ECN Forum Forums General Topics of the Trade General Discussion Area Fuse characteristics

Is there a difference between fast acting and current limtinig fuses?
Which is appropriate for protection of semiconductor based equipment (AC Drives, SCR type heaters, etc)?

Try visiting these sites for more info http://gouldshawmut.com/resources/index.html http://www.bussmann.com/library/docs/Spd.pdf

Thanks Tom.
I was actually looking for some open ended discussion on the topic. I was hoping there may be some fuse-type OCPD experts around.
Hello?!

Fuses can be either fast-acting or time-delay. Most fuses, such as Class CC, J, R, T, L and high-speed fuses are current-limiting. Class T fuses are fast-acting and are very current-limiting. High-speed fuses are fast-acting and extremely current-limiting (the most of any fuse). Class CC, J, R, and L have both fast-acting and time-delay versions and have good current-limiting abilities.


Fast-Acting fuses are typically used for heating and lighting circuits (Class CC and T) and have good current-limiting abilites.

A special type of fast-acting fuse is the high speed fuse. This is a very current-limiting type of fuse, specifically selected for the protection of drives and SCRs. Often drive manufacturers will recommend a specific type of high speed fuse for their drive. Replacement of this fuse, with another type or class is not recommended since it may sacrifice the needed protection. At Bussmann we have specialists for these applications and they can be contacted for assistance.

Time-delay fuses have the ability to pass temporary inrush currents from motors or transformers (primary), but can also be used for general applications. Time-delay fuses have the ability to be sized closer to the load than fast-acting fuses. Typically these fuses are applied in eletrical distribution systems (usually Class J, R, and L) and have good current-limiting abilitites.

The bottom line, for drives and SCRs, use the fuses recommended by the manufacturer, typically a high-speed fuse.

Fast-acting deals with how long it takes a fuse to begin to melt. Current-limiting deals with how long it takes the fuse to finish melting. By description a slow-blow fuse can also be current limiting, and a fast-blow fuse is not always current limiting.

There are three primary purposes for using semiconductor protection and traditional overcurrent is not one of them (taken from Bussmann SPD book).
1) Prevent device rupture - fuse needs to open before device does.
2) Isolate Failed Device - fuse is used to remove failed device from the system, and allow other, parallel, devices to keep operating. This situation is usually from an internal failure.
3) Protect the device - fuse is selected to protect against external short-circuits.

Protection for "fragile" semiconductors is obtained by using "high-speed" fuses which provide ultra-current limiting. High-speed fuses are often identified by their weird shapes (usually square) and their lack of fuse clips. Their selection requires specialized knowledge of the device characteristics.

In a nutshell,
Use the fuse the equipment manufacturer tells you to use. They are the only ones that can specify the correct semiconductor protection.
If no fuse is specified a good quality, time delay current limiting fuse (Class RK1) is often acceptable.