Rule of thumb for sizing fuses?

Thread Starter

danton133

Joined Mar 12, 2010
7
Assuming a load without significant inductance or capacitance am I correct in thinking the fuse should be about 1.3 times the expected operating current then use the next available sized fuse above this?

Additionally, at what point is the fuse size too large? What the general rule here? Is it at twice the operating current or 3, 4, 5 times?

I assume this depends on whether it’s being used to protect against short circuit or overload. How would you decided what to use based on protecting from:

  • Short circuit, and
  • Overload
Example, I need to protect a electronic device (a 12v counter) from overload. Operating current is 10mA and I’m using automotive style blade fuses. The smallest available is 1A, which is 10 times the operating current, is this too much to offer adequate protection?
 

MaxHeadRoom

Joined Jul 18, 2013
23,931
As a general rule, 1.5x the operating current of the device is the criteria for fusing.
But the type and fuse value depends on what style of protection you need, very fast or can the device tolerate an over current for a short period of time.
If yours is critical at 10ma, then you require a fuse rating close to the consumption of the device.
Use a different style fuse, or modify the automotive style to the suitable value , if needed
 
Sizing fuses according to electrical safety standards gives 150% to 167% of nominal load.
You want to protect your wiring/connectors/PCB traces, so that they do not melt and burn if there is a short.
You want to protect the load from overheating as well.

Automotive blade fuses look at the fuse curves but a 1A ATO will carry just over 1A forever, and 1.5A for 10 seconds. Fuse ratings are generally not when they will open, but what they will carry.
At low current say 50mA, consider using a polyfuse in your project instead.
 

Thread Starter

danton133

Joined Mar 12, 2010
7
What are you trying to protect with the fuse? Fuses are generally used to prevent fires, not to protect downstream devices.
Thanks for the reply Dennis. Could you explain why this is the case? (that they are fused for fire prevention rather than for device protection)
 

MrChips

Joined Oct 2, 2009
24,623
Thanks for the reply Dennis. Could you explain why this is the case? (that they are fused for fire prevention rather than for device protection)
Without a fuse to break the circuit, a continuous current could cause overheating which can start a fire.

Chances are that any sensitive component has already been destroyed by the over current.
 

dl324

Joined Mar 30, 2015
13,532
Could you explain why this is the case? (that they are fused for fire prevention rather than for device protection)
Regular fuses don't react fast enough to protect sensitive components. For that, you need to employ other, faster acting solutions.
 
Top