Log in to the online community

Want to post a reply? You'll need to log in

Earthing and the radio amateur

18 Replies

  • New Question

Earthing and the radio amateur

Posted by Stewart Mason on Feb 7, 2019 9:20 pm

I've recently joined the IET forums. I replied to a  topic regarding amateur radio and PME on the old forum. Unfortunately I cant access the old forum anymore so I thought it would be best to start a new topic.

So. On the bench there is a transceiver with a metal case. Next to the transceiver there is a antenna tuning unit which is connected to a antenna system which is using an earth rod. Am I right in saying there is a chance of a potential difference between the two metal cases of the equipment regardless if it is a TT, TNS or TNCS?

What would be the ideal solution?  Put the shack on it's own TT supply and bond all the radio equipment back to the MET of the TT and if a earth rod is used for the antennas connect that back to the MET also?

If the TT system was not an option what would be best if the supply was a PME and you didn't want to use balanced antennas removing the use of an earth rod? 

To be honest I've never really given PME's and amateur radio much thought which in hindsight was probably a mistake. I do remember when I did my training an earth rod was recommended for the radios.

Apologies for being so random.

Stewart M0SDM
Stewart

Re: Earthing and the radio amateur

Posted by gkenyon on Feb 8, 2019 9:46 am

Yes, you are correct that regardless of whether the installation earthing arrangements are TN or TT, the earthed side of the antenna connected to its own earth electrode is classed as an extraneous-conductive-part, and should be main bonded.

If the shack is a separate outbuilding, and there are no simultaneously-accessible extraneous-conductive-parts or exposed-conductive-parts of both installations, you can provide a separate earth electrode for the mains and provide a separate TT system ... or use the antenna's earth electrode if it meets the requirements for protective earthing.

If it's an attached building, then it may also be possible to provide a separate TT system provided again there are no simultaneously-accessible extraneous-conductive-parts or exposed-conductive-parts of the two installations, but depending on what materials are used for the construction and foundations, there could be undesirable effects in some cases, where the main installation is a TN system, under some circumstances of distribution network faults.

It's recommended that the antenna electrode and the electrode of the TT system are kept at a suitable distance from buried metalwork connected to the MET or protective conductors of the main installation - the guidance comes from Figure 16 of BS 7430, and indicates a separation distance of between 3.5 and 8 m ... although at distances less than 8 m you may need to take into account ground potential rise and reintroducing touch voltages in some circumstances.
Graham Kenyon, Managing Director, G Kenyon Technology Ltd www.gkenyontech.com G Kenyon Technology Ltd Logo © G Kenyon Technology Ltd 2015

Re: Earthing and the radio amateur

Posted by davezawadi on Feb 8, 2019 11:29 am

Stuart
Whilst the basic information given by Graham is perfectly correct, in a real situation you do not have the possibility of a (significant) potential between the Tx and ATU because the TX output coax screen connects the two directly unless it is of a curious design where there is deliberate isolation of the two items cases, perhaps by RF transformer. Connecting the two items to the RF earth electrode does provide an additional current path to earth for mains currents, but unless you manage a particularly good RF earth system, cannot carry very much current whatever the mains is doing. All of the shack equipment and the earth electrode are inherently bonded unless you deliberately decide to isolate them in some way, the only real question is the size of this bonding conductor. Depending on the type of aerial system you are using, long wire or dipole for example, there will be differing RF earth currents, and from the RF point of view the conductor to the earth electrode should be as short and large as possible. Unless your earth rod resistance to the MET is very low (measure it carefully, there may be a potential present, if so use an AC ammeter to calculate the resistance), the possible current will be able to be carried by the normal house wiring without problems. As you do not want to pass RF into the mains wiring, you may consider a suitable RF choke in the earth to the MET from the shack.

If you have a transceiver with an external PSU it may be possible to isolate all the radio equipment from mains earth fairly easily, in which case nothing accessible connected to mains earth must be accessible when in contact with the radio equipment, as again there may be a potential between them, which under fault conditions may be dangerous, even if the fault is in the next street! Under these conditions it would be wise to make the whole shack a TT "island" with its own RCD and no MET connected earth conductors present at all, although this may be difficult if there is any pipework, radiators etc present which may be connected to the MET.

The discussion is not so simple is it? Mains earths are also noisy when connected to long wire type unbalanced antennas, and verticals without radials, because the earth resistance is connected in series with the radiation resistance and has mains borne RF currents induced in it (in the HF range, 160 - 10m) particularly if the antenna feed resistance is also low at that frequency. BS7671 is not the ideal reference for HF radio installations because the RF requirements are also important. However the full range of safety aspects must be considered, along with the range of possible interference sources to make a satisfactory installation.

