UK Earthing system

[pistonvalve]

Can somebody tell me what type of earth and where is it connected to ground.

Can I have an independent earth bonded to ground for my Amateur radio equipment? Will this earth need to be connected to the domestic earth, if so can it be connected via an RF choke to prevent feed back to domestic equipment.

Comments please.

 

[telectrix]

looks like pme to me. the earth connection is via the suppliers incoming cable, staked to earth multiple times along it's route.

 

[Julie.]

That's PME, which is TN-C-S ; the primary earth is in the substation, but the combined neutral/earth conductor will be additionally earthed throughout the run from the substation to you, at which point the earth and neutral are separated and are now different wires throughout your property.

If you have any transceiver equipment, this will be earthed via this earth (cpc) wire when plugged in to a socket outlet.

As to earthing for your antenna, this really depends on the type used, and the equipment you have, sometimes they are regarded as separate, but within your transceiver there is usually an interconnection designed specifically for this, if you provide further earth connections, you may very well be bypassing this, plus sections of your atu if there is one.

It really depends on your equipment.

Now, there is an issue with bringing in a separate "earth" for your antenna and it's the same for water and gas pipes, which is that your local "earth" is not the same as the "earth" from the substation. Say your neighbour drops his electric drill/lawnmower /whatever in a puddle of water, this water could go to 230V, but would fall to zero towards the substation earth, if you are halfway between, then the outside earth could be around 115V - which your antenna "earth" (or water/gas pipe) would bring into your house; if you were to touch both the antenna "earth" and the transceiver box (earth) then you could get a shock.

For this reason all extraneous conductive parts - such as metal pipes coming in from your local earth/ground, must be bonded by no less than 10mm copper cable to the main earthing terminal - which must be on the other end of the green/yellow wire going through the wall.

The same would be true if you brought an "earth" in for your antenna!

There is an exemption if you provide proper insulation between the antenna "earth" and touch, along with devices to prevent voltage surges, but these are specialised installs.

Given its PME, and this is becoming the most common arrangement, I would expect rsgb to be able to provide exact guidance

 

[Lucien Nunes]

To expand on a couple of Julie's points, the potential between a local RF earth and the TN-C-S MET earth could be a problem for two reasons. One is that under normal conditions there is likely to be a potential of a few volts with very low source impedance due to drop along the CNE. If your RF earth is also of low resistance, any interconnection could end up carrying significant current all the time, hence the requirement for bonding of substantial cross-section.

Secondly, if the interconnection is too flimsy or disconnected, in the event of a fault in your property that passes a heavy current to earth, the potential on your MET will rise relative to the RF earth and the latter could (by not being part of the building's equipotential zone) present a hazard.

Agree, RSGB will have some standard recommendations. Yes, you could isolate the MET at RF with a choke, so long as the choke has satisfactory CSA to function as a bonding conductor at 50Hz.

 

[pistonvalve]

Thank you all for your most helpful comments. It is a 2 wire system, L - N with the neutral bonded to earth at several points along the run from the substation. At the domestic property this 2 wire system opens up to a 3 wire L-N -E. Not realised that, you learn something every day.

How can I find out where my nearest neutral bond point is?

Like the idea of linking my proposed RF earth (1M earth rod into ground) with the domestic earth via a choke. I take it that this choke could be a toroid wound with 10 mm cable to satisfy regulations.

 

[Julie.]

Thank you all for your most helpful comments. It is a 2 wire system, L - N with the neutral bonded to earth at several points along the run from the substation. At the domestic property this 2 wire system opens up to a 3 wire L-N -E. Not realised that, you learn something every day.

How can I find out where my nearest neutral bond point is?

Like the idea of linking my proposed RF earth (1M earth rod into ground) with the domestic earth via a choke. I take it that this choke could be a toroid wound with 10 mm cable to satisfy regulations.

It may sound a bit picky, but the terminology is important here, the cable from the substation is 2-wire, but not line-neutral, the combined neutral earth wire is just that - CNE (combined, neutral, earth) not neutral so line-CNE or L-CNE, they separate out within your property - in your case the grey box thing below the meter.

The distributor earths the CNE wherever the guys installing it decide, so depending on the nature of the ground conditions, at every split off to each property, every second, third... whatever.

If you have metallic gas or water pipes entering your property, these should be bonded to the MET, so these would actually end up being the closest bond to the local earth.

I would look at the rsgb website, or contact them about any choke you add in, it may not work well depending on the antenna as well as potential issues with the interconnection.

They will have already established the best way of doing this, both from a safety and operational point of view.

 

[pc1966]

You have two aspect here. The first and most critical is the safety aspect and for that you want to avoid the situation where a fault leads to a shock or fire risk.

