12v Electrical Grounding & RF Grounding confusion

[sveolo]

I have recently rewired the 12volt electrical system on my Fibreglass hulled Union Cutter 37. Believe it or not all seems to work well. However in doing this my attention has been drawn to the state of the grounding / bonding circuit, which seems to be in tatters (26 year old boat in need of attention).

I now want to sort this out but can't seem to get a practical description of exactly what should be connected up to what.

I understand that all underwater metal fittings should be connected up and understand that these should be connected to the engine block etc. Am I correct in saying that this should all be hooked up to the negative bus bar and the mast base etc? Presumably this should all be connected to a large hull fitted anode?

Does anyone know of a practical guide to a standard grounding / bonding circuit and exactly what should and shouldn't be connected to each other and the key do's and don'ts of all this or otherwise can anyone provide an overview?

I find myself further confused when it comes to my RF ground circuit (we have a large ground plate for the SSB) - should this be kept entirely separate from the electrical bonding circuit described above and should I hook all my RF grounds (VHF, Navtex, GPS etc) to this ground plate?

As you can probably tell, I am pretty confused by this subject and any advice would be greatly appreciated.

Many thanks!

 

[William_H]

My guess is that you are fairly correct in grounding everything to the engine assuming you don't use an insulated coupling to the shaft. ( If you do then get better advice than mine) Only the SSB needs the special eath preferably as short as possible from the antenna tuner (or Tx if it is built in) as short as possible using a multiwire or wide conductor. This is because the earth and its lead are part of the antenna.
The earthing of the mast is a good idea but use as heavy a cable as possible to conduct as much current as possible to minimise damage in a lightning strike. If you have a metal keel this is your best earth for SSB and lightning. If you go to a mains powered electric system then it gets more complicated regards olewill

 

[jerryat]

Re: 12v Electrical Grounding & RF Grounding confusion

Hi sveolo,

Your SSB (RF) ground should run from the ATC (automatic tuner) or manual tuner, direct to your ground plate or better still, a cast iron keel. The key to good perormance i.e. providing an excellent counterpoise, is to use 75mm (3") minimum wide copper tape. It's very easy to fit as it's only about 5 thous. thick.

Incidentally, multi wire will not be anywhere near as effective as wide tape. It is the surface area/width you are after, not density.

If this is done, it is not necessary to earth the radio unit itself as it is earthed via the ATU. Some here feel separate earthing is required, but whilst doing no harm, it is just superfluous.

Agree with ole Will re earthing back to main neg busbar and engine for the rest.

Cheers Jerry

 

[sveolo]

Re: 12v Electrical Grounding & RF Grounding confusion

Thanks for the thoughts above. Do you know what one should do with the "Ground" on other instruments - eg. GPS etc - should these be part of the electrical bonding through to the negative bus bar, engine block, anode etc or should they be wired into the SSB / Ground plate? Many thanks again.

 

[sveolo]

As you guessed I don't have an insulated coupling. Based on my understanding to date it seems that the name of the game is to connect as much of the metalwork on the boat together and ensure that this circuit is connected to both the negative bus bar and as large a sacrificial anode as possible! Since I have encapsulated ballast in a long keel there is no option to connect mast up to the keel bolts so I presume the best plan is to connect the mast into the same bonding ring. This will presumably offer the best path to ground in the event of a lightening strike - any thoughts on minimum guage wire for adequate protection? Fortunately, we are up on the hard out of harms way at the moment - so no risk until all is sorted out!

Many thanks for your help!

 

[William_H]

When you are considering lightning damage then the thickest wire possible would be the answer. However as your gearbox and shaft will be in the only current path to ground then anything with less resistance than the gearbox and shaft is perhaps superfluous. Use something like your starter cable size or a little smaller.
That reminds me of a story of a small turboprop plane that was struck on a propeller tip. There appeared to be no damage and the plane went back into service however fortunately shortly after it was found that the bearings of the prop blade (that allow pich change) were heat treated damaged and very nearly caused a disaster.
Jeryat is right about the earthing for the SSB. It seems that RF power tends to run only on the surface of a conductor. Hence flat tape is food. When I suggested multiwire I actuallyt meant strands insulated from one another as this increase surface area but ultimately it is probably not so critical. incidentally sacrificial anodes need to be near the metal they are protecting ie near the prop. good luck olewill

 

[jerryat]

Re: 12v Electrical Grounding & RF Grounding confusion

Hi sveolo,

Don't connect anything else to the SSB ground as this could cause interference with operation of the radio. Just take all other negatives back to the busbar/engine and all should be well.

