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Old 17th Jul 2008, 8:38 pm   #1
ChristianFletcher
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Default PME and RF Galvanic Isolation

Sorry in advance as this question maybe off topic but is probably a problem for many of us trying to operate modern or vintage transmitters.

My new house is wired with a PME system and I want to install a long wire with an Auto ATU. The ATU requires a good ground and hence the problem with the PME installation feeding back fault currents back to via coax screen to the earth spike in the garden.

Would it be possible to use a 1:1 transformer wound on a torroid in the shack to give gavanic issolation between the external earth spike and household PME.

I have been told that this is not really a problem as modern rigs employing a seperate switch mode dont have the 0 volt output tied to earth so are floating. However once you start connecting computer and similar to the rig it likely a earth will be introduced.

The correct solution is to bond the earth spike in the garden back to the consumer unit using 10mm cable, but this wont be easy for my installation.

Any Ideas.
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Old 17th Jul 2008, 8:49 pm   #2
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Default Re: PME and RF Galvanic Isolation

There is a series of leaflets by the RSGB, one covers
PME supplies.

http://www.rsgb.org/emc/leaflets.php
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Old 17th Jul 2008, 9:40 pm   #3
Brian R Pateman
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Default Re: PME and RF Galvanic Isolation

For the cost of a few feet of 24 gauge and a toroid why not give it a try.

I'm in the process of sorting aerials and earths out myself and I found that a simple balun made a considerable difference.

Even tying the consumer unit to a good earth spike is not necessarily a solution to some of the noise impressed onto the mains system.

It is probably still worth putting in a really good low resistance earth just for the shack. This should give you a clean earth.

(I'm in the process of upgrading our clean instrument earths at work because of similar noise problems caused by modern variable speed inverter drives).

Regards,
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Old 17th Jul 2008, 10:11 pm   #4
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Default Re: PME and RF Galvanic Isolation

Thanks for the replies. Yes I have read the RSGB advice and to cut a long story short it says bond the earth stake back to the consumer unit with a 10mm cable. The more I look into this idea of two wire feeds to houses with earth bonded to neutral the more I think its a really bad idea. It looks very possible to get a nasty belt using none double insulated equipment away from the house, ie a metal bodied drill in the garden.
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Old 18th Jul 2008, 2:21 am   #5
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Default Re: PME and RF Galvanic Isolation

You could run the shack from an isolating transformer, then run a true earth for the shack....

It is what I do here.....

Cheers
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Old 18th Jul 2008, 9:55 am   #6
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Default Re: PME and RF Galvanic Isolation

Sean

Have you bonded the isolated neutral side of the tx to the shack earth spike. The only problem I can see with this is my shack is in a bedroom and I will still have the problem of touch potential between the shack RF ground and house PME like a radiators.

I am surprised that I cannot buy a galvanic isolator for the incoming coax line off the shell. I have been told that a UNUN could be used but these have a autotransformer configuration and wont offer any isolation. You can by a number off Isolators off the shelf but closer inspection shows that these are simple choke baluns with a dc through path.

Regards Chris
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Old 18th Jul 2008, 11:07 am   #7
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Default Re: PME and RF Galvanic Isolation

Erm, no, i have not - I am using the transformer to give me the facility to use SNE in the shack.

As you say you will have issues with potential differences.

Is the bedroom upstairs?

If so then a seperate earth is not a good idea, as the earth lead may become resonant at certain frequencies.....
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Old 18th Jul 2008, 12:00 pm   #8
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Default Re: PME and RF Galvanic Isolation

Yes upstairs. every thing is against me on this.

I wanted to install the ATU and ground spike at the bottom of the garden off a tree and run the antenna wire back to the house chimney. Wire length 120 feet

The other option is to mount the ATU on the house and run the antena wire back to the tree and then and use counterpoises. That will not give a problem with the PME. But the hot end of counterpoise will be adjacent to my neighbors tv antenna.

I also wanted to work multi band 1.8 to 30MHZ and wont have room to install resonant counterpoise for each band. Just wondered if a non earthed insulated wire in a loop around the garden would make a good counterpoise.

Chris
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Old 18th Jul 2008, 12:14 pm   #9
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Default Re: PME and RF Galvanic Isolation

Well you don't really have any options, except to run a length of Green/Yellow
wire from your PME supply earthing point to your equipment. Once you do this, you
will need to bond everything together. (Equipotential Bonding).

