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Clean, filtered & isolated 230v mains Power Supply for those on PME (Rev. 1)

I am new to amateur radio, at the time of writing this I have only had my foundation ticket for a month, however since I am a ‘techy’, I tend to do loads of reading and research into things in order to arm myself with suitable knowledge before giving things a go.

With that said I quickly became aware of how important power supplies and particularly grounding can be to amateur radio. This actually has some crossover from audio production/computer issues I have experienced in the past due to mains interference and ‘dirty’ earths.

Full disclosure, I am neither a qualified electrician or electrical engineer, however I do have a firm grasp on basic electrical concepts as well as access to people who are highly experienced electrical engineers. What we all also have in common is access to the internet, aka the biggest and most rich source of information humanity has managed to create for us all to benefit from.

From the get go, I’ll say this:

Electricity, particularly in a domestic supply feeding home appliances is simple. Very very simple. What it is however, is very dangerous and therefore demands respect in the form of understanding and good safety practice.

What I am trying to say here is that its not some dark art or black magic that will instantly end anyone that is not an electrician or engineer. But we must remember it is dangerous and therefore you shouldn’t play with it if you don’t understand at least the basics and are not confident in what you are doing.

That’s the safety disclaimer over. Now lets get down to business…

What’s a PME Supply?

In my own words: A TN-C-S (or PME) supply brings two conductors to the incomer on your property. A ‘PEN’ conductor and the Live. The PEN conductor is a combined Earth + Neutral. There is no earthing system or ground rods etc located on your property. Instead, this PEN conductor is bonded to earth electrodes in MULTIPLE places upstream of your incomer. In simple terms, the network operator or supplier provides the path to earth for you, via the PEN conductor.

At the point where the supply enters your home, you will see something that looks like my supply on the right. Note how there is an earth bonding point on the PEN conductor where the normal 3rd wire safety earth is split out to give your standard 3 wire connection. This is connected to the house’s main earth block, usually right next or near to the incomer, to which the consumer unit and all metalwork in the house is bonded.

My TN-C-S ‘PME’ Incoming supply

The important thing to understand here is that the neutral in the form of the PEN conductor is bonded to earth several times upstream. There is no earthing electrode on your property.

For more detail on PME, I suggest you do some googling. Or look here for example.

PME & Supply noise?

PME works, as an electrical supply, its very common and also cost effective for the network operators to install. Simply put, from a power perspective, it is NOT a problem, it provides power, in a safe way to peoples homes.

However when using sensitive electronics including radio equipment, you may well find yourself the victim of a dirty / noisy supply, and particularly earth. By now it should be obvious to you why a PME system is more susceptible to this than other systems, but for everyone else’s sake I’ll say it. You are all sharing a path to earth via the PEN conductor. That business park/factory over the road running big ol’ noisy motors? That neighbour across the street with a house full of cheap phone chargers and TV’s etc? Yes… All of these ‘pollute’ the shared supply/earth and may result in noise/pickup through your equipment.

Why can’t I simply install my own earth & some filters?

Filters such as off the shelf mains EMI filters and Ferrite cores/clamps? Sure go for it, you can put those on all your power cables running your equipment and they will help. However I also see a lot of radio op’s that also like to provide their own clean and dedicated ‘RF Earth’ via a copper ground rod driven into the soil in the garden.

Providing your own earth for radio use is where the complications of having a PME electrical supply start. In a nutshell you CANNOT safely provide your own dedicated clean earth if your radio is directly powered by your house PME supply. In fact, if your radio room/shack has any metal surfaces that are bonded to the house ground, which is PME coupled, you also face a potential ‘cross bonding’ danger if your radio equipment is connected to your own ground separate from the house PME earth.

This danger is present due to how electrical fault current flows to earth. If the PEN conductor on your PME supply goes into a ‘fault condition’ (broken/damaged cables for example) before the first neutral>ground bonding point, this means there is no low resistance path to earth on the supply side.

If you happen to have installed a nice shiny earth rod or 2, and this is connected to the grounding lug on the back of your radio/braid of your coax, then current will now seek your path to earth, which happens to be via your house’s internal wiring, your radio etc and then to your ground rod.

Now we are talking enough current to melt wires and cause fires, let alone death if you happen to be hanging on to equipment in the path. Lets also not forget the risk if you’re holding onto the radiator for example at the same time as your radio…. You become the path for all that fault current. And that doesn’t end well.

You can see the risks here, and this is probably why this isn’t a popular subject.

So… What are the options for a clean supply & earth when on PME?

Option A, and this is the official guidance from the RSGB, is that if you must provide your own grounding system, either with one or more rods / buried contact plates etc, then that rod or system of electrodes MUST be bonded to the PEN conductor earth terminal at your incomer, with heavy gauge copper conductor, Think 10-16mm2. What this does is keep any fault current away from your internal wiring and you, should the upstream PME supply go into fault. It will now flow via this bonding connection, directly to your earthing system and not through you and your radio.

