sure the supply is TN-C-S?

 

[RF Lighting]

Hi,

The head is in box outside with the consumer unit on the other side of the wall. There was a 10mm cable from the neutral bar in the head to the cimnsumer unit. I thought this may have been broken so I placed my own link using 16mm cable and it made no difference. It now has an earth rod installed with a 16mm earth direct into the consumer unit. A 10mm cable from the board to an earth block and a 16mm from the earth block to the neutral bar in the head. (This link is what I was unsure of).

The original reading of Ze was 0.1 ohms.

Thats a really quite dangerous setup at the moment.

Test Ze in isolation. If it's a high or no reading ring the DNO emergency line and report it.

 

[riveralt]

Hi,

The head is in box outside with the consumer unit on the other side of the wall. There was a 10mm cable from the neutral bar in the head to the cimnsumer unit. I thought this may have been broken so I placed my own link using 16mm cable and it made no difference. It now has an earth rod installed with a 16mm earth direct into the consumer unit. A 10mm cable from the board to an earth block and a 16mm from the earth block to the neutral bar in the head. (This link is what I was unsure of).

The original reading of Ze was 0.1 ohms.

It sounds as if you have set up your own TN-C-S and TT earthing systems.

I would speak to the technical advisor of the competent persons scheme to whom you belong.

After all you don't want to complete the EIC and report to the LABC if its wrong.

 

[coys2006]

How do you know it was a PME supply?

There was an earth cable in the neutral bar at the head.

 

[electronicsuk]

I'm starting to wonder if this is a wiring issue with one of your circuits. When you run an RCD test, how are you doing this? Are you testing from a 13A BS1363 socket via an adapter lead to fit your meter, or via the test probes directly at the board? If it's the former, lack of an earth connection to the final circuit you're testing from would explain things. However, one would hope that you'd have thought to take a Zs reading from the final circuit at the same time. Maybe I'm just clutching at straws.

 

[coys2006]

If the property really does have a PME service, I find it hard to believe that the issues you describe could be caused by a fault on the supplier's side. After all, N and E are linked at the supply head, and we know the installation is working, so it's fair to assume that there is indeed a neutral present there. That doesn't leave a lot of places for a faulty earth connection to hide other than somewhere between the MET and consumer unit.

There was a 10mm cable from neutral bar to consumer unit. Test buttons did work.

I assume that the test buttons on the RCDs worked? After all, the majority of RCDs do not use a functional earth, so the test button should work regardless of the presence of a good, low-impedance earth. However, your tester has rightly alerted you to a potential fault.

Are you really, really sure the supply is TN-C-S?

 

[coys2006]

I'm starting to wonder if this is a wiring issue with one of your circuits. When you run an RCD test, how are you doing this? Are you testing from a 13A BS1363 socket via an adapter lead to fit your meter, or via the test probes directly at the board? If it's the former, lack of an earth connection to the final circuit you're testing from would explain things. However, one would hope that you'd have thought to take a Zs reading from the final circuit at the same time. Maybe I'm just clutching at straws.

Hi,

I still had the same issues with every circuit disconnected from the board

 

[electronicsuk]

If the property really does have a PME service, I find it hard to believe that the issues you describe could be caused by a fault on the supplier's side. After all, N and E are linked at the supply head, and we know the installation is working, so it's fair to assume that there is indeed a neutral present there. That doesn't leave a lot of places for a faulty earth connection to hide other than somewhere between the MET and consumer unit.

There was a 10mm cable from neutral bar to consumer unit. Test buttons did work.

I assume that the test buttons on the RCDs worked? After all, the majority of RCDs do not use a functional earth, so the test button should work regardless of the presence of a good, low-impedance earth. However, your tester has rightly alerted you to a potential fault.

Are you really, really sure the supply is TN-C-S?

You could make this a lot easier by putting your replies outside of the quote tags, or if you can't be bothered/don't understand how to do that, at least put your replies in bold.

 

[electronicsuk]

I'm starting to wonder if this is a wiring issue with one of your circuits. When you run an RCD test, how are you doing this? Are you testing from a 13A BS1363 socket via an adapter lead to fit your meter, or via the test probes directly at the board? If it's the former, lack of an earth connection to the final circuit you're testing from would explain things. However, one would hope that you'd have thought to take a Zs reading from the final circuit at the same time. Maybe I'm just clutching at straws.

Hi,

I still had the same issues with every circuit disconnected from the board

I assume that you were performing RCD tests directly at the consumer unit, then?

 

[coys2006]

If the property really does have a PME service, I find it hard to believe that the issues you describe could be caused by a fault on the supplier's side. After all, N and E are linked at the supply head, and we know the installation is working, so it's fair to assume that there is indeed a neutral present there. That doesn't leave a lot of places for a faulty earth connection to hide other than somewhere between the MET and consumer unit.

There was a 10mm cable from neutral bar to consumer unit. Test buttons did work.

I assume that the test buttons on the RCDs worked? After all, the majority of RCDs do not use a functional earth, so the test button should work regardless of the presence of a good, low-impedance earth. However, your tester has rightly alerted you to a potential fault.

Are you really, really sure the supply is TN-C-S?

You could make this a lot easier by putting your replies outside of the quote tags, or if you can't be bothered/don't understand how to do that, at least put your replies in bold.

apologies not posted that much before.

 

[coys2006]

I'm starting to wonder if this is a wiring issue with one of your circuits. When you run an RCD test, how are you doing this? Are you testing from a 13A BS1363 socket via an adapter lead to fit your meter, or via the test probes directly at the board? If it's the former, lack of an earth connection to the final circuit you're testing from would explain things. However, one would hope that you'd have thought to take a Zs reading from the final circuit at the same time. Maybe I'm just clutching at straws.

Hi,

I still had the same issues with every circuit disconnected from the board

I assume that you were performing RCD tests directly at the consumer unit, then?

I installeda temporary socket