Earthing and the radio amateur

A frfiend of mine did some earth bonding a few years ago and he was told on inspection that you were allowed to count copper pipes as an earth connection as long as the joints were soldered or compression fittings.


If that is still the case, could I bond my earth rod to an outdoor central heating drain tap just two metres away?

The advent of plastic (push fit) plumbing fittings used for alterations and repairs makes such assumptions a lot less safe than they used to be - so generally now we expect bonding (as distinct from earthing) connections to at the boundary of the equipotential zone (or as close as practical) - that way such plumbing changes can't undermine the effectiveness of the bonding. The regs so of do permit metallic pipework etc to be used as part of a protective conductor - but require precautions to be taken against its removal - which is going to be tricky in an uncontrolled environment like a domestic. If you had to expose and label every separately visible section of the pipe with a suitable warning, you'd probably find that dragging though a bit of G/Y wasn't so daunting after all.


I'm not sure like the idea of a central heating drain cock being outside anyway - how do you stop it freezing in winter and splitting the pipework/fittings?


  - Andy.

A frfiend of mine did some earth bonding a few years ago and he was told on inspection that you were allowed to count copper pipes as an earth connection as long as the joints were soldered or compression fittings.


If that is still the case, could I bond my earth rod to an outdoor central heating drain tap just two metres away? ?.


Mike

Thanks for that.


In my case, the antenna is an HF end fed half wave wire, connected to the transceiver via a voltage transformer and coax. Due to the constraints of our property it is only 12 feet above ground.


The ground rod is connected to the coax shield and is there to prevent static build up. It is not an RF ground (that function is provided by a section of the coax feeder which acts as a counterpoise).


When not in use I disconnect the antenna from the transceiver and its power supply from the mains. I'd like to know if any further precautions against lightning strikes are required. We haven't had one in this area for forty years, but you never know ...

The problem is that the radio ham, by putting bits of wire and metal outdoors, more often than not has more than one earthing system. Maybe not if if the station is VHF/UHF only and the antennas are all yagi designs looking like an overgrown TV antenna on the roof, as these do not need an RF earth to function, but a station for HF or even more so LF, is not really complete without a decent connection to the  earth beneath the antenna.

So given you have an earthed extraneous part to deal with, the options are somewhat restricted.


As far as lightning is concerned, any outdoor tower or mast, if not planted directly in the ground, should be arranged so that it can flash over  to an electrode if it is more than a few hundred volts off true earth.  A simple spark gap will do. Similar precautions to protect  external feed cables before they go indoors are also advised.

Sealed unit spark gaps with an inert gas filling are available. examples  though the breakdown voltages are a bit low for full legal power operation.

  For a direct strike, like the rest of the installation, such measures will of course be blown to smithereens, but for the far more likely induced few tens of KV from a strike within a km or so, they do greatly increase the chances of the indoor gear surviving, and of not injecting nasty voltages onto the local mains supply.

Gas discharge tubes and spark gaps allow a short to ground to be present only when it is actually needed, being open circuit the rest of the time! Howeverby far the best precaution to protect the house end is to disconnect the radio, from both antenna and mains supply, when not in use, and to short the antennas to ground if their design allows that.

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.

Although PME is often held up as a worst case example - there are similar dangers from having any two unconnected earthing systems within reach of each other - regardless of which type they are.


With any TN system a fault between a line conductor and true earth (but not a c.p.c) can result in the star point being pulled a long way from zero volts - which is then imposed on the DNO's earth equally in TN-S and TN-C-S systems.


Likewise standing leakage currents in a TT installation can raise the MET and everything connected to it significantly above zero volts - no more than 50V if the installation is in accordance with BS 7671 and all RCDs are functioning correctly, but another TT installation could be at held at a different voltage again. In the worst case if the other installation is on another phase, you could have as much as 50V √3 = about 87V between them.


Then there are a whole raft of faults (L-PE on any TN system, plus L-N on TN-C-S systems) that can temporarily raise the voltage on earthed conductors - most installation ones should clear within 5s, but DNO ones could take longer.


In short, try not to have any two different earthing systems within reach of each other, whether one of them is PME or not.


   - Andy.

As a radio amateur I'm interested in this discussion as I have the same situation as Stewart.


As has been suggested, there are ways to mitigate the risk of electric shock in the shack without bonding the antenna ground rod to the MET, but is there an issue with lightning protection? It has been suggested to me that a nearby strike could induce a voltage gradient in the surrounding ground that might cause arcing into the property. Would bonding reduce this risk?


Mike G8GYW

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.

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.

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