Cheers Jerry

 

[sveolo]

Re: 12v Electrical Grounding & RF Grounding confusion

Jerry,

Just to confirm - on the back of several instruments (GPS etc)I have 3 wires:

1) 12v +ve
2) 12v -ve
3) Earth

Clearly 1) and 2) above provide power to the instrument and 3) is connected to the negative bus bar to complete the 12volt circuit. However the third connection, referred to as earth is what I'm looking to understand. Per your suggestion above, all I need to do is connect this back to the negative bus bar and into the grounding circuit that can exit through the engine block / prop shaft etc. If this is indeed the case it seems strange that I need two connections from one instrument to the negative bus bar - is this right?

Hope I am not making a mountain out of a molehill but this has always perplexed me and this is why I had thought that perhaps this "Earth" connection might be better attached to the RF ground.

Sorry to labour this but very keen to understand once and for all.

Many thanks again.

 

[jerryat]

Re: 12v Electrical Grounding & RF Grounding confusion

Hi sveolo,

Yep, the 12v+ and - are obvious. I believe the 'earth' is usually used to ground the casing of the instrument/unit, though without seeing/knowing the type it's difficult to judge.

Others here are far more expert than me, so may well be able to offer a more positive technical response.

I would do the two main connections and operate the equipment. If interference becomes apparent, connect the 'earth' to the same neg. busbar since this is, or should also be the 'ground' on a boat.

Cheers Jerry

 

[MedMan]

Let us forget about RF grounding just for the moment.

Anodes are installed to prevent galvanic corrosion. This occurs when two chunks of metal are reasonably close to one another on the outside of the boat and they are connected to one another on the inside. The 'system' then acts like a simple battery and a current flows around the circuit, part of which is through the seawater. The most electrically active chunk of metal will corrode. This situation does not occur very often as there is not generally an electrical connection between two fittings on the inside, at least not deliberately so. However, if such a connection is made, let us say through copper plumbing pipes or through bilge water, galvanic corrosion can occur. There are two ways to prevent this from happening:

Isolation: Make certain that there is no electrical connection between through-hull fittings on the inside and make very regular checks to ensure that no connection has developed accidentally.

Deliberately connect all adjacent fittings on the inside and connect the whole lot to an anode. The anode is made of a zinc which will always corrode first thus protecting your fittings. (I will spare you a lecture on the galvanic series)

You can actually mix these two methods as galvanic currents are so small they will only flow through a few feet of sea water. If, for example, you have a whole cluster of fittings including the prop and rudder stock near the stern and a single loo seacock way forward you can bond all the stern metalwork to an anode placed at the stern and isolate the loo seacock.

 

[sveolo]

My fog of confusion seems to be lifting. It seems that I have several interwoven objectives:

1) Provide a safe "earth" for the 12volt system

Connecting the negative bus bar to the engine block provides this earth to the 12v system.

As further clarification, is anyone able to provide a practical illustration of when this "earth" might prove critical to my, the boat's, or the 12v system's safety?

2) Protect underwater metal boat parts from galvanic corrosion

As per the clear explanation above, underwater metal boat parts must be protected from galvanic corrosion through direct connection to a sacrificial anode or complete electrical isolation from other metal parts.

3) Provide a preferred path to "earth" in the event of a lightning strike

It seems that by connecting up the mast -- and presumably other large metal objects above deck such as stern and bow rails -- to the underwater "ground" that I am providing a "path of least resistance" for lightning to exit in the event of a strike.

If I have got this piece right, is it correct to say that connecting the above deck metalware (aluminium mast, stainless rails etc) to the below water bonding system (engine block, prop shaft, thru hulls, and sacrificial anode) will have no adverse impact on the galvanic protection that I am seeking for my below water parts?

4) Provide an RF ground for the SSB

A large copper ground plate as close as possible to the tuner / SSB connected with copper tape to maximise surface area provides an RF ground that is completely independent from the ground system discussed above.

Where navigation instruments such as Navtex, GPS, radar etc have a third wire referred to as "earth" or "ground" in addition to 12v +ve and -ve, this is to electrically ground the casing etc and should be connected back to the negative bus bar and the earthing system. This is not related to the "RF ground" described above for the SSB.

Have I got this right? Any further answers?
Many thanks for all the help.