The reason for the heavy bonding of external pipes using typically 10mm^2 cable to the Main Earth Terminal (should be on the other side of the wall where the various meter tales go through, but might be the earth rail inside the fuse box / Consumer Unit) is to deal with the low risk of an open in the common neutral/earth in the PME supply leading to tens of amps flowing to that metal work as the local earth becomes the return path for the current.

Your radio earth ought to be considered the same (external metalwork) though it is doubtful a 1m rod will be below several tens of ohms so you are unlikely to see a fire risk even with quite thin wire, but there might be a lot of volts around the rod in such a case!

The second point is your reason for wanting a separate earth in the first place: for good radio performance. In that case you don't want to see it connected to the supply ground due to the noise from power supplies, dimmer switches, etc, in use from the mains supply. You may not have the option to isolate the radio from the supply earth in any safe manner (unless it uses a double-insulated transformer arrangement or is battery powered).

The best option here is probably to use a balun or similar RF transformer or common-mode choke so the radio sees the difference between the external antenna and true ground, and very little of any noise difference between your local supply earth and the outside world. Some of them can provide electrical isolation, others do not.

 

[marconi]

I suggest you contact John Peckham - very helpful fellow and knowledgeable on your question.

Google using this term 'can the supply earth be used for radio earthing' and look at the IET piece '
IET Forums - Ham radio grounding,pme system - The IET'.

As you scroll down you will find JP a fellow radio ham.

 

[pc1966]

There are some details not given by Pistonvalve such as frequency range, antenna type, and if it is also for transmission (so high RF power). But it occurred to me that many of the old valve radios with a "live chassis" (chassis connected to neutral so potentially live for any one of many faults) would couple the antenna and ground terminals using small high-voltage capacitors, and something similar might be useful here.

This low frequency isolation approach avoids the risk of high currents in an external ground rod needing heavy wire, or alternatively high touch potentials around the rod needing it to be shrouded, in the rare event of a fault. But as pointed out on the IET board discussion you need such capacitors to be safe if the worst-case happened and the local (in house) PME earth went high voltage, and also you don't want static from thunderstorms, etc, causing a build up of charge.

The attached diagram shows a possible way to do this.

Here the wires from the external metalwork (i.e. earth rod and antenna) should be considered as potentially at mains voltage w.r.t. the internal PME earth under fault conditions so probably wire with 1.5mm^2 conduit cable / earth wire (e.g Toolstation part number 67609) or similar that is adequately insulated.

Similarly the electronics must be in a box to prevent touching of the antenna/external earth conductors. Simplest is plastic, but if the radio will be grounded via the coax connector then a metal box will be fine if it is also grounded via that connector. The capacitors shown are 4.7nF class Y1 rated (e.g. RS 875-2421) and so safe even if there is full mains voltage across them. At 50Hz they have around 677k impedance so less than 1mA leakage, but at 100kHz that are down to around 339 ohms and probably much less then the antenna's impedance then.

The resistors are present to discharge any static build up. As for the capacitors they should be able to stand high voltages, hence using two 1M 1W resistors (e.g. RS 131-924) in series for the safety-critical route across the isolation barrier to have a rated peak overload voltage of 2kV (5 sec max). Resistance is not critical, but care in construction/placement should be observed to keep the two sides of the isolation apart by at least 8mm. Basically treat them as if mains voltage will be present.

Finally there is a balun (common mode choke) so reduce the noise from the PME side appearing on the antenna's signal. Shown in the schematic is a 1:1 version but google will bring up many designs (say search for "VK6YSF choking balun for HF bands"). If working at the low end of the HF band and an electrically short antenna then a 4:1 or even 9:1 balun might work better.

 

[pc1966]

That schematic did not turn out very good, here is a PDF version instead.

 

[pistonvalve]

Thank you all again. I plan to use a common mode choke on the incoming supply to the radio shack ( a 2,5mm spur directly from the CU), winding l-n-e onto a toroid (about 8 turns) I wondered if there would be sufficient isolation from the PME as I have no idea where the neutral/earth grounding point is! I have asked the network provider for this info and they have refused to tell me! - as I have no need to know! So to be sure can I (qualified electrician) arrange for the MET to be connected to ground (10mm conductor) via a 1m rod driven into 'Terra Ferma' adjacent to the incoming supply.

 

[static zap]

Have you (burried-some random metal in the garden)
made an attempt to make an earth for the antenna system.
This may give you an understanding of the conditions for
"electricains 1m rod"
Your antenna choices and atu use are also factors preventing RF sneaking back into the house/shack.
(Other Hams may have more hints if you have any particular bands /antennas in mind ie Ground mounted vertical. )

 

[davesparks]

So to be sure can I (qualified electrician) arrange for the MET to be connected to ground (10mm conductor) via a 1m rod driven into 'Terra Ferma' adjacent to the incoming supply.