Regards
David G8FNR

Re: Earthing and the radio amateur

Posted by Zoomup on Feb 8, 2019 8:50 pm

Purely from an electrician's point of view, a P.M.E. supply to a radio shack that has earthed equipment with conductive parts such as earthed cases that can be touched can present shock risks. The P.M.E. earth is connected directly to the neutral of the supply cable at the main intake meter position in the house. The neutral can attain a Voltage above true earth, as the neutral carries current and supply conductor cables' resistance cause a Voltage rise on the neutral. So, a P.M.E. earth terminal in the house can attain a Voltage above true earth, and this Voltage can appear on earthed equipment in the radio shack.

If you have conductive parts in the shack that are earthed via an earth rod at the shack, there can be a Voltage appearing between TT earthed equipment and P.M.E. earthed equipment. This is undesirable, and could be dangerous.

If the supply neutral was broken due to a fault before the house, the P.M.E. earthed equipment could reach a potential approaching full mains supply Voltage of 240 Volts in the shack. That coule be very dangerous.

TT earthing  is the safest method for a radio shack using a suitable earth rod and R.C.D.s. Normally an earth rod (electrode) would be at least 4 feet long, or two may be joined together to extend the length. Alternatively two rods may be used spaced at least the length of a rod apart.

It would be a good idea to get a qualified electrician to advise you and undertake the work.

If we bond metalwork in an outbuilding, such as a metal radiator or metal water pipe which are extraneous-conductive-parts, to comply with B.S. 7671, we would run a 10.0mm2 green and yellow cable back to the main earthing terminal in the house.

Generally it is best to keep TT earthed installations completely separate from P.M.E. earthed installations. So a fully TT earthed shack is ideal if it meets your radio requirements.

Test any R.C.D.s regularly using the test button to confirm their operation.

C.

 

Re: Earthing and the radio amateur

Posted by Stewart Mason on Feb 8, 2019 9:59 pm

Thanks for all the input. It is much appreciated. Personally I've always used dipoles and loops so no need for earth rods to be a part of my station! 

Clive I'm a bit ashamed to say I am a qualified electrician!! As I said in my opening post I have never given PME's and radio much thought. In 20 years I've never had anything to do with TT installations.

The main reason for all the questions is I have volunteered myself to give a talk at radio club on earthing in the shack. In my ignorance I thought it would be straight forward. Obviously it isn't straight forward as David pointed out. I can see on the night I am going to be met with one of my favorite lines  "It's been like that for years and never been a problem". I'll carry on with my questions on here and all being well when the time comes in April I'll be an expert.

One point that does need getting over is how dangerous a PME can be in fault conditions where a radio shack is in use.

A few thoughts on a shack that has got an antenna using an earth rod, in a property that is supplied by a PME
  1.  An earth bar is installed in the shack all equipment is bonded to the earth bar and a 10mm2 back to the MET along with a 10mm2 from the earth rod to the MET
  2. Bond the piece of equipment that is connected to the earth rod antenna system where it enters the shack with a 10mm2 back to the MET?
  3. Supplementary bond with a 4mm2 across all the gear and back to the main earth terminal.
  4. Rely on the coax braid to keep at the metal gear at the same potential
I don't know if it's me making this be more complicated then it is but it certainly appears to me that the more I think about earthing in a radio shack the more complicated it becomes.
Stewart

Re: Earthing and the radio amateur

Posted by gkenyon on Feb 9, 2019 9:38 am

Whilst these points might look great on paper, broadly follows BS EN 50310 practice we use for large electronic installations in buildings (e.g. computer rooms and data-centres etc.), and would certainly make things safe (electrically) in the shack, there are a couple of considerations:
  1. The PME broken-neutral danger is moved from the shack to accessible earthed parts of the antenna outdoors (similar to the outside tap requiring a plastic insert if fed with metal pipes connected to PME earthing terminal) - the lower the antenna earth rod resistance, the lower the touch voltage and the smaller the risk.
  2. No-one has mentioned surge protection at all yet ... but if the main installation has SPDs, there should be a separate SPD on the feed to the shack ... and perhaps  query whether a separate SPD scheme is provided in the shack itself.
    (Or alternatively, whenever thunderstorms are forecase, simply disconnect and separate the antenna connector from the equipment and earthed / bonded parts in the shack if you want to save cash.)
Graham Kenyon, Managing Director, G Kenyon Technology Ltd www.gkenyontech.com G Kenyon Technology Ltd Logo © G Kenyon Technology Ltd 2015

