Reverse polarity

[Fluid36]

Hi guys,

Tiredness and in the dark resulted in a brief moment of hooking the battery up reverse polarity. Managed to blow some fuses in the switch panel and the inline fuse in the outboard, all replaced and working now.

The only issue I have is the VHF, - as soon as the switch on the switch panel is turned on it blows, seems to be drawing too much current and my assumption is that its cooked.

Thoughts?

[Rebel]

Your right.

But,take it out and test it away from the boat.

Cheers.

Edited September 12, 2021 by Rebel

[Rebel]

If the radio is ok, make sure you have in line fuses fitted.

Cheers.

[Fluid36]

Thanks Rebel,

It's fused individually at the switch panel which blew, guess that didn't quite do it's job quick enough.  Are you saying an extra inline fuse from switch panel to unit?

The stereo has one of these and that blew (along with the individual switch panel fuse) so I guess the individual switch panel fuse for the stereo didn't really do it's job either.

Also have an inline breaker from the battery switch to the switch panel but that didn't trip either.

How do I test it when out of the boat?

Edited September 12, 2021 by Fluid36

[zmk1962]

To me it sounds like it’s cooked and has developed a short circuit. But this could be a planned safety mechanism  to direct reverse polarity current straight to ground. Check the manual. There may be an explanation or a procedure to reset after reverse polarity situation. 
cheers Zoran 

[Fluid36]

Thanks also Zoran, will check and provide feedback to help others going forward.

 

[Fluid36]

Manual states:

 

Reverse Polarity & Overvoltage Protection  - Shunt diode and voltage regulator.

 

will need to take apart and check.

[zmk1962]

Yup shunt diode - that’s it - was trying to think back 40yrs to uni electric circuit theory 

[Rebel]

The wiring has to be at least 8mill. I use 30amp fuses. Bit over the top but the system works.

[zmk1962]

1 hour ago, Rebel said:

The wiring has to be at least 8mill. I use 30amp fuses. Bit over the top but the system works.

Not sure what you're saying there @Rebel.  I agree in a DC circuit you need bigger diameter cables for longer distances so that you don't experience current (AMP) and voltage (V) drop... or don't overload the wire.

An 8mm cable can carry a 40AMP load over 1 meter or 10AMP load over 10m length.

But putting in a 30AMP fuse in that circuit will provide no protection to the VHF radio that usually has a max 2 to 5AMP load. Infact the $300 VHF radio will be the weakest link in the electrical circuit and will burn out first to protect the 20c 30AMP fuse.... the radio should have its own fuse correctly rated to protect it.

If you are using the 8mm wire with a 30AMP fuse  to feed other electrical devices whose combine load might reach 30AMPs  then the 30AMP fuse will protect the 8mm wire at max of 2m length.... beyond that at say 5m length the wire will only be able to safely carry 20AMP so it would start to burn before the 30AMP fuse blew.  Also if you are using the cable as a feed, then each device should have it's own correctly rated fuse protecting it. 

Cheers Zoran

 

[Rebel]

I have had problems in the past.

The system I use works ok.

Different people have different ideas when it comes to electrics.

Cheers.

 

[zmk1962]

12 minutes ago, Rebel said:

I have had problems in the past.

Still not sure what to make of that mate. But it sounds like you had a problem with fuse blowing so you upped the fuse rating... that will certainly stop the fuse blowing, but will expose the wiring to heating up unless the cause of the original problem is rectified.

14 minutes ago, Rebel said:

The system I use works ok.

Yup. It would allow things to run as you have a continuous electric circuit .... and all is good while everything is ok. But what happens when it stops being ok like a short circuit from a loose terminal connection. If the fuse is too high rated and doesn't blow ... then the weakest component in the circuit will. If that weak component is the wire then it can overheat, catch alight and burn back toward the battery.

17 minutes ago, Rebel said:

Different people have different ideas when it comes to electrics.

Yup. And some ideas are safe and others are not.

Cheers Zoran

[Rebel]

Haven't had a problem in over 20 years.

Cheers.

[dunc333]

a fuse is useless. if it is not the weakest link in the system and i'm no sicenctist.

Edited September 13, 2021 by dunc333

[kingie chaser]

Smaller inline fuses are good to protect individual appliances & as a secondary defense but if you had have had a circuit breaker between say the main battery to a bus board then you wont fry your individual appliances(or wiring) if a surge occurs.

CB's are alot cheaper than a shunt diode but I might be missing something on what the diode can do that the CBV cant??

 

I thought a shunts measures the current, maybe its the diode that cuts the circuit if the current is to high? 

Edited September 13, 2021 by kingie chaser

[zmk1962]

54 minutes ago, kingie chaser said:

 

I thought a shunts measures the current, maybe its the diode that cuts the circuit if the current is to high? 

If I remember my ECT correctly a diode only lets current through in one direction. So it can be incorporated in a circuit to prevent current flowing in opposite direction like the reverse polarity case. 
Cheers Z

[dmck]

"Reverse Polarity & Overvoltage Protection  - Shunt diode and voltage regulator."

typically a shunt diode will be connected across the +   & - leads at the point they connect to the radios circuit board. 