However…. This defeats the object of having a dedicated and clean earth. Since its now bonded to the PEN conductor, probably with an even better electrical conductor than your consumer unit is. So you satisfy the safety concern, but you don’t achieve what you probably wanted to acheive. This is also for the majority, difficult or out of the question due to the location of our radio shacks vs the PEN bonding point. Running heavy gauge earth cable may not be possible.

Option B, is where we finally come to my solution to this problem and is based on transformer isolation to break any existing neutral>earth bonds, derive a new entity of L+N, then bond N with our own earth ourselves. Thus replicating what a standard supply looks like, but with total galvanic isolation from the house PME mains. Any ‘thru’ earth must be lifted from the transformer as well.

See this article on Galvanic Isolation.

Now that you’re all probably bored to death reading this, lets get down to business and I’ll show you what I have built, using simple off the shelf products to achieve what I’d call just about the cleanest mains fed power supply you’re going to get. Specially built to overcome the limitations imposed by PME supplies.

LETS DO IT: Building the supply

You don’t need very much to do this, but its not supposed to be cheap and cheerful… If you’re going to this effort then you’d think spending some money wouldn’t be an issue right?

Parts List:

  • 230v Isolation transformer, I used a Carroll & Meynell unit rated for upto 3kW.
  • Length of quality flex, I used arctic type, rated for 16 amps.
  • A selection of single pole terminals, I used the clamp Wago type. 3 ways is enough. You should also have some spade connectors/crimps on hand.
  • An couple of enclosures / IP Rated boxes in which to house the Neutral>Earth bonding connection and incoming filter. You’ll also want cable compression glands to fit the knockouts.
  • Large Mix31 type Ferrite clamp & 230v EMI filter.
  • Heavy gauge earth cable and earth terminal block for connection between this supply and your own grounding electrode/system. I started with 10mm2 earth conductor at first, but then changed to 6mm2 solar PV cable with MC4 plugs so I could easily move the set up between two locations, each with their own terminal for connection to earth.
  • double RCD protected socket which will be your connection point for all things on your radio bench/in your shack.
  • Of course you must also have your own grounding rod/electrode. It is suggested you use a copper rod, driven 8ft into the ground. You can use multiples of and bond them together, but they should be 8ft at least. 4ft rods do not provide the same low impedance path to ground, even if you use more than one. Ideally a pair of 8ft 5/8 copper rods driven all the way in and bonded with at least 10mm2 cable, buried a few inches down. Hell if you can drive 12ft rods in, even better. I use these.

**Make sure you have a reliable multi meter, and a socket tester as well**

STEP 1: Build the primary side filter

Feed some 3 core flex with a standard UK plug on one end into one of the IP65 enclosures and either solder or crimp the L + N onto spades and connect to the input side of the EMI filter. The earth is a ‘thru’ connection but the chassis of the EMI filter must be grounded, so as I have done in my picture, using a Wago terminal, take the earth in, take 1 to the EMI filter chassis, and leave one to be the ‘thru’ connection.

Next you need to take a length of your 3 core flex and strip off the outer sheath, don’t skimp on this, you need to allow for several turns through the Large Mix31 Ferrite clamp. I went with 4 turns but you can do more. Be sure to leave enough length on both ends for the connections. Take the L + N from the output of the EMI filter, and the Earth from the spare way you left on the earth terminal connection earlier.

The internals of our primary side mains filter.

I used 3M double sided mounting table to hold the filter and ferrite in place, but I’ll leave that to your intuition. Also note how I have used cable/zip ties to hold the clamp shut. This is to ensure the split core always has good contact to form a complete core. Not sure I would trust the little plastic latches it relies on normally.

Finally you need to cut the plug off the transformer input lead and connect it using terminals blocks like the Wago’s to the L+N+E coming out the bottom of the ferrite clamp. Pro tip here, even though we are using compression glands on the knockouts, I secure a couple of cable ties round the sheath of the cables on the inside of the box so they cannot be pulled out.

Our completed mains filter, in line of the transformer primary side.

If all has gone well, you should now be left with a shiny new mains filter inline on the transformer primary side input connection as follows. Excuse the tape on my transformer, it was knocked about in transit, but as was still functional, I kept it for some money back and gorilla tape fixed the rest.

Mains filter attached to the transformer primary side input lead.

STEP 2: Recreate standard supply from transformer secondary side

At this point all we have is a standard isolation transformer with a filtered input into its primary. What we need to do now is take the newly derived L+N from the outlet and bond the Neutral with our own Earth, as is done several times on a standard supply coming into anyone home.