Re: Earthing and the radio amateur

Posted by Zoomup on Feb 9, 2019 6:06 pm

TT earthing for an outbuilding is quite easy. You just do NOT export the earthing system from the TN-C-S of the house supply. TT is good for outdoor supplies such as caravans or farms etc. The supply is just Line and Neutral to the outbuilding, with say an earthed armouring of a supply cable from the house. At the outbuilding the earth is insulated back and does NOT continue to exposed conductive parts or socket earth terminals. The outbuilding is then earthed via a local earth electrode (rod) A 30mA double pole R.C.D. is normally used to provide shock protection. The earth fault loop impedance Zs can be up to 1666 Ohms. (Table 41.5 B.S. 7671) But anywhere around a stable 200 Ohms is good.

TT earthing of the outbuilding is good as no metalwork can be raised to a dangerous Voltage by an external fault on the supply cable to the house.

Exporting a TN-C-S earth to an outbuilding can introduce potential shock risks and should be avoided if possible, especially if there is metalwork that is in contact with the ground, true earth.

C.

Re: Earthing and the radio amateur

Posted by Zoomup on Feb 9, 2019 6:24 pm

The Radio Society of Great Britain has a publication entitled "Earthing and the Radio Amateur".

http://rsgb.org/main/files/2012/11/UK-Earthing-Systems-And-RF-Earthing_Rev1.3a-.pdf

C.

Re: Earthing and the radio amateur

Posted by gkenyon on Feb 9, 2019 11:42 pm

Clive Brittain:
TT earthing for an outbuilding is quite easy. You just do NOT export the earthing system from the TN-C-S of the house supply. TT is good for outdoor supplies such as caravans or farms etc. The supply is just Line and Neutral to the outbuilding, with say an earthed armouring of a supply cable from the house. At the outbuilding the earth is insulated back and does NOT continue to exposed conductive parts or socket earth terminals. The outbuilding is then earthed via a local earth electrode (rod) A 30mA double pole R.C.D. is normally used to provide shock protection. The earth fault loop impedance Zs can be up to 1666 Ohms. (Table 41.5 B.S. 7671) But anywhere around a stable 200 Ohms is good.

Agreed

TT earthing of the outbuilding is good as no metalwork can be raised to a dangerous Voltage by an external fault on the supply cable to the house.

As long as the installation earth electrode is separated by a suitable distance from buried metalwork connected to the PME earth. See Figure 16 of BS 7430

Exporting a TN-C-S earth to an outbuilding can introduce potential shock risks and should be avoided if possible, especially if there is metalwork that is in contact with the ground, true earth.

C.

Agreed
Graham Kenyon, Managing Director, G Kenyon Technology Ltd www.gkenyontech.com G Kenyon Technology Ltd Logo © G Kenyon Technology Ltd 2015

Re: Earthing and the radio amateur

Posted by Mapj Test on Feb 10, 2019 6:45 am

Some pragmatism is also needed; there is the world of difference between a fixed shack, that is  to say a permanent shed or converted room in the house where many hours will be spent and maybe the station runs receiving software unattended, and a temporary hobbyist installation where someone has taken their radio along to use on holiday, or in a nursing home or another sub-optimum where TT conversion  is not so easy.  In such an expedient set-up antennas may be little more than bamboo and wire in the garden or even fishing poles.
If the whole installation can be made to plug in via  one 4 way block type extension lead, operating hours are a few per week, and the kit is all left unplugged when not in use, the risk in practice is very low.  Particularly if it is made clear that operation must not occur if there is any irregularity to the supply - dimming or surging of the lights for example often precedes PME problems.
Amateurs do all sorts of things that would not be acceptable to unskilled persons, using a coax connector with 100W of RF on it where it could be unplugged and the centre pin could be touched, tuning antennas with kV of RF at the tips within reach of the ground, climbing trees, using generators on field days  the hobby, like many (mountain climbing, go kart racing ... ) is not without some added risk, but equally, hopefully, the skills to manage that risk responsibly. I so have a slight beef that more of  this really ought to be in the exam syllabus .
Also there are common earthing tricks that are well outside the normal 'british standard' approach - fuses in series with antennas and or their earths may be an lightning reduction measure. you may find capacitors in series with the RF earth and or chokes in series with the mains earth.

