Riverguy
Senior Member
- Joined
- Feb 17, 2013
- Messages
- 285
- Location
- USA
- Vessel Name
- Serendipitous
- Vessel Make
- Mainship 390, Bayliner 3258, Bayliner 4788
Re: "The Hall effect type meter, as Riverguy suggested, would be fine IF the correct range is picked. I do not believe a 30A unit is anywhere near adequate. Maybe most loads will be under that but it would only take once to possibly damage it."
The device I recommended absolutely cannot, ever, be damaged by overcurrent. This is because the Hall Effect Sensor simply measures the magnetic field around the wire. The output of this thing is measured in millivolts and microamps. The sensor itself is good up to 1,000A, and even that limitation is only there because the size of the ring cannot accomodate a larger gauge wire running through it!
Think of a compass. Put it next to a simple, straight wire carrying 400A of current. Can you "overload" the compass? If you think so, try putting a compass next to the wire feeding your starter motor and crank away... ;-)
Re: "But I still maintain that what you described before, a 30A INTERNAL SHUNT type, is inadequate. You MIGHT get away with it but there is no way I would use it. All it would take is one instance of overload to wreck it and to boot you would lose your electrical. It could catch fire which may be alarmist but it is possible."
No. First off, remember that the meter itself is just a little millivolt meter, connected across a shunt. To "burn out" this 'meter', you have to burn out the shunt.
The I-squared-R math for that 30A shunt (0.00167 ohms) tells us that it will dissipate exactly 16.7 watts at 100A. You would need to be running 100A through it continuously for a long time before that little 16.7 watt 'heater' is going to damage anything except itself. This simply can't happen if the upstream overcurrent protection for the circuit is properly sized -- typically 50A for this circuit. As long as it's wired according to the diagram I showed from Blue Sea System, none of these bad things can happen.
Re: "It is the wrong device in the wrong use. Buy the proper device."
Look back at the O.P and review what Phil wants to accomplish. Assuming it was made by a reputable manufacturer, it is just fine (and proper) for measuring house battery loads on a typical 35 foot powerboat.
Now...there are more modern options (as I've said) that are easier to install...
Getting down into the weeds...it is true that you want to size a shunt-based ammeter for no more than 80% of the continuous load. This is true of both internal and external shunts. The reason has to do with accuracy, not safety. The special metal alloy the shunt is made of can 'drift' in terms of it's resistance if it is running above a certain temperature for an extended period of time. This again is not a safety concern. The internal-shunt ammeter is designed to fail-safe. If grossly overloaded (say, 5x rated current) for more than several minutes, the alloy will melt -- and you will be glad it did, because it means you really messed up installing it, and it probably saved your A$$.
Re: "That's why these external shunt and Hall effect types are produced and used but again they too must be chosen with enough range to take care of all eventualities such as someone turning on an unintended load."
If someone 'turns on an unintended load' (larger than intended for the circuit), it will trip the breaker(s). If the breakers don't trip, it's the fault of the person who over-sized (or removed) them. In this case it will be the wiring itself that will start a fire and not the ammeter.
A shunt meanwhile is a low ohm resistor, sized to match the size of the current it is intended to measure. Shunts are designed to dissipate heat efficiently. To "overload" a shunt, you need to overheat it for a LONG time. Brief overloads are meaningless. If you overload a shunt for too long, it will simply melt, but this is not like a fuse. If you want to use a fuse analogy, think of a very, very VERY slow-blowing fuse, aka a "fusible link".
Re: "ALmost always the same wiring that feeds the loads is the same wiring that the alternator uses to feed the batteries..."
No...never. Current from the alternator(s) does not ever go directly to the panel that feeds the house loads, and this wiring must NEVER be 'shared'. The alternator output(s) attach to positive at the battery switch on the down-stream (switched) side. From here, alternator current flows either upstream into the battery (through the BIG cables), or it flows downstream to the overcurrent device(s) that protect the (further downstream) house loads wiring. In any case, there will always be an overcurrent protection device between the alternator output and the house loads. At the same time this device protects the wiring, it will also protect the ammeter from all of the scenarios you describe. See alternator wiring photo from Yanmar attached.
Re: "Most ammeters need to be capable of handling at least 100A to cover eventualities, at least more than the alternator can output."
No...they need to be capable of handling the current going to the loads. This ammeter needs to be sized to handle as much current as the nearest upstream breaker will pass before it trips. This will be based on the size of the wiring that feeds the panel and the loads downstream. It will have nothing whatsoever to do with alternator maximum outputs, etc.
Remember, the device in question is a 0->30A meter, not a -30 -> 0 -> +30 amp meter. It is not intended to show charge rates, and (in any case) that's not what Phil is looking for. So the alternators are not in the picture here anyway.
Re: "Most good units for these purposes should be even up to 400 or 500A capacity."
No. If the circuit is designed according to ABYC or international standards, current will be limited by the upstream overcurrent device(s). By the time you find a way to get 400-500A through your house-loads ammeter to it's downstream loads (without tripping the upstream breaker), the boat will have burned to the waterline because you will have turned your supply wiring into a 30 feet of red-hot, glowing fire-starting filament. By the way, you will never find a shunt that is sized for (lets say) a 0-100A application that will be rated to handle 500% overloads. That's not how they work.
Again, the "ask" here is to measure load currents, not charging currents.
re: "Since the meter that goes with it is digital the accuracy is still good even on low A uses."
No. The accuracy of the meter that measures the volts across the shunt is not the problem. The problem is that (like any other resistor), the shunt has a tolerance...in this case it is +/- 2% or so. For a 500 amp shunt, this means the reading you get will be off by +/- 10 amps. For a 50A shunt it will be +/- 1 Amp.
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