RF quiet 12 V 2 A power supply


David Cutter
 

Chris

I've been puzzling over the electrical regs for tying antenna earth systems to mains earth and I see that the above psu has mains earth tied to the negative output. 

I haven't checked all my supplies but I know the one for the main transmitter is not. 

It might be a fine point but for some electrical installations an outdoor antenna "earth" should not be tied to earth/ground since this will also join it to mains earth.  I am on PME which may change in the not too distant future when we are up-dated.  

I would be obliged of your expert opinion on the subject. 

73

David G3UNA


David Wilkins
 


Hi - just my quick thoughts -  the antenna feed line / connector from your tower is likely an extraneous conductive part and as such should likely be bonded to the electrical installation’s Main Earth Terminal as per various Regulations in Chapter 54 of BS7671. 

It might be a fine point but for some electrical installations an outdoor antenna "earth" should not be tied to earth/ground since this will also join it to mains earth. 

 As far as I know, there is no regulatory prohibition to adding an earth rod to a PME / TNCS electrical installation MET, it just needs to be designed for possible fault currents. 

Hope that helps,
David.


On 8 Oct 2021, at 17:07, David Cutter via groups.io <d.cutter@...> wrote:



Chris

I've been puzzling over the electrical regs for tying antenna earth systems to mains earth and I see that the above psu has mains earth tied to the negative output. 

I haven't checked all my supplies but I know the one for the main transmitter is not. 

It might be a fine point but for some electrical installations an outdoor antenna "earth" should not be tied to earth/ground since this will also join it to mains earth.  I am on PME which may change in the not too distant future when we are up-dated.  

I would be obliged of your expert opinion on the subject. 

73

David G3UNA


Chris Moulding
 

Hello David,

I originally trained as an electrical engineer with the local Electricity Board so studying the IEE Regs was one of the areas of study. I also had the fine points of PME (protective multiple earthing) explained to me by the engineers working on installation and fault finding the PME networks.

I will explain the situation in the UK, other countries may do this differently so please get local advice. I'm going to use British terminology so earth is ground and earth rods are ground rods in the US.

Bear in mind that it's many years since I was a trainee and I specialised in radio engineering on VHF radios and microwave links after I was qualified.

To save on running a separate earth conductor the neutral conductor in the mains distribution cables is used as earth. It's tied to the substation earth and also with earth rods at each house fed by the distribution cable hence protective MULTIPLE earthing.

As usual this works well in normal conditions but you have to consider what happens when an appliance has a fault where the live conductor touches earthed metal work. A high fault current will flow from the live conductor into the earth conductor of the appliance back to the main neutral conductor of the distribution system. If the appliance is fused correctly a fuse or circuit breaker will eventually trip making the appliance safe.

Consider the same situation but with a fault causing an open circuit connection to neutral in the distribution cable. This is possible when cables are damaged by diggers. The circuit will now be made using the earth rods connected to the main distribution cable after the neutral fault rather than back through the neutral conductor to the substation. Even a collection of many earth rods has a significant resistance to ground so the fault current will try to find the path of least resistance.

Meanwhile our local radio amateur has erected a vertical antenna and run many buried radials to make it efficient. This also makes it a very low resistance path to earth. If the neutral fault and the appliance fault occur in his neighbourhood it's likely that most of the fault current will try to flow to earth through for example his transceiver, antenna tuning unit, SWR bridge and coax cable to his antenna. Obviously it's not designed for this and something has to give. It may not be the fuse or circuit breaker if the fault current is not high enough to cause them to break the current. Fuses and circuit breakers take a significantly longer time to trip if the fault current is only just above the rating of the fuse or circuit breaker. There is a high possibility of fire, damage to the equipment and wiring and possible electric shock off the radio equipment while the fault current flows.

In areas equipped with a PME mains system it's advisable to isolate antenna feeders from the mains earth with an isolation transformer to break the possible circuit between the mains earth and the antenna earth.

We make a suitable transformer for this. It can also be used with loop counterpoises to isolate them from the mains earth.

The web page is: http://www.crosscountrywireless.net/hf_antenna_isolator.htm

Regards,

Chris


Radio Guy
 

This concern comes up at repeater sites, a lot of people disconnect the mains grounding wire inside the rack mounted Astron power supplies as the power supply is metal and is secured to the metal rack.The rack is earth ground. The wire is terminated with a push on connector, giving the user the option. I pulled out an Astron RM 50m to discover the negative is connected to the case.


On Fri, Oct 8, 2021 at 12:07 PM David Cutter via groups.io <d.cutter=ntlworld.com@groups.io> wrote:

Chris

I've been puzzling over the electrical regs for tying antenna earth systems to mains earth and I see that the above psu has mains earth tied to the negative output. 

I haven't checked all my supplies but I know the one for the main transmitter is not. 

It might be a fine point but for some electrical installations an outdoor antenna "earth" should not be tied to earth/ground since this will also join it to mains earth.  I am on PME which may change in the not too distant future when we are up-dated.  

I would be obliged of your expert opinion on the subject. 

73

David G3UNA



--

-----------
Ken


Martin - G8JNJ
 

Hi All,

Station earthing is endlessly debated but it is a serious issue, and If you have an outside antenna, it's one thing that you really can't skimp on

Beware as regulations vary around the world and there is a lot of information on the web (mainly originating from America) that is not applicable in the UK.

The RSGB have at least two good information sheets regarding UK regulations and best practice.

http://rsgb.org/main/files/2019/12/EMC07-v4-Earthing-and-the-Radio-Amateur-Basic.pdf

http://rsgb.org/main/files/2019/12/UK-Earthing-Systems-And-RF-Earthing_Rev1.4.pdf

If in doubt consult a registered electrician, as they will probably be required to undertake any work in order to comply with building regulations and household insurance requirements.

