Electric boats 'safe' voltage

I'm building a 72 volt electric outboard using commonly available 72 volt moped/ escooter battery, motor and controller.

I have read in various places that 48 v is the generally agreed 'marine safe' voltage.

Obviously I'm going to be super careful plugging and unplugging batteries with wet hands etc., but does anyone know where the 48 volt voluntary limit came from, what the actual dangers are, how they can be simply mitigated?

 

I'm not sure where the 48V "safe" definition came from; current DC propulsion technology goes to 144V and probably beyond.

 

A quick google of IEEE 48 volts shows that this is widely accepted as safe for people working with it and used for telecomms equipment but I don't know if there is legislation about it. Cars etc. will be going to 48v for everything except traction power because it means much lighter wiring and some systems such as air conditioning can be replaced with 48v electrics. Some car batteries are 288v and probably more. In a past life I was a telecomms apprentice and had to sometimes top up the 50v battery in a telephone exchange; each of the open cells probably had a 1000 litres of dilute sulphuric acid in them...

 

48 V is a standard set by Thomas Edison. He started the Western Union Telephone company, which run their system at 48 V. He had enough political influence to get legislation to set 50 V as "safe". The standard is still in place, even though we know 50 V can be lethal. In short, anything working at 50 V or below has less safety requirements and is therefore cheaper.

 

I think I'm going to get a pair of big rubber gloves with gauntlets for plugging and unplugging the battery, and crack on.

 

Never be too trusting of the word safe, because it's all relative. A nine volt battery can kill you if you do the wrong thing with it, though you do have to use pins as electrodes and push them through the top layer of skin. Someone was trying to make himself part of a safe electrical circuit and did this on purpose with the two electrodes attached a finger on opposite hands. That put enough current across his heart to stop it.

 

At METS last year ePropulsion showed a pod drive at 96V. The salesman said it was required for high-power motors as losses in wiring were too high at lower voltages, as far as I recall. That was on YouTube (sailing Uma channel).

 

Electrical circuits that have an inductor, such as an electric motor, can be dangerous when switching on or off. There can be some serious arcing at the instant of switching contact or breaking of the contact. Fire, even if instantaneous, as in inductive circuits, are bad news especially on a boat.

I already knew about that phenomena and I once had an episode that made me certain of the reality. I was in my workshop doing some fiberglass work. Acetone was involved for cleaning up. I was ready to quit for the day. I had a rag with some acetone content in my hand when I flipped the light switch. The rag instantly burst into flame. I reacted by flipping the burning rag away from my hand. Where do you suppose the flaming rag landed. In a pan where there was some acetone residue....of course. Fortunately I managed to extinguish the flames without injury or fire losses. Lesson learned! Two lessons actually. The second one is; Don't get careless with acetone or other volatile potions.

Why would the switch shoot a spark like that? The lights were florescent types that are driven by a transformer. That is an inductive circuit which means that a back EMF is involved. The transformer was innocently trying to discharge itself. Any circuit with a coil behaves like that, including electric motors. The problem can be mitigated by using switches such as mercury types or incorporating capacitors in the circuit. Voltage regulators such as the speed controller for the motor will usually have some protection from switching hazards.

How many gasoline motorboats have blown up when pushing the starter button? Gas vapor in the bilge and sparks from the starter solenoid will do the trick.

 

In ABYC Standards and the USCG regulations, 50V or less is considered Low Voltage. They do not use the term "safe".

For instance "33 CFR §183.430 Conductors in circuits of less than 50 volts." sets regulations for conductors in DC electrical systems. less than 50 Volts.

 

I agree that they don't use the term "safe". However, a level of safety is implied by the less stringent regulations.

 

I agree.

 

The engineer in me is struggling with this. So I did some digging; and I don't fell any better than before.

 

There appears to be agreement that DC voltage under 50 volts is generally safe and will not cause injury from significant injury. I remember hearing in the 1990s when there was interest in the auto industry in going to higher voltage electrical systems (compared to the standard 12 volts) for electric power steering, higher volume cooling fans, etc. and the industry adopted a nominal 42 volt standard because it was considered sufficiently safe.

Higher voltage systems are not prohibited by the 50 volt (or 42 volt) standard. Higher voltage systems do require additional safety features to prevent accidental contact. Electric propulsion systems in autos usually have much higher voltages.

 

While any voltage will spark if presented the right conditions, IIRC "low voltage" was designated because it wouldn't jump the air gap in most dusty and humid industrial conditions. This is a far cry from the turbo-electric DC generator on the USS Glenard P. Lipscomb (SSN 685). You did not pass fore and aft when running at full power...you would close the air gap.

 

Could some kind of GFCI be rigged up to help the shock or Spark situation?