You can connect a 1m rod to your MET via 10mm cable if you wish, but it will almost certainly be a pointless exercise.
Depending on soil type and ground conditions you would normally need multiple long rods to achieve a sufficiently low resistance to earth to hold the installation neutral and eaetb at or near true earth potential during a broken PEN fault.

 

[static zap]

( A little research reminded me why they were called , Ham Shacks , as far away from dirty PME , electrically noisy (QRM) )
If possible ! TTed and SPDs are topical. (Local electrician ?)

 

[pistonvalve]

As a matter of interest just where and how is the neutral bonded (or connected) to Terra Ferma by the electricity network company and to what spec.

 

[pc1966]

Usually N and E would be common back at the centre of a star-connected three phase transformer at the local substation, so I guess that would be the obvious place for "true ground".

I think that the PME type (common earth/neutral to each home) has multiple earths around the network to reduce the risk of an open N+E conductor causing a major voltage on the earth/metalwork of any homes. Any more knowledgable folk able to cover this?

Also in most systems each home would have water and gas pipes bonded to the local earth at the main earth terminal. They might not go far in to the ground as metal these days (so are not usable for a proper protective ground connection), but it does result in cases like a block of flats having each flat's earth bonded to the neighbours via the service pipes, even if there was an open on the supply (DNO) connection.

 

[davesparks]

As a matter of interest just where and how is the neutral bonded (or connected) to Terra Ferma by the electricity network company and to what spec.

In all earthing systems for small supplies the neutral is connected to earth via an earth electrode network at the substation, this will have a very low resistance to earth (usually less than an ohm as far as I know)
If it is a TT system there will be no further earth connection by the DNO, the customer installs their own earth rods to get their own earth connection.

If it is a TNS system the DNO will not make any further connections to earth but supply a seperate earth conductor to the property.

If it is TNCS the neutral (technical PEN or CNE) is further connected to earth by additional earth electrodes along the distributing main. These will be located at the far end of the distributing main and at key points along it's length. On overhead supplies these rods are usually easy to identify by either a PME label in the pole or just seeing the earth cable. On underground supplies they tend to be a little bit more random as earth rods are added with each joint.

 

[pc1966]

Thank you all again. I plan to use a common mode choke on the incoming supply to the radio shack ( a 2,5mm spur directly from the CU), winding l-n-e onto a toroid (about 8 turns) I wondered if there would be sufficient isolation from the PME as I have no idea where the neutral/earth grounding point is! I have asked the network provider for this info and they have refused to tell me! - as I have no need to know! So to be sure can I (qualified electrician) arrange for the MET to be connected to ground (10mm conductor) via a 1m rod driven into 'Terra Ferma' adjacent to the incoming supply.

Using a common mode choke on the supply makes a lot of sense. You will need a big toroid (one of the 60mm or more internal diameter) and apply a bit of sense to avoid damaging the twin & earth by excessive bending. Also you might want to de-rate it a bit due to the reduced ability for heat to escape (maybe just a 16A breaker, probably more than you should be using for the shack anyway). If you can't get enough turns on in any sane way, just use a 2nd toroid to increase the series impedance.

While there is no electrical reason to prevent you simply adding another earth via a rod to the MET please think and check before driving it in. Is there any chance of meeting a supply cable, water pipe, or gas pipe in that area?

Also when you talk of the "shack" is it just part of a normal room, is it a whole room, or is it an isolated area like a shed? The reason for asking is the suggestion by Static Zap that you could have a TT supply for it. In this case your power comes via a double-pole RCD and all of the earths in the shack are local, depending on your own earth rod. You only need < 200 ohms for the rod, as earth fault disconnection is met via the 30mA RCD trip and not the MCB over-current. But I suspect you would need all earths in the shack area to be part of the TT set-up (so lights and sockets) so you don't have the small risk of a PME open causing a big difference between you local earth and the supply "earth".

It is the sort of arrangement typically used for powering caravans. For example, search for "IET electrical-installations-in-caravancamping-parks-caravans-and-motor-caravans" (for some reason any links I post get borked so they don't work - part is replaced with dots)

 

[pc1966]

Note that even if you had a TT supply installed you would still need a common mode choke on the supply to reduce the injection of noise coming from the supply side via mains filters in your equipment. What the TT earth system break would offer is greater isolation at lower frequencies than you can easily get with a ferrite choke.

Also if the planned radio earth rod becomes part of the system (as it would in a TT arrangement) then you should wire it with at least the same CSA as the supply cable, so 2.5mm in your proposed arrangement.