Any sort of electrical problem or worse still a fire, you may find that you are out on your own and totally liable.

Regards,

Martin


David Cutter
 

Thanks to Martin and Chris and all.  The leaflets and the regs could go further with more diagrams to explain the various combinations. 

I can see the point of the regulations but I'm missing why some power supplies have negative output and mains earth connected inside the supply and whether or not this is a good or bad thing and whether it is a safety requirement.  I don't remember it from BS3456 but that was an age ago.    

It is recommended that antennas should float, ie no connection to ground or mains earth.  I want to ground my antennas to minimise common mode current. 

In the event of a break in the neutral at street level  (what are the statistics on this?) isn't it the case that the fault current will be the accumulation of currents from all the properties downstream of the break and thus be large enough to vapourise my antenna connection before it overcomes the house neutral cable?  

The route for the fault current will be from the street,  through my neutral conductor, around my ring main, up into a power supply, through the radio negative/chassis connection, through the coax and out to the antenna where it meets real earth, then back to the sub station via a fairly lossy route.  It's across that last link (via lossy earth) that the voltage will be developed, until a thin bit of wire melts, then everybody down stream has high voltage on their equipment unless they have a good local earth to neutral.  I feel I'm missing something obvious, please point it out to me.       

Notwithstanding this small risk, Is the joining of mains earth to antenna earth a way of bringing in mains-borne interference?

I like Chris's isolation transformer for receive-only installations.  

David G3UNA

On 09 October 2021 at 13:16 "Martin - G8JNJ via groups.io" <martin_ehrenfried@...> wrote:

Hi All,

Station earthing is endlessly debated but it is a serious issue, and If you have an outside antenna, it's one thing that you really can't skimp on

Beware as regulations vary around the world and there is a lot of information on the web (mainly originating from America) that is not applicable in the UK.

The RSGB have at least two good information sheets regarding UK regulations and best practice.

http://rsgb.org/main/files/2019/12/EMC07-v4-Earthing-and-the-Radio-Amateur-Basic.pdf

http://rsgb.org/main/files/2019/12/UK-Earthing-Systems-And-RF-Earthing_Rev1.4.pdf

If in doubt consult a registered electrician, as they will probably be required to undertake any work in order to comply with building regulations and household insurance requirements.

Any sort of electrical problem or worse still a fire, you may find that you are out on your own and totally liable.

Regards,

Martin


Martin - G8JNJ
 

On Mon, Oct 11, 2021 at 03:21 PM, David Cutter wrote:
I can see the point of the regulations but I'm missing why some power supplies have negative output and mains earth connected inside the supply and whether or not this is a good or bad thing and whether it is a safety requirement.
It's simply a design choice, depending upon what it's intended to power, and how it's designated in terms of electrical safety regulations.

For example double insulated power supplies generally have no safety earth, but they could have, and also the DC negative (or positive) connected to it.

Power supplies with mains safety earth, may not have the DC negative (or positive) connected to it.

All of these examples are unusual, but not impossible.

Regards,

Martin


David Wilkins
 

“but I'm missing why some power supplies have negative output and mains earth connected inside the supply”


Hi - afaik - if the appliance has an external conductive part (ie a battery charger in a metal box) then that conductive part must be connected to the circuit protective conductor (which in turn will connected to the supply earth). This is for “automatic disconnection of supply” to function as intended during a fault and protect us.

Whether the power supply output is floating, +ve earthed or -ve earthed is a design choice.

Cheers,
David.

On 15 Oct 2021, at 15:39, Martin - G8JNJ via groups.io <martin_ehrenfried=yahoo.com@groups.io> wrote:

but I'm missing why some power supplies have negative output and mains earth connected inside the supply


David Cutter
 

Thanks David and Martin

The supply in question is in a plastic box, so there are no exposed metal parts (that I can see). Wouldn't that make it a Class 2 equipment? My other power supplies (apart from wall warts) are in metal boxes, ie Class 1 with earth to the chassis.

I can understand why an earth is supplied to the mains side for filtering purposes, but I haven't fathomed out the mains earth connection to the dc output.

David G3UNA

On 15 October 2021 at 15:50 David Wilkins <davidrwilkins88@gmail.com> wrote:


“but I'm missing why some power supplies have negative output and mains earth connected inside the supply”


Hi - afaik - if the appliance has an external conductive part (ie a battery charger in a metal box) then that conductive part must be connected to the circuit protective conductor (which in turn will connected to the supply earth). This is for “automatic disconnection of supply” to function as intended during a fault and protect us.

Whether the power supply output is floating, +ve earthed or -ve earthed is a design choice.

Cheers,
David.

On 15 Oct 2021, at 15:39, Martin - G8JNJ via groups.io <martin_ehrenfried=yahoo.com@groups.io> wrote:

but I'm missing why some power supplies have negative output and mains earth connected inside the supply




Martin - G8JNJ
 

On Fri, Oct 15, 2021 at 03:08 PM, David Cutter wrote:
Wouldn't that make it a Class 2 equipment?
Not necessarily, it depends upon what the certification mark indicates it as being.

Regards,

Martin


David Cutter
 

Over to you, Chris

Is it Class II or III (Roman numerals, ie double insulated, SELV, et al)?

What is the purpose of the mains connection to output negative and how does this impact electrical earthing requirements?

73 David G3UNA


On 16 October 2021 at 13:02 "Martin - G8JNJ via groups.io" <martin_ehrenfried@...> wrote:

On Fri, Oct 15, 2021 at 03:08 PM, David Cutter wrote:
Wouldn't that make it a Class 2 equipment?
Not necessarily, it depends upon what the certification mark indicates it as being.

Regards,

Martin