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Push switch and safety cover

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Tony K

Member
Hi all,

I've decided to fit a stop solenoid on my yanmar diesel engine, that has been ordered no problem. It draws 30amps (12 volts) when activated, so I suppose I either fit a relay or buy a switch that is rated 30amps or more.

Should I buy a 40amp relay? or should I buy a 40 amp switch. Is it a good idea to overrate these items to ensure they don't burn out, ie 40amps?

Also, if I fit a relay what is the minimum amp rating for the switch?

Cheers Tony
 

cowboybob

Well-Known Member
Most Helpful Member
Hi all,

I've decided to fit a stop solenoid on my yanmar diesel engine, that has been ordered no problem. It draws 30amps (12 volts) when activated, so I suppose I either fit a relay or buy a switch that is rated 30amps or more.

Should I buy a 40amp relay? or should I buy a 40 amp switch. Is it a good idea to overrate these items to ensure they don't burn out, ie 40amps?

Also, if I fit a relay what is the minimum amp rating for the switch?

Cheers Tony
I'm going to assume that the relay would also be a 12VDC device.

If the stop solenoid is a "momentary" device (i.e., once the engine has stopped, the solenoid is released), then a 40A relay should be sufficient. A 40A switch is unwieldy (better off with a 40A breaker, except they're not designed to be used as a "switch").

A switch for the relay needs to be able to handle the current needs of the relay electromagnet, which you have not yet provided.
 

JimB

Super Moderator
Most Helpful Member
A solenoid that draws 30amps at 12volts ?
Are you sure about that?
Sounds like a lot of current for a solenoid (valve?)

JimB
 

cowboybob

Well-Known Member
Most Helpful Member
A solenoid that draws 30amps at 12volts ?
Are you sure about that?
Sounds like a lot of current for a solenoid (valve?)

JimB
Thought the same thing, Jim, but didn't address the issue. Generally (as you probably know), a diesel is stopped by removing fuel access, most often a simple valve shut-off.
 

Tony K

Member
Yanmar list their solenoid for fuel shut of as drawing 30 amps. Quite a clunk I think!

I have found that on a 40 amp relay the current required by the winding to activate it is 150 - 200 milliamps, is that equal to 0.15 - 0.2 amps? So quite a small switch would do.
 

JimB

Super Moderator
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Tony K

Member
Thanks for that.

I have a new one for you.

I have decided that I would like two batteries on the boat for obvious reasons. Each battery is 100ah. One new the other about a year old. The alternator is a standard 55 amps with the engine. I have purchased a Blue Sea charging relay and switch to ensure the batteries are both charged and not discharged! In the wiring diagram for the Blue Sea device it shows two midi-link fuses, one for each battery in the wiring adjacent to each + terminal. The starter motor on the engine draws 200 amps when engaged, if only for a second or two. Do I really need these big fuses? A 100amp fuse, according to the specs, blows at 200 amps, therefore if I use two 125 amp fuses it should be well protected up to 250 amps? I say two fuses because if I ever had to combine the batteries for starting then presumably both will need protecting. The blue Sea Relay has a maximum rating of 120 amps.

What are your thoughts on a good set up for this please?
Cheers Tony K
 

Diver300

Well-Known Member
Most Helpful Member
The fuses protect the wring. If you ever get a short from the battery lead to ground, the fuse should save the wiring.

There is some good advice here:- http://www.pbase.com/mainecruising/battery_fusing

As you are intending for either battery to start the engine, each battery wire, and the fuse protecting it, will have to take the starter current, but only for a short time.

There are plenty of fuses larger than 100 A available. Large fuses tend to be quite slow.

http://www.littelfuse.com/~/media/a...bolt-down-fuses/littelfuse_mega_datasheet.pdf

According to that, a 100 A fuse of that type will hold 200 A for 10 seconds.
 

Tony K

Member
Many thanks I would like to say I find all the members info really helpful.

Are you saying that two 100 amp fuses will be OK or would 125 amp be better?

Cheers T
 

Diver300

Well-Known Member
Most Helpful Member
Many thanks I would like to say I find all the members info really helpful.

Are you saying that two 100 amp fuses will be OK or would 125 amp be better?

Cheers T
I would say that a 100 A fuse on each battery would be fine. They will take 200 A for just under 10 seconds. However it might be a bit close and a 125A fuse will take 200 A for 100 seconds.

You should fuse each battery as close to the battery as practical. The fuse and wire rating needed isn't affected by having two batteries, as you will only be using one at a time.

If there is a way of cranking from both batteries at the same time, any wire, including earth wires, that can take the combined current from both batteries should be large enough to stand the combined fuse blowing current from the two fuses.
 

JimB

Super Moderator
Most Helpful Member
Fuses in the starter motor circuit?
I have never seen that done in a car. As the starter motor is near enough a short circuit anyway when it is stalled and the wiring to the starter motor is sized to cope with that, I see no advantage in fusing the starter motor circuit.

What is a fuse?
It is a resistor which gets hot and melts, it has volt drop.
The last thing you need when starting an engine is extra volt drop across a fuse.

JimB
 

Tony K

Member
Thanks Diver300 and JimB for that,

In fact the Blue Sea system works on the assumption that if the main starter battery (1) is low, you can combine it with the other house battery (2) (which is usually used for navigation lights etc.) So the question is if we go for 2 x 125amp fuses what size battery/starter cable should I use please?

Then I think I'm home and dry!

T
 

Diver300

Well-Known Member
Most Helpful Member
Many modern cars have fuses in the starter motor circuit. Fuses in boats can be considered more important as the passengers can't walk away if it catches fire.

It can be more difficult to select a fuse for starter motor circuits, as the difference between a short and normal running isn't big. Some cars have a squib to disconnect the battery in case of a crash, which shows that the manufacturers consider it important.

A 250 A megafuse (see link above) has a resistance of 0.2 mΩ, or about 45 cm of 35 mm2 cable. All fuses suffer some voltage drop, so I don't think that starters motors are a special case for that reason.
 

Diver300

Well-Known Member
Most Helpful Member
Thanks Diver300 and JimB for that,

In fact the Blue Sea system works on the assumption that if the main starter battery (1) is low, you can combine it with the other house battery (2) (which is usually used for navigation lights etc.) So the question is if we go for 2 x 125amp fuses what size battery/starter cable should I use please?

Then I think I'm home and dry!

T
The cable size is probably defined by current draw, cable length and what voltage drop is acceptable. I would aim for 0.5 V total, but that is just an estimate. Do you have a wired earth, or is there a hull or engine to provide the earth path?

Having a cable large enough to not burn out if there is a short is probably not going to be a problem. As far as I can tell from:-
http://www.schneider-electric.com.a...-guide/EIG-G-sizing-protection-conductors.pdf
then any cable over about 2 mm2 will blow a 125 A Megafuse without burning out. I'm not sure about that, as it seems too small, and I certainly wouldn't use a 2 mm2 cable with a 125 A fuse.
 

Tony K

Member
The batteries will be either in front or behind the engine so nothing should be more than one metre maximum. Could I get away with the next size down if it's only carrying the start current for a few seconds?
 

cowboybob

Well-Known Member
Most Helpful Member
The batteries will be either in front or behind the engine so nothing should be more than one metre maximum. Could I get away with the next size down if it's only carrying the start current for a few seconds?
Just an opinion, but from my experience with engines (especially marine), there is always those times when they are reluctant to start, requiring more than a "few seconds" of starter effort.

So, I'd suggest sticking with the current gauge wire.
 
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