Earthing arrangements for Radio Workshop.

[64bits]

Are there any special earthing considerations when feeding a TT system in an outbuilding from a domestic (TNC-S) supply that will be used for low power RF equipment and sensitive electronics?

Whats there now...

The building already has a DC power system (5-36V DC) for various RF gear with a 36V battery bank charged from two solar panels.
The DC system is not charged from Mains supply.
0V rail of DC system is tied to 1.6m ground spike and housings for RF gear, amps etc.

RF equipment, Mast, Antennas and RF feeders are earthed to the same 1.6m ground spike as the DC system.

What I am planning to do...

I intend to use the domestic supply (dedicated MCB in CU through 4mm CSA SWA buried in ground) to feed the radio workshop building.
The buried CSA SWA cable sheath will be earthed at the CU. The sheath will not be used as earth for outbuilding, only as earth protection for swa feeder cable. I intend to use TT system in outbuilding.

I will be adding a larger antenna mast in the near future, this will have lightning arrestors and equipment cabinets earthed near the base of the mast. Power to masthead amps etc, will be fed via coax RF cable from equipment cabinets at base. The cabinets and all RF gear in them will be powered from the DC system in the workshop.

Now the questions..

- Would it be sensible to use the existing 1.6m ground spike (earthing the RF gear and DC system) for the AC TT system earth? or would it be preferable to use a dedicated (possibly longer) copper ground spike for earthing the AC power system?

If a separate earth spike is preferrable is there a minimum separation distance for the two earth spikes? Would this depend on soil resistivity? or is there some sort of 'rule of thumb' when dealing with power and RF system earthing that I have not yet found?

Anyone with experience in power/RF installations? Your views would be greatly appreciated..

 

[ericmark]

It is common to use a TT system for radios because the radio earth has to be so good that faults with a TN-C-S system can cause huge currents to flow.

1.6M seems rather small for a radio earth I will guess Amateur Radio which means 400 Watts of RF. Much depends on the aerial being used long wires need good earth but Yagi beams really don't need an earth as such.

Lighting is a real problem if a mast is earthed it will attract lighting strikes but if not earthed the operator can get a very nasty shock when plugging in the aerial. The norm is a row of resistors to leak away the voltage around 6 100MΩ in series with a spark gap across them similar idea to found in master telephone sockets.

This is really something for the Radio guy to decide as he is the one connecting and disconnecting aerials and so one would treat the installation as being in the control of skilled users. Assuming he does hold a RAE or what ever replaces that now.

I have seen some really odd set ups including use of an isolation transformer to stop RF getting into the supply line. Using a pre-built radio likely today any power supply will isolate but with home brew could be using a resistor to drop current where using valves. So you need to know more about the radios in use. Live chassis is also a problem it may mean RCD is a must but it could also mean can't use a RCD some old sets need the old ELCB-v as can have 30 or so volts casing to earth.

With repair shops the RCD for sockets is often 10ma rather than 30ma again you really need more info.

 

[64bits]

Thanks You for the constructive reply!

I have some info re; radio equipment that will be in use.

It is mostly good quality modern gear, Icom, AOR etc, There will be three transceivers (Max TX 25W) and several receivers including SDR hardware with its associated computer equipment.

All the radios (except two) are in casings designed to be earthed, the odd ones out are an old Hacker VHF receiver (wooden box) and a Westminster AM receiver (Bakelite Case) both have terminals earthing them to existing earth spike.

Proposing to have Workshop CU with MCBO,MCBs with 10ma RCD on bench sockets.

Thanks for pointing out live-chassis issues. On further research I will offer option of having ELCB-v protected socket(s) for working on live chassis gear.

New copper earth spike for TT system earth. (1.6m is not adequate) soil core sample shows clay layer about 1.5m down about 0.5m thick with hard soil/ ironstone below it. Existing earth spike may not have penetrated clay layer. looking to install 3m spike

Have proposed replacing existing 1.6m earth spike with 3m copper to improve radio earth.

 

[ericmark]

Personally I would ask the guy what he wants go it's TT that is the right way to go but just sticking in a spike is not good enough it needs measuring. There are two common methods either compare against DNO earth or use special meter with probes. The problem with latter is getting access to ground 30 foot from the rod to put test probes into.

The corona that developed around an earth rod being used means an old rod is often better than a new rod when I had to fit rods in a gas terminal 8 ohms was required for each rod. The bars were 1.2 meters long and could be connected together with coupler I had a paddies motorcycle to put them in with. I found between 3 and 8 rods were required. This is a huge difference for an area which seemed the same I just did not expect such a huge difference throughout the site.

At the incoming earth resistor a pit with 4 rods one each corner and earth tape joining them was installed. Testing that was a real problem needed around 50 meters of cable for the furtherest probe. Resistance before back filling was 3 ohms.

However the DNO earth only has to be 16 ohms so with an 8 ohm rod you could still read 24 ohms with a earth loop impedance meter. With a maximum of 400W using a long wire one can expect voltages around 100 so 4 amp flow to earth so the rod can rise quite a few volts. Having an earth pit so you can't touch the rod is very important.

But with a dipole hardly any current flows to earth. What is the main factor is not the make of radio but type of aerial and you have no idea what aerial may be installed in the future.

There is a good reason why Radio Hams take the RAE exam and that is to ensure safety and non interference as an electrician we don't cover the same things so I would get the licensed operator to tell you want he wants. He should know what he is doing.

Personally I would fit rod for supply and bond it to rod for radio I would not remove the original just add to it. I would fit all 30ma RCBO's all pole in a three phase box or adaptable box to get all pole RCBO's to fit that way if a special supply is wanted easy to add.

Remember this shed is under the supervision of a skilled or instructed person as the ham has his RAE so you don't need to use type tested distribution units.

Use brass screws not steel for earth connections and loads of petroleum jelly around connections then self amalgamating tape as electrolysis can quickly cause erosion and steel bolts can cause harmonics rust acts like a diode.