Fuses for 24V DC circuits


Thread Starter

Chimalkar, Sumeet

Does anybody have any idea if a fuse rated for 230V ac, 2A can be used in a circuit using 24V dc and still blow at the same fault level? Thanks in advance Regards --------------------- Sumeet Chimalkar Jacobs H&G - Mumbai (9122) 8208075

Marco C. Mason

I believe it works like this: The fuse's power loss is I*I*R, where R depends solely on the construction of the fuse. So all things being equal (ambient temperature, etc.) the fuse should melt at a specific current irregardless of the voltage of the circuit. (Note that many fuse holders are enclosed. It may be an intentional design feature preventing stray air currents from cooling the fuse and allowing them to dissipate more power without melting...)
Responding to S. Chimalkar's query dated Wed, Mar 28, 1:31 pm on the use of a 230 Vac fuse in a 24 Vdc circuit: For overloads the fuse will respond to currents in excess of 2 Amperes. However, whether or not it can safely clear short circuits is dependent on the available dc fault-current at the fuse location. If the fuse is clearly marked (approved?) for ac, then don't use it for dc. Regards, Phil Corso, PE (Epsicon Inc)

Ramer-1, Carl

The short answer is "No". Fuses are power dissipaters that destruct when enough voltage and current are applied to melt the wire. There may be a conversion table available from a manufacturer, but with the price of fuses that small being so low, I'd be surprised to find much information on the subject. If your application calls for 24V D, 2A rated fuses, just use them. In an emergency, where life or property aren't in danger, you can get the machine running with the 230VAC model but you're over fusing the circuit. Whether a slow blow 24VDC and a quick blow 230VAC operate anywhere close to the same is yet another issue. Carl Ramer, Engineer Controls & Protective Systems Design Space Gateway Support, Inc. Kennedy Space Center, Florida Unsponsored professional posting

Alan Rimmington

The AC voltage rating is the maximum voltage that the fuse can safely break. You should find that this fuse will work fine at 24v DC, although a DC rated fuse would be needed for higher DC voltages
Sumeet, I don't think this will work. Most trip devices are really rated for power. So, the fuse from your example will have a power rating of P=IE (230 * 2 = 460 watts) This can be reverse calculated for the current capacity at 24 volts by using I=P/E. (460 watts / 24 volts = 19.17 Amps). Typically it is the heat generated inside the device that causes the tripping of a fuse or breaker. Hope this helps. Dale

Curt Wuollet

Incredible, Simply incredible. Fuses rated in watts. Ok, Here's how it really works. Until they blow, fuses never see much more than a fraction of a volt. They are current devices. The fuse link does open because of the power it dissapates but that has nothing to do with it's voltage rating. The current rating is typically how much current it will carry continuously. Often other factors are considered and uprating and derating applied. A 1 amp fuse will not open at 1.01 amps but is guaranteed to open within a certain time at a certain overcurrent. This is the basis for "slow blow" "quick acting" and other qualifiers. There are many types with different time/current ratings. The voltage rating on a fuse is how much voltage it will consistently withstand after blowing without sustaining an arc. In other words the highest voltage that it will safely and definitely interrupt. For general classes this is considered to be with a non-reactive load. Inductive loads present special problems as the di/dt of interrupting the load generates very high voltages and tends to sustain an arc. Fuses for this service are often filled with chemicals, sand, etc. in order to extinguish this arc. There's a lot more to fuses and fusing and circuit breakers are even more complex but, it is generally safe to apply fuses at less than their rated voltage providing the other characteristics suit the application. Regards cww

Ramer-1, Carl

The logic is right, but the voltage to use is the actual voltage drop across the fuse element. Occasionally you can come across high resistance fuses that develop enough drop to reflect problems in sensitive circuits (like radar calibration reference circuits). The difference between a 100 ohm and 0.1 ohm fuse is dramatic in those circuits. The protection parameters for the two also varies even if the markings are the same. Quite commonly the resistance of identically rated fuses from one manufacturer will vary from the next manufacturer and act differently in circuit as well. Carl Ramer, Engineer Controls & Protective Systems Design Space Gateway Sipport, Inc. Kennedy Space Center, Florida Unsponsored professional comment
There is not a 24VDC voltage drop across a 24VDC fuse element when it is intact. Nor is there a 120VAC voltage drop across a 120VAC fuse element when it is intact. The energy that ultimately melts the fuse element is the integral of (the heat) current (RMS) squared times the resistance of the fuse element minus the (cooling) heat loss. If I remember correctly this may be called I**2t * R. If I have a fuse in series with a lamp on a 24VDC circuit, I can't drop all or even significant voltage across the fuse element - there would be a dimly lit lamp. I think the critical parameter here is if the 24VDC fuse will open the 120VAC circuit in the face of the high voltage and probably lower source impedance (more power to keep the current flowing through an arc as the fuse blows). I think this is the interrupting rating (I have also seen it called rupturing capacity?) I guess this is related to distance between the end caps of the fuse and whatever gas is produced in the barrel as the element vapourizes. Maybe it depends on the material the barrel is made of since nobody would want it to explode. Perhaps a fuse designer is lurking and can shed some light here. I have wondered about the voltage rating on glass fuses myself. If a look at glass fuses, a 12VDC automotive fuse and a 120VAC fuse *look* pretty much the same. What is the difference? I wondered if the automotive fuse may be able to interrupt more current from a large battery than a control power fuse sourced from a small control (high %Z) transformer. Bill Code. MPM Engineering Ltd. Surrey, B.C.
Sorry Dale - this is a load of rubbish. A fuse will melt if the energy dissipated in the fuse exceeds the limit. The limit depends on current alone. So a 2 A fuse will blow if the current exceeds 2 A by a suitable margin. (I think that the various standards will dictate what margin and how long, but these may be specified as "25% overcurrent will cause the fuse to blow within 2 hours".) The voltage rating refers to 1) the insulation level of the fuse holder 2) the ability of the fuse to withstand and clear an arc when it does blow. As has been pointed out, one item of concern is that an AC fuse will generally have to be derated for use on DC, because of the extra problems of interrupting a DC arc (no current zero). The same applies to switches. However, the derating is generally to about 1/3. So there is nothing wrong with using a @ A 230 V AC fuse for 2A 24 V DC service - in most cases. As always, information received over the Internet is worth what you pay for it. Cheers, Bruce.