Re: [electricboats] Battery Fusing

I use a class T-fuse at the battery, the primary criterion was ability to safely and effectively interrupt a potentially massive short circuit from the LiFePo bank.  There's a notional rating for fuse capacity (trip current) which you've referenced, but there's another rating which is the absolutely maximum that the fuse can safely and effectively interrupt.  This rating is tied to the physical design and construction of the fuse/CB. 

If a short circuit exceeds this interrupting capacity, the current could arc over the device and/or other catastrophic failure modes.  Please note that the interrupting capacity drastically changes on whether the current is AC or DC, so check that on the datasheet.  AC can self extinguish on polarity reversal, this can't happen with DC.

This is largely an issue for Lithium chemistries, I don't believe that (most?) LA batteries are physically capable of generating the very intense short circuit currents which is necessary to overcome the interrupting capacity.

Kai

On 28/5/21 7:00 am, john via groups.io wrote:

It would still be really nice to have a breaker right at the battery to protect in the event of a short somewhere along the line.

Looks like Amazon has 48V breakers up to 300A.  I saw a fuse for 500A or 600A, but they're only rated to 32V.  

John

On Thursday, May 27, 2021, 03:43:31 PM CDT, Dan Pfeiffer <dan@pfeiffer.net> wrote:

Very useful info.  Thanks. 

Does a controller (like the Sevcon4) handle current limiting to the motor?   It does have thermal overload control which is related.  Would the "overleads" device you describe between the controller and motor (sounds like a circuit breaker) be redundant if the controller can do the same thing?   

Dan Pfeiffer

 

On 2021-05-27 1:56 pm, Dave Yamakuchi via groups.io wrote:

 

NEC generally calls for 175% of Full Load Ampere rating for dual element time delay fuses for motors.  Fast acting fuses, inverse time or instantaneous breakers are all rated differently for the short circuit protection.

 

However, this only covers short circuit protection, and not an overload condition. Thermal or electronic 'overloads' located after the control circuit directly before the wires to the motor is the generally accepted practice for protecting against an overloaded motor. Fishing line or seaweed maybe binding the shaft and preventing it from spinning freely, for instance.  Or prop stuck in the mud.  This is in addition to the fuses.  Depending on the motor's service factor and temperature rise specs, the overloads usually shut off at no more than 115% or 125% of the FLA.

 

Overloads have a much tighter current limit range, but they're somewhat slower acting to allow for inrush at startup. (fuses and breakers often vary widely in the actual trip current)  They're meant to recreate the motor's windings' temperature to trip the control circuit.  Also, they're resettable.

 

See Article 430 of the NEC for more info.

 

On Thursday, May 27, 2021, 12:39:03 PM CDT, Dan Pfeiffer <dan@pfeiffer.net> wrote:

 

 

For those running 48V 8-20kW systems, what are you doing for fuse protection on your battery bank?  Brands and sizes?   My 12kW motor has a constant capacity of 250 amps.  The peak capacity is 600.  I don't know what I might actually see for short duration peaks but I was thinking a 300 amp fuse might blow in normal operating if I had a momentary peak on the motor draw.   What are others using? 

Dan Pfeiffer