The isolation transformer method sounds attractive, but in the end, you will end
up spending more money buying a transformer than you would on buying some lengths
of 10mm and 6mm Green/Yellow wire.

You should also fit a RCD which cuts the power off to the whole area of your shack.
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Old 18th Jul 2008, 2:43 pm   #10
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Default Re: PME and RF Galvanic Isolation

Not sure installing an RCD offers any additional protection. The fault current is flowing in the earth wire circuit from the neutral earth bond point. I am only really concerned with the fault situation where neutral line to my house ruptures and current find its return through the shack earth rod and not the normal neutral return path.

Hence my original question about coax inline RF isolation transformer. If we can break the coax shielding from the earth stake outside the house we remove this potentially large fault current.

Thanks Chris
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Old 18th Jul 2008, 3:09 pm   #11
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Default Re: PME and RF Galvanic Isolation

If the fault situation is what bothers you then what about this:

1) Put an RCD on the supply to your gear for safety, so that you can...
2) Put a fuse in the mains earth with a relay in parallel with it arranged to cut the mains supply (e.g. by tripping the RCD) if energised.
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Old 18th Jul 2008, 4:10 pm   #12
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Default Re: PME and RF Galvanic Isolation

Christian, you are correct in having reservations about being totally "PME". However, all UK supply companies are governed by the "ESA" - Electricity Supply Act" and associated PME Regulations. I can only speak regarding Scottish Regulations( For a number of years prior to retirement I was a Hydro-Board(SSE) Installation Inspector). Up here, the IEE (now IET) 16th Edition Regulations are statutary under the terms of the "Buildings Scotland Act",and we must also comply with the PME Regulations. All that has been advised regarding earthing and bonding is correct. Certainly, if a radio shack or workshop is outside the "equipotential zone", it's mains supply MUST be fully bonded & earthed back to the supplier's earth terminal via the consumer unit's earth connector. Sheds, garages etc MUST be supplied using split-concentric cabling or 3 core SWA(steel wired armoured) cable(the steel armouring must not be relied on as the main earth conductor - use the spare "brown" conductor - suitablty sleeved in gr/yell sleeving). Min x-section of all conductors = 10mm.sq. Also, the "outbuilding" MUST be protected by an RCD(ELCB). It boils down to the fact that all fault current devices - RCBO's, MCB's etc have to break connection in the fastest time possible - to avoid damage to persons & installations. The supply network's PSCC(prospective short circuit current) can be thousands of amps (for a few micro-seconds).
But, and its a big "but" - I have strong reservations about using the supply company's PME earth as an "earth" connection for a ham or similar private domestic radio installation. I've seen several wierd and strange fault conditions - particularly in rural locations. Although folk's domestic supply is known to be 230/240V single phase, 50 Hz - the supply company's network is usually 11 & 33KV, in "star or delta" configuration. And in some rural locations - are straightforeward single spurs with farms & domestic premises sharing a 4 wire(415V) or 3wire(split phase)(480V) supplies from the same transformer.Weather it was PME or CEW earthing - I've measured some alarming voltages at the mains"cut-out" isolating fuse, after lightning damage to pole insulators & transformers.
I would suggest that you consult with other radio enthusiasts in your near locality and see what "spiking" or "over-voltage" conditions have occured. Possible solutions could be :- An isolation transformer, a battery powered static inverter, or insert a current shunt( a virtual short circuit) between a PME earth conductor & a "real" earth conductor just as they connect to your Tx/Rx - then monitor any transient currents. If you make the connection switchable - then you can also monitor transient voltages. This is where a good old AVO 8, &/or a decent oscilloscope would come in handy.
Here endeth the lesson.
Regards, David
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Old 18th Jul 2008, 5:09 pm   #13
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Default Re: PME and RF Galvanic Isolation

Thanks David

For a very complete response. I think I will try a counterpoise first with a number of long and ungrounded radials. I only use QRP power and if the counterpoise works so far so good. If it does not work I will be installing a 16mm cable from the remote ATU located at the end of the garden back to the consumer unit.

