Figuring out which earthing system in use, and how to use it

[nuked]

Hi all,

Disclaimer: I'm not a qualified electrician .

I'm trying to figure out what sort of earthing system our house has, looking to replace a very
dated voltage operated RCD and consumer unit with something more recent/sensible (BCO
notified and all that). What's confusing me slightly is that there are two earth leads emerging
from the wall in the house, along with the live and neutral conductors. One of these earths
was/is connected to the "auxiliary earth electrode" of the voltage-RCD and nothing else, the
other goes straight into the CU, whose earth is in turn connected to the "earth continuity
conductor" of the voltage-RCD. With everything disconnected, I get a resistance of around
1.2k-ohms between them, no apparent potential difference between them or neutral, and the
expected between all three and live. The voltage-operated RCD is a Chilton 60A type D,
labelled as "voltage operated earth leakage circuit breaker BS 842".

So, does anyone have any ideas as to what sort of earthing system might be in use here?
TT? and what these earths might be connected to outside the house -- unfortunately the
wiring out here is old and mostly covered in render/paint so it's hard to tell what's going on!
Since we have water and gas supplied in plastic pipes I'm assuming it's not these.

In replacing this dated stuff with something more modern (split consumer unit with dual-pole
RCDs feeding MCBs, plus a couple of dual-pole RCBOs), which of these earth leads should
be hooked up to the earth block in the unit?

I did try calling UK-power who are the DNO for the area, but they happily stated that they
don't keep records on this and that I should ask an electrician..

Your years of wisdom appreciated!

-- nuked

 

[nikmet]

Do you have any experience of electrical installation work and how are you planning to test and certificate?

 

[malcolmsanford]

As Nik says are you an Electrician to start with. If it is a ELCB-V it would point to being a TT system. The normal connection would be from your earth rod via the ELCB-V to the CU or an external MET. The ELCB-V could be well over 25 yrs old and a lot of things could have happened to that earth rod in the intervening years, and with you getting a 1200ohm (???) reading on an earth then there is a problem.

I would have a good look round to see if you can find a rod, and see if the earth conductor is still attached. If you can't find one, then for safety I would fit a new rod and test the Ra. Once you have stabilised the earthing arrangements, I would contact the local DNO and ask what the possibility of getting a PME fitted.

Once the earthing as beed sorted, then you can think about removing the ELCB-V and bringing the installation up to date.

 

[Knobhead]

For gods sake get that voltage operated ELCB out as soon as possible! At least you’ve got plastic gas and water pipes, the problem comes if you have another parallel earth path.
For your own and your families sake get it sorted quick.

 

[Octopus]

I'd recommend you get a sparky or 2 round to give you estimates.

Where are you?

 

[telectrix]

as murdoch says. sure there will be a forum member close to you to pop round and have a look see if you post your location.

 

[nuked]

Hi all,

Many thanks for the detailed replies! The earth loop/PSC tester that I had ordered arrived, hooking that up gives 47 ohms for one of the earths and 419 for the other, and 43 ohms for them combined (which makes sense I assume going on DC parallel resistance maths). I had a brief stare at the wiring outside, but it's not at all clear what goes where unfortunately. I'm in the Boughton/Faversham area of Kent. With regard to the work, I'm taking the plunge and doing it myself -- the BCO have appointed a suitable company to inspect and test the installation once complete, though I'm to provide test results as the work progresses. The first bit, getting rid of the ELCB is now done, and existing circuits are wired into the new CU all through 30mA RCDs/RCBOs. They all trip right when the test buttons are pressed, which gives me at least a little confidence in its safety! That said, the existing circuits are mostly very old (I would guess 60s/70s/80s, with non-copper twisted cores insulated in plastic in places, unearthed wires in others); replacing these bit by bit is the next stage.

My experience with electrical installations is pretty limited, mostly to installing new lights/sockets and that sort of (non-notifiable) stuff. I do have a reasonable amount of electronics experience, and (I hope!) a reasonable understanding of electrics in general -- I build microcontroller/microprocessor based control boards for fun and partly work, DC/low-voltage/embedded/robotics applications. Also have a reasonable understanding of RF electronics, licensed radio amateur, etc. However my knowledge/understanding of how domestic earthing is arranged was (until recently) pretty non-existent, having just previously dealt with the in-house side of things, not what's on the other side of the meter. But I've not any more clue about what's on the remote ends of those earth leads ). At the moment, just the 47 ohm earth is connected -- would it make more sense (and be safe) to hook up the other in parallel to drop the overall impedance?

Thanks!

-- nuked

 

[IQ Electrical]

It's been pointed out already that if there is an electrode on this installation then there's no guarantee of it's suitability or condition.

It's one thing adding a few sockets/lights to your house but when it comes to the means of earthing, especially if it's a TT system, you need to swallow your pride and call a competent electrician.

 

[malcolmsanford]

. I had a brief stare at the wiring outside, but it's not at all clear what goes where unfortunately


I fully appreciate that you have involved the LBO and they have advised you that they will be monitering the work through an approved contractor, which in esscence means you will need to carry out your work to the BS 7671-2008, but I'm sure you will already understand this.

As you know a TT system is basically an earth that is provided by the consumer and not the electricity supplier as a TN system is, so I would now be advising you to contact your local network provider, which in Kent could be EDF, and asking them for the possibility of connecting you to a TNC-S system.

I've highlighted above your post and have to again advise you, that if you wish to continue to use a TT arrangement you are going to have to confirm the use of an electrode and in a Domestic TT it is normally via a rod as Regulation 542.2.1 outlines in the BS 7671-2208. As your services are plastic, which is amazing for a property that as been around since the 60's, I can only assume that you must be connected to a rod, or the very least some Earthy metalwork, but that approved contractor will want to see and test your electrode and so therefore you need to establish one.