**Its important to realise here that the Live & Neutral coming off the transformer secondary are brand new entity’s. They do not exist anywhere else, they are not coupled, or related to the Live & Neutral feeding the transformer primary side. They are isolated from the primary side connections and have been induced by magnetism in the transformer. Therefore any N>E bonds that exist on the primary side of the transformer are not reachable from the secondary and hence do not exist. This is what an isolation transformer serves to do, remove the reference to earth so that there is no L>E potential. Typically used in building sites where there is risk of contacting a single live cable. Only having potential between L>N means that the operator, even when in contact with earth does not become a path for current to flow. They would have to form a path between L+N meaning touching separate L + N cables with different body parts, this is much more unlikely than simply bridging live to earth with your body.**

As I mentioned earlier, most isolation transformers such as this one, do carry a ‘thru earth’ for safety reasons as a this is still used as a chassis ground on metal equipment etc. This is simply a direct electrical connection between the transformer input (primary) and output (secondary). 

Its is VITAL that this thru earth be removed.

Lifting this thru earth is easy, the easiest way is to not connect the 3rd wire earth in the plug that we use to connect to the secondary outlet on the transformer. Just cut it off where you have removed the outer sheath on both ends. I would suggest labelling this all clearly so there can be no mix ups in future. Alternatively you can open the transformer and cut the thru earth inside.

The internals of the neutral>earth bond. Thus replicating the reference to earth on a standard supply.

Take the length of flex with the plug on one end (intended to be connected to the transformer output) that is now only carrying L+N and bring it into an enclosure. Straight join the live conductor to the outgoing live. Do the same for the neutral, connect it to the outgoing neutral. Note how my incoming yellow flex at the top has no earth. This is just the L+N from the transformer.

Now take your heavy gauge earth connection from your own earth rod/system and bring that into an earthing terminal in the enclosure.

Connect the earth from the outgoing flex into this earth terminal. At this point we now have a functioning supply with a safety earth. However there is still no reference to ground and the supply can be considered ‘floating’.

To create the Neutral > Earth bond, simply connect a spare way on the neutral terminal and connect it to the earth terminal. I used some spare 2.5mm2 wire to do this but 1.5mm2 will do. Congratulations we have now replicated essentially what any standard mains supply looks like, however it is a totally new entity of L+N with your own clean earth, not shared with anyone else, not connected to the grid.

Safety first: You could now simply just connect some outlets or directly wire this into your DC PSU etc… However a modern ‘normal’ supply uses RCD’s for safety, we will do the same.

You have the option here of using our outgoing connection from the NE bonding box to feed a small ‘garage’ style consumer unit with a couple of RCD’s and breakers in it if you wanted to. You should remember though that your transformer is rated for 3kW MAX and is also being fed by a 13A socket in your home. The fuses in the plug on the primary side, as well as the secondary side should provide adequate over current protection. You could just have well had fed the transformer from a fused spur or via a dedicated MCB. But then you’re approaching the realms of a certifiable permanent install rather than what could be considered an ‘appliance’.

I stuck with the easy option and fitted a double socket with built in RCD as shown here:

This is the completed power supply. Note the RCD protected sockets and the PV rated earth cable connection on MC4 connectors.

STEP 3: Load test & check with meter/testers

As with any project, good testing is vital, especially when we are dealing with 230v like this. This is a simple check of the potential and earthing conditions basically and is done with a simple multimeter and socket tester. What we are aiming for is exactly normal conditions that match the results you get from a known working outlet on your house mains.

Eseentially we need to see mains voltage between Live>Neutral and Live>Earth. We should see no potential between Neutral & Earth. Our socket tester should read correct with no earth or swapped wiring faults. I use a Fluke 325 meter and Tacklife EST03 socket tester. The socket tester also has an RCD test button that we can use for ensuring our RCD trips as expected.

Here you can see the complete system along with test results proving that it replicates the behaviour of a standard mains power supply as expected.

That concludes this article and I hope it provides an option to those that are seeking the ultimate in clean power/earthing systems, yet still want to run from the mains rather than doing something similar with batteries. I have said it before and I’ll say it again… SAFETY FIRST! This supply would be ideal for a dedicated radio shack such as a garden shed where there is no PME mains present as well. However should you choose to use it elsewhere, be aware that should you cross bond between your own ground and a PME connected device, you have potentially created a return path to earth for dangerous levels of fault current should the PME go into fault.

This system doesn’t automatically provide said dangerous return path under normal running conditions as it is not mechanically connected to your PME mains at all so is safe to use and be left connected all the time. Just be aware of the risks if you choose to use this in the vicinity of PME powered items and other bonded things such as metal fixtures, radiators etc…

Thanks all!

Tom – M7CTO

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