GOOD MARINE ELECTRICAL INSTALLATION PRACTISE.

1. Keep cables as short as possible EXCEPT for tuned transducer circuits of echo-sounders and similar equipment. You can shorten VHF antenna cables without causing any problem - excessive cables reduce performance. If you have a particularly long run of VHF cable consider replacing it with the larger diameter type 50 Ohm coax to improve performance. MF and HF antenna cables have to be handles differently and may require the transmitter re-tuning if shortened. Secure cables properly to stop movement and insulation damage.

2. Separate receiving cables from those that carry RF signals such as VHF antenna cables and echo-sounder transducer cables. Try and route transmitter cables separately from others as otherwise interference can be induced into the oeth cables. On a new installation, plan and install earthed cable conduits to help with layout - but remember that ferrous metals cause rust so use copper or stainless. This can be difficult to plan and all installations are to some extent a compromise.

3. DO NOT coil excess cables in loops. Shorten as much as possible and if you have to run them past other cables, try and cross at 90 degrees instead of running alongside.

4. Design your system using an insulated earth return system (negative isolated from ships metalwork or engine) and ground metalwork to an underwater specialist signal earth plate, fitted with sealant such as Sikafles or Bondit PU18 below the waterline outside the hull and through bolted with bronze bolts. Use an internal pad for strength, with more sealant to guarantee watertight integrity. Most commercial vessels use this type of design.

5. If you have enclosed items (e.g. Outboard motor covers) through which interference is being transmitted, line them internally with foil, overlap and use burglar alarm foil connectors to connect sections together and then connect to a signal earth point. An alternative for small items is to use electrically conductive spray paint, although this is less efficient.(Used by many manufacturers inside plastic instrument cases)

6. Fit a purpose made carbon brush assembly to the stern tube assembly on inboard shafts, so that the brushes connect tube and shaft together, ensuring equal potential at all times. Ensure that the shaft and engine are connected to the cathodic protection system.

7. Solder the centre connector of VHF radio R.F. connectors. Some connectors allow the outer braid to be soldered as well. Do not think of a VHF cable as being screened - although it has an outer braiding, it radiates electromagnetic waves for the entire length.

8. When earthing conduit enclosures, use only one cable as otherwise you can create an earth loop which can increas both interference and/or corrosion. Try and use flat copper strip or braid if possible for earthing to the signal earth plate in preference to single conductors.

9. Use D.C. circuit breakers (NOT fuses or A.C. circuit breakers) on D.C. circuits and select the correct size for each application. Provide a suitably sized MAIN fuse to prevent fire in case of the main ship's services cable shorting out, but don't fit this on heavy engine starter cables.

10. Do not install mains A.C. supplies on board a boat without RCD (Residual Current Device) and MCB (Miniature Circuit Breaker) protection. (Both can be combined into one unit). Without the use of an RCD you could die if you accidentally come into contact with 230 volts mains. Salt water and A.C. mains voltage is a potentially lethal combination.

11. Do not use domestic solid cored PVC cables for marine installations. Use marine grade (e.g. Polysulphide insullation) flexible cables and tin the ends with electrical multi-core solder before you connect up if the cable is not pre-tinned.

12. Make use of suitable materials that repel moisture and seal efficiently, such as self-amalgamating tape (you can then cover with ordinary PVC tape) and high modulous Dow Corning silicone sealant. Use Polyurethane sealant (e.g. Sikaflex or PU18) below the water-line as silicone will often seperate from the substrate in wet conditions.

13. If you have both A.C. and D.C. supplies on board your boat, consider using a galvanic isolator to reduce excessive wear on your zinc anodes - particularly if you plug into a marina or shorepower facility, otherwise your boat will act as an anode for the entire marina. Through the use of a galvanic isolator my own anodes last several years in my own marina and yet I suffer no electrolysis on my propellor, fittings or shaft.

 

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