By all means I have no problem with a competent person carrying out their own work on their house, and I would imagine you have a time table of what stages this approved contractor will come in and inspect your work, but as IQ stated some areas of the install are more important than others, and means of earthing IMO is the most important, I would advise you to perhaps for this very important part you do get a qualified electrician in, perhaps the contractor that is going to oversee your installation, to help you with it.

 

[UKMeterman]

Hi all,

. Also have a reasonable understanding of RF electronics, licensed radio amateur, etc.



-- nuked

As a radio amature, you have a need for a technical earth for your antenna set up particularly if are going to use HF/MF and will have lots of bits of wire going to outside your house!. In your case it make make sense not to go for PME but to stay with TT You will want to bond your HF earth to your TT earth rod, but only do this via a piece of 6mm or 10mm with a number of ring chokes fitted. This is so they are connected at 50Hz, but your RF is kept out of the local mains and you don't send 5 and 9s to the local switch mode power supplies. The RSGB do have notes explaining this. The US military have a good book on how to do grounding shielding and bonding http://www.wbdg.org/ccb/FEDMIL/hdbk419a_vol2.pdf

In your case I would suggest calling the DNO tomorrow and ask them to fit an isolator, then you can fit in a suitable enclosure an 80/100A 30mA rcd of the same make as your intended consumer unit. NEDL charged me £50 to supply and fit such an isolator a few years ago. This will improve the safety of the installation whilst you get the rest done. Do a quick check to ensure that you can't draw more than the 80/100A.

I would also suggest that you may wish to do a little reading such as http://www.******************/Products/TLGUIDE.html or Guide to the Wiring Regulations 2008: IEE Wiring Regulations BS 7671:2008: Amazon.co.uk: Darrell Locke: Books

Regards

 

[Engineer54]

As a radio amature, you have a need for a technical earth for your antenna set up particularly if are going to use HF/MF and will have lots of bits of wire going to outside your house!. In your case it make make sense not to go for PME but to stay with TT You will want to bond your HF earth to your TT earth rod, but only do this via a piece of 6mm or 10mm with a number of ring chokes fitted. This is so they are connected at 50Hz, but your RF is kept out of the local mains and you don't send 5 and 9s to the local switch mode power supplies. The RSGB do have notes explaining this. The US military have a good book on how to do grounding shielding and bonding http://www.wbdg.org/ccb/FEDMIL/hdbk419a_vol2.pdf
Regards

Are you being serious here Meterman?? You would sacrifice having a TN earthing system in favour for TT system because it's better for ham radio gear???

I think someone needs to get their priorities right, and that means house electrical installation saftey comes FIRST, and that means the PME earth, if available.

If the OP wants a suitable earth for his ham radio equipment ...nothing stopping him driving a rod electrode in, to provide that suitable ground path...

 

[UKMeterman]

Hi,
You have to consider what happens under fault conditons, the op could have kit conected to his techical earth, which is then connected possibly via 0.75mm2 green and yellow in the plug top to the safety earth. In the event of a TNCS break of the PEN conductor which does happen, a large current could flow via this 0.75mm2 wire with the fire hazards associated with this. So which either earthing system it is important that the technical earth is bonded to the safety earth. Such a bond would be constructed out 10mm or larger depending on the earth system in use.

Have a read of http://www.rsgb.org/emc/docs/pdf/leaflets/emc-leaflet-07.pdf

 

[Engineer54]

Hi,
You have to consider what happens under fault conditons, the op could have kit conected to his techical earth, which is then connected possibly via 0.75mm2 green and yellow in the plug top to the safety earth. In the event of a TNCS break of the PEN conductor which does happen, a large current could flow via this 0.75mm2 wire with the fire hazards associated with this. So which either earthing system it is important that the technical earth is bonded to the safety earth. Such a bond would be constructed out 10mm or larger depending on the earth system in use.

Have a read of http://www.rsgb.org/emc/docs/pdf/leaflets/emc-leaflet-07.pdf

Not the broken PEN thing again, and how often it happens!!!


Well give me PME system any day of the week over a TT system. Especially in UK these days, where your lucky if you get anything over a single 3/8'' 1.2 m rod.

 

[malcolmsanford]

I have to agree with Engineer here I would not be wanting to sacrifice safety that a TNC-S gives you over a TT system.

It's not the argument for me that TNC-S is better or a TT is not safe, what swings this kind of thing for me is 40 yr experience of knowing that 5+ plus after I have installed a perfectly sound TT system, there will have been no checks, maintenance or testing on it. I have lost count the amount of times I have found an unconnected piece of 4-6mm Earth wire that once went to a rod, that was installed, but in later years was dug up by the home homer who had no idea what the rod done, or the house had been sold and a new owner wanted to change the garden around. I once saw a rod literally stuck in the ground like a stake, with perhaps just inches in the soil.

This post itself is testimony to what happens to TT systems over the years. Can't find a rod, don't know where cables go, or are connected to etc etc. With a DNO earthing system they are bound by law to maintain that earth, it is there easy to see and test to confirm that it is still operational, and if not a call will have them out ASAP to sort out a problem.

If the OP is at all worried about converting to a TNC-S if it was available, that it could loose a neutral then fit SPD in the case of a rise in potential, for me the safety of my family over a few hundred/thousand pounds of equipment is more important.

 

[Knobhead]

I’ve often wondered why these haven’t been made compulsory. Makes it obvious to the house holder what’s there.
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