In short there is no substitute for well informed analysis.
73 G7VZY
 
regards MAPJ2 that is MAPJ1 under his other alias

Re: Earthing and the radio amateur

Posted by AJJewsbury on Feb 10, 2019 12:42 pm

Clive Brittain:
The Radio Society of Great Britain has a publication entitled "Earthing and the Radio Amateur".

http://rsgb.org/main/files/2012/11/UK-Earthing-Systems-And-RF-Earthing_Rev1.3a-.pdf

C.

Some of that needs to be read rather carefully - e.g.
It used to be the case that all extraneous-conductive–parts in a house with  TN-C-S needed to be bonded, but with the recent changes to the IET Wiring Regs if you have a supply totally fed from RCD or RCBO protection devices, this requirement is no longer mandatory.
could be very misleading (it's not obviously within the context of supplementary bonding within bathrooms),

   - Andy.

Re: Earthing and the radio amateur

Posted by davezawadi on Feb 10, 2019 5:39 pm

I have to take significant difference with Clive Brittain on this one, simply because he does not appear to understand the difficulties with TT should there be other extraneous parts (such as central heating pipes or radiators, storage heaters etc.) in the shack. It is simple to TT an outbuilding but not part of an internal installation in a house. I think the worry about lost neutrals is excessive, there are many more serious significant dangers with high power radio equipment, and often much higher potentials available, as well as seriously dangerous RF potentials. The word shack does not mean shed, it is a technical term for the radio operating space! It is possible to TT the whole building installation, but this is not always wise where there are buried metal services which are extraneous to the building, or everything around the site is a TNC-S installation which must always be assumed. RCD protection in line with BS7671 2018, is suggested whatever the installation type. Unfortunately advice from average electricians about this kind of installation is very unlikely to be satisfactory, most struggle with normal TT installations in my experience. Items and the external earthing system may be separately bonded to an Earth marshaling terminal if desired in the shack, and thence to the MET. RF voltages on the house electrical system are very undesirable, and could be a fire risk, hence a series choke.
Even if one has beam antennas on a tower or mast the best solution is undoubtedly a good external earth system, and it is worthwhile from an operating point of view if any unbalanced antennas are used. As I mentioned low radiation resistance is important and makes the antenna pattern of directional aerials much more predictable, which can only be a good thing. TVI is also likely to be less likely. Burying a length of copper wire, pipe or tape the length of the lawn may be more rewarding than expected, and is likely to be much more effective than the normal earth electrode. It will also reduce the danger from lightning, by controlling the potential surrounding the antenna system. I have no difficulty bonding this to the MET, perhaps with a high current RF choke, in TNC-S installations, again serious mains voltages are much less likely than RF ones, which can cause both shocks and severe burns to the unwary. The household should be aware of this danger.

I have a good copy of Dud Charmans Aerial Circus DVD available which you may like to borrow as there is a lot of good antenna (and earthing for RF) material discussed. PM me if interested. The one on youtube is obviously ex 6 generations of VHS and useless! I made this many years ago for Dud, with the Bristol TV group, so as good as we could manage at the time, the youtube one is an unauthorised rip-off, I hold the copyright.

Regards
David CEng MIET G8FNR.

Re: Earthing and the radio amateur

Posted by Stewart Mason on Feb 10, 2019 6:50 pm

Thanks for all those that have replied. It is all very interesting. It's clear to see there are lots of different opinions on the subject. I have read the RSGB leaflet.

If you were going to right a list of do's and don'ts regarding earthing for the radio amateur what would it contain?
Stewart

Re: Earthing and the radio amateur

Posted by Zoomup on Feb 11, 2019 8:38 am

In reply to David Stone I have this to say. I was initially considering an external outbuilding as a radio shack. I have seen several garden sheds or old wash houses used as an amateur radio enthusiast's radio room. Ships' radio rooms may have confused me not being called shacks.  Now I understand that a radio shack can be an internal room in a house.

The items mentioned like radiators, storage radiators or metal pipes may or may not be extraneous-conductive-parts within a house. It depends on how well they are connected to earth. If plastic pipes are used a central heating radiator may not be connected  with earth at all, or very poorly. An electric storage heater will be earthed via its circuit protective conductor, but if the house is TT earthed then that is not a problem. So many new properties are supplied by plastic main pipes these days, such as gas and water so no main equipotential bonding within the house will be required. 

A whole house can easily be converted to TT earthing from PME earthing if required. That would just need the PME earth conductor disconnecting and a separate earth electrode installed in a suitable location and connected to the house main earthing terminal. RCD protection then is essential on all circuits.

