Current Limiting Fuses

[Britt]

I was reading the info on Victron's 12/200 "BMS" which has an alternator input. This AB input path can accept different values of fuses which correspond with the current limiting of the device.

https://www.victronenergy.com/upload/documents/Datasheet-BMS-12-200-EN.pdf

https://www.victronenergy.com/upload/documents/Manual-BMS-12-200-EN-NL-FR-ES-DE.pdf

Are the fuses themselves limiting current or is the BMS somehow reading the resistance of the fuse to limit the current through other means?

It does look like current limiting fuses exist, as they heat up their resistance increases. I'm not sure if these would be good for continuous use though. But maybe they could be useful? Any thoughts?

Thanks

 

[Zil]

All fuses limit current. That is their job. Too much current and they open (blow) and stop the current. A resistance device limits voltage and that results in lower amperes. I do not see the wisdom of increasing the resistance in a charging circuit. Use fuses to protect the wire awg you use. That Victron uses the fuse as a shunt, a resistor. It must have some circuity that then drops the current. Do you have that large of an alternator?

 

[efficientPV]

PTC fuses are used in a lot of consumer applications wher people are likely to short out something. These increase in resistance and drop the current substantially. It takes about 1/3 Watt to keep them alive. Power must be totally removed for them to reset. I use a number of them around the power shed to protect from loose test leads shorting things out. Fuses have gotten quite expensive. That wouldn't be helpful if you weren't there to see it happen and reset the load.

Interesting idea about using fuses as shunt resistors. I have used them that way, but only when soldered in. Simple plug in fuses can have considerable contact resistance and fuses themselves will vary greatly from mfg to mfg depending on the alloy. But, they have bolt on fuses and from the wording they appear to do just that. A higher fuse will have lower resistance and the electronics will read that. Imaging there are warnings to use exact fuses.

 

[Britt]

Thanks for the responses, that makes sense. I do have a pretty large alternator but will be using some sort of dc-dc converter to regulate voltage and drop charging current down to around 0.2C.

 

[Keith C]

In the electrical field when you say "current limiting fuse" they are not talking about what every fuse does 'interrupt during an over current event'. They're talking about something completely different!

The electrical protection devices in all equipment have a limited ability to interrupt current during a fault. If the SOURCE is above a certain capacity then an underrated interrupter will fail to interrupt the fault. What transpires next is not pretty and can sometimes lead to fire trucks, hose streams, and other unpleasantries.

Because of this, electrical equipment is designed for sources up to a certain capacity. You will note on things that interrupt faults, an interrupt capability listing. You will see numbers like 10kA or 30kA or 100kA. Look at any circuit breaker you find in home panel. A device capable of interrupting 10kA will utterly fail to interrupt 100kA of available current.

"I only have 200A service so I don't have to worry about 10kA", you may think, but you'd possibly be wrong.

What sets the available fault current is the power network supplying the power. If the utility transformer has a certain current rating and the cables bringing that supply into the facility are short and large then the instantaneous delivery ability could well be much much higher than 10kA. This has to be worked out by an engineer typically so that either the system can be modified to reduce the available fault current or everything downstream needs to be rated for interruption of the available current.

Back the 'current limiting' fuse question. In many situations the use of special 'current limiting' fuses means they themselves can be used to remediate the available source fault current to something much lower than what's otherwise available. If a system states it needs to be protected via current limiting fuses then that means the 'thing', whatever it is, likely would not be able to interrupt 'typically available' fault currents by itself. Hence the specified special fuse type.

 

[efficientPV]

Not even remotely the same subject. These fuses act as current shunt resistors to sense current. Lower current fuse has higher resistance than a high current one. The voltage is sensed across the fuse. No need for any adjustment because the fuse sets th limiting current for the electronics. Pretty clever. And if all hell breaks loose, the fuse will eventually open.