Many thanks and best regards Chris.
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Old 18th Jul 2008, 11:41 pm   #14
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Arrow Re: PME and RF Galvanic Isolation

Quote:
Originally Posted by Sean Williams View Post
You could run the shack from an isolating transformer, then run a true earth for the shack....

It is what I do here.....

Cheers
Sean
And I do similar.

All the radio gear is mains-powered via an isolation transformer that has a big Belling-Lee RF filter on its output. All the metalwork and the E connections of all the radio kit are wired directly to a dedicated earth stake, using thick copper wire. The case of the mains RF filter is similarly run direct to the same earth stake.
This arrangement keeps the RF earth seperate from the (noisy) utility mains supply.

The rest of the gear & equipment in the shack / workshop is powered from conventional mains. A little thought in designing the initial equipment layout and its subsequent usage ensures that the two earths never come into contact with each other; general utility earth and seperate, clean RF earth.

Al / Skywave.
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Old 19th Jul 2008, 7:15 am   #15
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Default Re: PME and RF Galvanic Isolation

Fortunately I don't have PME here.....

The usual reason given (by the RSGB and elsewhere) for bonding the two earths together is that if the incoming neutral breaks for any reason your earth return carries the neutral current of everybody else in the street. Probably unlikely in urban areas with buried cables but obviously possible with overhead lines. Some have suggested putting an rf choke in the bonding wire to isolate them at rf. My feeling is that a suitable fuse in your added earth wire (and not bother with the bonding wire) would offer the same sort of protection. After all the sorts of earth most of can achieve with earth stakes is unlikely to carry the whole current of your estate, so maybe this danger has been over done.

But it is good to have someone on here with real experience of what happens 'in the field'.

Dave
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Old 19th Jul 2008, 11:43 am   #16
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Default Re: PME and RF Galvanic Isolation

Just a thought the fact that the shack is upstairs there is a strong risk of tvi with a long earth lead according to rsgb handbook.I am in same postion and worry about about future use of a linear i have
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Old 19th Jul 2008, 6:02 pm   #17
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Default Re: PME and RF Galvanic Isolation

Without providing an earth you will always be limited in what antenna installation
you would perfer to operate. In the end, once the earth wiring has been installed
the hard work has been done.

One concern about the isolation transformer method is that you will still need
to provide an earth, else you will have a 240volt supply with no means providing
protection around the shack. Remember the whole idea of an earth is to
provide a method of isolating you from electric shock if a wire became disconnected
and touches the metal case of a item of equipment.
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Old 20th Jul 2008, 11:54 am   #18
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Default Re: PME and RF Galvanic Isolation

Hello again, I must point out to John Robson - that the provision of an "Earth" in a domestic electrical circuit is not necessaraly to prevent a person getting an electrical shock - it is to ensure that the overload current protection devices operate as quickly as possible. The only dedicated device to protect a person is an RCD. These are rated at 30 milli-seconds. ie 1& a half cycles of a 50 hertz supply. When the IEE & BS experts originally consulted cardioligsts - they replied that this was the max peak to peak mains voltage waveform that one's heart muscles could take before incuring stopage or fillibration.
Returning to Christians's original PME thread(apologies to moderator), - Not many folk ken this - these days most if not all PME supplied premises are fed off their local supply networks by a "straight concentric" u/g cable of 16 or 25mm cross section. ie just two conductors - live(solid aluminium these days) & neutral(mulistranded insulated copper). No "Earth" as such.Back at the urban sub-station, or rural transformer the "Neutral" & all metalwork is bonded to terra-firma via a large loop or radial of usually 25 - 50mm sq. copper conductor burried in the sub-soil.This "earth" could be several hundred meters away from your house.When you think about it - in a domestic premises these days - nothing is at earth potential naturally. All water supply company's pipework is usually alkathene or poly-propelene.Radiators & copper pipework are screwed to stud-framing through plasterboard etc. Hence the requirement to "bond" everything. But - you are just bonding & "earthing" back to a supply company's straight-concentric neutral conductor!
True "earth" - as required for terrestial EM transmission & reception, as explained by others contributing to this forum thread, requires different parameters than those required by electricity companies,IEE Regs, and Government regulations.All of which as designed to protect human life & the buildings we live & work in.And quite rightly so.
Here endeth the Sunday lesson. My apologies for "banging on" in my two submissions.

