grounding confusion

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waynesworld
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grounding confusion

Postby waynesworld » Sat Oct 13, 2012 1:27 am

Hi all,
I have a couple of questions that I need some feedback on. I am a novice so please don't laugh :oops:
1.0 I am in the process of wiring a little of my build. I have grounded all my through hull fittings etc to a common copper ground as per "Inboard motor installations" by Glenn Witt. Eg. I have run individual wires from the water intake, rudder, exhaust outlets, fuel fuel & tank, metalic fuel filter & pump etc back to a copper bar. From what I understand this "Bonding" should be completly seperate from any electrical system. My question is, can I not tie this in with my engine block to create a ground for my electrical system? This is a trailer boat and will not by moored in the water so I am not to worried about corrosion at this stage.
2.0 I have installed two blowers which I have connected together. Individually they are rated at 4.5 amps and have a recomended 8 amp fuse. So by joining these two units together does this change? Am I going the wrong way about this? Do I still use a 8 amp fuse?
On a side note I have these set to exaust rather than intake and are wanting to wire them direct to the ignition rather than a switch. I am concerned about someone forgetting to turn them on or off. Any advice here for that?
Little hard to see but below the blowers is the common bond.
Kind regards

Wayne
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falconer100
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Re: grounding confusion

Postby falconer100 » Sat Oct 13, 2012 5:46 am

Hi Wayne,

Firstly let me compliment you on what looks like a very neat installation.

I am in a similar position as yourself as regards bonding, I have been led to believe that keeping it seperate from other systems meant to keep it seperate to a mains electrical instalation ie a shore based power supply, not necessarily your 12 volt system, I would appreciate some feedback on this myself.

As regards to your blowers, again I have done the same with two blowers. Your blowers take 4.5 amps each, meaning your pulling 9 amps with a possible start up draw of slightly more than 9 amps. You may find that an 8 amp fuse will blow at this rating, or at least get rather warm. Remember that a fuse is there to protect your wiring as much as anything else, so you need to know exactly what rating your wiring is, if the wiring is only capable of handling 8 amps, then you may consider fitting seperate fuses for each blower with a feed wire capable of handling maybe 21 amps supplying those fuses so that you only have to run one wire from your fusebox. Fuse this wire at 15 amps and you should have a good safety margin.

I would strongly advise you NOT to wire these to your ignition, they should be run for 4 or 5 minutes prior to switching the ignition on, if you have any fuel vapours in the bilge, a stray spark, say from the alternater for example, (and yes, it can happen) when switching on the ignition could result in most unfortunate situation, they should also be used when moving at low speeds, try fitting a warning light to show when they are running.

I hope this helps, and hopefully, someone will jump in with some more info for you.

Good luck with your build.

Martin

jcallends
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Re: grounding confusion

Postby jcallends » Sat Oct 13, 2012 6:00 am

Hi Wayne, You had to learn to crawl before you could learn walk and is true for everyone so don't feel bad if you have not learned yet.
Your question about bonding I can't answer as I don't know much about this in regards to boats but corrosion of metal can occur even when a boat is out of water using moisture in the air and sometimes occurs at the joint of like metals. About your blowers, each blower should have an individual fuse and wired on the hot side just before the blower. As for leaving the key on we have all done that and is a pain. It is practical to wire a relay in your system that would sound an alarm when the motor was stopped with the key on. You just have to find a circuit or get someone to do it for you.

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Caber-Feidh
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Re: grounding confusion

Postby Caber-Feidh » Sat Oct 13, 2012 11:19 am

circuit protection should be as close to the power buss as possible, NOT at the device end. Putting the fuse at the blower end leaves the whole wire as a possible point of short circuit without a breaker. A short is going to turn your electrical system into a poor version of an Edison light bulb. Secondly fuse holders are a source of ignition, especially when they have a tiny bit of oxidation causing arcing. It will become equivalent to putting spark-plugs in your bilge. Not the best idea in the world.
Some plan to stroll through Saint Peter's Gates, I plan to go through them at 150mph... backwards... in a screaming ball of flame, with a glimmer belt wrapped around my head, and a NOx button in my hand.

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galamb
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Re: grounding confusion

Postby galamb » Sat Oct 13, 2012 5:32 pm

I have read quite a bit on bonding and there seems to be two different schools of thought on the matter.

Some say "don't bond at all", but since this flies in the face of ABYC guidelines I paid more attention to the second opinions.

Most state that your common bond should terminate at the negative post of one of your batteries. Bonding to your engine block could cause a "water circuit" which would allow stray electricity (that travels along the bond wire) to pass from your lower unit or prop (as the case may be) through the water back to your through-hulls which could lead to corrosion.

By terminating at the battery, the (circuit) would end there. In fact, if you also have 120 volt installed on-board, your ground (green) can tie into your common bond if terminated at the batteries negative post.