They are not conducting under correct voltage connection but are conducting (virtually a short circuit) if the wiring is reversed... hence the radio fuse blows.

If you have only a heavy duty fuse protecting the radio, there is a good bet that the surge in current has burnt out some of the copper track on the radio circuit board.

Fuses should be the MINIMUM value that allows the device to operate correctly.  ..... and...... every device should be fused separately... you dont want to lose your important electronics (radio/GPS/FF etc ) just because one of them develops a fault.

Ignore the basic rules and you will eventually get caught out.

[zmk1962]

9 minutes ago, dmck said:

They are not conducting under correct voltage connection but are conducting (virtually a short circuit) if the wiring is reversed... hence the radio fuse blows.

Perfect explanation.... Thanks !

Cheers Zoran

[Fluid36]

Thanks or all the replies.

 

There is a 50 amp breaker between the battery switch and the front switch panel but this did not blow. (battery switch feed goes to rear positive busbar then to front fused switch panel via this breaker).

There is a 10 amp fuse in the front switch panel for each individual device, the fuse for the VHF did blow.

There is no inline fuse between the front switch panel and the VHF itself.

The fuse in the switch panel that blew did not protect the VHF from damage so I don't think another inline fuse between the switch panel and device would have worked but who knows.

Cheers

[zmk1962]

1 hour ago, Fluid36 said:

so I don't think another inline fuse between the switch panel and device would have worked but who knows.

The problem you have is that a 10A fuse is too big... it allowed too much current to run through the VHF radio. You need to protect the radio with a much smaller fuse inline between the radio and the switch panel 10A fuse.  The VHF manual should tell you what size fuse is correct for your radio... usually 2A and I would be really surprised if it was more than 5A.

At the risk of over simplifying here is the theory that relates current, voltage and power and a rough and ready real life comparison ...

P=VI   P=watts (power), V=Volts, I=Amp (current)

So on a 12volt system, running 10A the power is  P=10 x 12 = 120W  !!!  For comparison, a typical soldering iron uses 60W power delivered as heat to melt solder. So at 120W you are running 2 x soldering iron power (heat) through the radio.

If you limited the current to 2A using a smaller fuse, the max power that could have run through the radio would have been P=12x2=24W ... ie a very dim lightbulb of heat vs 2 x soldering irons.

So yes another inline fuse (smaller 2 - 5A) between the switch panel and the device would have protected it.

Cheers Zoran

 

[kingie chaser]

1 hour ago, zmk1962 said:

The problem you have is that a 10A fuse is too big... it allowed too much current to run through the VHF radio. You need to protect the radio with a much smaller fuse inline between the radio and the switch panel 10A fuse.  The VHF manual should tell you what size fuse is correct for your radio... usually 2A and I would be really surprised if it was more than 5A.

At the risk of over simplifying here is the theory that relates current, voltage and power and a rough and ready real life comparison ...

P=VI   P=watts (power), V=Volts, I=Amp (current)

So on a 12volt system, running 10A the power is  P=10 x 12 = 120W  !!!  For comparison, a typical soldering iron uses 60W power delivered as heat to melt solder. So at 120W you are running 2 x soldering iron power (heat) through the radio.

If you limited the current to 2A using a smaller fuse, the max power that could have run through the radio would have been P=12x2=24W ... ie a very dim lightbulb of heat vs 2 x soldering irons.

So yes another inline fuse (smaller 2 - 5A) between the switch panel and the device would have protected it.

Cheers Zoran

 

Yep.

When I installed the GME UHF CB in my ute I used a 3A inline fuse(even though GME states 2A), pretty sure the positive power cable to the unit is only 10 or 15amp max

Radio remains ON when
the ignition switch is OFF-
Connect the radio's negative (black) lead to the vehicle's
chassis, or if preferred, directly to the battery's
negative terminal.
The radio's positive (red) lead should be connected via the
2 Amp fuse to the battery's positive terminal. Alternatively,
the positive lead could be connected into the fuse box
at a point that has +13.8 Volts continuously available
(preferably the battery side of the ignition switch) via
the 2 Amp fuse.

Radio turns OFF with the ignition switch-
Connect the radio's negative (black) lead to the vehicle's
chassis, or if preferred, directly to the battery's
negative terminal.
The radio's positive (red) lead should connect to an
accessory point in the vehicle's fuse box via the 2 Amp fuse.
This point should supply +13.8 Volts only when the ignition
switch is turned ON or in the ACCESSORY position.

 

Dont see an install in a boat being any different really?

Edited September 29, 2021 by kingie chaser

[Fluid36]

Thanks guys.

 

Issue is some other individual wired the VHF up prior to myself purchasing the boat so it would have been what I would have done and what I will do with the new one. 

I have pretty much rewired the entire boat front to back apart from the lines from the new switch panel to the devices and have now learnt my lesson on those.

Thanks for the details both of you, really helpful.

I wouldn't say I'm an expert but I'm pretty good at 12 volt having wired multiple cars over the years, boats are so much easier with far more room.

ps. you wouldn't believe some of the wiring I pulled out of this boat, really unbelievable what some people consider to be a job well done.

 

Any tips on best place to buy tinned copper wires at decent prices btw?

Cheers

 

 

Edited September 29, 2021 by Fluid36