With PME earthing that is why electricians are required to install 10mm2 main-equipotential-bonding
conductors to extraneous-conductive-parts in a house and other locations, to mitigate potential shock risks in case of a lost neutral on the incoming supply cable. And that does happen as has been reported on the IET electrician's forum.

I know little about amateur radio so am just speaking from the point of view of an electrician and BS 7671.

C.
 

Re: Earthing and the radio amateur

Posted by davezawadi on Feb 11, 2019 9:39 am

Thank you for your reply Clive.
As I attempted to make clear, this is not a simple problem, and needs considerable background on more than the basics in BS7671. At the present time we do appear to have a problem with our mains supplies, and the reliability of neutral integrity. It is wise to consider all domestic supplies to be TNC-S unless TT is definitely installed, and reasonably isolated from surrounding properties by distance. I find from teaching the 18th edition update courses that many electricians have quite a knowledge gap around TT as a concept, unless they normally work outside of towns. Whatever the situation I would strongly advise against trying to TT one space in an otherwise TNC-S property as the chance of bringing an appliance connected to the other earthing system into the wrong space is considerable because any fault on the TN system will produce a dangerous potential between the appliances on differing earthing systems.

Regards
David CEng MIET G8FNR

Re: Earthing and the radio amateur

Posted by Mapj Test on Feb 11, 2019 1:15 pm

I would also add that there is no need to strive for 100% safe in all unlikley fault  conditions,  to make the outdoor metalwork at least as  safe as  PME street furniture such as a bus stop or lamp post on a PME supply, is really probably  enough. After all, threre are plenty of situations with electric lights and appliences take in to the garden that also are a comparable risk.

Given the ham will have passed a technical exam, albeit one that is not putting mains wiring at it's core, they ought to be in a slghtly better position than the average punter to  evaluate the risks and make decisions about what is best, and for the beginner, there are more often seasoned hands at local radio clubs well placed to advise.
Given that a 'shack' may be set up in anything from a folding card table in the back bedroom of someone else's house or flat to a dedicated summer house style cabin with permanent bench and equipment  and a high mast all the end of the garden,  there is no simple one size fits all rule.

It is also possible to monitor the differnce voltage between mains CPC and terra-firma earth outside, and raise an alarm if some danger threshold is exceeded, perhaps 25V, as half of the permitted 50V , would  be a good early warning level. For non amateur use, I have used  low wattage 12V lamps N-E as a tell tale for wandering neutrals - if I come back and the filament is blown, then there was a fault while I was away.

 
regards MAPJ2 that is MAPJ1 under his other alias

Re: Earthing and the radio amateur

Posted by Stewart Mason on Feb 11, 2019 5:05 pm

David Stone:

At the present time we do appear to have a problem with our mains supplies, and the reliability of neutral integrity. It is wise to consider all domestic supplies to be TNC-S unless TT is definitely installed, and reasonably isolated from surrounding properties by distance.

The more I read about PME failure it makes me think we would be better off without it.
 

Stewart

Re: Earthing and the radio amateur

Posted by Zoomup on Feb 11, 2019 7:09 pm

Yes Stewart,
                         I was once called to a mid 70s neighbour's house where electrical items were not working correctly. I measured 240 Volts L to E and L to N. The supply was an underground TN-C-S. Eventually I twigged that when heavy loads were turned on the supply Voltage dropped considerably, and I suspected a bad N joint in the road. Jointers arrived and the joint was found. Indeed the fault was due to a corroded connection on an aluminium cable. I talked to the jointers and was told that this happened on a regular basis with old underground aluminium joints. The aluminium is compressed with a crimp joint but loses its strength and then allows the joint to become loose. Then I squared R heating takes place and the situation gets worse.

Z.

Re: Earthing and the radio amateur

Posted by Scott Dakin on Feb 13, 2019 12:42 am

I would happily export a TNCS earth to a remote shed as long as all metalwork was bonded or earthed as appropriate.

The radio equipment will perhaps run off DC via an internal AC/DC transformer to reduce the voltage and bridge rectifier to change the AC to DC so i would expect an earth fault path from the AC system is perhaps impossible until you attach your home made lighting rod radio aerial to the MET.

I would be tempted not to bond the aerial at all as that might introduce a shock risk that would otherwise not be there and could introduce interference from DNO network currents flowing through the aerial to the MET but would of course bow down to greater knowledge.

Share:

Log in

Want to post a reply? You'll need to log in