Bilge Control Via Relay

Discussion in 'OnBoard Electronics & Controls' started by IronPrice, Jul 25, 2017.

  1. IronPrice
    Joined: Jul 2017
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    Location: NZ

    IronPrice Senior Member

    TLDR: Can I reliably use relays at the transom to switch the manual circuit on my bilge pumps and control these from the helm?

    Why? To reduce the length of the manual supply circuit and therefore reduce required cable gauge.

    The manual circuits run power from the battery to the (forward) helm and back again, just so I can switch from the helm. That's a lot of power to push around the boat and the cable gauge required to keep voltage drop under 3% is heavier than the factory fitted cable on the pump.

    If I switched the manual circuits with Relays (protected by fuses) located in the transom but controlled from the helm that would reduce the manual circuit length and therefore cable gauge. I'd add bypass switches at the transom in case of relay failure. This could all go in an IP68 rated box for protection.

    Some multi-core trailer-light leads would easily carry the alarm and control currents to the helm.
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yep, your plan sounds fine and is a wise way to conserve gauge requirements on long runs. How long is the run?
     
  3. IronPrice
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    Location: NZ

    IronPrice Senior Member

    The run from battery to helm switch is about 6 meters with corners then about 4 metres back to the pump.

    I have a dedicated DC main circuit for the bilge pumps.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Find a safe, high up and dry place for the relays and your current wiring, will likely be enough to get it done.
     
  5. IronPrice
    Joined: Jul 2017
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    Location: NZ

    IronPrice Senior Member

    Thanks for you advice, I have a plan for that:-
    - IP67 box for the relays etc in the battery locker;
    - Pump wiring runs to IP68 junction box on the underside of the bilge inspection hatch hinged;
    - Supply wires run to the junction box in flexible PVC conduit that terminates on water proof cable glands.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Bingo, and Bob's your brother . . .
     
  7. Craig Miles
    Joined: Aug 2017
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    Location: Lincoln, UK

    Craig Miles New Member

    Out of interest , are you considering 'normal type' electro-mechanical relays, or solid state types?
     
  8. IronPrice
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    IronPrice Senior Member

    I would consider either. Provided they can sustain the required draw. Is there an advantage of one or other?
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Solid state is the way to go for several reasons, though no open contacts is the primary reason.
     
  10. IronPrice
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    Location: NZ

    IronPrice Senior Member

    OK thanks I'll see what I can find.
     
  11. IronPrice
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    IronPrice Senior Member

    Found these. they look like they would do the job. My only concern is they look like they are designed to dump a lot of heat. I had planned to house all the relays etc in sealed plastic box. Dissipating heat could be an issue.

    I'll be running at 13A on the biggest pump.
     
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    How often do your pumps run and can you install a heat sink?
     
  13. BertKu
    Joined: May 2009
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    BertKu Senior Member

    I am puzzled. Normally those solid state relays uses Mosfets. P(ower) = resistance from the Mosfet in conducting state and could be for a good relay 0.007 Ohm multiplied by current square. = 13 x 13 x 0,007 = 1,83 watt and is in my view not really a hot spot. Except if they are using a cheap mosfet and the conducting resistance is 0.02 Ohm then the power dissipation would be 3,4 watt which is also not a real exceptional hot. I am questioning whether your 6 x 2 x 2 = 24 meter copper wire will dissipate more than the solid state relay. Using a normal relay the resistance often is also around the the 0.1 Ohm and that is even worse. P = 0.1 x 13 x 13 = nearly 17 watt heat disipation. I am really puzzled and question your decision. Bert
     
  14. BertKu
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    BertKu Senior Member

    A 3 mm copper cable in diameter has o,252 Ohm per 100 meter and can draw up to 18 Ampere without problems. Your length is 24 meter all in all which would mean 0.063 Ohm at 13 Ampere your power dissipation would be 10.6 watt and is definitely less than a normal relay. Yes if you go lower in copper diameter to save money, then you will indeed run into problems like voltage drop etc. If you go up in diameter, it will cost you money and weight added. Only you can make a good decision for yourself. Bert
     

  15. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    I found some information on a normal relay:

    With a new relay and contacts this ON-resistance will be very small, generally less than 0.2Ω’s because the tips are new and clean, but over time the tip resistance will increase.

    For example. If the contacts are passing a load current of say 10A, then the voltage drop across the contacts using Ohms Law is 0.2 x 10 = 2 volts, which if the supply voltage is say 12 volts then the load voltage will be only 10 volts (12 – 2). As the contact tips begin to wear, and if they are not properly protected from high inductive or capacitive loads, they will start to show signs of arcing damage as the circuit current still wants to flow as the contacts begin to open when the relay coil is de-energized.

    This arcing or sparking across the contacts will cause the contact resistance of the tips to increase further as the contact tips become damaged. If allowed to continue the contact tips may become so burnt and damaged to the point were they are physically closed but do not pass any or very little current.

    If this arcing damage becomes to severe the contacts will eventually “weld” together producing a short circuit condition and possible damage to the circuit they are controlling. If now the contact resistance has increased due to arcing to say 1Ω’s the volt drop across the contacts for the same load current increases to 1 x 10 = 10 volts dc. This high voltage drop across the contacts may be unacceptable for the load circuit especially if operating at 12 or even 24 volts, then the faulty relay will have to be replaced.

    If you really like to cut on cost and weight on cable, go for a good quality solid state relay. Bert
     
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