Regards, David
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Old 20th Jul 2008, 2:39 pm   #19
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Default Re: PME and RF Galvanic Isolation

Quote:
Originally Posted by David Simpson View Post
...Returning to Christians's original PME thread(apologies to moderator), - Not many folk ken this - these days most if not all PME supplied premises are fed off their local supply networks by a "straight concentric" u/g cable of 16 or 25mm cross section. ie just two conductors - live(solid aluminium these days) & neutral(mulistranded insulated copper). No "Earth" as such.Back at the urban sub-station, or rural transformer the "Neutral" & all metalwork is bonded to terra-firma via a large loop or radial of usually 25 - 50mm sq. copper conductor burried in the sub-soil.This "earth" could be several hundred meters away from your house.
Out here in overhead rural distribution-land, the house-supply is two wires from the 11kV / 230V pole-mounted transformer (where the main earth-point is tied to the neutral) to the house feeder-pole, where it is led overhead to the Lucy-box by either a cable of the type you describe, or a 'D'-section twin cable. The neutral conductor at each pole is tied to earth via a down-lead with a nut-and-bolt connection above tampering height (maintaining the PME). Because of this, our house 'earth' is about ten metres away.

This poses an interesting dilemma for antennae (I, too, was contemplating a long-wire from the 'shack' upstairs), as it would put any aerial cable at-or-near the same height as the earth, being as it is fed to the premises overhead! I have a small field and access to large trees above and below chez Barnes, so I'm tempted to go for a dipole with balanced feeders and an ATU (when I get around to it...) - as soon as I find a route where I don't have to cross the 230V overhead rural distribution!

- And here's my question... What form does the earth take on the OH distribution poles as opposed to the earth on the main transformer? A plate in the ground? A rod? How deep? How big?

Just curious...
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Old 20th Jul 2008, 4:09 pm   #20
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Default Re: PME and RF Galvanic Isolation

Russel, I was just generalising when mentioning house supplies. Obviously, out in country locations(where I did a 6 year stint as a linesman), various combinations of conductors & earthing are used.Too many to boor folk with,& I'd probably get in trouble with the moderator for transgressing too far from the forum subject.
When HV transformer poles are errected,various measurements are taken with a bridge megger from test earth rods back to the t/f metalwork & N conductor - to determine the size & placement of the PME or CEW earth conductor in the ground.In the old days - a few gash lengths of 3142 copper conducter were connected together with a linetap(nut & bolt connector) & dumped in the hole prior to back-fill. However, these days care is taken to place a loop of maybe 35mm bare conductor in a circular zone of a couple of meters or so around the t/f pole, or one or more radials. These would be approx between 1 & 2 m down in the ground. Should ground resistance values require it(as indicated by the initial measurements as mentioned), - suplementry earth conductors of 16 or 25mm are connected to the LV pole's Neutrals & steel cross-arms which run up to the house. Again, these would be approx 1 or 2 m down. Should an Electrical Inspector or authorised NICEIC contractor observe high resistance values when carrying out line-loop impedance tests,PSCC tests etc, then this might indicate poor PME or CEW earthing from the supplier. 11 &33KV fault voltage zones(of just a few hundred volts) can exist for up to a mile, and 240/415/480 V faults can supply zones for several meters. ie, I've seen someone boil a kettle with 100V in half an hour - even when one of the two 11KV conductors to his t/f was completely broken & lying on the ground - half a mile away ! Also, I've seen cows "dancing"(& lucky to be alive) when subjected to a ground voltage zone.
If its a dark stormy night & the lightning is going zing zing zing in your rural location, dissconnect all aerial, earthing & supply connections to your Tx/Rx & associated equipment for safety. As you would with your computer anyway. Litigation with your supply company could prove costly & difficult to prove, should you experience damage or injury from ground zone voltages.
Here endeth today's 2nd lesson. Prahaps,to please the moderator, if anyone wants more detailed advice on PME & associated circuitry - send me a PM, & I'll try & help. I'm nae wiser than anyone else, just fortunate in having been trained in Electronics(Radio Comms & Test Equipment), and HV & LV Electricity Supplies.

Regards, David

Last edited by Brian R Pateman; 20th Jul 2008 at 9:57 pm. Reason: Potentially offensive word removed.
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