You just can't use your bond wire as a "convenient" hook up in place of actually running a negative wire (black or the "now recommended" YELLOW) in any of your 12 volt circuits.

So at the very least, terminating at the battery may give your anodes a few extra years of life - and at current prices for replacements, that alone is worth hooking it to the battery....
Graham

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waynesworld
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Re: grounding confusion

Postby waynesworld » Sun Oct 14, 2012 6:41 pm

Thank you for your replys.
Ok so I am intending to procede with bonding and to terminate it at the battery. So not only can I bond all my through hulls to this common bond but also ground any electrics to this as well? ie-connect all my negetive wires to this?
So maybe one aft and one forward and have these two connected by way of a decent sized wire. Would this be the correct way to do this?
Thanks again

Wayne
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galamb
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Re: grounding confusion

Postby galamb » Sun Oct 14, 2012 7:01 pm

No, that's the point I was trying to stress there - you can NOT connect your negatives to your bond.

You can run a heavier gauge negative bus all the way back and connect into that (but it must be heavy enough to handle the collective amps of everything piggy-backing on it), and it must be "separate" from your bond.

You CAN connect any 120 volt (shore power) ground wires to your bond circuit..
Graham

Yes, Plywood is "real" wood :)

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

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DrBryanJ
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Re: grounding confusion

Postby DrBryanJ » Mon Oct 15, 2012 3:50 am

Wayne

Have you read this? http://newboatbuilders.com/pages/electricity1.html I know nothing about electric systems and it helped me understand alot. Doesn't take long to read through all the pages.

Bryan
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jcallends
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Re: grounding confusion

Postby jcallends » Mon Oct 15, 2012 5:53 am

The point of where the fuse goes in the circuit, in this case, is moot. The real problem is the builders desire to provide overcurrent protection with one device for two motors and since he got a quick reply from another builder only confirms that this is a problem that others have. It is interesting to see what is going to happen if we follow this course of thinking. We double the size of the fuse to 15 amps for 2 blowers, now the only protection this circuit has is if both blowers suffer the same fault at the same time exceeding the current of 15 amps, a very unlikely event. If one blower develops a fault that exceeds the the 8 amps that one fuse would provides nothing will happen as the existing fuse is for currents exceeding 15 amps. If this fault is fatal then the blower becomes useless. Now we have one blower left and it is also protected by this same 15 amp fuse and if it develops a fault it faces the same result unless the fault causes a current draw in excess of 15 amps. The end result of this thinking is going to be an expensive repair, don't do it.
The statement about putting the fuse at the source or buss is in fact correct. Most circuits in electrical systems have more than one fuse and some have many and protection of wiring is provided by having a separate large fuse at the source. Putting overcurrent protection on devices to just protect the device from destroying itself is common practise in industrial and commercial applications but is rarely seen on consumer devices for obvious reasons. My choice of words about where to put the fuse was wrong for the average person not familar with electrical circuits. The statement was meant to mean the fuse would be the last thing in the circuit before the blower, phyiscally it could 10 inches or 10 feet away does not make any difference and probably would be near the helm.

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Caber-Feidh
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Re: grounding confusion

Postby Caber-Feidh » Tue Oct 16, 2012 1:51 pm

From USCG docs:

183.455 - Overcurrent Protection: General

(b) A manually reset, tripfree circuit breaker or fuse must be placed at the source of power for each circuit or conductor except:

(1) If it is physically impractical to place the circuit breaker or fuse at the source of power, it may be placed within seven inches of the source of power for each circuit or conductor, measured along the conductor.

(2) If it is physically impractical to place the circuit breaker or fuse at or within seven inches of the source of power, it may be placed within 40 inches of the source of power for each circuit or conductor, measured along the conductor, if the conductor is contained throughout its entire distance between the source of power and the required circuit breaker or fuse in a sheath or enclosure such as a junction box, control box, or enclosed panel.


(c) The current rating of each circuit breaker or fuse must not exceed:

(1) For circuits of less than 50 volts, 150% of the value of the amperage in Table 5 for the conductor size it is protecting; and

(2) For circuits of 50 volts or more, the value of the amperage in Table 5 for the conductor size it is protecting. If this value does not correspond to a standard size or rated circuit breaker or fuse the next larger size or rated circuit breaker or fuse may be used if it does not exceed 150% of the allowed current capacity of the conductor.


Simply put, it's gotta be at the buss, or it's not going to make 'da man happy. Besides, hunting down a blown fuse in a nasty bilge on a moonless night is no fun at all.

The blowers should not be mounted to the hull bottom. Water will not drain out of them if they are. I don't know of a CG directive specifying such, but it makes sense.
Some plan to stroll through Saint Peter's Gates, I plan to go through them at 150mph... backwards... in a screaming ball of flame, with a glimmer belt wrapped around my head, and a NOx button in my hand.


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